Download Geology and offshore resources of the Solomon Islands, joint cruise

GEOLOGY
AND OFFSHORE
THE SOLOMON
RESOURCES
ISLANDS
JOINT CRUISE
REPORT
R/V S. P. LEE
1982
,
Editors
J.
G. Vedder
and
K. S. Pound
1984
U.S. Geological
345 Middlefield
~nlo
california
OF
Survey
Road
Park
94025
U.S.A.
•
'!'ABLB
OF COIfl'BHTS
Introduction to Geology and Offshore Resources
J. G. Vedder, F. I. Coulson
of the Solomon Islands
1
PART 1
Navigation for CCOP!SOPAC Cruise, Leg 3, Solomon Islands
W. C. Steele, K. L. Kinoshita
17
Submarine Topography of the Solomon Islands Region
T. E. Chase, B. A. Seekins, K. E. Lund
18
Single-Channel Seismic, Uniboom, and 3.5-kHz Systems Used in Solomon
D. L. Tiffin
Multichannel
Seismic Operations
for CCOP/SOPAC
Islands
.20
Cruise, Leg 3, Solomon Islands
D. M. Mann
22
Wide-Angle Seismic Reflection and Refraction
A. K. cooper, R. A. Wood
Data from the Solomon
Islands
24
Sampling Methods, Solomon Islands
J. B. Colwell, J. G. Vedder
30
PART 2
Geology of the central and western Solomon
F. I. Coulson, J. G. Vedder
Islands
Correlation of Rock Units in the Solomon
K. S. Pound
Islands
Regional
Islands
Offshore
J. G. Vedder,
Geology of the Solomon
F. I. Coulson
36
67
Tectonics of the Southeastern Solomon Islands:
Anticlinorium
L. W. Kroenke, J. M. Resig, P. A. Cooper
76
Formation
of
the
Malaita
Tectonic Implications of Seismicity Northeast of the Solomon Islands
P. A. cooper, L. W. Kroenke, J. M. Resig
Gravity Anomalies
L. A. Beyer
88
95
of the Solomon Islands Between 1560 and 161°E Longitude
101
ii
Crustal Structure of the Solomon Islands
seismic studies
A. K. Cooper; T. R. Bruns, R. A. Wood
Intra-Arc Basins
Deep
Structure of the central and Southern
Implications for Tectonic Origin
A. K. Cooper, M. S. Marlow, T. R. Bruns
from
Sonobuoy
112
Solomon
seismic Stratigraphy and Structure of sedimentary Basins
Islands Region
T. R. Bruns, A. K. Cooper, D. M. Mann, J. G. Vedder
Islands
Region:
127
in
the
Solomon
142
Description and Interpretation of Dredged Rocks, Solomon Islands
J. B. Colwell, J. G. Vedder
168
PrelLminary Descriptions of Gravity Cores from New Georgia Sound
J. B. Colwell
177
Recent Depositional Patterns in the central Solomons Trough of the Solomon
Islands
J. B. Colwell, D. L. Tiffin
178
Foraminiferal Stratigraphy and paleobathymetry of Dredged Rock, R/V s. P. Lee
Cruise, Solomon Islands
J. M. Resig
184
Elevation of the Pacific Province, Solomon Islands, at the Pacific and IndoAustralia Plate Boundary
J. M. Resig, L. W. Kroenke, P. A. Cooper
188
Source-Rock Evaluation of OUtcrop Samples from Guadalcanal, Malaita, and the
Florida Islands, Solomon Islands
B. Buchbinder, R. B. Halley
195
Offshore Petroleum Potential, Solomon Islands
J. G. Vedder, T. R. Bruns
202
Summary of the Geology and Offshore Resources of the Solomon Islands
J. G. Vedder
214
iii
IN'l'RODDCTION TO GEOLOGY AND OFFSHORE RB$CXlRCKS OF THE SO:t.C:K)N
J.
U. S. Geological
Institute
of Geological
Survey,
G. vedder
Menlo Park,
California,
F. r , Coulson
Sciences,
Nicker Hill,
ISLANDS
94025
Keyworth, Notts,
England
PURPOSE
ANDSCOPE
This report
is intended
to serve a dual purpose:
to review the geology
of the Solomon Islands and to present
new findings from a multinational
marine
surv-ey in 1982•.. ,A brief
summary of geography •. hiStory,
and culture
proVides
additional
background on the physical
features
of the islands
as well as their
socia-political
aspects.
A two-part format is used for the entire
report.
The first
part consists
of
general
descriptions
of
shipboard
operations,
data
acquisition
and
processing,
analytical
methods and tabular
material.
Large bathymetric
maps
that accompany the first
part are included in a separate
packet.
The second
part
contains
topical
and interpretive
papers.
Because each contribution
to
the
second part
is
intended
to
stand
alone,
some background information
reappears
throughout,
particularly
that concerning regional
tectonics.
Collectively,
these reports
are a direct
outgrowth of a program of marine
geoscience
and mineral resource
investigations
arranged under the auspices of
the ANZUs (Australia,
New Zealand, United States)
Tripartite
Agreement with
CCQP/SOPAC
(United Nations Committee for the Coordination
of Joint Prospecting
for Mineral Resources in South Pacific
Offshore Area).
The U.S. Department of
State was instrumental
in negotiating
the agreement and establishing
the support for cooperative
research.
Funding for implementation and operation
was
?rovided by the Office of Energy of the United States Agency for International
Development (AID).
Additional
funds were furnished by the Australian
government.
The various
facets
of research
were coordinated
by the office
of
CCOP/SOPAC
in FiJi.
Scientific
personnel were supplied by the U.S. Geological
Survey, Australia,
New Zealand, and other CCOP/SOPAC
member nations.
The investigations
were made aboard the U.S. Geological
Survey R/V ~
l:!§. during 23 days at sea.
The cruise began at Honiara, Guadalcanal,
Solomon
Islands on May 19, 1982, and concluded at Rabaul, Papua NewGuinea on June 11,
1982.
The principal
study area was the Central
Solomons Trough; although
additional
tracklines
were run across
adjoining
basins,
intrarc
ridges
and
flanking
trenches in order to help resolve regional structure.
Veddex , Coulson:
Introduction
1
GEOGRAPHY,
HISTORY
ANDCULTURE
Geographically,
the northwest-trending
Solomon Islands
archipelago
(Fig.
1) stretches
across more than 900 km of the southwestern Pacific
OCean between
5"S and 11"s latitude,
and 154"E and 163"E longitude.
As a political
eneiey,
however, the Solomon Islands
are spread over a considerably
different
area.
The Eastern
Outer Islands,
which are geologically
related
to the Fiji
Basin
and the
Vanuatu
(formerly
New Hebrides)
archipelago,
are
governmentally
managed by the Solomon Islands.
Bougainville,
which is geologically
part of
the Solomon Islands,
is administered
by Papua New Guinea.
Tectonically,
the
Soloman Islands
form a segment of the Melanesian island-arc
complex, which
extends
as a sinuous
belt
southeastward
from the islands
of the Admiralty
Group in Papua New Guinea through Vanuatu and Fiji to Tonga and the Kerrnadec
Islands.
Names that have been applied to this region include Outer Melanesian
Zone (Glaessner,
1950), Melanesian Re-entrant
(Coleman, 1970), and Melanesian
Borderlands
(Coleman and Packham, 1976).
The
six
large
islands
of
Choiseul,
New Georgia,
Santa
Isabel,
r.uadalcanal,
Malaita,
and San Cristobal
form two chains that are separated
by
cw Georgia Sound at the northwest
and Indispensable
Strait
at; the southeast.
Smaller
islands
within
the
double chain
include
Nggela (Florida
Is-lands-), Russell Islands,
Ulawa, and Uki.
Each of the large islands
is elo~gate and oriented
northwest except Guadalcanal,
which is broadly sigmoidal and
oriented
nearly
east-west;
and Malaita,
which has a north-northwest
trend.
The islands
of the northeastern
chain are arranged in a left-stepping
echelon
pattern.
OUtlining the northwest
end of New Georgia Sound are the Shortland
Islands,
the largest
of which are Alu, Fauro, Mono, oveu , and Cerna. Nearly
400 Jan of open ocean separates
the main islands
from the Easeern OUter
Islands,
which include
the santa Cruz group (Nendo, Vanikolo,
Utupua, Reef
Islands,
Duff Islands).
Still
farther
east are Tikopia, Anuta, and Fatutaka,
which lie
within
the North Fiji
Basin.
None of these
eastern
islands
are
described
in this
report.
The large
atoll
of Ontong Java is about 275 km
north of santa
Isabel,
and remote Sikaiana is about 200 kJn east of northern
Malaita.
The low-lying
islands
of Bellona and Rennell are about 175 km south
of GuadalcanaL
Guadalcanal
is the largest
(-6,000 kJn2 in area - 150 km long, 45 km wide)
and highest
(Mt. Makarakombuo, 2,447 m altitude)
of the Solomon Islands.
The
total
land area of the Solomons is estimated
to be about 27,750 smz ,
The
••ndigenous population
is approximately
235,000: a number that is expected to
double by the year 2000.
Most of the large islands have steep relief
and are
covered by tropical
rain forest,
except for the grasslands
along the northern
coastal
belt on GuadalcanaL
The archipelago
is outside
the usual hurricane
belt and lies within a zone of oceanic equatorial
climate,
where temperatures
usually
range
between 22" and 32"C and the humidity
averages
about
80
percent.
Southeast
tradewinds
generally
prevail
between March and December.
As much as 635 cm of rain per year falls
on the southern coast of Guadalcanal
(Hackman, 1980).
Although sea and air services
link the main islands,
limited
roads make travel
on the islands
difficult.
Beyond the
influence
of the
larger
towns, Melanesian
tribal
culture
dominates.
The earliest
inhabitants
probably moved into the archipelago
more
than 3,000 years ago.
First
contact with Europeans was in February 1568 when
a two-ship Spanish expedition
under the commandof Alvara de Mendana de Neyra
made landfall
at Santa Isabel and spent the next six months exploring
some of
the larger
islands.
An attempt at colonization
on a second voyage in 1595
Vedder,
Coulson;
Introduction
2
ended in failure
when Mendana died,
and the encampment at Graciosa
Bay on
Nendo was abandoned.
Other early explorers
were Quieros (1595-1606), Carteret
( 1767) ,
Bougainville
( 1768) ,
Surville
( 1769) ,
Short land
( 1788) ,
O'Entrecasteaux
(1791), and La Perouse (1799).
Visits
by Europeans virtually
ended until
the period
between 1840 and 1860, when the trochus
shell
and
sandalwood trade,
together
with whaling, brought ships to the islands.
The
arrival
of planters
(after
1870), blackhirders
(slavers)
between 1850-1900,
and missionaries
(after
1850) had severe
cultural
impact on the
native
populace.
Up to 1920, the population was sparse and actually
declined in certain areas as a result
of blackbirding,
introduced
diseases,
and tribal
headhunting.
Since World War II, the number of Melanesians liVing in the islands
has increased
more than twofold.
English and pisin
(pidgin) are the languages
commonly used although numerous dialects
are spoken on individual
islands.
The Solomon Islands
were a virtual
political
nonentity within the sphere
of influence
of both Germany and Great Britain
until
1893. At that time, the
British
established
the Solomon Islands
protectorate,
which included only the
southeastern
part of the group.
In 1900, santa Isabel,
Choiseul,
and Ontong
Java were transferred
by the Germans to the British
Protectorate.
In May
1942, some of the islands
were occupied by Japanese troops,
but most were
driven out after
heavy fighting
against
American forces between August 1942,
and February 1943.
Remaining Japanese soldiers
starved in the jungle or WP~'3
killed
or captured by New Zealand troops in 1944 and 1945.
After the war, a
self-governing
movement called
"Marching Rule" developed in the islands,
but
was suppressed
by the British
in 1947.
On July 7, 1978, the Solomon Islands
became an independent
member of the British
Commonwealth and elected
its oen
parliament.
REVIEW
OF GEOLOGIC
INVESTIGATIONS
Before the establishment
of the British
Solomon Islands Geological
Survey
in 1950, very little
was known about the geology of the islands.
In fact,
the
only substantive
published
account was that
by Dr. H. B. GUppy, Royal Navy
Surgeon
(GuPPY, 1887),
who visited
the
Short land Islands,
Choiseul,
the
Florida
Islands,
San Cristobal,
and seve in 1881 while serving on the hydrographiC survey vessel H.M.S. ~(Grover,
1965).
Some of the kinds of difficulties
experienced
by early
workers are
graphically
recounted
by Grover
(1965) in the following:
In
1896
the
"Albatros"
Expedition,
sponsored
by
the
Geographical
Society of Vienna, landed on Guadalcanal.
Eighteen
armed sailors
and scientists,
••••
ith four beach natives
as guides,
crossed
the Guadalcanal Plains,
and five days later
reached the
steeper
slopes of the sacred mountain 'raeuve ,
The local natives
informed the newcomers of their
belief
that if anyone climbed the
mountain of their
Great Spirit
all
their
people would die.
To
which the Austrians
replied
that they had come a long way to climb
it,
and could not go back without doing so.
On the morning of the
attempt
the party
was quietly
surrounded while breakfasting,
but
as "great pity was felt
for the white men who were about to die",
the natives
decided that the least
they could do would be to let
them fight
on a full
stomach. 1 They waited awhile for the party
to split
into two, before attacking
simultaneously
on a signal in
Vedder,
Coulson:
Introduction
3
overwhelming numbers: naked painted
warriors
ar~d
with battle
axes.
One mate lot bravely hurled his attackers
one by one over
the nearby precipice
until
he was chopped down. Others went down
beneath weight of numbers.
The attackers
were driven off.
Five
of the Europeans lost
their
lives:
their
leader,
the brilliant
Austrian
geologist,
Henrich Foullon von Norbeeck, badly wounded,
refused medical attention
so that his sailors
might have it,
and
died before midday.
And so ended the second attempt to study the
geology of the Solomons.
1
Told to
attackers,
Grover in
1950
a very old blind
by the
man.
last
survivor
of
the
Onshore Geology Programs
The Geological
Survey, which began with one geologist
and later
(1954)
increased
to three,
initiated
a program of reconnaissance
geological
mapping
t a scale of 1: 200,000 in a collaborative
effort
with the University
of
Sydney.
The first
University
of Sydney expedition
started
field
work on
Guadalcanal,
Malaita,
and Santa Isabel in 1950 and 1951 (University
of Sydney,
1956; Rickwood, 1957; and Stanton 1961) and in 1954 was followed by a joint
survey of Guadalcanal
(Pudsey-oawson and Thompson, 1958; COleman, 1960a). In
1956, reconnaissance
surveys
of both San Cristobal
(Thompson and pudse y-'
Dawson, 1958) and the Florida
Islands
(Thompson, 1958) were completed. work on
Choiseul began in 1957 (Coleman, 1960b) and in New Georgia in 1959 (Stanton
and Bell,
1965).
In 1962, after
12 years of collaborative
effort,
the early
reconnaissance
work was compiled into the first
geologic map of the Solomon
Islands
(Coleman, 1965a, Coleman et al.,
1965) at a scale of 1:1,000,000.
A
second edition
showing additional
geology,
and corrections
to the original
geology was published
in 1969.
In
1963,
after
acquiring
1: 50, OOO-scale topographic
maps of
the
protectorate,
a regional
geological
mapping program commenced, initially
on
the island
of Guadalcanal
(Dennis and Hackman, 19721 Hughes, 1977a, 1977b;
Hackman, 1979, 1980; Turner and HacJanan, in press).
This regional
program
continued at a reduced level from 1963-1975.
By the end of 1975, south Malaita (Hughes and Turner,
1976), the Eastern
Juter
Islands
(Hughes et; ai , 1981), northwest
San Cristobal
(Jeffery,
1976),
seve (Proctor and Turner,
1977), the Russell
Islands
and Mborokua (Danitofea
and Turner, 19B1), western Florida
(Taylor,
1977), and Ulawa (Danitofea,
1978)
were mapped at 1:50,000.
Commencing in 1976, the regional
mapping program
regained momentumas a British
Technical cooperation
Project,
and senior staff
were supplied
by the Institute
of Geological
Sciences,
United Kingdom.
By
1979, geologic
maps of the Shortland
Islands
and Choiseul
were finished
(Ridgway and Coulson,
in press).
Mapping of the New Georgia group was completed
in 1983 (Dunkley et aI, unpublished mapping).
Upon completion
of the New Georgia project
in 1983, approximately
66
percent
of the
Solomon Islands
was mapped geologically
at
a scale
of
1:50,000.
Santa
Isabel,
Malaita,
san Cristobal
and the eastern
Florida
Islands
remain to
be surveyed
and constitute
a significant
gap in the
knowledge of island
geology.
Of particular
importance
is the need for
detailed
mapping of santa
Isabel,
where two tectonostratigraphic
terranes
apparently
are juxtapose d.
Vedder,
Coulson:
Introduction
4
Complementing the mapping program, topical
research
has been done 'at a
number of .•••
or-ke z ej
for example,
paleontology
(Coleman, 1965bi Coleman and
McTavish, 1964; Hughes 1982), ultramafic
petrology
(Stanton and Ramsay, 1975;
Neef and Plimer,
1979), alnoites
(Nixon and Coleman, 1978; Nixon, 1980) and
isotopic
dating
(Richards
et
aI,
1966; Snelling
et
aI,
1970; Neef and
McDougall 1976).
Onshore Geophysics
Programs
The first
gravity survey, made in 1960 across the plains of north-central
Guadalcanal,
discovered
a broad gravity
high that
strikes
north .•••
ard and has
gradients
of as much as 9.5 m gal/km on its eastern
flank (Coleman and Day,
1965).
In 1961, T. S. Laudon of the university
of Wisconsin linked
the
Solomons airfields
into
the .•••
orld
gravity
survey net .•••
ork and subsequently
undertook a regional
land graVity survey (Laudon, 1968).
From 1965 to 1968, a major aerogeophysical
survey .•••
as made as a joint
venture
of the United
Nations
Developnent
Program and the Government of
citish
Solomon Islands.
In addition
to the aerogeophysics,
projects
in
photogeology,
reconnaissance
stream-sed1ment
geochemistry,
and
follOW'up
geological
and geophysical
investigations
were contracted
'at the United
Nations
to
ABEM Company of
Stockholm,
Sweden, .•••
hich
combined airborne
magnetometer, electromagnetometer,
and scintillometer
surveys (ABEM,1967).
A
total
of about 40,000
line kilometers
.•••
as flown,
and about
2,800 line
kilometers
of
inter-island
magnetometer
survey
.•••
ere
added in
order
to
establish
the regional
magnetic field.
The line spacing was mostly 400 m, in
some areas 800 m, and mean terrain
clearance
was 125 m,
A regional
interpretation
of the results
was published
by Winkler (1968).
Offshore
Geophysics
Programs
Geophysical
surveys
and drilling
in the seas adjacent
to the Solomon
Islands
have been carried
out since 1964 by a variety
of naval, institutional,
and oil company vessels.
After the discovery of oil seeps in Tonga in 1968,
the petroleum industry
began marine geophysical investigations
in the Solomons
region.
The Tongan occurrence
stimulated
hydrocarbon exploration
on shallo .•••
.narine shelves throughout the southwest Pacific,
.•••
here buried carbonate reefs
were the prospective
targets.
Surveys were made either
'at "ships of opportunity"
transiting
bet .•••
ee n southeast
Asia and the Tonga-Fiji
region,
or by
world-wide
reconnaissance
programs such as those of Mobil, Gulf, and Shell.
Commercial and CCOP/SOPACsurveys
in the Solomons after
October 1969, are
listed
in Table 1.
Selected
marine seismic data have been integrated
into
topical
syntheses of parts
of the Solomon Islands
(navenne et a L, 1977; Katz,
1980; Maung and Coulson,
1983), but no work of a comparable nature has been
published
for offshore magnetic and graVity data.
Following a short test
run of a gravity meter aboard the USS WANDANK
in
1964 (Rose et a L, 1968), a cnree-ecnen
marine gravity
survey was run by the
HMSDAMPIER
in 1965.
In 1966, the Hawaii Institute
of Geophysics launched a
three-ship
seismic-refraction
survey
in the SolomonS Sea (Woollard et a L,
1967).
The R/V CONRAD
and the R/V VEMAalso
did geophysical
work in the
region during 1966 and 1967 (Ewing et aI, 1969; Houtz et aI, 1968).
The findings of these early cruises
gave impetus to investigations
of the Ontong Java
Vedder,
Coulson:
Introduction
5
Plateau
in 1967, 1968, 1970, and 1971 (Kroenke, 1972, Fig. 13).
As part
of
the Deep sea Drilling
Program, the D/S GLOMAR
CHALLENGER
drilled
Site 64 on
the Ontong Java Plateau
in 1969 (Winterer et a L, 1971) followed by two holes
at Sites
2S7 and 288 during Leg 30 in 1972.
Site 286 in the Coral Sea also
was dri}.led on Lag 30 (Andrews et aI,
1975).
In 1971, the R/V CHAINof Woods Hole Oceanographic Institution
cruised
the WOodlark Basin region
(Leg S of Cruise No. 100).
In 1972, HMSHYDRA
conducted a detailed
bathymetric-gravity-magnetic
survey of New Georgia Sound~
as well as Bougainville
and Manning Straits.
ORSTOM
from Noumea carried
out a
seismic survey using AUSTRADEC
III IN 1975.
As part
of the
Solomon Islands
work prOgram of the
Committee for
Coordination
of Joint
Prospecting
for
Mineral Resources
in South Pacific
Offshore Areas (CCOP/SOPAC),two marine geophysical
cruises
were undertaken
in
1979 and 1981 using the vessel
MACHIAS. In 1982, two cruises
were sponsored
as part of a Tripartite
Agreement between CCOP/SOPAC,
Australia,
New zealand,
and the United States.
The R/V S. P. LEE Leg 3 cruise was concentrated
in New
Georgia
Sound (the
"Slot")
(Fig.
2),
using seismic-reflection,
refraction,
-rr av.Ltiy , and magnetics systems.
R/V KANAKEOKI investigations
were done
..
!inly
in the
Woodlark Basin
and New Georgia
regions,
where heat-flow,
seismic-reflection,
bottom-sample,
and magnetics data were collected.
REGIONAL
RELATIONS
Geomorphic Setting
The Solomon Islands
archipelago
generally
is described
as a fragmented
island
arc
that
is
situated
along
the
boundary between the
Ontong Java
Plateau/Central
Pacific
Basin and the Solomon Sea/Woodlark-Torres
Basins (Fig.
3).
An intra-arc
basin separates
the two chains of large islands
that form
the main part
of the archipelago.
This basin,
called
the central
SOlomons
Trough by Katz (19S0), is about 350 Ian long, as much as 90 Jan wide, and l,SOO
m deep in its deepest part.
To the south of the double island
chain is a well
defined
but discontinuous
linear
trench that
is more than 6,000 m deep Sa Jan
south
of san Cristobal
and more than
8,500 m deep 110 Jan southwest
of
Bougainville.
The western
part
of the trench
is known as the New Britain
~rench, the eastern
as the San Cristobal
(South Solomon) Trench. The trench is
Jbscure
south of the New Georgia group where northeast-trending
features
in
the Woodlark Basin abut the Solomon chain.
The limits
of the linear
segment
of the trench
system are clearly
defined
by sharp bends in the Solomon Sea
Basin west of Bougainville
and in the Torres Basin southeast
of San Cristobal
where the San Cristobal
(South Solomon) Trench meets the North New Hebrides
Trench.
South of the trench
system, a complex area of troughs,
basins,
and
ridges underlies
the Coral Sea.
To the north
and east
of the Solomon Islands
archipelago.
a less well
defined
trench
system includes
the Kilinailau,
North Solomons, cape Johnson,
and Vitiaz Trenches (Fig. 3).
This system is as much as 6,000 m deep near the
eastern
end of the Solomons where it forms an acute-angle
bend 50 Jan east of
the island
of Ulawa.
Flanking
this
trench system on the north is the broad,
anomalously shallow Ontong Java plateau.
Vedder,
Coulson:
Introduction
6
Tec~onic Set~ing
Complex interactions
between the Australia-India
plate
and the Pacific
plate
created
the sinuous chain of island
arcs that extend from the Bismarck
Sea of Papua ~w Guinea to Tonga and the Kermadec Islands
(Coleman and
Packham, 1976).
At present,
the Australia-India
plate is moving northward at
an absolute
rate of 7 cm per year (Minster and Jordan, 1978) and is being consumed beneath the Pacific
plate and its subsidiary
microplates
along a series
of trenches that flank the southern and western sides of the island arcs.
The
Pacific
plate has a northwestward absolute rate of motion of 10.7 cm per year
(Minster and Jordan,
1978).
The Solomon Islands
are on a northwest-trending
segment of the leading
edge of the Pacific
plate where the rate
of oblique
convergence may exceed 10 em per year (Johnson and Molnar, 1972).
presumably
a combination
of
subduction
and sinistral
shear
accomodates
this
rapid
convergence.
Intense
seismicity
characterizes
the northwestern and southeastern
parts
of the Solornon Islands
arc where the trench system is well developed (Fig. 4).
Conversely,
seismic activity
is much reduced along the central part of the arc
Jpposite
the Woodlark spreading
center
where the trench
system is weakly
expressed
bathymetrically.
Regional patterns
of seismicity
are reviewed and
interpreted
by Cooper et al
(this
volume).
Another relatively
-quiet zone
between the main island
group and the santa Cruz group is interpreted
as a
transform
(Ravenne et aI, 1977, Fig. 6-2a).
Clearly
defined Benioff zones dip steeply
northeastward
in the vicinity
of Bougainville
and the
Santa Cruz Islands
(Figs.
4A, 4B).
Near San
Cristobal,
the Benioff zone is not well defined and may be steeply
inclined.
Deep-seated earthquakes
in the range of 500-700 kIn occur beneath Bougainville
and the Santa Cruz group.
It has been suggested that these deep shocks (Fig.
3) may originate
along a detached lithospheric
slab derived
from an old,
southwest-directed
episode
of subduction
(Halunen and von Herzen,
1973;
Kroenke, 1972).
Early
studies
(Denham, 1969, 1971) indicated
that
seismicity
could be
ascribed
largely
to the
northward movement of the Australia-India
plate
coupled wi~h an east-to-west
shear along its northern edge.
Interpretation
of
focal mechanisms of large earthquakes
for the period 1963 to 1967 showed that
slip vectors wez e.vr-ouqh Ly orthogonal to the NewBritain Trench, a pattern
that
was not in accordance with simple underthrusting
of the Australia-India
plate
oenea t.h the Pacific
plate
in the Solomons sea area (Ripper, 1970).
In order
to explain
the pattern
of hypccenee r-a and the direction
of slip
vectors
in
terms of plate
tectonics,
minor plates
are believed to have been sandwiched
along the boundary zone of the two major plates,the
minor plates
deriVing
their
relative
motion from the collision.
Different
authors have identified
various microplates,
the names and locations
of which vary.
The nomenclature
of these microplates
is summarized below and in Figure 5.
1.
2.
Solomon sea plate
(Johnson and Molnar, 1972; Johnson, 1979; Curtis,
1973; Ramsey, 1982), or simply the Solomon plate
(Luyendyk et aL,
1973; Weissel et aI, 1982) lies between the Woodlark spreading ridge
in the Solomon Sea and the NewBritain
Trench.
South Bismarck plate
(Johnson and Molnar, 1972; Connelly,
1976;
Johnson,
1979; Taylor,
1979) is in the South Bismarck sea between
the
New Britain
Trench and the
Bismarck Sea seismic
lineament
(Denham, 1969; Ripper,
1975a, 1975b, 1977; Connelly,
1976; curtis
1973; Krause, 1973) and thus includes the Island of New Britain.
Vedder, Coulson:
Introduction
7
3.
4.
5.
North Bismarck plate
(Johnson and
Bismarck sea seismic lineament and
NewBritain plate
(Curtis,
1973) is
Bismarck plate.
Manus plate
(Curtis,
1973) lncludes
as the Solomon Island archipelago
Coleman (1965a).
Molnar, 1973) lies
north of the
south of the Manus Trench.
almost synonymous with the South
the North Bismarck plate as well
south of the Pacific
Province of
Crustal
structure
in the Solomons region was first
investigated
by Rose
et a L, (1968).
In the central
Solomons, crustal
thickness
was estimated
to
vary from about 27 kIn under Indispensable
Strait
to 17 Ian southwest
of
Guadalcanal.
From five other profiles
across the entire
Solomons/northern New
Hebrides region,
crustal
thickness
was estimated to vary from 9 to 29 Ian, the
higher
figures
being
associated
with New Georgia Sound and Indispensable
Strait.
Thus, the Solomon Islands
appear to have little
or no "root",
and
crustal
thickness
seems to be similar
to that of the oceanic region to the
north.
seismic refraction
studies
by Furumoto et al (1970) suggest a linear,
"lock-like
character
for the Solomons and a crustal
thickness
that varies from
. 5 to 20 Jan.
The crust .beneath the Ontong Java Plateau
is as much as 40 kIn
thick
(Furumoto et aI, 1976) but only 10 to 12 kIn in the Solomon Sea south of
the arc.
A complex set of relations
including current
underthrusting
of the
Pacific
plate
beneath
San cristobal,
the
occurrence
of
deep remnant
lithospheric
slabs
beneath
the
central
part
of
the
island
chain,
and
underthrusting
of the Pacific
plate beneath Bougainville
and San Cristobal
was
proposed by Denham (1975).
Interpretations
of crustal
structure
from sonobuoy, seismic,
and gravity
data (A. Cooper et; a L, this
volume; Bruns et a L, this
volume) indicate
that
the Central Solomons Trough contains three subsidiary
basins in which as much
as 6 Jon of Cenozoic sediment has accumulated.
The sedimentary sequence consists
of lew velocity
(1.6-2.6
km/sec) strata
in the upper part and higher
velocity
(2.8-4.4
kIn/sec) strata
in the lower part.
Volcanic rocks (5.0-5.5
kIn/sec) and lewer crustal
rocks (6.2-7.0
km/sec) underlie
the basins. On the
basis
of preliminary
interpretations
of seismic and gravity
data,
two different
arc reconstructions
are plausible.
One implies a rootless
arc in which
high-velocity
upper mantle rocks 12 to 15 Ian beneath the intra-arc
basin are
juxtaposed against
deformed oceanic crust along the faulted
northeast
side of
';he arc.
The other,
which is preferred,
suggests
(1) low-density
lithosphere
under the Woodlark Basin,
(2) a tongue of lithosphere
subducted to a depth of
150 km beneath the modern arc, and (3) shallow, possibly
relict
mantle beneath
the
intra-arc
basin,
and (4) a remnant of subducted
Pacific
lithosphere
faulted
against old arc rocks.
High heat flow is reported in the Woodlark Basin and Solomon Sea (Halunen
and von Herzen, 1973; Macdonald et al, 1973; Taylor and Exon, 1983).
However,
heat
flew values in the central
Solomons Trough are low (Taylor and Exon,
1983).
According to Coleman and Kroenke (1981) and Kroenke et al (this volwne)
the northeastern
flank of the Solomon Island arc is an abducted piece of the
oceanic Ontong Java Plateau,
and the central
large islands
are the remnants of
an early Tertiary
northeast-facing
arc that collided
with the plateau about 8
m.y. ago.
The modern southwest-facing
arc is marked by the late cenozoic
volcanic
centers
that
extend from Bougainville
to cuada LcenaL,
This postulated
reversal
in arc polarity
(Karig and Ma.mmerickx, 1972; Halunen and von
Herzen, 1973) presumably was a direct
result
of the older arc-plateau
colli-
Vedder,
Coulson:
Introduction
8
s Lon,
Coleman and Kroenke (1981) explained the absence of subduction-related
volcanism east of Guadalcanal as an effect
of cool, thick,
depleted oceanic
lithosphere
of the Ontong Java Plateau being juxtaposed against
the downgoing
Australia-India
slab and the resultant
nongeneration of arc magmas. Dunkley
(1983),
however, stated
that
volcanism is not to be expected in the area of
inactive
subduction directly
east of San Cristobal;
the descending slab probably is not in contact
with the lithosphere
of the Ontong Java Plateau at
depth; and the absence of volcanism in eastern
Guadalcanal and San Cristoba"l
does not represent
an unusual gap in spacing of volcanoes along the entire
arc.
ACKNOWLEDGEMENTS
To those energetic
and prescient
individuals
who drafted
and implemented
the Tripartite
Agreement that
led to the R/V S. P, LEE cruise
in the Solomon
Islands,
we owe our gratitude.
Without the skillful
supervision
and enterprising
initiative
of H. Gary Greene, program director,
the cruise might not
"<ave transpired
or attained
its objectives.
Captain vernon Pilgrim
and Chief Engineer Aaron Dunwald, together
with
their
respective
crews, provided the expert
seamanship and mechanical proficiency
needed for trouble-free
ship operation as well as cooperative
assistance for the shipboard scientifiC
staff.
Shore-based logistic
support was in
the
capable hands of Stephen L. Wallace and Robert J. Ramstad, who were
instrumental
in expediting
the handling and transport
of equipment, records
and personnel at ports-of-call.
Much of the credit
for the success of the cruise
goes to the research
scientists
and their
assistants
as well as to the expert technicians
who were
assigned
to the Solomons leg.
The names of these cruise
participants
are
listed
in the follOWing section of this report.
PARTICIPANTS
Listed below are the names of the
position
and shipboard responsibilities)
Solomon Islands portion of the three-leg
scientists
and technicians
(and their
who participated
in the third leg or
Tripartite
expedition:
Larry
•
A. !\eyer,
USGS, Menlo Park,
california,
U.S.A.;
Geophysicist;
navigation watch.
Donna K. Blackman, USGS, Menlo Park, California,
U.S.A.1 Physical Science
Technician;
multichannel watch.
Terry
R.
Bruns,
USGS, Menlo
Park,
California,
U.S.A.;
Marine
Geophysicist;
multichannel watch.
GUy R. Cochrane, USGS, Menlo Park, California,
U.S.A.; Physical
Science
Technician; multichannel watch.
James B. Colwell,
Bureau of Mineral Resources,
Canberra,
Australia;
Sedimentologist;
general geophysics watch.
Alan
H.
Cooper,
USGS, Menlo
Park,
California,
U.S.A.;
Marine
Geophysicist;
Chief, sonobuoy watch.
Frank I. Coulson, Chief Geologist,
Solomon Islands
Geological
Survey;
Geologist;
general geophysics watch.
vedder,
Coulson:
Introduction
9
David
J. 8ogg. USGS Marine Facility,
Menlo Park, California,
U.S.A.;
Electronics
Technician.
Kay L. Kinoshita,
USGS. Menlo Park, California,
U.S.A.; Chief; navigation
watch.
Lawrence D. KOoker. USGSMarine Facility.
Menlo Park. California.
U.S.A.;
Electronics
Technician
Loren W. Kroenke,
Research Associate,
Hawaii Institute
of Geophysics;
Geophysicist;
general geophysics watch.
Gregory Lewis.
USGS. Menlo Park,
California.
U.S.A.;
Physical
SCience
Technician;
naVigation watch.
Michael
S.
Marlow,
USGS, Menlo Park.
California,
U.S.A.;
Marine
Geophysicist;
Chief. general geophysics watch.
J. Kevin O'Toole, USGS, Marine Facility,
Menlo Park, California.
U.S.A.;
Marine Technician
Donald L.
Tiffin,
CCOP/SOPAC. Suva;
Marine Geophysicist;
Co-chief
Scientist.
John G. Vedder. USGS. Menlo Park. California,
U.S.A.; Marine Geologist;
Co-chief Scientist.
Paul A. Wenberg. USGS Marine Facility,
Menlo Park, California,
U.S.A.;
Marine Technician.
Raymond A. Wood, New Zealand Geological
Survey. rower Butt, New zealand;
Geophysicist;
general geophysics watch.
.,Other scientists
participated
by taking
lead roles
in post-cruise
studies
of
collected
data, or by contributing
maps, charts
or other information
necessary
for interpretation
of the data collected.
These lead contributors
are:
Tau
Rho Alpha - USGS, Menlo Park,
California,
U.S.A.;
physiographic
diagram.
Binyamin Buchbinder
- University
of Israel.
Jerusalem,
Israel;
onshore
hydrocarbon study
David Bukry - USGS. La Jolla,
California,
U.S.A.; calcareous
nannofossils
Thomas E. Chase - USGS. Menlo Park, California.
U.S.A.; maps and charts
Patricia
Cooper. Hawaii Institute
of Geophysics; tectonics
Robert B. Halley - USGS. Denver, Colorado,
U.S.A.; onshore hydrocarbon
study.
Katherine S. Pound - USGS, Menlo Park, California,
U.S.A.; correlation
of
rock units.
Vedder,
Coulson:
Introduction
10
REFERENCES
A.B.E.M.,
1967, Report on an airborne
geophysical
survey in the British
Solomon Islands:
Aktiebdag Elektriks
Malmletning (Stockholm) 1965-1966,
v. 1 and 2, 316 p.
Andrews, J.E.,
G.H. Packham, and others,
1975, Initial
Report~ of the Deep Sea
Drilling
Project:
U.S. Government prining
Office,
Washington, D.C.,
v.30, 753 p.
Coleman,
P.J.,
1960a,
North-central
Guadalcanal,
an
interim
geological
repcr-es
British
Solornon Islands Geological Record (1957-1958), v, " p.
4-13.
-----1960b, An introduction
to the geology of Choiseul
in the western
Solomons, 1957;
British
Solomon Islands Geological Record (1957-1958),
v, 1, p , 16-26.
-----1965a, Stratigraphical
and structural
notes on the British
Solomon
Islands
with reference
to the first
geological
map:
British
Solornon
Islands Geological Record (1959-1962), v. 2, Report no. 29, p. 17-31.
-----1965b, Tertiary
assemblages
of larger
Foraminifera
in the Solomon
Islands
and New Hebrides
Archipelago:
Contributions
to the Annual
Report, NewHebrides Geological Survey, p. 48-51.
-----1970, Geology of the Solomon and NewHebrides Islands,
as part of the
Melanesian re-entrant,
Southwest Pacific:
Pacific
Science,
v, 24, p.
289-314.
-----and A.A. Day,
1965, Petroleum
possibilities
and marked gravity
anomalies
in
north-central
Guadacanal:
British
Solomon Islands
Geological Record (1959-1962), v, 2, p , 112-119.
-----and L.W. Kroenke, 1981, Subduction without volcanism in the Solomon
Islands arc: Geo-Marine Letters,
v, 1, p , 129-134.
-----and R.A. McTavish,
1964, An association
of larger
and planktonic
Foraminifera
in
single
s ampLes
from
middle
Miocene
sediments,
Guadalcanal,
Solomon Islands,
southwest Pacific:
Journal of the Royal
Society of Western Australia,
v. 47, p. 13-24.
-----and G.H. Packham, 1976, The Melanesian Borderlands and India-Pacific
Plates'
boundary:
Earth-Science
Reviews, v. 12, p. 197-233.
-----et a L, 1965, A first
geological
map of the British
Solomon Islands,
1962, .i£.
Reports
on
the
geology,
mineral
resources,
petroleum
possibilities,
volcanoes,
and seismiscity
in the Solomon Islands:
Record
of the Geological
Survey of the British
Solomon Islands
(1959-1962), -r,
2, Report no. 28, p. 16-17.
Connelly,
J. B.,
1976, Tectonic
development of the Bismarck Sea based on
gravity
and magnetic
rnodeling:
Geophysical
Journal
of
the
Royal
Astronomical Society,
v. 46, p. 23-40.
Curtis,
J.w.,
1973, Plate
tectonics
and the Papua New Guinea-Solomon Islands
region:
Journal of the Geological Society of Australia,v.
20, pt. 1, p.
21-36.
Danitofea,
S., 1978, The Geology of Ulawa Island:
Solomon Islands Geological
Survey Bulletin
No.4
(unpublished).
-----and C.C. Turner,
1981, The geology of the
Russell
Islands
and
Mborokua:
Solomon
Islands
Geological
Survey
Bulletin
No.
12
(unpublished) •
Denham, D.,
1969, Distribution
of earthquakes
in the New Guinea-Solomon
Islands region:
Journal of Geophysical Research, v. 74, p. 4290-4299.
Vedder, Coulson:
Introduction
11
------
1971, seismicity
and tectonics
of New Guinea and the Solomon Islands:
Recent crustal
movements:
Royal Society N. 2. Bulletin
No.9,
p. 31-38.
-----1975,
Distribution
of
underthrust
lithosphere
slabs
and focal
mechanisms--Papua Ne••••Guinea and Solomon Islands region (aba.):
Bulletin
of the Australian
Society of Exploration
Geophysicists,
v. 6, p. 78-79.
Dennis, R.A., and B.D. Hackman, 1972, Geological map of Cape Esperance-Ndoma,
Guadalcanal:
Geological
Survey, Honiara, Guadalcanal,
Solomon Islands,
1:50,000.
DUnkley, P.N.,
1983, volcanism
and the evolution
of the ensimatic
Solomon
Islands
Are, in D. Shinozura
and I.
Yokoyama, ede , , Arc volcanism:
physics and tectonics:
Terrapub, Tokyo, p. 225-241.
Ewing,
M.,
R.
Houtz,
and
J.
Ewing,
1969,
South
Pacific
sediment
distribution:
Journal
of Geophysical Research, v. 74, p. 2477-2493.
Furumoto, A.S.,
D.M. Hussong, J.F.
Campbell, G.H. SUtton, A. Malahoff,
J.e.
Rose, and G.P. Wollard,
1970, Crustal
and upper mantle structure
of the
Solomon Islands
as revealed
by seismic
refraction
survey of Nov-Dec
1966: Pacific
Science,
v, 24, p. 315-332.
-----J.P. Webb, M.E. Odegard, and D.M. Hussong, 1976, Seismic studies on the
Ontong Java Plateau,
1970:
Tectonophysics,
v. 34, p. 41-90 •
.•.
laessner,
M.F.,
1950,
Geotectonic
position
of
sew Guinea:
American
Association
of Petroleum Geologists
Bulletin,
v. 34, p. 856-881.
Grover,
J.C.,
1965,
A brief
history
of
geological
and
geophysical
investigations
in the British
Solomon Islands
1881-1961:
British
Solomon
Islands Geological
Record (1959-1962), v. 2, Report no. 27, p. 9-15.
GuPPY, H.B., 1887, The Solomon Is lands,
their
geology, general features,
and
suitability
for
colonization:
S••••
ann,
Sonnerscheim,
Low rey and Co.,
London, 152 p.
Hackman, B.D.,
1979, The geology of the Honiara area,
Guadalcanal:
Solomon
Islands Geological
Survey Bulletin,
no. 3, 40 p.
-----1980, The geology of Guadalcanal,
Solomon Islands:
Overseas Memoirs of
the Institute
of Geological
Science,
Her Majesty'a
Stationery
Office,
London, no. 6, 115 p ,
Halunen, A.J.,
and von Herzen, R.P.,
1973, Heat flow in the western equatorial
Pacific
Ocean:
Journal of Geophysical Research, v. 78, p. 5195-5208.
Houtz, R., J. Ewing, and X. re Pichon,
1968, Velocity
of deep sea sediments
from Sonobuoy data:
Journal
of Geophysical Research,
v, 73, p. 26152641.
~ughes,
G.w.,
1977a, The geology of the IA.ir1'ggaBasin area,
Guadalcanal:
Solomon Islands Geological
Survey Bulletin
No.6
(unpublished).
-----1977b, The geology and foraminiferal
micropaleontology
of the Lungga
and Itina
Areas, western Guadalcanal,
Solomon Islands:
Unpublished Ph.D.
Thesis,
University
College of Wales, Aberystwyth.
-----1982, Stratigraphic
correlation
between sedimentary basins of the ESCAP
region,
Volume VIII,
Solomon Islands,
ESCAPAtlas of Stratigraphy
III:
Mineral Resources Development Series,
United Nations,
New York, no. 48,
p.115-130.
-----P.M. Craig,
and R.A. Dennis,
1981, The geology of the Eastern OUter
Islands:
Solomon Islands
Geological
Survey Bulletin
No.4,
108 p.
-----G.W., and C.C. Turner,
1976, Geology of southern
Malaita:
Solomon
Islands Geological
Survey Bulletin
No.2,
80 p.
Jeffery,
D.H., 1976, The geology of north ••••
estern
San Cristobal,
Uki Ni Masi
and Pia,
and the
Three
Sisters:
Solomon Islands
Geological
Survey
unpublished
Bulletin
No. 10.
Vedder,
Coulson:
Introduction
12
Johnson,
R.W., 1979, Geotectonics
and volcanism in Papua New Guinea; a review
of the late Cainozoic:
Bureau of Mineral Resources Journal of Australian
Geology and Geophysics,
v, 4, p. 181-207.
Johnson, T., and P. Molnar, 1972, Focal mechanisms and plate
tectonics
of the
southwet Pacific:
Journal of Geophysical Research, v. 77, p. 5000-5032.
Karig, D.E., and J. Mammerickx, 1972, Tectonic framework of the New Hebrides
island arc:
Marine Geology, v. 12, p. 187-205.
Katz,
H. R.,
1980,
Basin development
in the
Solomon Islands
and their
petroleum potential;
CCOP/SOPAC
Technical Bulletin
No.3,
p. 59-75.
Krause,
D.C.,
1973, Crustal
plates
of the Bismarck and Solomon seas,
in R.
Fraser,
compiler,
Oceanography of the
South Pacific:
New zealand
National Commission for UNESCO,wellington,
p. 271-280.
Kroenke, L.W., 1972, Geology of the Ontong Java Plateau:
Hawaii Institute
of
GeophySics, Report no. HIG-72-5, University
of Hawaii, 119 p.
Laudon, T.S.,
1968, Land gravity
survey of the Solomon and Bismarck Islands,
in The crust and upper mantle of the Pacific area:
American Geophysical
Union Mongram No. 12.
Luyendyk, B.P.,
K.C. MacDonald, and W.B. Bryan, 1973, Rifting
history
of the
Woodlark Basin in the Southwest Pacific:
Bulletin
of the Geological
Society of America, v. 84, p. 1125-1134.
Macdonald, K.C., B.P. Luyendyk, and R. Von Herzen, 1973, Heat flow and plate
boundaries
in Melanesia:
Journal of Geophysical Research,
v. 78(14), p.
2537-2546.
Maung, T. v., and F.I. Coulson, 1983, Assessment of petroleum potential
of the
central
Solomons Basin:
CCOP/SOPAC
Technical Report No. 26, 68 p.
Minster,
J.B.,
and T.H. Jordan,
1978, Present
day plate
motions:
Journal of
Geophysical Research,
v. 83, p. 5331-5345.
Neef,
G.,
1978, A convervent
subduction
model for
the
Solomon Islands;
Bulletin
of the Australian
Society of Exploration
Geophysicists,
v. 9, p.
99-103.
-----and I. McDougall, 1976, potassium-argon
ages on rocks from Small Nggela
Island,
British
Solomon Islands,
s.w. Pacific:
Pacific
Geology, v, 11,
p. 81-96.
-----and I.R.
Plimer,
1979, Ophiolite
complexes on Small Nggela Island,
Solomon Islands;
summary: Geological
Society of America Bulletin,
part
1, v. 90, p , 136-138.
Nixon, P.H.,
1980, Kimberlites
in the southwest Pacific:
Nature,
v; 287, p.
718-720.
----and P.J. Coleman, 1978, Garnet-bearing
Iherzolites
and discrete
nodule
suites
from the Malaita alnoite,
Solomon Islands,
and their
bearing on
the
nature
and origin
of the Ontong-Java
plateau:
Bulletin
of the
Australian
Society of Exploration
Geophysicists,
v. 9, no. 3, p. 103-107.
Proctor,
w.O., and C.C. Turner,
1977, The geology of save Island:
Solomon
Islands Geological
Survey Bulletin
No. 11, 44 p.
Pudsey-oawson,
P.A.,
and R.B. 'nlompson, 1958, The detailed
geological
survey
of western Guadalcanal,
1954:
Geological
Survey of the British
Solomon
Islands
Memoir No.2,
p. 43-56.
Ravenne, C., C.E. de Brain,
and F. Aubertin,
1977, Structure
and history
of
the Solomon-New Ireland
region, l:.!:.. International
symposium on geodynamics
in
the
SW Pacific,
lew caledonia,
August-September
1976:
Editions
Technip, Paris,
p , 37-50.
Vedder,
Coulson:
Introduction
13
Richards,
J.R.,
A.W. Webb, J.A. Cooper, and P.J.
Coleman, 1966, Potassiumargon measurements of the ages of basal schists
in the British
Solomon
Islands:
Nature, v, 211, p. 1251-1252.
Rickwood, F.K.,
1957,
Geology of
the
Island
of
Malaita
.i!!.. Geological
Reconnaissance
of parts
of the central
islands
of the British
Solomon
Islands Protectorate:
Colonial Geology and Mineral Resources,
v. 6, no.
3, p. 300-306.
Ridgway, J.,
and F.I.E.
Coulson, in press,
The Geology of Choiseul and the
Shortland
Islands,
Solomon Islands:
Solomon Islands
Geological
Survey
Division Bulletin
No. 16.
Ripper,
1.0.,
1975a, Some earthquake
focal
eechenfsme
in the New GuineaSolomon Islands
region,
1963-1968:
Bureau
of
Mineral
Resources
Australia,
Report 178.
------
1975b, Seismicity
and earthquake
focal eechani.ees
in the New GuineaSolomon Islands
region
(extended abstract),
Bulletin
of the Australian
Society of Exploration
Geophysicists,
v. 6, p. 80-81.
-----1977, Some earthquake
focal
mechanisms in the New GUinea/Solomon
Islands
region,
1969-1971:
Report of the Bureau of Mineral Resources,
Geology and Geophysics, Australia,
no. 192.
Rose, J.C.,
Wollard, G.P., and Malahoff, A., 196B, Marine gravity an~ magnetic
studies
in the Solomon Islands,
in 'Ihe crust
and upper mantle of the
Pacific
area:
Monogram of the A;;rican
Geophysical Union, no. 12, p.
379-410.
Snelling,
N.J.,
I.H. Ingram, and K.P. Chan, 1970, KlAr age determinations
on
samples from the British
Solomon Islands
Protectorate:
Institute
of
Geological
SCiences,
Geochemistry Division,
Isotope Geology Unit Report,
no. 17-14.
Stanton,
R.L.,
1961, Explanatory
notes to accompany a first
geological
map of
Santa Isabel,
British
Solornon Islands Protectorate:
Overseas Geology and
Mineral Resources,
London, v. B, no. 2, p. 127-149.
-----and J.D.
Bell,
1965, New Georgia Group, a preliminary
geological
statement:
British
Solomon Islands Geological
Record (1959-1962), v, 2,
Report no. 31, p. 35-36.
-----and W.R.H. Ramsay, 1975, Ophiolite
basement ccep Lex in a fractured
island
chain,
Santa Isabel,
British
Solomon Islands:
Bulletin
of the
Australian
Society of Exploration
Geophysicists,
v. 6, no. 2/3, p. 61-65.
Taylor, B., 1979, Bism.q.rck Sea: evolution
of a backarc basin:
Geology, v. 7,
p. 171-174.
-----and N.F. Exon, 19B3, 1982 R,!V~
Keoki cruise
in the WoodlarkSolomons region
(abs.):
Basic geo-scientific
marine research
requr re o
for assessment
of minerals
and hydrocarbonsa
in the South Pacific,
A
workshop, Suva, Fiji,
October, 1983.
----------1984, An investigation
of ridge subduction
in the WOodlark-Solomons region:
introduction
and background,
.i!!.. N.F. Exon and B.R.
Taylor,
compilers,
Seafloor
spreading,
ridge subduction,
volcanism and
sedimentation
in the offshore
WOodlark-Solomons region
and Tripartite
cruise report
for ~
Keoki cruise 82-03-16, Leg 4, CCOP/SOPAC
Technical
Report no. 34, p , 1-42.
Taylor,
G.R.,
1977, Florida
Islands
Geological
Map Sheet FL 1:
Geological
Survey, Honiara, Guadalcanal,
Solomon Islands,
1:50,000.
Vedder,
Coulson:
Introduction
14
Thompson, R.B.M., 1958, The geology of the Florida Group, 1956, in The Solomon
Islands-geological
exploration
and research,
1953-1956:
Memoir of the
Geological Survey of the British
Solomon Islands,
no. 2, p. 97-101.
-----and P.A. Pudsey-Dawson, 1958, The geology of eastern
San Cristobal:
Memoir of the Geological Survey of the British
Solomon Islands
1955-1956,
v» 2, p. 90-95.
Turner,
C.C., and B.D. Hackman, in press,
The geology of the Beaufort Bay
area, Guadalcanal:
Solomon Islands Geological Survey Bulletin
No.9.
University
of Sydney, 1956, Geological reconnaissance
of parts
of the central
islands
of the British
Solomon Islands
Protectorate:
Colonial Geology
and Mineral Resources, v.6, p. 267-306.
weisse I, J.K.,
B. Taylor,
and G.D. Karner, 1982, the opening of the Woodlark
Basin, subduction of the Woodlark spreading system, and evolution
of the
northern
Melanesia since the mid-pliocene
time:
Tectonophysics,
v, 87,
p. 253-277.
Winkler,
H.A., 1968, Regional
geophysical
structure
of the British
Solomon
Islands
Protectorate:
UN Special
Development
Programme Aerial
Geophysical Survey Project Report (1965-1968), Solomon Islands Geological
survey Report A12•
• nterer,
E.L., W.R. Reidel~ and others,
1971, Initial
reports
of the Deep sea
Drilling
Project,
v.7.,
U.S. Government Printing
Office,
Washington, D.C.
Wollard,
G.P.,
and others,
1967, Crusie
report
on 1966 seismic
refraction
expedition
to the Solomon Sea:
Hawaii Institute
of Geophysics, Report
HIG-67-3, 31 p.
Vedder,
Coulson:
Introduction
15
TABLE 1
SUMMARY
OF OIL EXf'LORATlQol IN rHE
DATE
SURVEYEDSY
OCt.oberNovembel:'
1969
M"qell"n ~tl:'olewn
{AulSt.l:'alial Lt.d.
SOLOfl:lN
ISLANDS (!"ROM
SURVEY'LOCATlQol
Shol:'t.land Shelf
Bouqainville
Sl:.rait.
Manning St>;ait.
Indillpen •• ble strait.
Reqion bet.veen ~lai~a
Guacialcanal
MA~G ANDCOIJt,.StW, 198J)
L!NGTH
769 kill
O£SCRIPTIOl
seismic ellrvey
(ail:'qun "nd ap"l:'ke>;)
di.cont.inllolls
t.raver.ell
and
19651·1970
Bataafse Int.el:'nat.ional.
Pet.roleum ",aat.schappij
N .v ,
Short.lands
Choiselll
sant.a babel
Malait.a
Guadalcanal
Field examination of
elCpolled.edilllentary
l:'ocks
HIIy
1971
Shell Int.e>;nat.ional
Pet.roleum
Short.lands, Nev Geol:'gia
Sound, Malait.a
Indispensable
strait.,
San Cristobal,
and
RiIInnell I.land
Oct<:.
1971
W=stern Geophysical
and Teledyne Explorat.ion
International,
Inc.
for SOl:lthern Pacific
Petroleum
OCtoberNovember
1971
2,764 kill
Mult.ichannel .eismic
reflection
Maillet.OlIlet.er, Gravimeter
Manning St.rait.
141 kill
12-fold diqital
l:'eflection
Teledyne Exploration
Company
Shortlands
aouqainville
St.l:'ait
Oloiselll,
Malai t.a
santa Isabal,
Ulawa
Florida Islands
N. eoalSt Guadalcanal
2,458 Ian
seismic
June
1972
Mobil Oil
Corporation
N. eoast $an Crietobal,
Malaita, NE eoast Guadalc:anal
NWand SWof Russell Islands
Manninq Stl:'ai t
N. eoast of OIoillllul
Bougainv111e strait
South of New Georgia
4,447 Ian
Multichannel SlIismic retlettion
SOnobl:loyseillmic refraction
(7)
197·
""stral.ian
Gulf
Oil COqpany
Shortlands
Shelf
Bouqainville
Strait
N. coast Oloiseul
New Georgia Sound
S. coast Santa Isabel
RiIIqionbet ••.•en Malait.a,
Guacialcanal, and san Cristobal
2,480 kill
S1nqle and multichannel
seismic
reflect.ion
Sonobuoy refraction
profile
(1l
Mailletometer, Gravimeter
December
1978
Western Geophysical
for
P.cifi~et'gy
and
Minerals,
Ltd.
;\;rea between Guadalcanal,
seve , and the
Florida Islands
100 kill
Multichannel
1979
CCOP/SOPAC
New Geor••ia
4,064 lcm
Sinqle-channel
Sound
,lIiSlll.ic
survey
(sparker)
seismic
seismic
reflee-tion
refleetion
t.rackline
spacing
15-25 Ian
-----------------------",-yJune
1982
CCOP/SOPAC
FVVs. P. LEE Leg' J
Shortland
blanda
New Georgi. Sound
Indispensable
Strait.
Iron BottOlll Sound
Mala1t.a fold belt
Ne" Georgia-Guadale-anal
tor •• ee area
5,500 lcm
track line
lIpacinq
5·10 lcm
COPMultichannel
(24 fold)
H1gh-resolution.uniboom
seism.i.c
reflection
3.5 kHz hiqh resolution
seismic
reflection
12 kHz bat.hymet.ric profiles
Wlde-anqle 8ei~c
reflection/
l:'efract1on
SOnobouy profiles
(J6)
Maqnetometer, Gravimet.er
PART 1
This
is
Cruise Report.
the
first of
two parts of
the Joint
The purpose of the study, previous
geologic investigations, regional geology, and other
topics are summarized in the Introduction.
The
remainder
of
Part
1 consists
of
general
descriptions of shipboard operations, data acquisition
and processing, analytical methods and tabular data.
Large
bathymetric
diagrams
and
maps
trackline
and
profiles, physiographic
plots
are
included
in
an
accompanying map packet.
16
NAVIGATION FOR CCOP/SOPAC CRUl:SE, LEG 3,
SOLOMAN'ISLANDS
w. C. Steele. L
U.S. Geological
L. nnoshita
Survey, Menlo Park, California
94025
Navigational
control
of the survey was by satellite
fixes.
These fixes
averaged about one every two hours and had an estimated accuracy of 8400 m,
All
fixes
were integrated
with dead-reckoning
and radionavigation
(bridge
radar) •
The computer processing
was done both aboard ship and on shore.
)igi tal navigation
data were recorded on magnetic tape and were processed on
shore by a multi-purpose
Honeywell 68/80 MULTICS
system.
After completion of the cruise,
the digital
navigational
data were demultiplexed
to extract
satellite
data, naVigation shot points,
operator
requests
and comments, and position
updates.
The position
updates
consisted
of
satellite-induced
position
corrections
and position
corrections
based on
radionavigational
information.
Allowable updates were restricted
to satellites
whose azimuths were between 15° and 75° and that had fewer than 4 iterations and a latitudinal
standard deviation
of less than .005 and a longitUdinal standard
deviation
of less than .010.
Navigation shot points
were adjusted assuming linear
deviations
with time between adjacent position
updates
and not on satellites
directly.
Deviant navigational
shot points
based on
noncruise-specific
criteria
were then edited
using an interactive
graphics
editor.
These data were plotted
for review to determine deletions,
omissions,
and modifications
based on cruise-specific
criteria.
Appropriate
corrections
were made, and naVigation
and satellite
fixes
were statistically
compared.
The mean discrepancy
between navigational
and satellite
fixes is 360 m. with a
standard deviation
of 569.
A track chart was generated,
a reduced and simplified version of which is shown in Fig. 1.
The long-term
storage
media for the naVigational
data consist
of:
(1)
microfilm of the Shipboard navigational
computer output;
(2) paper copies of
naVigational
logs prepared
during the cruise;
OJ magnetic tapes
(digital
data) of naVigation
and operator
commands during cruise;
(4) magnetic tapes
(digital
data)
of
processed
data.
These data
are
kept
in
permanent
U.S.Geological
Survey library
archives.
Steele,
Kinoshita:
Navigation
17
SUBMARINE TOPOGRAPHY OF THE SOI..CKJH ISLAHDS
REGION
T.I. Cha.se, B.A. seekins.
K:.B. ~d
U.S. Geological
Survey, Menlo Park, california
94025
INTRODUCTION
Included in the map packet that
accompanies this
report
is a series
of
maps, physio.graphic diagrams, and bathymetric
profiles
of the Solomon Islands
and adJacent regions.
Sources of information,
including that collected
during
'the 1982 cruise
of the R/V S.P. Lee (Cruise L7-82-SP), are noted on the maps
and diagrams.
Acquisition
and pr~ssing
of navigation
and trackline
information
for this
cruise
is described
by Kinoshita
and Steele
(this
volume).
Additional
information
regarding
procedures
used in preparing
the maps, diagrams, and profiles
listed
below can be obtained by contacting
the authors.
SUMMARY
OF MAPS,DIAGRAMS,
ANDPROFILES
(see map packet)
Figure
1.
Index map of submarine topographic maps and companion physiographic
diagrams
in
the
Solomon Islands
and adJacent
regions
of the
southwestern
Pacific.
Figure
2.
Submarine topography of the Vanuatu and southeastern
Solomon Islands region.
Contour interval
is 200m (based on depth-corrected
acoustic
soundings).
Detailed
topography
of the
southeastern
Solomon Islands region is shown in Figure 7.
Figure
3.
Submarine physiography of the Vanuatu and southeastern
Solomons region.
This diagram is scale-matched
to the topographic
map of the
same area (Figure 2) to assist
the user in rapidly
perceiving
the
general configuration
of the sea floor.
Figure
4.
Submarine topography of the Solomon Islands-papua
New Guinea region.
Contour interval
200m (based on depth corrected
accoustic
soundings).
Detailed
topography of the region is shown in Figures
10andl1.
Figure
5.
Submarine physiography
of the Solomon Islands-Papua
NewGuinea region.
This diagram is scale-matched
to the topographic
map of the
same area (Figure 4) to assist
the user in rapidly
perceiving
the
general configuration
of the sea floor.
Chase,
Seekins,
Lund:
Topography
18
Figure 6.
Detailed submarine topography of the southeastern Solomon Islands
region.
Contour interval is 100m (based on depth-corrected
acoustic soundings).
Figure
Detailed submarine physiographic diagram of the southeastern
Solomon Islands region. Companion sheet for Figure 6.
7.
Figure 8.
Detailed submarine topography of the northwestern Solomon Islands
region.
Contour interval is 100m (based on depth-correctea
acoustic soundings).
Figure
9.
Detailed submarine physiographic diagram of the northwestern
Solomon Islands region. companion sheet for Figure 8.
Figure
10.
Detailed submarine topography of the southeastern New Guinea
region.
Contour interval is 100m (based on depth-corrected
acoustic soundings).
igure 11.
Detailed submarine physiographic diagram of the southeastern Papua
New Guinea region. Companion sheet for Figure 10.
Figure 12.
Trackline map of R/V .g. ~ cruises L6-82-SP and L7-82-SP in the
Vanuatu and southeastern Solomon Islands region.
Figure 13.
Trackline map of the R/V ~
~
cruise L7-82-SP in the northwestern Solomon Islands-Papua New Guinea region.
Figure 14.
Detailed trackline map of the R/V ~
southeastern Solomon Islands region.
Figure 15.
Detailed trackline map of the R/V S.P. Lee cruise L7-82-SP in the
northwestern Solomon Islands region.
~
cruise L7-82-SP in the
Figures 16-20. Bathymetric profiles of the Solomon Islands-papua New Guinea
region made from enlargements of microfilms of original bathymetric
recordings. See Figures 12-15 for locations of profiles.
Chase,
Seekini,
Lund:
Topography
19
SIlGLB-cHAHHEL
SBISMIC, 'ORIEKX)M. AIm 3.S-kHz
USED IN SOLOfI)N ISLANDS
D. L. Tiffin
U.N. ESCAP, CCOP/SOPAC,c/o Mineral Resources
SYSTEMS
Dept.,
Suva,
Fiji
INTRODUCTION
Single-channel
airgun
(SCAG)equipment, Uniboom, and 3.S-kHz systems were
used on the USGS R/V S. P. Lee during
the Solomon Island
phase of the
Tripartite
program.
These are only three of the wide selection
of geophysical
, systems
carried
on
the
ship.
All
geophysical
data
were
acquired
simultaneously.
SINGLE-eHANNEL
AIRGUNSYSTEM
The SCAGseismic-reflection
system utilized
the acoustic
impulse generated by the rrultichannel
seismic airgun array as the sound source.
The tuned
array
consisted
of five
Bolt air
guns totaling
1,326 in.3,
towed from the
after
deck of the
vessel.
Reflections
from bottom and sub-bottom
were
received
on channel
24 of the multichannel
hydrophone streamer
(the channel
closest
to the ship),
which was used as the SCAGreceiving
array.
Bottom and
sub-bottom
reflection
signals
from the streamer
were recorded
on a Globe
Universal
Sciences
(GUS) multichannel
seismic
recorder,
and, after
a slight
delay
read off the tape by a read head before being amplified
by a Geospace
III
seismic arrplifier.
After
passing
through a Krohnhite
band-pass
filter,
the signals
were displayed
on two Raytheon Line Scan Graphic 19-in. dry-paper
recorders.
Direct water arrivals
were usually
muted in the GUSrecorder.
The
SCAGsystem pz-ov.Lde d the necessary
monitoring
of the multichannel
operation
and of the GUS recording
equipment.
Because the multichannel
seismic
(MCS) array was fired
at a rate of 16 19, the SCAGgraphic recorders
were operated in a START-STOP
mode in which the
first
few seconds of data follOWing the beginning of the shot were recorded,
after
which the recorders
stopped to await the next shot.
The sweep rate
ordinarily
was 4 sec, but along 13 lines in the western part of the area, a 5sec sweep was recorded.
Sweep delays were adjusted
to de-emphasize the water
column when necessary,
and to record
the maximum sub-bottom
penetration,
usually to and beyond the first
bottom multiple.
Band-pass filters
were normally
in the range of 15 to 101) Hz, rot sweep rates
and filter
settings
were
different
for each recorder.
Vertical
exaggeration
on the principal
recorder
was about 5.5 for 4-sec sweep times, and about 4.5 for 5-sec sweeps.
The large
sound
source
resulted
in excellent
penetration
and good
resolution,
until
the time of arrival
of the bottom multiple
which effectively
I
Tiffin~
Single
Channel
20
obscured most SCAGarrivals
beyond that time.
Often, almost 2 sec of travel
time in the sub-bottom was clearly
recorded.
The weather during the survey
usually
was fair and the seas calm, aiding in the recording
of low-noise data.
The single-channel
airgun records were photographed aboard ship using a
Polaroid
camera mounT.edover one of the recorders,
and overlapping
photographs
were trimmed and taped together
to make a continuous
record of each seismic
line.
These spliced
photographs
proved extremely
useful
for
reference,
discussion,
and planning,
particularly
in selecting
bottom sampling sites.
UNIBOOM
HIGH-RESOLUTION
SYSTEM
The Uniboom system
consisted
of four
hull-mounted
EG&Gtransducers
mounted back-to-back,
with a total
energy of 1,200 joules.
The return signals
were received by two side-by-side
short hydrophone streamers
(about 15 m long)
towed just be Lew the water surface beside the Ship.
The echoes were processed
through a Raytheon Correlation
Echo Sounding Processor
(CESP II)
amplifier,
filtered
outside
the pass-band
of 250 - 1,500 Hz, and recorded on a Raytheon.
t a-c n, graphic
recorder.
Sweep rate
was 0.5 second, firing
was programmed
according
to depth, and a delay was adjusted
to maintain the bottom and subbottom echoes w;lthin the 0.5 - second gat.e~ Exagger"ation on the records is
apprOximately
10X.
The Uniboom system consistently
gave extremely clear
and highly detailed
records on the sub-bottom sediments to depths in excess of 100 m below the sea
floor,
even in water depths of 1.7 Jan in New Georgia Sound.
The records
reveal
substantial
and significant
infomation
on the Holocene near-surface
sedimentary
structures,
discussed
by Colwell and Tiffin
(this
volume), and
Resig, Kroenke, and Cooper (this volume).
3.S-kHz SYSTEM
The 3.S-kHz system consisted
of four hull-mounted
transducers,
all
of
which transmit
the 3.S-kHz acoustic
wave, and receive
the returning
echoes.
The returns
were processed
and amplified
with a Raytheon CESP II correlation
processor
using 25-ms or 100-ms delays on the cor-r-e Let or-,
The output was
displayed
on a Raytheon 19-in. recorder
using a 1-sec sweep.
The 3.S-kHz records show details
of depositional
features
in the shallow
sub-bottom,
but penetration
is limited,
usually
about 10 m or less,
and on
many lines
does not provide
mrcb more information
than
the
12-kHz echo
sounder.
After completion of the survey, the SCAGand high-resolution
data records
were photographed
from the t s-dn,
graphic record onto continuous
35-mm film
strips.
Copies were made on vellum and paper.
Fifty-two
lines
were recorded
on the SCAGand high-resolution
seismic
systems,
as well as througl1-turns
between nultichannel
lines
which were not
recorded
on the GUS system.
A total
of epproximaee Iy 3,500 krn of data was
obtained
in the Solomon Islands
region.
Tiffin:
Single
Channel
21
JlDLTICBANHEL SBISMI.C OPERATIONS
FOR CCOP/SOPAC CRUISE. LEG 3. SQLOtI)N ISLANDS
D. M. Mann
U.S. Geological
Survey,
Menlo Park,
California
94025
EQUIPMENT
ANDDATAFORMATS
Multichannel
seismic-reflection
data
were
collected
on
the
]eological
Survey R/V S.P. lee using a Globe Universal
Sciences
Inc.
model 4200 seismic
recording
system,
a Seismic
Engineering
(SEI)
acoustic
array,
and a seismic source of five; Bolt airguns.
U.S.
(GUS)
towed
The acoustic
array by SEI consists
of 48 active
sections,
each 50 m long
and each containing
30 Teleeyne MD-Shydrophones; two 66-m stretch
sections
at
the head end, and one at the tail
end; ten bird transducer
sections
spaced at
regular
intervals
along the streamer;
and a 476-ft.
heavy lead!n
cable to
isolate
the streamer
from ship noise.
The depth of the streamer
sections
is
determined
by transducers
in each of the variable
wing birds that are placed
on the bird sections.
A control
panel in the multichannel
lab allows each
bird to be indiVidually
controlled
to a desired depth.
The data enters
the multichannel
lab through a deck leader,
and into a
SEI-DSSV signal
conditioner/line
equalizer.
In this unit the signal
strength
is balanced to a set level to correct
for the offset
difference
in each section
of the streamer.
The 48 sections
are combined into 24 channels by a
patch panel before the data enters the GUSrecording system.
Data enters
the GUS recording
system through a bank of gain ranging
amplifiers,
high (110Hz)
and. low (5 Hz) cut filters,
and an A-D (analog to
digital)
converter.
The output is sampled every 2 ms by the MCU(Main Control
Unit).
Data is then recorded on AMPEX-DMA
high speed tape transports
in BDDR
format,
14-track,
4000-ft reels at 8000 bpi.
Each track is written
separately
with the odd tracks
in the forward direction,
and the even tracks
in the
opposite
direction.
A complete data record consists
of a 256-bit preamble, 50
bit
sync check,
data words of 24 bits
each plus a sign bit,
and a 256-bit
postamble.
Quality
of the data •.•.
as monitored using a Raytheon LSR recorder
for a near-field
single-channel
playout,
and an SEI wiggle trace
camera for
all 24 traces.
Seismic energy •.•.
as provided by five Belt airguns of the following
sizes
(in cubic inches):
309, 466, 194, 194 and 148.
Total volume was 1311 in3 at
an air pressure
of 1700 to 2000 psi.
The airguns were timed to within 0.5
milliseconds
of each other using a Litton
Industries
model LRS-100 gun timing
computer and an oscilloscope
to monitor the signals
from sensors
in the firing
solenoids.
Mann:
Multichannel
22
DATAPROCESSING
Data tapes
were brought
frorn the srip
ta the U.S. Geological
Survey
Marine Multichannel
Processing
Facility
in Menlo Park, California.
The first
stage of processing
consists
of reading the tapes on an AMPEX
HDDR
tape drive
similar
to those on the ship.
Data is read into a Data General Model 2QO
Eclipse computer which checks quality
and prints
out the header information.
At this
time the tapes were found to have many bad tracks
due to tape drive
problems on board the Ship.
Using high-density
14-track tape drives allows
for fewer data tapes to be written,
but also causes a greater
loss of data
when problems occur.
Many of the lines
collected
on the Solomon Islands
Cruise have record gaps caused by bad tracks on the tapes which cannot be read
with the present processing
system.
Arrangements are being made to have these
tracks read, and most of the gaps will be filled
in the future.
Using the information
provided by the header listings,
an editor makes up
a deck of processing
parameters
for the demultiplexing
program.
Some of the
-ar emeeexs include the static
delays,
streamer
geometry, water blanking,
and
various
in-house
infarrna,tion.
The demultiplexing
is also done on the Data
General 200 Eclipse
computer.'·
Output tapes are 9-track,
1600 bpi, and in_~."
Phoenix I format.
Further
processing
is
done on a Data General
model 230 Eclipse
computer.
First
the demultiplexed
tapes are sorted
to obtain COP (commondepth-point)
data to be used in Velocity
Spectrum Analysis.
Data are also
stacked using a set of brute velocities
to give the analyst a seismic section
to work with when picking spectrum Analysis.
After the velocity
file is made
from this
analysis
the data are stacked into
24-fold
CDP integer
data and
output on a 9-track Phoenix format reel.
The stacked data can then be plotted
an a Versatec model 2160 plotter.
Post-stack
plotting
information
is derived
using the stack
tapes
and a Power Spectrum Analysis
program which uses a
Fourier transform
routine
on the data.
'n1e data in this
paper have had a
predictive
deconvolution
and a band-pass filter
applied.
The band-pass filter
was 5-10, 5--60 (hz), and the deconvolution used a gap of 32 ms, an operator
of 100 ee , and the derive gate length was 2000 ms,
Final plotting
was also an
the Versatec model 2160 electrostatic
plotter.
Mann:
Multichannel
23
WIDE-A!CLE
U.S.Geological
SEISMIC RBFLECTION AND REFRACTION
FROM THE SOLOMON ISLANDS
Survey,
New zealand Geological
A. It. Cooper
345 Middlefield
Road,
Menlo
R. A. Wood
P.O. Box 30368, Lower
Survey,
DATA
Park,
Hutt,
Ca.
New zealand
INTRODUCTION
In May 1982, the U.S. Geological Survey (U.S.G.S.) recorded 35 seismic
sonobuoy stations in the Solomon Island region and 1 sonobuoy in Rabaul
harbor, New Britain of Papua New Guinea (Fig. 1, Table 1). The survey was
part of a larger offshore hydrocarbon
research project undertaken in
cooperation with the united Nations Committee for Coordination of Joint
Prospecting
for Mineral Resources in South Pacific Offshore Areas
(CCOP/SOPAC) •
During the survey (U.S.G.S. cruise L7-82-SP), sonobuoy data were
collected along geophysical tracklines, in conjunction with multichannel
seismic-reflection,
high-resolution
seismic-reflection,
gravity, and magnetic
data. The purpose of this paper is to describe the data-collection,
reduction,
and interpretive procedures that were used for the sonobuoy data and to note
some of the analytical methods and geologic factors that may introduce errors
into the results.
Based on these data, further studies have been done on the
crustal structure of the Solomon Islands intra-arc basins (Cooper, Bruns, and
Wood, this volume) and on the regional tectonics of the Solomon Islands region
(Cooper, Marlow, and Bruns, this volume)
Other sonobuoys, recorded in 1972 by Mobil Oil Corporation and by Gulf
Oil Corporation (Mobil, 1972; Maung, 1983), were obtained from CCOP/SOPAC and
interpreted for this study (Fig. 1, Table 1).
SONOBUOY
DATA COLLECTION
Sonobuoy station sites were selected during the cruise on the basis of
several criteria.
Where possible, the sites were
(1) in areas where geologic
or bathymetric features were thought to be either flat or uniformly dipping
over the length of the sonobuoy station, (2) in areas where the sedimentary
section was believed to be thickest, and (3) in areas adjacent to islands
where geologic units mapped onshore could be correlated with the acoustic
units in the offshore section.
Two types of seismic sonobuoys were used for the study.
U.S. Navy
sonobuoys (176 mhz, type 41-8) were used at all but one station;
at station
31, a commercial REFTEK sonobuoy (76 mhz), with a lengthened antenna for
A.
Cooper,
Wood:
Wide-Angle
Seismic
24
extended range, was deployed.
The offset range in which useful seismic data
were recorded by the Navy sonobuoys averaged between 25 and 40 km;
the offset
of the REFTEl< buoy was about 35 km , Horizons at depths of 12 to 14 km
and
apparent refraction velccities of 7.42 to 8.2 kIn/sec were
recorded at the
greatest offset ranges.
The seismic source was a 5-airgun tuned array with a total volume of
1,326 cu in. that was fired every 50 m (approximately every 17 sec) at an air
pressure of 1800 psi.
During normal station operations,
the Navy sonobuoys were launched from
the ship to a distance of 50 m abeam to avoid entanglement of the sonobuoy
hydrophone in the multichannel
seismic streamer. The wide-angle seismic data
transmitted from the sonobuoy were recorded unfiltered on analog magnetic tape
and were filtered and displayed in real time on a Raytheon Line Scan
Recorder.
The sonobuoy station ended when seismic returns were no longer
visible in real time on the graphic recorder.
Data on magnetic tape were
later replayed at different filter and gain settings to enhance the lower
frequencies, and to better display the deep refraction horizons.
Real-time
cords were normally recorded in the 5-64 hz frequency band and replays were
filtered 5-30 hz.
SONOBUOY
DATA REDUCTION
The data reduction procedures and computer algorithms used by the
U.S.G.S.
for the analysis of the wide-angle reflection and refraction data
(Childs and Cooper, 1978) have been applied to the data both at sea for
preliminary
interpretation
(Cooper and others, 1982; Tiffin and others, 1983),
and
onshore for final adjustments to the data (Table 2). A typical sonobuoy
recorded over the sedimentary
section in the New Georgia Sound is shown in
Figure 3 with the interpretation
added.
Re fraction
Da ta
The refraction velocities for each sonobuoy record were determined by
selecting and digitizing segments of the seismic first arrivals, fitting these
egments by linear least square lines, and using the slope and intercept of
the fitted lines to determine the thickness and velocity for each layer.
Because a refractor is not generally observed for the sea floor, a
hypothetical
refractor with an assumed velocity (1.6 kIn/sec) was used.
On most records, refractors with velocities less than 6.0 km/sec are seen
as distinct linear segments, separated by sharp breaks in slope, that indicate
distinct sub surface velocity boundaries.
At higher velocities, the
refractors are weak and often wavey, which indicates either lateral or
vertical velocity gradients, or geologic structure at great depth.
Where
geologic structure is not evident on the seismic reflection data, curved
refractions have been approximated
by a series of linear segments. In the
presence of geologic structure, only those refraction events occurring away
from the structure are used.
In the initial selection of refractors, only those seismic events that
were easily identifiable
were used; we did not attempt to establish regional
continUity of refraction horizons.
Later interpretation added picking weaker
and second arrivals, correlating
refractors between sonobuoy stations,
verifying picks for the deeper high-velocity
refractors, adding a refractor
A.
Cooper,
Wo~d:
Wide-Angle
Seismic
25
with an assumed velocity for the sea floor and applying slope corrections
based on the dips of reflection horizons determined from the single-channel
seismic-reflection
profiles.
Wide-angle
Reflection
Data
Wide-angle reflection arrivals on sonobuoy records are hyperbolas with
zero-offset times equal to the corresponding reflection horizons in the
vertical-incidence
seismic data. Interval velocities between selected
reflection horizons have been determined by digitizing each of the hyperbolic
wide-angle reflection arrivals, calculating the root-mean-square
(rms)
velocity for each hyperbola, and iteratively solving for the interval velocity
between successively
deeper pairs of hyperbola by the methods of LePichon et
aI, 1968.
Normally, the deepest reflection horizon used in the solution is
the acoustic basement
(velocity of 4.5-5.5 km/sec) and thus the interval
velocities are all within the sedimentary section.
Several criteria are used to evaluate and select the reflection hyperbola
used for the interval velocity solutions:
1. The reflection horizon on the vertical-incidence
seismic profile
should be continuous and dip uniformly over the first 4-6 km of the sonobuoy
station. This is the distance over which the wide-angle reflection is
digitized and the rms velocity is determined.
2. Reflection horizons should be separated, in depth, by at least 200 to
300 m (0.2 to 0.3 sec two-way time); this is the general limit of resolution
for stable velocity solutions in intermediate water depths (1,000-2,000 m
(LePichon et al,196B).
3. When refractors are present, the wide-angle reflection that is
associated with the refractor has been selected; if a refractor is not clearly
associated with a wide-angle
reflection then the closest identifiable
reflector has been used.
4. Prominent reflection horizons, such as unconformities,
tops of
acoustic units, and basement reflectors, that are present in the verticalincidence seismic profiles are also selected in the sonobuoy data.
Slope corrections,
based on correlations with reflection horizons
identified in the single-channel
seismic reflection profiles, have been
applied to the data in areas of steep dip near local geologic structures and
along the flanks of sedimentary basins.
These corrections are generally
small, normally less than 10%.
SOURCES
OF ERROR
The sonobuoy velocity determinations
for both wide-angle reflection and
refraction data are affected by errors from three sources: field operations,
data-reduction
procedures,
and
geologic structures:
Field Operations
The largest field errors arise from uncertainties
in the distance from
the ship to the sonobuoy during the sonobuoy station.
Variations in ship
heading and speed, as wel+ as sonobuoy drift, preclude the use of navigational
information for finding the exact range from the ship to the sonobuoy.
This
A.
Cooper,
Woo,d:
Wide-Angle
Seismic
26
distance has been estimated by multiplying the direct-arrival
(D-wave) travel
time by the velocity of sound in water, a value derived from surface-water
temperature measurements.
Surface temperatures
ranged from 29 to 3J degrees Celsius (acoustic
velocities of 1,539 to 1,541 mise c) throughout the study area.
If lower water
temperatures
(hence lower acoustic velocities) are present over the D-wave
travel path, then the calculated velocities in Tables 2 will be low, in direct
proportion to error in water temperature (about 0.13\ low per degree
celsius).
In most cases, this error has negligible effect on data accuracy.
Data Reduction
Procedures
Errors in picking and digitizing the D-wave and the reflection and
refraction arrivals yield the largest source of computational
errors in the
velocity solutions.
Least-square
calculations for refraction slopes and
\ntercepts as well as reflection rms velocities are sensitive to these
.igitizing errors and cause subsequent errors in the solutions for layer
thicknesses and interval velocities.
The magnitude of the error varies-usually larger at higher ve·locities and greater depths--and is difficult to
estimate1 comparison of velocities from a few buoys that have been digitized
more than once indicate that the error is a 1-2 ,.
Geologic
Structures
Sonobuoy solutions are based on the assumption that layers have uniform
velocity, thickness, and constant dip over the extent of the sonobuoy
station.
These assumptions
are violated when anomalous geologic features are
encountered during the sonobuoy station.
The largest uncertainties
in the
layer velocity and thickness determinations arise from the presence of
geologic structures
(such as faults, folds, basement uplifts, and intrusives),
irregular basement relief, and lateral depositional variations
(such as
wedging, onlap, overlap, termination, and development of localized bodies such
as reefs and channels).
These geologic features manifest themselves on the
sonobuoy records as disrupted, wavy, and offset reflectors and refractors,
that often have associated
sidelobes, crossovers, and diffractions.
we minimized the errors from geologic structures by using the verticalincidence seismic-reflection
data to select reflection and refraction events,
that are away from-obvious
structures.
The amplitude of Wide-angle
reflections and refractions at large
horizontal offset distances varies systematically with geographic area and
sometimes causes uncertainty
in the picks.
In areas with volcanic-rich
and
limestone sediment, the sonobuoy seismic amplitudes are usually weak.
The
strongest amplitudes from the largest offsets are achieved along sonobuoy
lines parallel with regional structures and in areas away from chaotic
acoustic units (such as the southern Russell Basin).
Amplitudes are
diminished in part by structural relief, but mainly by lateral and vertical
compositional
variations.
For the Solomon Island region, the type of sediment at the sonobuoy
station appears to have a larger effect on the seismic amplitudes than the
total thickness of sediment; strong basement refractors are often recorded
beneath thick (5-6 km) sediment south of Santa Isabel Island away from active
A.
Cooper,
Wood:
Wide-Angle
Seismic
27
volcanic centers whereas weak refractors are found beneath
krn) near the active New Georgia Island volcanos.
Other
thin sediment
(2-3
Corrections
Slope corrections have been made for dipping events.
However, in the
presence of large lateral variations, especially te~nation
of layers, the
slope corrections are not fully effective in correcting the velocities.
The
errors in refraction velocities caused by geologic structures generally
increase with greater sub-sea floor depths because of a greater uncertainty
in the velocity structure and unknown geometry of deep structures.
For highvelocity sub-basement horizons (greater than 5.5 km/sec) the velocities are
highly sensitive to small changes in the slope of the horizons.
Consequently,
the high-velocity
refraction horizons have the greatest potential errors.
A few sonobuoys and selected velocity solutions have been rejected
because of equipment malfunctions,
uncorrectable operational problems (such as
major changes in course, speed, and airgun fire rate), poor data quality, and
unmanageable
geologic conditions.
Field data have been collected on many of
these buoys and in sorne cases are shown in Chase et aI, (this volume) and in
reports by Cooper et aI, (1982) and Tiffin et aI, (1983).
However, the final
results are included here (Table 2) only for those sonobuoys that are deemed
useable and reliable after all corrections have been applied.
SUMMARY
Wide-angle reflection and refraction data from seismic sonobuoys have
been used to compute slope-corrected
crustal velocities at 43 sites in the
Solomon Island region shown on Figure 2 and 1 site near New Britain.
velocities representative
of sedimentary
(v=1.6-4.2 krn/sec) and igneous
(v-4.5-8.2 km/sec) rocks, at maximum sub-sea-floor depths of 14 km, were
measured throughout the region.
In general, the most reliable velocity and
thickness solutions are derived from sonobuoy stations where crustal layers
are flat lying and where concentrations
of coarse debris in the overlying
sedirrentary section are minimal.
A.
Cooper,
Wood:
Wide-Angle
Seismic
28
REFERENCES
Childs, J.R. and A.K. Cooper, 1978, Collection, reduction, and interpretation
of marine seismic sonobuoy data: U.S.G.S. Open-file report 7f-442, p. 1219.
Cooper, A.K., R.A. Wood, T.R. Bruns, and M.S. Marlow, 1982,
Crustal structure of the Solomon Islands arc from sonobuoy refraction
data: EOS, v, 63, p , 1120.
Furumoto, A.S.,
D.M. Hussong, J.F. Campbell, G.H. Suttan, A. Malahoff, J.C.
Rose, and G.P. Woolard, 1970, crustal and upper mantle structure of the
Solomon Islands as revealed by seismic refraction of November December, 1966 :Pacific Science, v. 24, p. 315-332.
LePichon,X., J. Ewing, and R.E. Houtz, 1968, Deep sea sediment velocity
determination made while reflection profiling: Journal of Geophysical
Research, -r, 73, no. 8, p , 2597-2614.
Maung, Tun U, 1983, Assessment of petroleum potential of the central Solomon
basin: CCOP/SOPAC, Surna-Fiji, Technical report no. 26, p. 1-57.
Mobil oil Corporation,
1972, Sonobuoy data, Open file, Geology DiVision,
Ministry Natural Resources. Solomon Islands.
Tiffin,D.L., J.G. Vedder, and A.Cooper, 1983, Multichannel seismic and
geophysical survey of "The Slot" and adjacent areas in the Solomon
Islands, CCOP/SOPAC,
cruise report no. 71, p. 1-16.
A.
Cooper,
Wood:
Wide-Angle
Seismic
29
SAMPLING lCBTHODS, SOLOMON ISI.AJmS
Bureau of Mineral
Resources,
J. B. Colwell
Geology and Geophysics,
J.
u. S. Geological
Survey,
canberra,
Australia
G. Vedder
Menlo Park,
california
94025
INTRODUCTION
In order to correlate
seismic
stratigraphy
with composition
and age of
ea-if Loor rocks,
bottom. samples must be acquired.
Time limitations,
bcwever ,
precluded
a thorough sampling program on Leg 3 of the cruise.
Nevertheless,
rocks and unconsolidated
sediments
representing
some of the shallC7W seismic
sequences were obtained at nine of the eleven stations
occupied {Fig. 1, Table
1}.
Sampling stations
were selected
by choosing previously
noted areas of
interest
on the multichannel
monitor records
and then checking against
the
high-resolution
seismic
records
for the best possible
targets.
Final
site
selection
also took into account the time available
on site and the distance
between stations.
Because of the widespread
occurrence
of Virtually
flatlying sediments
in the deeper parts
of New Georgia Sound, the dredge sites
were restricted
to the margins of the basin.
However, a horst-like
feature
in
the central
part
of New Georgia Sound between Kolombangara and Choiseul was
sampled during ~
Keoki tripartite
cruise
KK82-03-16-4 (Exon and Taylor,
1984, Figure 1).
SHIPBOARD
ANDPOST-CRUISELABORATORY
TECHNIQUES
Cored and dredged material
was photographed,
briefly
described,
and split
.nto subsamples onboard.
Representative
sample splits
were distributed
to the
U.S. Geolog.l.cal Survey, the Bureau of Mineral Resources (BMR), Australia,
and
the sc i.oecn Islands
Geological
survey
for analysis
and az-ch Lv.i.nq ,
After
lengthwise
splitting
and subsampling,
the unsampled half splits
of the cores
were frozen
for storage
in the U.S. Geological
Survey's
refrigerated
core
archives.
Post-cruise
laboratory
work was done mainly at BMR. Chemical analyses
for calcium carbonate
and organic
carbon content were made at the Australian
Mineral Developnent Laboratories
in Adelaide.
Standard laboratory
techniques
used at BMRincluded
preparation
of thin
sections,
wet-sieving,
smear-slide
analysis,
staining
of carbonate components (Alizarin
Red-S and Titian
Yellow),
X-ray diffraction,
and point-counting
(carver,
1971).
Subsamples were taken
at the laboratories
of all three countries
for micropaleontological
examination
by various
investigators
inclUding David Bukry, U.S. Geological
Survey,
George Chaproniere,
BMR, and Johanna Resig, University
of r.:awaii.
Colwell,
Vedder:
Sampling
30
Grain-size
Analyses
Grain-size
data
were obtained
on all
samples by standard
wet-sieving
techniques.
Samples were sieved into <45 , 45-63 , and >63 fractions.
No
attempt was made to subdivide
the <63 (silt
and clay) fractions.
component Analyses
Analyses of the constituents
in the rocks and sediments were made using
smear slides,
thin
sections,
x-ray diffraction,
and a binocular
microscope.
Abundances of components in the cored sediments were estimated,
whereas in the
dredged rocks, point-counts
were made (500-600 points per thin section).
Smear-slide
estimates
of component abundances. which were made using the
<45 fraction,
were based upon the area of the slide covered by each component.
The only major difficulty
encountered was estimating
the ratio
of calcareous nannofossils
to terrigenous
clay.
Nevertheless,
there
generally
was
good agreement between the estimated abundance of calcareous
nannofossils
and
the expected abundance based upon the total
carbonate content of the sediment
(determined chemically),
the relative
proportions
of the three size fractions,
and the composition
of the two coarser size fractions
detennined
by using a
binocular
microscope.
X-ray diffraction
analyses were made by J. Fitzsimmons
of BMRusing standard
techniques.
Organic Carbon and Carbonate
Analyses
Carbonate-carbon
(C02) was determined by acid evolution.
Samples were
digested
for 3S min in phosphoric
acid.
The evolved CO2 was then passed
through a series
of chemical filters
and driers
and finally
absorbed into a
pre-weighed Micnale tower containing
soda asbestos
and magnesium perchlorate.
The increase
in weight percent
of the tower was then expressed as a percentage
of initial
sample weight,
that
is.
percent
CO2'
Percent
caco3 was then
calculated
by using molecular weights.
Organic carbon was determined by using a LECO induction
furnace.
The
samples were first
boiled in HCI to remove any carbonates.
The residues
were
then filtered
onto Whatman glass-fiber
pads and the pads transferred
to LECO
crucibles.
The crucibles
were then burned in the induction
furnace.
A direct
readout of percent
C was thus obtained.
The instrument
was calibrated
using
standard reference
samples, LECOstandard carbon rings,
and pure dry CaC03'
SAMPLE
SITE DESCRIPTIONS
Locations.
coordinates,
and approximate water depths for all sample sites
are shown in Figure
, and Table 1. Brief annotations
on the retrieved
rocks
are given in Table 2.
The following descriptions
include seismic line number.
bathymetric
configuration,
expected rock type. and uncorrected
water depth or
range of depth.
colwell,
Vedder:
Sampling
31
;)redge Stations
Station
1.--(Gravity
core).
Station
2.--Station
2 was selected
to sample a short ridge that crosses
Line 26 on tht. southern margin of Russell basin (Fig. 1).
Acoustic basement
is shown on the seismic profile
at this
ridge.
The ridge forms part of a
series
of isolated
elongate knolls that extend from the Russell Islands to the
New Georgia Group.
The site
is on the shallOW'est segment of Line 26, ranging
from approximately
1,200 to 750 m of water.
Its
shape {Fig. 2Al, seisrni;c
character,
and pOSition between the islands
of Mborokua and Nggatokae suggested a volcanic
origin.
Rapid southeastward
drift
prevented
the ship from
maintaining
course directly
on Line 26 with the result
that the steepest
and
highest parts of the target
were missed.
Station
3.--Station
3 was intended to be on Line 27 approximately 30 Jon
southeast
of Station
2, where a prominent south-facing
scarp truncates
northdipping strata,
features
which shOW'up as well-defined
reflectors
on the seismic profile
(Fig. 2B).
Because of drift
and problems in locating
the target,
the selected
site
was overrun,
and the feature was dredged about 3 Jan south'ast
of the line.
Water depth at the dredged site
is approximately
980 m,
considerably
less than that at the proposed site.
Stations
4 and 4A.--Station
4 was an unsuccessful
dredge site that was
repeated
as 411.. Both were located on Line 13 on the northern
edge of the
relatively
shallow-water
platform.
that
joins
Guadalcanal
and the Russell
Islands.
The aim was to sample the rock sequence directly
below the upper
surface of the platform (Fig. 2C).
Station
5.--Station
5 was selected
because it was on an apparent fault
scarp on Line 13 about 3 km northeast
of Station 4A. The aim of the haul was
to retrieve
rocks from a sequence along the deeper part of the GuadalcanalRussell
Islands
platform.
where well-defined
reflectors
are
shown on the
seismic profile
(Fig. 2C).
Although no rocks were recovered after
several
"bites"
at 8,000 lb of tension,
small amounts of gritty
mud adhered to the
dredge jaws.
Station
islets
that
the station
the aim was
and that at
6.--Station
6 was sited
on Line 30 just outside
the line of low
forms the northern margin of the Russell Islands.
Water depths a
range from approximately
1,000 to 400 m. As at Stations
4A and 5,
to sample the sequence o~ rocks that forms the southern margin of,
some places may extend beneath, the Russell basin (Fig. 20).
Station 7.--Station
7 was on Line 31 approximately
15 km southeast of San
Jorge Island.
The intent
was to sample the base of the prominent scarp that
forms the southwestern edge of the shallow-water
platform joining Santa Isabel
and the Florida
Islands.
A raised
rim is present
along part of the southern
side
of the platform
(Colwell and Tiffin,
Fig.
1, this
volume).
At this
station,
the steep scarp ranges from approximately
400 up to 60 m of water.
Large slump deposits
shown on the seismic profile
extend from the base of the
scarp do.in slope to the basin floor in approximately
1,500 m of water (FJ.g.
2E) •
Colwell,
Vedder:
Sampling
32
Station
8.--(Gravity
core).
Station
9.--Station
9 was sited
on Line 22 northeast
of the island
of
Vangunu at the southeast
end of the NewGeorgia group.
It was chosen to sample a low, double-crested
ridge that appears to be upfaulted acoustic
basement
on the seismic profile
(Fig. 2F).
On this line, the ridge crest
is in about
1,100 m of ••••
eee r,
A probable southeast ••••
ard continuation
of the same ridge
crosses Line 24 in about 1,300 m of water.
Station
10.--Station
10 was on Line 51 approximately 35 Jan west
Lavella near the western end of the survey area.
It was selected
in
sample a ridge beneath ••••
hich seismic reflectors
are deformed.
These
reflectors
possibly
extend under the Shortland basin (Fig. 2G).
The
flank of the ridge was sampled in approximately 700 m of water.
of Vella
order to
deformed
southern
Station 11.--Station
11 was chosen on Line 51 about 4 km north of Station
10 to sample beds that appear as a series
of well-defined
reflectors
on the
seismic profile
(Fig. 2G) and apparently
extend beneath the Shortland basin.
The reflectors
indicate
a northward-dipping
sequence that suggests uplift
of
he southern side of the basin.
Water depths of the sampled interval
range
irom approximately 550-600 m,
Gravity-Core
Stations
Station
1. --Station
1 ••••
as used to check the condition
of the wire before
beginning dredge operations,
as well as to sample the top of the Holocene section in the deep part of the Central Solomons Trough of Katz (1980).
It was
on Line 26 on the axis of the Russell basin in about 1,800 m of ••••
ater
(Fig.
2A) •
Station
8.--Station
8 was on Line 31 in a zone of faults
and slumps at
the base of the slope leading
up to the shallo ••••
-water platform
that
joins
Santa Isabel and the Florida Islands.
It ••••
as selected
in order to sample the
top
of
the
Holocene section
northwest
of Savo.
The water
depth
was
approximately
',280 m (Fig. 2E).
MATERIAL
SAMPLED
Brief
descriptions
of the amount and type of material
recovered at the
sampling stations
are given in Table 2.
Four main types of rock and sediment
were reexaeveds
1) reef limestone
and coral;
2) unconsolidated
hemipelagic
clay, silty
clay, and sandy mud; 3) sedimentary rocks of mixed biological
and
Volcanic origin;
and 4) hornblende-bearing
crystal
tuff.
Detailed
descriptions
of the materials
recovered
are given in Colwell
(this
volume), and
Colwe11 and Vedder (this
volume).
Results
of chemical and paleontologic
analyses
on selected
samples are
summarized in Tables 2 and 3 and are
described
in more detail
in Resig
(this
volume), Colwell and Vedder (this
volume), and Colwell (this volume).
Colwell,
Vedder:
Sampling
33
REFERENCES
carver,
R. E.,
1971, Procedures
in sedimentary
petrology:
New York, Wiley,
Interscience,
653 p.
Exon, N.F.,
and B.R. Taylor,
1984, Seafloor
spreading,
ridge
subduction,
volcanism and sedimentition
in the offshore
Woodlark=Solomons region and
Tripartite
cruise
report
for .!:!!!.!.
Keoki cruise
82-03-16,
Leg 4:
CCOP!SOPAC
Technical
Report no. 34, 386 p.
Katz,
H. R.,
1980, Basin
Development in the
Solomon Islands
and their
Petroleum Potential:
~ UNESCAP, CCOP!SOPAC
Tech. Bull. 3, p. 59-75.
Colwell,
Vedder:
Sampling
34
TABLE2.--BRIEF DESCRIPTION
OF MATERIAL
RECOVERED
AMOUNT
OF
MATERIAL
STATION
RECOVERED
ROCIC
AND(OR) SEDIM£NT
1 192 cm Light yellowish-qrey
a9'ic aud conu,ining
2 1/4 dredge bag
calcarenite
(reef
to <urk olive-"rey
hemi~la few sandy turbidite
lllyers.
90\: ~limestone)
Ql.illternllry (l",u
and Mn-.oxide-st",ined,
lind coral.
10\,
Dark olive-g-rey,
slightly
volcaniclastic
Sllndy siltstone.
J 1/4 dred9'" boa"
bored siltstone.
ForAminiferA.
AGE
heavily
bored
Ql.iaurnary2
Pliocene or OUlltern",ryJ
cllicareous,
98\,
Ol.i.ve-"rey,
weakly calc;u'eou....
IIlOderately indurated,
Trace of
partly
Qu.lIternllry (N:23,
CN.14l3,2
Quaternary
CN.14alJ•2
(N.23,
1\
Olive-91'ey. _ll_indurated,
carbonate-free
siltstone.
Clllyey. Finely bedded.
-
4 a
Lost dredge.
4A
1/2 dred9" bolo"
100\,
Fe- and Mn-oxide-stained
coral.
D~a98netically
altered.
Well cemented.
S Trace
Smears of Silndy IlUd on dredge jaws.
volcanic or volcaniclastic
material.
6 1 pc.
One piece of highly bored,
reef. limestone
(calcarenite
7 Full
8 262
dredge b",,,
detrit\ls.
=
reef
limestone
100\,
Coralline
!'lainly rhodoliths.
algae-bound
and encrusted
of l\Wlnganese-oxide-stained
"
Full
ane rounded cobble,
reef
sandy hemi-
Q1,l4ternary (late
IlUddy
fine-
to mediwn-
I\'Clderately indurated,
light-grey.
crystal
N.22-N.23l'
QUllter",ary (t<.23, (;N14a)3.2
Q1,1aternary
(late N.22, (;!'J.14lJ,2
Quatern ••.
ry
lear Ly N.22. CN.14"'lJ,2
tuff.
dredgll bag
90\'
Fe- lind Mn-.oxide-eo",t=d, light
orange auc:ritic
ealcarenite
(biol!licrite)
eeee ••.
ining minor volcanogenic
materi",l.
!:lttensively
bored.
Evidence of s".b",el:'ial expos".re.
10\,
c;rellnuh-gl:'ey.
sl~ghtly
sandy hellUplIlagJ.c:
mudstone.
bte
phocene
(14te",t N.21)3
N. - pl&nktlC: for&m.inifllral 'l'one
CN. - CalC4l:'eOUIln",nno!ossil
acne
3 RlIsi" (thill volume) - pl",nktic:
QUllurnary2
branching
10 1/5 dredge bag
70\'
Olive-grey,
fossilifero1,ls,
grained volcaniclastic
sandstone.
M1,ldd'j. Bored.
Coated with lNlnganese Oxide.
1\,
Quaternary2
J
»hocene or Quaternary
Po5sibly
9 1/8 dred"e bag
95\:
Buff. fos5iliferou5.
fine-91'4ined
sandstone an<i sandy mudst one ,
Generlllly thinly
coated wit!,! llllIn"anese OXide. ExtenSively bored.
29\,
Light yellowish-grey,
sandy calcareous
mudstone.
and
llllInganese-oxide-stained
l.
Dus'"y-yellow to olive-grey •.•• lightly
pelag~c mud. Sandy in part.
S\,
Pieces
corili.
N.22-N.2J)
yellowish-
QI1ater"ary 3,2
Quaternary
(CN.14a)2
1 Ch4proniere (pers. comm., ,ge2) _ pla"ktic
Foraminifer",
2 Bukry (writ ••.•n c:omm., 1982) _ eall;areoull ",annofossils
",nd benthic Foraminifera
,
PART
This
topical
section
of the Joint
and interpretive
from
the R/V S. P. LEE
that
are
based
data and earlier
2
upon
a
papers
Cruise
Report
contains
that resulted
directly
1982 Cruise
combination
published
as well as articles
of the
new
cruise
work.
- -"0'
35
GEOLOGY OF TIlE CENTRAL AND WESTERN SOLOMON ISLANDS
Institute
of Geological
U.S. Geological
F. I. Coulson
Sciences,
Nicker
J. G. Vedder
Survey, Menlo Park,
Hill,
Keyworth, England
california
94025
ABSTRACT
The central
and western Solomon Islands
are composed of an exceptionally
thick succession
of Lower Cretaceous to Holocene rocks.
Although the stratigraphic sequences and rock compositions
vary from island
to island
and show
local
complexities,
regional
correlations
indicate
an orderly
pattern
of
island-arc
genesis from northeast
to southwest across the archipelago.
On Malaita
and the northeast
flank
of santa
Isabel,
the oldest
rock
sequences consist
of Cretaceous and Paleocene pillowed tholeiitic
to alkalic
basalt
that
is intercalated
and overlain
by pelagic
mrdecone and limestone.
These pelagic
deposits
are succeeded by Tertiary
deep-water strata,
in part
turbidites,
that contain
increasing
amounts of terrigenous
material
upward in
the Miocene and Pliocene
parts
of the section.
Along the chain of islands
that
includes
Choiseul,
southwest
flank
of Santa Isabel,
Florida
Islands,
Guadalcanal,
and San Cristobal,
so-called
basement rocks consist
chiefly
of
basaltic
flows
that
range
from unaltered
to
amphibolite-facies
schist.
Embodied in this pre-late
Eocene (?) basement complex are intrusions
of gabbro
and diorite
as well as diapirs,
thrust
sheets,
and rrelanges of ultramafic
rocks
that
possibly
represent
dismembered ophiolite
or
arc-root
rocks.
Southwest-directed
subduction that may have begun as early as late Eocene time
created a northeast-facing
island arc in which pillowed tholeiitic
basalt
and
basaltic
andesite
of Oligocene and early
Miocene age were extruded
together
with their
pyroclastic
counterparts.
These arc-related
volcanic
rocks are
succeeded by marine volcaniclastic
and carbonate
strata
of middle Miocene to
Holocene age.
Along the southwest
edge of the archipelago,
magmatism resulting
from
northeast-directed
subduction followed a reversal
in arc polarity
near the end
of Miocene tirre.
Calc-alkaline
effusive
rocks and cogenetic
volaniclastic
strata
of late Miocene to Holocene age predominate.
These volcanogenic
rocks
extend from the Shortland
Islands
through the New Georgia island
group into
Guadalcanal
where they
overlap
eastward
onto
older
rocks.
Pyroclastic
deposits
are less extensive
than epiclastic
strata.
These partly
subaerial
volcanogenic
rocks are largely
andesite
and basaltic
andesite
except in New
Georgia,
where porphyritic
olivine
basalt
and picrite
commonly are present.
Aphyric magnesium-rich basalt
also occurs in NewGeorgia.
Intrusive
complexes
that range in coceos ae i.cn from gabbro to quartz monzonite are exposed in the
Shortland Islands,
NewGeorgia, and Guadalcanal.
Coulson,
Vedder:
Island
Geology
36
Thicknesses
of
stratigraphic
sequences
are
variable.
The exposed
Cretaceous to Holocene sequence on southern Malaita,
including
the basaltic
lavas, is as much as 3,750 In thick; and the correlative
sedimentary section on
northern
Malaita may be as much as 2,200 m thick.
Pre-Qligocene
basaltic
flows on northeastern
Santa Isabel
are reported
to be as Ill.l.chas 3,500 m
thick.
So-called basement rocks are overlain by at least 2,500 m of Oligocene
and early Miocene volcanogenic rocks on Guadalcanal and by nearly 1,000 m of
equivalent
sedimentary section on Choiseul.
Middle Miocene strata
are nowhere
more than a few hundred meters thick,
but late Miocene strata
in the northern
Shortland Islands are about 1,000 m thick.
The thickest
section
of post-Qligocene
clastic
strata
1s in east-central
Guadalcanal,
where more than
5,000
m of
late
Miocene,
Pliocene,
and
Pleistocene ( 7) conglomeratic
sandstone and mudstone form a lenticular
body of
intertongued
lithofacies.
Miocene and Pliocene
volcaniclastic
strata
on
northwestern
Choiseul
have a composite thickness
of about 1,050 m,
Even
though the volcano that
forms Kolombangara in New Georgia is deeply eroded,
pyroclastic
material and flows still
rise nearly 1,700 m above sea level.
Faults are the dominant post-Miocene structures;
most are high-angle normal and reverse faults
that trend northwest.
A subordinate
set trends north-as c ,
Thrust faults
of uncereaan age displace
basement rocks on oieaeec i ,
.:ianta Isabel,
and Guadalcanal.
The thrust
planes are cut by younger highangle faults.
Folds, where present,
are chiefly
of Pliocene and Pleistocene_
age and generally
are broad,
parallel,
and northwest
trending,
except
on
Malaita where they are en echelon and relatively
steep flanked.
A major fault
system that may involve large amounts of left slip transects
Santa Isabel from
northwest to southeast.
This fault
system may be the surface expression of an
inferred
tectonic
suture
between unmetamorphosed Cretaceous
and Paleogene
ocean-floor
rocks of the Ontong Java Plateau and highly tectonized
basement
rocks that underlie
late Eocene (7) to early Miocene island-arc
rocks and that
were metamorphosed approximately
35 to 50 Ma.
INTRODUCTION
This paper is a summary of the findings and conclusions of previous geologic investigations
in the Solomon Islands.
It is an attempt to synthesize
and integrate
the
available
geologic
literature,
which in many cases
is
disseminated
in obscure
or out-of-print
publications
that
are not easily
-bna Lneb Le ,
The arrangement of the text is unconventional in that regional patterns
of tectonic
evolution
are
reviewed before
the
rock sequences and local
structural
features
are described.
This departure seems justified
as a simple
means of introducing
the causes of the contrasting
stratigraphy
of the various
island
groups.
The Eastern
Outer Islands
are not discussed,
as they are
outside
the purview of the marine geologic study for which this report
was
••••
ritten.
An introductory
statement
on the inferred
origins
of the islands
is
followed by a brief
description
of the traditionally
used geologic prov1.nces
and shortcomings of this
usage.
The synopsis of rock units is organized into
three
chronostratigraphic
divisions
that
best reflect
the major phases of
island-arc
development.
Primary sources for stratigraphic
names and rock
compositions for each unit are parenthetically
referenced
where the name is
introduced in the text.
Coulson, Vedder:
Island
Geology
37
Origin
of the Islands
Interaction
bet .••.
een the Australia-India
and Pacific
plates
since Eocene
time presumably resulted
in the creation of the complex island arc that now is
manifested by the Solomon Islands
(Fig. 1).
The absence of allochthonous
continental
material
indicates
that
the Solomons formed entirely
in an ocean
environment.
Cretaceous and early Tertiary
oceanic basement rocks, generally
lacking
terrigenous
material
and extensively
metamorphosed, contrast
with
overlying,
predominant.ly volcaniclastic
sequences that
are characterized
by
abrupt facies
changes and relatively
simple deformat.ion.
Subduct.ion-related
volcanism occurred between late Eocene (1) and early Miocene time and again
between the late
Miocene time and the present.
Late Oligocene and younger
strata
are dominated by thick volcaniclastic
wedges that are interspersed
with
reef
and shelf
limestone
bodies,
which were constructed
during volcanic
quiescence.
Although it is na.l generally
agreed that south-west directed
subduction,
impingement of
an oceanic
plateau,
and reversal
of
arc
polarity
were
controlling
factors
in the tectonic
development of the Solomon Island region,
·ecapitulation
of the evolVing ideas about the geologic history
points
out
aoce unsolved problems.
Early concepts stressed the taphrogenic
nature of the
region (Colemanr 1965, 1975, 1976; Coleman and Packham, 1976; Hackman, 1973)
and proposed
that. volcanism
may have originated
along major transcurrent
fractures.
Substantial
strike-slip
was invoked in order
to accommodate
oblique
convergence
between the Australia-India
and Pacific
plates.
The
foregoing models interpreted
the origin of the islands as late Mesozoic linear
geanticlinal
welts that progressively
broke up by tension
and shear into an
echelon arrangement.
Terms such as "fractured
arc" (Coleman, 1970; Hackman,
1973) and "non-arc"
(Coleman, 1975) were used to describe
the island chain.
Taylor
(1976) emphasized the
importance
of carey's
(1958) Tethyan Shear
System, which includes
the Solomons Megashear, and interpreted
the primary
tectonic
element to be a westward-migrating
rhombochasm in a zone of sinistral
shear wit.hin the system.
Although it has been suggested (Curtis 1973, Neef 1978) that the Solomon
Islands might be the site of double subduction, most models involve a polarity
reversal
of the are, probably within the last
10 m.y. (Kroenke 1972, 1984;
Karig and Mammerickx, 1972; Packham, 1973; Falvey, 1975; Ravenne et aI, 1977;
Coleman and Kroenke, 1981; Dunkley, 1983).
The consensus of the polarity-ever-ee r advocates is that a large part of the Solomon Islands arc was built
on late
Mesozoic oceanic basement by Paleogene southward-dipping
subduction
along what is now the northeastern
flank and that this episode was followed by
nort.heast-directed
subduction beginning in late Miocene time.
Coleman (1975),
however,
contended
that
several
conditions
apparently
contradict
the
hypothesized
northeast
- facing Paleogene arc.
Amongthese conditions
are 1)
t.he comparative
dearth
of calc-alkaline
rocks and absence of a tholeiitecalcalk.aline-high
K progression;
2) the aseismic gap and absence of a trench
between Bougainville
and Guadalcanal;
3) the anomalous near-trench
location,
abnormal lava composition,
and high heat flow in the New Georgia island
group.
On the other hand, most of these apparent anomalies are reconcilable,
particulary
if subduction and local occlusion of the active Woodlark spreading
axis (Fig. 1) are taking place, as seems likely
(Weissel et a L, 1982).
Further work is required
to fully explain a variety
of geologic features
such as 1) the absence of late Miocene and younger volcanism adjacent to the
San cristobal
Trench in the region between Guadalcanal and the Eastern OUter
Coulson,
Vedder~
Island
Geology
38
Islands,
2) the origin
and evolution
of the central
Solomons Trough, 3) the
position
and configuration
and apparent disassembly of the Solomons segment
within
the Melanesian arc system, and 4) the nature and origin
of the preOligocene basement rocks and their
suspected juxtaposition.
Without detailed
mapping of Santa Isabel
and san Cristobal
and until the rocks throughout the
region are more completely dated and genetically
correlated,
many questions on
the origin of the islands will remain unanswered.
Geological
Provinces
of Coleman
Coleman (1965, 1970) subdivided the central
and western modern Solomon
Islands arc (Fig.
1) to geological
provinces
(Fig. 2) whose value as a means
of
identifying
contrasting
stratigraphic
and structural
domains has been
accepted by many subsequent workers.
Because of this
commonusage, brief
descriptions
of
these
provinces
are
given
below.
However, increased
understanding
of the
geology of the Solomon Islands
limits
usage of the
province
concept
as originally
presented
by Coleman. Some of these
newer
interpretations
are
included
in the province
descriptions.
Stratigraphic
-o tumne shown in Figure 3 and the correlation
chart of Pound (this
volume)
_llustrate
some of the complexities
among the rock sequences of the three main
provinces defined by Coleman (1965, 1970).
Pacific
Province.--Malaita,
Ulawa, and the northeastern
flank of Santa
Isabel
are included
in this province.
It is composed of ocean-floor
rocks
that may have formed a segment of the leading edge of an anomalously thickened
portion
of the Pacific
plate
(Ontong Java Plateau),
part of which may have
been abducted onto the northeastern
front of the Solomons arc during Miocene
time (Kroenke, 1972; Kroenke et a L, this volume).
The basement rocks consist
of unmetamorphosed tholeiitic
basalt
that occurs on Malaita as pillowed and
massive flows of Early (1) and Late Cretaceous age.
The overlying sedimentary
rocks include thick sequences of pelagic carbonate and range in age from Late
Cretaceous to Holocene (Fig. 3).
The entire pre-Quaternary
sequence is folded
along northwest-trending
axes into
a broad anticlinoriwn.
Because they
contain small amounts of volcanic material,
post-Eocene strata
in parts of the
Pacific
Province possibly
represent
deformed and uplifted
remnants of distal
forearc deposits.
Increasing
amounts of volcanic detritus
and shoaling upward
'n the stratigraphic
succession tend to support the concepts of an evolving,
.ortheast-facing
arc
and incipient
obduction
of part
of the Ontong Java
Plateau.
Central
Province.--Within
this province
(Choiseul,
southwestern
side of
Santa Isabel,
the Florida Islands,
Guadalcanal and san Cristobal),
the islands
have intensely
faulted
cores of pre-late
Eocene (?) mafic lava and associated
intrusions
of gabbro and diabase,
in part metamorphosed to greenschist
and
amphibolite
facies.
Bodies of serpentinized
ultramafic
rocks
that
may
represent pieces of arc roots or ophiolite
are Widespread within this basement
complex.
The overlying
sedimentary succession ranges in thickness
from more
than
5,000
m in
east-central
Guadalcanal
to
less
than
700 m on San
Cristobal.
The diverse
sedimentary
rocks
include
biogenic
limestone,
calcarenite,
and volcaniclastic
sandstone that range in age from early Miocene
to Holocene.
In general,
the sedimentary
sequences display
shallow dips,
Coulson, Vedder:
Island
Geology
39
extensive
block faulting,
and 10000-amplitudedrape folds that usually
reflect
basement structures.
On several
islands within the central
Province (Florida
Islands,
Guadalcanal),
tholeiitic
basalt
of late Oligocene to early Miocene
age together with dioritic
intrusive
complexes are interpreted
as representing
an initial
phase of arc volcanism above a southwest-directed
subduction zone.
Volcanic Province.--This
province includes the reversed,
southwest-facing
modern arc and incorporates
the New Georgia
island
group, the Shortland
Islands,
parts
of Choiseul,
the Russell
Islands,
northwest Guadalcanal and
seve,
The volcanic
rocks
presumably
were
generated
by northeastward
subduction
of the active
WOodlark spreading-axis
segment of the AustraliaIndia plate
beneath
the overriding
Pacific
plate.
This ongoing subduction
constitutes
the phase of arc volcanism that
began in late Miocene time.
The
province,
which is typified
by rocks in the NewGeorgia group island,
forms a
series
of emergent volcanic
centers
and lava piles
that consist
largely
of
subalkaline
basalt
and smaller
amounts of andesite.
These islands
are
surrounded by fringing
and offshore
reefs
that
provide a framework for the
accumulation of varied volcaniclastic
and biogenic sediments.
Diorite
stocks
~re present on Guadalcanal and NewGeorgia.
Kavachi submarine volcano in the
;outheastern
NewGeorgia island group is currently
active.
Sikaiana,
Roncador Reef, and Ontong Java to the north of the main island
chain were assigned to the Atoll Province by Coleman (1965).
To the south,
the uplifted
atolls
of Rennell and Bellona were placed in the same province
even though they form a distinct
and separate
geographic entity.
None of
these
atolls
is
discussed
in detail
in this
report.
The northern
group
consists
of barely emergent Quaternary reef limestone and calcareous sand atop
the
Ontong Java Plateau.
Rennell and Bellona
are composed of uplifted
Pleistocene
reefs that are built on remnants of a possible
Eocene island arc.
CRETACEOUS
TO LOWER
OLIGOCENE
(1):
Regional
OCEANIC
BASEMENT
ROCKS
Relations
In the
Solomon Islands,
the oldest
known exposed rocks
consist
of
unmetamorphosed ocean-floor
tholeiites
of Early (1) and Late Cretaceous age on
Malaita (Fig. 3).
Elsewhere, on Choiseul,
southwestern santa Isabel,
Florida
tslands,
san
Cristobal
and
Guadalcanal,
possibly
correlative,
largely
~etamorphosed igneous and sedimen~ary rocks form a basement complex upon which
later
arc volcanism and sedimentation
were superimposed.
The distribution
of
these rocks is shown on Figure 4.
Similar metamorphosed basement rocks may be
deeply buried
beneath
late
Miocene to Holocene volcanic
rocks in the New
Georgia
island
group,
the
Shortland
Islands,
and Bougainville.
The
unmetamorphosed, basement rocks on Malaita
typically
consist
of deep-water
nonvesicular
pillowed and massive tholeiitic
basalt and associated
pyroclastic
and volcaniclastic
strata
that are intercalated
with and overlain
by pelagic
limestone and mudstone beds of Late Cretaceous and Paleocene age.
At places,
the flows are cut by intrusions
of gabbro and diabase.
A younger set of
basalt
flows in southern Malaita is alkalic
and apparently
ranges in age from
Late Cretaceous to Eocene (Hughes and Turner,
1977).
Pillowed to massive
basalt
flO'N's on northeastern
santa Isabel
are intercalated
in the upper part
with Paleocene pelagites.
On Guadalcanal and Choiseul, unmetamorphosed basalt
Coulson,
Vedder:
Island
Geology
40
of probable Cretaceous
and (or) early Tertiary
age grades into metabasalt
of
greenschist
to amphibolite
facies
(Hackman, 1980; Stanton and Ramsay, 1975;
Ramsay, 1978; Arthurs,
1981).
The schist
and unmetamorphosed basalt
commonly
are
in fault
contact.
On southwestern
Santa Isabel,
the basement
rocks
consist
mainly of greenschist
and amphibolite
that probably were derived
from
andesitic
flows and pyroclastic
strata
(Stanton,
1961; Coleman, 1965).
Radiometric
(K!Ar) dates on the basement metamorphism on Choiseul
give a
range of 32.4 :!; 6.8 to 51.5 :!; 6.8 Ma, with a mean of 44 :!; 18 Ma and a
preferred
date of 50 Ma (Richards
et a L, 1966).
On the Florida
Islands,
samples of two metamorphic rocks give dates of 35.2 ± 1.4 and 44.7 =- 2.1 Ma
(Neef and McDougall, 1976).
Presumably this Widespread Eocene and (or) early
Oligocene metamorphism began during the initial
phase of southwest-directed
subduction
of the Pacific
plate
beneath the Australia-India
plate
(Dunkley,
1983) •
Tectonically
emplaced
Alpine-type
ultramafic
complexes,
consisting
predominantly
of serpentinized
harzburgite
disrupt
the
basement rocks
on
Choiseul,
Santa Isabel,
the Florida
Islands,
Guadalcanal,
and San Cristobal.
The large ultramafic
bodies of the Solomons generally
are arranged in a linear
fashion,
parallel
to the elongate
trends of the major islands.
Exceptions
are
the slab-like
thrust
sheet on eastern
Choiseul and one northeast-trending
mass
in central
Guadalcanal.
Although one body in the Florida
Islands has gabbroic
constituents
that
are as young as latest
Eocene or earliest
Ollgoce'ne,
both
the age range (K!Ar, 38.4 ± .07 and 36.7 ±. 04 xe , Neef and McDougall, 1976)
and mechanism of emplacement of the ultramafic
complexes are imprecisely
known.
On Guadalcanal,
Eocene to late
Miocene emplacement ages have been
assigned to them (Thompson, 1960; Coleman, 1965; Hackman, 1980).
On Choiseul,
the
nearly
horizontal
thrust
sheet
overlies
deformed
and metamorphosed
basement rocks and is unconformably overlain
by strata
of early Pliocene
age
(Hughes,
1981).
Its
age of emplacement therefore,
is Miocene or older.
According to Coleman (19661, the earliest
erosion products
from the ultramafic
complexes are early
Miocene.
Possibly,
the ultramafic
complexes represent
multistage
protrusions
of long duration.
Some geologists
assert
that
the
tectonically
emplaced
ultramafic
rocks
are
ophiolite
suites
on Choiseul
(Arthurs,
1981; Hughes,
1981 ) , Florida
(Neef and Plimer,
1979), and Santa
Isabel
(stanton
and Ramsay, 1975).
Alternatively,
they could be remobilized
fragments of arc roots.
Cretaceous
to Lower Oligocene
Stratigraphy
Malaita.--on
the
northern
half
of
Malaita,
mafic
lava
flows
and
pyroclastic
rocks at the base of the section
(Alite
Volcanics
of Rickwood,
1957) are overlain
by a sequence of Cretaceous
mudstone and limestone
beds
(lower part of Malaita Group of Rickwood, 1957) (Fig. 3).
The volcanic
rocks
are composed of unrnetamorphosed pillow lava, diabase and subordinate
andesite
(Fiu Lavas of Rickwood, 1957).
The pyroclastic
strata
include
thin-bedded
tuff and probable
ignimbrite
(Fo'ondo Clastics
of Rickwood, 1957) that are as
much as 600 m thick;
they may be correlative
with the lava flows.
Siliceous
and locally
limy mudstone beds (Kwara'ae Mudstones of Rickwood, 1957) as much
as 270 m thick overlie
the basaltic
flows and pyroclastic
beds. The oldest
of
these beds are Albian{?) and cenomanian (van Deventer and Postuma, 1973).
The
siliceous
mudstone beds represent
a lithified
deep-sea ooze that was deposited
at depths greater
than 4,000 m (Hughes and Turner,
1976).
Coulson,
Vedder:
Island
Geology
41
In southern Malaita,
Early (1) Cretaceous oceanic tholeiitic
basalt flows
(Malaita Volcanics
of Hughes and Turner,
1976)
are equivalent
to the Alite
Volcanics
of northern
Malaita.
The overlying
rrudstone correlates
.••.
ith
the
siliceous
mudstone unit of northern Malaita but is increasingly
calcareouS up
section.
The nudstone is eucceede d by pelagic
limestone (Are'are
Limestones
of Hughes and Turner,
1976)
that
include
zones of chert
and intercalated
peperite.
The limestone
sequence ranges in age from Late Cretaceous
to
Eocene.
On Small Malaita
(Maramasike),
the siliceous
mudstone section
is
absent;
and as IlD.Jchas 550 m of pelagic
limestone and minor chert beds rest
conformably upon tholeiitic
basalt
("older
basalts"
of Hughes and Turner;
1977).
At places,
this
limestone
sequence (Apuloto Limestone of Hughes and
Turner,
1976) contains
thickly
interbedded
flows of alkalic
basalt
("younger
basalts"
of Hughes and Turner,
1977).
As in southern Malaita,
the limestone
sequence ranges
in age from Late Cretaceous
to Eocene and 1s large ly a
lithified
calcareous
ooze that was deposited in an open-ocean environment.
Ulawa.--A stratigraphic
succession
s~lar
to that on Malaita is exposed
on Ulawa.
Basement rocks consisting
of unmetamorphosed massive and pillowed
ceeru,c tholeiitic
basalt
(Oroa Basalts
of Danitofea,
1978)
are pre-Late
retaceous
and probably correlate
.••.
ith the mafic lava and diabase of northern
Malaita and the "older basalts"
of southern Malaita.
These basalt
flows are
conformably overlain
by a pelagic
limestone and chert sequence (Arau Limestone
of Danitofea,
197B) that
is as much as 400 m thick and that ranges in age from
Late Cretaceous to late Eocene.
San Cristobal.--Basement
rocks exposed on San Cristobal
(San Cristobal
Basement Complex of Jeffery,
1977)
.••.
ere briefly
described
by Thompson and
Pudsey-Dawson (1 958)
and Coleman (1 965) •
The complex includes
pre-Miocene
pillow basalt,
massive lava and pods of gabbro (Warahito Lavas of Coleman et
a L, 1965) that
are extensively
fractured,
sheared and altered
by low-grade
metamorphism.
At places,
lenses
of lo .••.
er Eocene pelagic
limestone
are
incorporated
in the flow sequences.
Discrete
limestone masses as much as 200
m thick (Ravo Limestones of Thompson and PUdsey-oawson, 1958) rest directly
on
the flow rocks and are probably
Paleocene or Eocene (Coleman in Hackman,
1980).
Assemblages of ultramafic
rocks occur in fault-boundedtracts
in
eastern
San Cristobal
and apparently
were tectonically
emplaced.
Thrust
. locks of serpentinized
ultramafic
rocks structurally
overlie
basalt
in the
estern part of the island
(Hughes, 1982).
The widespread alteration
of the
basement lavas and the emplacement of the ultramafic
rocks probably occurred
during Eocene and Oligocene time.
Guadalcanal. --The pre-Miocene
basement rocks on Guadalcanal
have been
diVided into
two main groups (Mbirao Group and Guadalcanal
Ultrabasics
of
Hackman, 1980).
The Mbirao Group consists
of a thick
sequence of mafic
volcanic
rocks,
subsidiary
limestone,
diabase sills
and local
intrusions
of
gabbro.
The volcanic
rocks (Mbirao Volcanics of Hackman, 1980) are composed
predominantly of relatively
unaltered
pillowed and massive basalt
that include
small bodies of recrystallized
limestone,
and chert.
As nuch as 1,200 m of
pillow
lava occurs along the south side of the island.
Minor, but discrete
belts
of recrystallized
pelagic( 1) limestone
(Tetekanji Limestone of Hackman,
Coulson,
Vedder:
Island
Geology
42
1980} as auch as 200 m thick are present
in the Mbirao Group in the eastern
part
of the island
where they commonly coincide
with faults.
A belt
of
greenschist-facies
metamorphic rocks (Mbirao Metabasics of Hackman, 1980) is
included
in the Mbirao Group and forms a major east-west
zone on eastern
Guadalcanal
where it
consists
primarily
of brecciated
and schistose
mafic
rocks.
Bodies of gabbro dated at 92 ;l- 20 Ma (Guadalcanal Gabbro of Hackman,
1980) that intrude
the mafic volcanic
sequence are considered
part
of the
Hbirao Group.
The gabbro occurs in sheared
and faulted
contact
with the
metamorphic rocks.
.
The other
group of basement rocks is composed of three roughly linear
belts of ultramafic
rocks that were designated
the Marau, suea , and GhausavaItina
Ultrabasics
by Hackman (1980) •
All are predominantly
serpentinized
harzburgite
and probably
were emplaced during Eocene or possibly
Oligocene
time.
Choiseul.-~oleman
(1960a; 1962; 1965) described
the basement complex on
Choiseul as faulted
schistose
and granulitic
rocks (Choiseul Schists),
that
are overlain
by' a sequence of pre-Miocene andesitic
and basaltic
volcanic
-ccxe (Voza Lavas).
Ramsay (1978) Arthurs (1981) ,and Ridgway and Coulson (in
""ress) showed that some of the schistose
rocks are intensely
deformed equivalents of the ~~trusive
and extrusive
rocks and that the volcanic and sohistose
rocks are juxtaposed
in thrust
sheets and fault-bounded
blocks.
The flows are
pillowed
to massive,
brecciated
and sheared
ocean-floor
tholeiites
that
display
alteration
that
ranges from zeolite
facies
through greenschist
to
amphibolite
facies.
The relatively
unaltered
flows are unconformably overlain
by lower Miocene strata
and generally
are assigned Oligocene and older ages.
Ramsay (1978) infers
a Cretaceous age for part
of the flow sequence,
the
thickness
of which is unknown. A minimumthickness
of 800 m was estimated by
Smith (1980) for incomplete exposures in the east-central
part of the island.
The Choiseul
Schists
are mainly amphibole
schist
that
has dominant
northwest-to
southeast-trending
foliation.
One body of
schist
in
the
southeastern
part of the island yielded a K/Ar mean age of metamorphism of 44
18 Ma (Richards,
et aI, 1966).
Igneous intrusives
into the basement complex
include
diabase dikes and a body of altered
microgabbro (the oaka Metamicrogabbro of Hughes, 1981) that
invades the flows in east-central
Choiseul.
In
southeastern
Choiseul,
a slab-like
mass of serpentinized
harzburgite
as much
as 560 m thick
(Siruka Ultrabasics
of Thompson, 1960) structurally
overlies
...•
oth the unaltered
basalt, flows and the schist
as a nearly horizontal
thrust
ahe et; (Hughes, 1981).
*
Santa Isabel.--Little
detailed
geologic information
is available
on santa
Isabel.
Basement rocks composed largely of schistose
amphibolite altered
from
protoliths
of andesitic
lava and pyroclastic
rocks form a belt
along the
southwest
side of the Kaipito-Korigole
fault
zone (Stanton,
1961; Coleman,
1965).
The schistose
rocks
are intruded
by gabbro and diorite
(Vitora
Microgabbro of Stanton and Ramsay, 1975) that
range from massive,
slightly
retextured
bodies
to
completely
recrystallized
amphibolite-facies
grade
rocks.
Stanton and Ramsay (1975) described
the basement on southeasternmost
Santa Isabel
as an ophiolite
and noted that
the basalt-gabbro
part of the
sequence is at least
6.5 Jan thick.
Alternatively,
these rocks may represent
tectonically
thickened
fragments
of arc
roots
rather
than
an ophiolite
complex•.
Coulson,
Vedder:
Island
Geology
43
On the
southeastern
part
of the
island,
ultramafic
rocks
(Kolomola
Ultramafites
of Stanton and Ramsay 1975) form a series
of elongate
pods of
serpentinized
harzburgite
that
appear
to
be genetically
related
to
the
Kaipito-Kongole
fault
zone.
On the island of san Jorge,
a plug-like
body of
ultramafic
rocks (San Jorge Ultramafites
of sc aneon and Ramsay, 1975) may be a
diapir.
Overlying the belt
of schistose
basement along the southwest coast are
about 200 m of pillowed basalt
flows of unknown age.
To the northeast
across
the Xaipito-Korigole
fault
zone, as nuch as 3.5 }an of pillow lavas and thin
flows are sparsely
intercalated
with volcaniclastic
strata
(Sigana Volcanics
of Stanton,
1961).
Coleman et al (1978) described
this
thick sequence of
volcanic
rocks
as
deep-water
pillowed
and massive
lava
and tuff
that
chemically are oceanic tholeiitic
basalt.
These tholeiitic
rocks are assigned
a Late Cretaceous to Paleocene age on the basis of a 66 z. 3.0 Ma K/Ar date
(Hackman, 1980).
Stanton
{1961} recognized
that
the tuffaceous
sandstone
beds overlying
the so-called
Sigana Volcanics
southwest of the Kaipito-Korigole
fault
zone
are
fundamentally
different
than
the
limestone-muds tone-tuff
sequence
overlying
the Sigana Volcanics
along the northeast
side
of Santa Isabel.
'ec.reover , the oldest
tuffaCeo\ls
sandstone
beds southwest
of the fault
are
early
Miocene (Coleman, 1965)"
Furthermore,
the _sedimentary
succession along
the northeast
coast (Tanakau Group of Stanton,
1961) probably is thicker
than
1,800 m (Fig.
3), and is highly deformed in the lower part
(Coleman, 1965).
Lenticular
and
massive
pelagic
carbonate
beds
containing
chert
are
intercalated
at places in the volcanic
sequence northeast
of the fault
zone.
Planktic
foraminiferal
assemblages
indicate
that these intercalated
pelagites
range in age from late
Paleocene to early Oligocene (Coleman et at.,
1978).
This pelagic
sequence resembles the Paleogene sedimentary section
in northern
Malaita and probably is genetically
related
to it.
Stanton (196 1) suggested
that
the
Sigana
Volcanics
and
overlying
beds
possibly
were
thrust
southwestward
over the
basement
complex along the Kaipito-Korigole
fault
zone.
It
is
not certain,
hccevez-,
that
this
fault
zone is entirely
a
compressional
feature
(see Summary).
Florida
Islands.--Basement
rocks of the northwestern
Florida island group
were described
by Taylor
(1976,
1977) as an in-situ
island-arc
ophiolite
suite,
although they equally as well could represent
dismembered, deep-seated
ar-c roots.
'!he basement complex consists
of repetitive
flows of oceanic
pillow
basalt
more than
2,800 m thick
(Kasika Metabasics
and Naghotano
Volcanics
of Taylor 1977).
Much of this mafic sequence is metamorphosed to
zeolite
and greenschist
grade.
All the mafic basement rocks are intruded by
ultramafiC
rocks
(Nggela
Ultrabasics
of
Coleman,
1965)
that
include
harzburgite,
serpentinized
peridotite,
and a swarm of diabase dikes, as well
as gabbroic rocks (vatilau
Gabbro and Vatilau Microgabbro of Taylor, 1977).
Basement rocks on small Nggela are cceeceed of serpentinized,
deformed
harzburgite,
dunite and wehrlite
together
with minor gabbroic rocks and mafic
pillow
lava (Hanuvaivine ultramafite
belt of Neef and McDougall, 1976).
At
places
the
ultramafic
rocks
are
tectonically
admixed with
a
younger
sedimentary
sequence.
A gabbroic
body within the ultramafic
belt pz-ovd des
K/Ar apparent
ages of 38.4 :II: 7 Ma and 36.7 :I; 0.4 Ma (Neef and McDougall,
1976).
Small patches of ultramafic
rocks also occur as melanges in northern
Small Nggela (Siota
Ultrarnafite
Belt of Neef and McDougall, 1976).
Both
Coulson,
Vedder:
Island
Geology
44
groups of basement rocks were called
incomplete parts of an ophiolite
(Neef and Plimer, 1979) and both probablY are diapiric.
sequence
OLIGOCENE
TO UPPERMIOCENE
ROCKS
Regional
Relations
Presumably,
the widespread Eocene and (or) early Oligocene tretamorphic
event
that
is
recorded
in basement rocks from Choiseul
to san Cristobal
occurred
in response
to the initiation
of southwestward subduction
of the
Pacific
plate beneath the Australia-India
plate
(Dunkley, 1983).
The onset of
subduction-related
arc volcanism
led to the emergence of ancestral
islands
during the Oligocene.
Some of the magmatism may have overlapped metamorphism
and vice versa.
Widespread extrusion
of tholeiitic
lavas on Guadalcanal the
Florida
islands,
and the Shortland
Islands
marks the first
episode of islandarc
volcanism related
to subduction.
Oligocene and lower Miocene volcanic
rocks on Bougainville
(Blake and Miezitis,
1967) probably represent
the sarne
episode
of volcanism.
Subduction
also seems to be indicated
by intrusive
igneous
complexes on Guadalcanal,
the
Florida
islands,
and the Shortland
Islands.
On Guadalcanal,
a body of diorite
is dated at 24.4 -: 0.3 Ma (Chivas
and
McDougall,
1979).
The active
arc
environment
provided
abundant
volcaniclastic
material
to
localized
Miocene depocenters
now exposed on
Guadalcanal,
san Cristobal,
the Florida
Islands,
Santa Isabel
and chot eeul ,
Subsequent widespread deposition
of early Miocene limestone,
calcarenite
and
foraminiferal
marl across the shelves of Guadalcanal, the Florida Islands,
and
San
Cristobal
attest
to
waning
volcanism
in
these
areas.
Pelagic
sedimentation
continued
undisturbed
in
the
Malaita
region.
Although
increasing
amounts of fine-grained
volcanogenic
detritus
occur upward in the
Miocene sequence on Malaita,
the source is unknown.
Igneous
Rocks
Subduction-related
mid-Tertiary
volcanic
rocks throughout
the Solomons
consist
mainly of flows that contain intercalated
volcaniclastic
and carbonate
strata.
The distribution
of these
rocks is show-non Figure S.
Tholeiitic
pillow
lavas
are
common; and most of the
associated
volcanogenic
rocks
'Jrobably
are
seafloor
extrusives,
although
some volcanism
may have been
.subaerial
on Choiseul
(Hughes, 1982) and Bougainville
(Blake and Miezitis,
1967>.. Basalt
and basaltic
andesite
predominate,
whereas andesite
and more
sa lie rocks account for only a small proportion
of the total
volume (Dunkley,
1983).
Intrusive
rocks are mostly calc-alkaline
and dioritic.
Guadalcanal.--Extensive
volcanic-arc
activity
is indicated
by exposures
on GUadalcanal
where at
least
2,500
m of basaltic
andesite
flows
are
intercalated
with pyroclastic,
volcaniclastic,
and carbonate
strata
(Suta
Volcanics
of Hackman, 1968, and Marasa Volcanics of Thompson, 1960).
These
pillowed and massive flows are assigned a late Oligocene and early Miocene age
on the basis
of benthic
foraminiferal
assemblages in interbedded
limestone.
Intrusive
diorite
(Poha Diorite of Hackman, 1980) has been dated at 24.4 : 0.3
Ma (Chivas and McDougall, 1978).
COUlson, Vedder:
Island
Geology
4S
Florida
Isiands.--<oncurrent
arc volcanism is evident
in the Florida
Islands
where s irnilar
rocks are exposed (Soghonara Lavas of Taylor,
1977,
GhwnbaBeds of Plimer and Neef, 1980).
Santa
Isabel.--A
narrow belt
of volcanic
rocks
overlying
schistose
basement southwest
of the Kaipito-Korigole
fault
zone is overlain
by early
Miocene strata
(Coleman, 1965).
Although these partly
fragmented basaltic
flows were correlated
"'ith a thick flow sequence northeast
of the fault
acne
(Sigana Volcanics
of Stanton,
1961), they may be unrelated
(see preceding
section).
Possibly
this
southwestern
belt of flows represents
Oligocene arc
volcanism.
Choiseul.--Pillowed
and massive to brecciated
tholeiitic
basalt
flows and
intrusive
diabase
(Voza [Vosa} Lavas of Coleman, 1960a) are widely distributed
on Choiseul.
Although Coleman (1960a) suggested that these rocks are middle
Oligocene to early
Miocene in age, they are more likely
to be older
(see
discussion
in
preceding
section:
Cretaceous
to
lower
Oligocene
(1)
';tratigraphy)
•
Short land Islands.--Volcanic
rocks on Fauro and Alu are of uncertain
age
but some of them probably are related
to nearby Oligocene and lower Miocene
igneous rocks on Bougainville
(Kieta Volcanics of Blake and Miezitis,
1967).
More than 500 m of altered
volcanic flows form the so-called
basement lavas on
Fauro,
where the
sequence
consists
of massive,
pillowed,
and brecciated
basalt,
icelanditei
and tholeiitic
dacite
(Masamasa Volcanics of Turner, 1978;
Turner and Ridgway, 1982).
On Alu, more than 400 m of altered
massive and
brecciated
lava flows are composed mainly of basalt and basaltic
andesite
(Alu
Basalts of Turner,
1978).
Unusual Intrusive
Rocks
In north-central,
Malaita a pipe-like
body of brecciated
alnoite
intrudes
early
Tertiary
mudstone and apparently
is overlain
by mid-Tertlary
strata.
Pb/U dates obtained
from zircons enclosed in the alnoite
are 33.9 and 34.1 Ma
'Oavis l!L Nixon, 1980). The alneite
contains
xenoliths
that are characteristic
of kim.berlites
in continental
cratonic
settings
(Nixon and Coleman, 1978).
The occurrence of garnet lherzolite
xenoliths
interpreted
as unmodified mantle
material
is especially
unusual in a supposed island arc setting.
Mineralogic
data from the alnoite
and calculations
based upon the pyroxene geothermometer
imply a lithospheric
thickness
of 110 kIn (Boyd in Nixon, 1980).
Isotopic
stratigraphy
studies
by Bielski-Zyskind
et al
{1984) concluded that
these
rocks and associated
basalt
were derived
by partial
melting
of relatively
undepleted
mantle under older depleted
upper mantle and that
the undepleted
mantle occurs deeper than 100 km,
They further
state
that
the results
of
their study are incompatible
with the ontong Java Plateau being a piece of old
continent.
Coulson,
Vedder:
Island
Geology
46
sedimentary
Rocks
Upper Oligocene and Miocene sedimentary sequences in the Solomon Islands
are dominated by volcanogenic
clastic
strata
that were deposited chiefly
in
deep-water basins and extensive carbonate deposits that were built in shallowwater shelf
areas.
Rapid erosion
of the newly emerged and tectonically
unstable
island
blocks led to the denudation of both the basement and early
arc
volcanic
rocks.
The resulting
voluminous
volcanogenic
sediments
accumulated in fault-bounded
basins and troughs that have since been uplifted
and tilted.
These strata
probably began to be deposited locally
as early as
late Oligocene time;
their
distribution
is shown on Figure 5.
Choiseul.--As
much as 2,500 m of volcaniclastic
breccia,
sandstone,
and
mudstone (Mole Formation of Coleman, 1960a) unconformably overlie the basement
rocks on chc.rseut ,
At several
places on the western part of the island,
a
rudaceous
facies
(Komanga Grit
of Coleman, 1960a, and Koloe Brecc.ias of
Hughes, 1982) forms a basal zone of breccia and conglomerate as much as 250 m
thick.
Its sporadic distribution,
restricted
occurrence,
and abrupt thickness
changes suggest
that
the
coarse
detritus
fills
hollows or
small faultcontrolled
basins
within· the basement.
.The br$ccia
consists
of angular
basalt.ic
clasts
set in a basaltfc
grit
matrix and both the clasts
and the
matrix
we-re derived
from nearby exposures' of volcanic
rocks and ~eefs,
possibly
as debris
flaws.
The breccia
grades upward into
finer-grained
clastic
strata
that constitute
the bulk of the formation.
In ••••
est-central
Choiseul,
lower to middle Miocene limestone (Mount Vuasa
(Vasu, Vavasa] Limestone of Coleman, 1960a) forms a series
of discontinuous
lenses as much as 50 m thick
within the lo ••••
e r , coarse-grained
part of the
volcaniclastic
sequence.
These carbonate
lenses
include
calcisiltite,
calcarenite,
and biocalcirudite
that
represent
both
reef
and fore-reef
deposits.
Most of the Miocene sedimentary section consists
of repeated sequences of
interbedded
microbreccia,
and well-bedded,
color-banded
marine sandstone,
siltstone
and mudstone.
The succession
generally
is
increasingly
finer
grained and calcareous
upsection.
Clasts within the breccia beds were derived
mainly from the basaltic
and schistose basement, although clasts
of limestone,
mudstone and siltstone
occur higher in the section.
Color banding is well
developed in the finer
grained and thinner bedded strat-a-.
Dark bands contain
abundant basalt
and black pumice fragments; light bands, abundant plagioclase
grains.
Sedimentary
structures
including
graded and cross-bedding,
loadcasts,
ripple
marks and scour are evident
(Coleman, 1962)•
Much of the
sequence probably reflects
mass-flow depositional
processes
onto the slope,
although some parts
may represent
shallo ••••
-marine and estuarine
environments
(Hughes, 1981).
Deposition
of these strata
commenced in late Oligocene time
in north-eentral
Choiseul, but did not begin until the middle or late Miocene
in the northwestern
part of the island where the beds are only a few hundred
meters thick.
According to Hughes (1982), beds as young as early Pliocene
occur at places in the uppermost part of the sequence.
Guadalcanal.--A
variety
of sedimentary rocks were deposted on an irregular surface of heterogeneous pre-Qligocene basement rocks on Guadalcanal. The
variable
nature of the stratigraphic
sequences is illustrated
in Figure 3 and
Coulson,
Vedder:
Island
Geology
47
in Pound (this
volume,).
In the southwestern part of the island,
graywacke
and sparsely
calcareous
clastic
sediments accumulated in late
Oligocene to
Miocene deep basins as marine turbidites
and debris
f Lcws,
In the southcentral
part
of the
island,
a series
of thick,
lenticular
poorly sorted
graywacke and conglomerate
beds
(Kavo Graywacke Beds of Hill,
1960 and
Coleman, 1965) are more than 2,500 m thick.
Within this
sequence,
dark
massive shale beds possibly
indicate
a euxinic environment (Hill,
1960).
The
graywacke sequences both overlie
and interdigitate
with Oligocene to Miocene
volcanic rocks (Suta Volcanics)
from which they were principally
derived.
Biogenic
limestone
and back-reef
calcarenite
(Mbetilonga
Group of
Hackman, 1980) are the predominant sedimentary rock types directly
above the
Oligocene and early
Miocene Volcanogenic sequences.
These carbonates
are
sporadically
distributed
over llJJ.chof the island and are mainly of early and
middle Miocene age, although some are reported by Hughes (1981) to be as young
as Pliocene
in eastern
Guadalcanal.
From west to east,
five stratigraphic
units
have been differentiated
(Mbonehe [Bonegi] Limestone of Coleman, 1957,
Mbetilonga
[Betilonga]
Limestone
of Coleman, 1960b, Tina calcarenite
of
Hackman, 1980, Lake Lee calcarenite
of Coleman, 1960b, and Valasi Limestone of
Coleman, 1960b).
These units,
which rest
unconformably upon older rocks,
range in minimum thickness
from 100 to 400 m and have a maximumestimated
.hickness
of 1,000 m (Valas.i Limestone).
The group is composed largely
of
impure shelf-carbonate
rocks of variable
texture,
grain
size
and organic
content
and- characteristically
contain a admixtures of terrigenous'
ma:terial.~
The biostromal
limestone,
which consists
of pure carbonate,
is massive and
partly
recrystallized
and constitutes
as mrch as 70 percent
of some units
(Mbonehe Limestone).
Sinlilar recrystallization
is well developed in other
units
(upper half of the Mbetilonga Limestone, basal beds of the Lake lee
Calcarenite).
A coralgal
reef facies
forms the western part of the youngest
unit
in
the
group
(Valasi
Limestone).
At
places,
a
foraminiferal
biocalcarenite
facies,
usually
thick bedded, is intercalated
with or grades
laterally
into massive recrystallized
limestone.
These biocalcarenite
facies
are commonin some units
(Lake Lee Calcarenite,
and the eastern
exposures of
the Valasi Limestone).
One calcarenite
unit
(Tina calcarenite)
is composed
mainly of well-bedded flaggy strata
that contain large amounts of terrigenous
material
inclUding sporadic carbonized wood fragments.
Where present,
the terrigenous
content of the carbonate
deposits
generally is greatest
in the lowermost parts of the sequence (Mbetilonga and Valasi
Limestones).
Basal conglomerate beds contain locally
derived clasts
of volcanic and ultramafic
rocks.
Benthic foraminiferal
assemblages suggest water
~pths in the range of 40 to 80 m for most of the impure shelf-carbonate
units
in the group. However, the purer biostromal
limestone probably accumulated in
quieter
back-reef
conditions
or as fringing
reefs sheltered
from terrigenous
contamination
(Hackman, 1980).
In western
Guadalcanal,
a succession
of late
Tertiary
volcaniclastic
arenite
and wacke beds (Lungga Beds of Wright, 1968) overlies
the pre-middle
Miocene volcanic-graywacke-limestone
sequence.
These volcaniclastic
strata
contain
subordinate
conglomerate,
mudstone and andesitic
lava flows.
The
diversity
of lithic
fragments in these coarse clastic
deposits
is greater than
in the underlying graywacke beds.
Near the west coast,
these strata
rest upon
oceanic basalt
(Turner and Hackman, 1977); and directly
south of Honiara, they
overlie
biostromal
limestone
and are as much as
1,200 to
1,500 m thick
(Wright, 1968, Hughes, 1977). According to Hackman (1979), this volcaniclastic
sequence ranges in age from middle Miocene to late
Pliocene ( ? ), but Hughes
Coulson,
Vedder:
Island
Geology
48
( 1982) assigns
an age range of late
late Miocene to late Pliocene or early
Pleistocene.
Abrupt
facies
changes
and internally
deformed beds
are
characteristic
of this
sequence
indicating
active
tectonism
and slumping
during accumulation.
Santa Isabel.--Most
of the island of Santa Isabel has been mapped only in
reconnaissance
fashiom
consequently,
little
is known about facies
relations
and ages of the strata
that
overlie
the volcanic basement complex.
The
distribution
of sedimentary
facies
on the island
is
complicated
by the
apparent
juxtaposition
of two tectonostratigraphic
terranes
along a major
northwest-trending
fault
system.
These contrasting
rock
sequences
are
illustrated
in Figure 3 and in Pound (this volume, Fig. 1).
Stanton (1961) described the sedimentary rocks and incorporated
them in a
single informal group (Tanakau Group) in which stratigraphic
and facies connotations
were implied but not named. Nevertheless,
the contrasting
sedimentary
successions
on opposite
sides of the axial
volcanic spine were recognized.
Along the
northeast
coast
and southeast
end of the
island,
a stratal
succession
containing
an abundance of pelagic
limestone in the lower part was
distinguished
from a sequence containing
terrigenous
material and no limestone
along
the
southwest
coast.
The early
Miocene and younger part
of the
sedimentary section
along the northeast
coast includes zones of pink chert,
minor sedimentary
breccia,
bedded ~alcisiltite
and calcilutite,
and rare
tuffaceous
sandstone.
These strata
probably
are
conformable
with
the
underlying
volcanic
and
pelagite
sequence,
although
large-scale
penecontemporaneous
or
post-depositional
slumping
created
apparent
unconformities
in some areas.
Volcanic wacke, consisting
of an admixture of
noncalcareous
volcaniclastic
sandstone
and
siltstone
and
tuffaceous
calcarenite
form the remainder of the succession,
which may be nonmarine in
part.
carbonized
plant
remains
representing
the
first
incursion
of
terrigenous
material
occur
in
strata
as old as late
Oligocene
in
the
northeastern
Santa
Isabe I
successions.
Abundant terr igenous
detritus,
however, first
appears in late Miocene strata
(Coleman et aI, 1978).
The post-oligocene
section
has been locally
subdivided into named units
in southern santa Isabel,
where the oldest strata
(ser-e Beds on the southwest
coast,
Loguhutu Beds on san Jorge)
consist
of early Miocene coarse-grained
volcaniclastic
and tuffaceous
sandstone that grades up section into clay-rich,
tuffaceous
fine-grained
sandstone
(Rob Roy Beds) of early and middle Miocene
age (Coleman, 1965).
Middle and late Miocene graywacke beds overlie the fineJrained tuffaceous
sandstone (Hughes, 1982).
The post-oligocene
succession
in southwest santa Isabel
is at least
2.2
km thick,
and locally
includes interbedded
shale, mudstone, and minor amounts
of conglomerate.
Ultramafic detritus
commonlyoccurs in the early Miocene and
younger strata
along the southwest coast.
The section along the southwest
coast is slightly
deformed in contrast
to that in the northeastern
part of the
island where dips are steep and folds verge northeastward (Stanton,
1961).
Florida
Islands.--Because
detailed
mapping has been completed only on
parts
of the Florida
Islands;
stratigraphic
relations
are not completely
resolved.
Coleman (1965) described the sedimentary succession above the preMiocene volcanic
rocks as commencing with calcareous
coarse-grained
lithic
sandstone
and subordinate
siltstone
of early
Miocene age.
These basal
Coulson, Vedder:
Island
Geology
49
sandstone beds apparently
were derived entirely
from volcanic
rocks as they
contain
no ultramafic
detritus.
calcarenite,
fine-grained
volcaniclastic
sandstone,
and siltstone
succeed the basal strata
and in turn, are overlain
by
a prominent zone of thick-bedded
foraminiferal
calcarenite
(Anuha calcarenite
of Coleman, 1965), also of early M.iocene age.
This calcarenite
unit forms a
discontinuous,
north-trending
belt of outc:;rops in eastern
Big Nggela and is
the first
in the succession
to contain ultramafic
detritus.
The lower Miocene
strata
are overlain
by a sequence of middle and upper Miocene calcareous
sandy
siltstone
and fine-grained
tuffaceous
sandstone that probably was deposite;d
rapidly in shelf environments.
These sandy strata
are increasingly
calcareous
upsection
and, on small Ng'gela, grade upward into a Pliocene
fringing-reef
deposit (Florida Limestone of Coleman, 1965).
The Miocene sequence may have a
composite thickness
of as much as 2,200 m thick.
The northwestern
part
of the
Florida
island
group contains
a late
Oligocene to Pliocene sedimentary succession
(Mboli Beds of Taylor,
1977) that
rests unconformably on altered
basaltic
flows.
Within this succession,
which
has a minimum thickness
of 600 m, a turbidite
sequence forms the dominant
member (the
Kombuana Sandstone of Taylor,
1977).
These turbidites
are
composed largely
of volcaniclastic
arenite,
lutite
and epiclastic
rudite.
Although direct
correlations
have not been made, the Mholi Beds are probably
in part
equivalent
to the Anuha calcarenite
and Florida
Limestone farther
east.
On small Nggela, IC7llller
Miocene sedimentary rocks on the north end of the
island
(Siota Beds of Neef, 1979) consist
of massive sandstone and sporadically inter layered mudstone that represent
deep-water mass-flow deposits.
The
sequence is
as much as 850 m thick.
These sandstone-mudstone
beds are
geographically
separated
by an east-trending
ophiolitic(?)
wedge from a lower
Miocene sequence of arenite,
rudite
and pillow
lava (Ghumba Beds of Neef,
1979) on the
southern
part
of the
island.
Neef (1979) attributes
the
deposition
of the
lenticular,
coarse strata
to deep-water
debris
flow and
slumping; the sequence is progressively
finer grained upsection and as much as
1,500 m thick.
In western Small Nggela, a thin-bedded
to laminated
finegrained sandstone
(Ndandala sandstone of Neef, 1979) forms the youngest part
of the section.
These beds are of middle and late Miocene age and are as much
as 280 m thick.
The late Oligocene to Pliocene
strata
throughout the Florida
group are
affected by numerous high-angle
faults
and, minor folds.
San Cristobal.--Because
san Cristobal
probably is geologically
the least
known of all
the major islands,
the stratigraphic
relations
are uncertain.
Lenses and irregular
masses of upper Oligocene (?) pelagic
limestone
(Ravo
Limestones
of
Coleman,
1965)
and partings
of
calcareous
siltstone
are
intercalated
with and directly
overlie
the flows to form bodies as much as 200
m thick.
Fault-bounded blocks of coarse-grained
strata
less than 700 m thick
(Cristobal
Group of Coleman, 1965) are believed
to be underlain
by basaltic
flows.
These
strata
constitute
one
of
the
thinnest
post-oligocene
depositional
sequences in the Solomon Islands.
On the
eastern
part
of
the
island,
conglomeratic
strata
(Hariga
Conglomerates of the Cristoval
Group of Coleman, 1965) both interdigitate
with
and overlie
pillowed
basaltic
lavas.
Included in the conglomeratic
sequence
are coarse agglomerate and sandstone lenses that have a calcareous
tuffaceous
matrix.
The sandstone
contains
early Miocene foraminiferal
assemblages,
al-
Coulson, Vedder:
Island
Geology
50
though the entire
sequence probably ranges in age from early to late Miocene.
Stratigraphically,
the conglomeratic
beds are partly
equivalent
to foraminiferal
calcarenite
beds that occur in northeastern
san Cristobal.
On northwestern
san Cristobal,
a middle Miocene succession
consists
of a
basal pelagic
limestone
(Hautarau Limestone of Jeffery,
1975a) and calcareous
mudstone, volcaniclastic
wacke and slump breccia
(Ruawai Beds of Jeffery,
1975a).
A minimum thickness
of 410 m is estimated
for the Miocene section
in
this area.
A volcanogenic sequence (the Waihada Volcanics of Jeffery,
1975a),
which is in part equivalent
to and in part younger than the Miocene sedimentary rocks, consists
of tUff,
agglomerate and pillow breccia
that show evi:"
denee of submarine reworking and slumping.
Faults
cut all
of the Miocene sequences;
folds have not been mapped,
except on Uki N1 Masi.
The serpentinized
ultramafic
bodies in the western
part of the island have been emplaced along thrust
faults
(Hughes, 1982).
Malaita. --In
the
northern
part
of Malaita,
late
Eocene(?)
to middle
Miocene strata
(Alite Limestone of Rickwood, 1957) consist
of as much as 900 m
of bedded limestone
that
contains
zones of chert.
These limestone beds are
'ucceeded by as much as 760 m of thick-bedded
chalk (Sauba Chalk of Rickwood,
,957), .•••
hich has an age range of middle to late Miocene and possibly
early
Pliocene.
In the southern part of the island,
the Cretaceous to Eocene pelagic
limestone
beds
are
overlain
by
hard
calclsiltite
beds
(Haruta
calcisiltites
of Hughes and Turner, 1976) that contain an increasing
number of
brown llUdstone beds upsection.
The rrudstone beds may represent
distal
turbidite
deposits
or primary products of nearby volcanism.
These beds are believed to range in age from late Eocene to middle Miocene.
On Small Malaita,
a sequence of calcilutite,
calcisiltite,
and minor calcarenite
beds (Hada
Calcisiltites
of Hughes and Turner, 1976) gradationally
overlies
the Eocene to
middle Miocene strata
and is as much as 200 m thick.
Sporadic,
thin noncalcareous mudstone beds are present at places in this upper Miocene to Pliocene
sequence. Most faults
and folds
on Malaita
post-date
the
late
Miocene to
Pliocene
depositional
sequences
(see section
on upper Miocene to Holocene
rocks) •
Ula.•••
a.--A
380 m-thick
succession
of calcisiltite
and calsilutite
beds
containing
thin layers of mudstone and intercalations
of pillowed and massive
lkalic
basalts
(Waipaina Calcisiltites
and Haumela. Basalts
of Danitofea,
.978) overlies
the Cretaceous to Eocene beds on Ula.•••
a.
These strata
range in
age from early Oligocene to late Miocene and probably are lateral
equivalents
of the Oligocene and Miocene calcisiltite
beds of South Malaita.
A gradual
change from deep-vaee r to shallower environments and an increasing
influx
of
volcanic
detritus
are evident. According the Coleman (1965) the island
is a
faulted
anticine.
UPPERMIOCENE
TO KOLOCENE
ROCKS
Regional
of
Relations
Upper Miocene to Holocene largely calc-alkaline
eruptive
centers
along a broad,
irregular
belt
Coulson,
Vedder:
Island
Geology
volcanism built
that
stretches
a series
from the
51
Short land Islands
to northwest
Guadalcanal.
This volcanic
activity,
which
seems to have peaked in Pliocene
and Pleistocene
time,
was eccompanLed by
empLecement; of several igneous intrusive
ccmpLexes ,
This episode of volcanism
was preceded by middle to late Miocene tectonism and presumably was generated
by a r-ever-aaI in arc polarity
and incipient
northeastward
subduction about
eight million
years ago.
At the same time, renewed uplift
heralded a period
of tectonism and intense alluviation,
particularly
on Guadalcanal.
Ultramafic
bodies, probably initially
emplaced in late Eocene or Oligocene time, began to
be protruded,
overthrust,
and extensively
exposed to subaerial
erosion.
Early
Pliocene
and older
strata
generally
are warped by northwest-trending
ope n
folds;
exceptions are on northeastern
Santa Isabel and Malaita where folds are
relatively
closely
spaced and steep
limbed.
According to Coleman (1965),
there
are no recognizable
folds on San CristobaL
Intersecting
high-angle
normal and reverse
faults
form northeast-and
northwest-trending
sets in prePliocene strata
on most of the islands.
Renewed subduction created
a new pattern
of sedimentation
at the end of
Miocene time.
In the region of the volcanic
axis,
rapid uplift
and instability
curtailed
reef growth, and high erosion rates
led to the shedding of
huge quantities
of volcanic
detritus
into adjacent
newly developed intra-arc
basins.
Farther
behind the new are,
however, sheltered
marine conditions
allowed the deposition
of extensive
platform carbonates.
Igneous
Rocks
General features.--Volcanic
rocks that are related
to northeast-directed
subduction occur chiefly
along the southwest side of the arc from Bougalnville
to Guadalcanal.
The distribution
of these rocks is shown on Figure 6.
Lava
and epiclastic
breccia
derived
mainly from flows generally
predominate over
pyroclastic
deposits.
Ash-flow
tuff
and pumice flows
of
intermediate
composition are limited
in extent
and are associated
with andesite
domes on
some quae scene centers.
Shallow-intrusive
complexes are present
at places in
these suites
of effusive
rocks.
Coleman and Kroenke (1981) described a 75 ~ km spacing of volcanoes along
the southwest side of the Solomons arc.
Even though they fall
outside
the
general trend,
two late Cenozoic volcanoes on Choiseul (Maetambe and Kornboro)
are included within the belt
of southwest-facing
arc magmatism.
Other offtrend
"displaced"
volcanism
is indicated
by the sites
of some New Georgia
volcanoes,
which seem to be anomalously near the trench.
For example, the
southernmost volcanic center on Simbo lies almost directly
above the inferred
axis of the clogged trench south of Vella Lavella.
Compositionally,
the second-episode
volcanic rocks are variable,
although
they are chiefly
calc-alkaline
throughout the arc.
In the NewGeorgia Group,
large
volumes of olivine-rich
basalt
and picrite
have been erupted and may
reflect
unusual or abnonnal subduction processes.
Excluding the apparently
anomalous NewGeorgia volcanism,
the upper Miocene to Holocene volcanic rocks
generally
are more salic
than the Oligocene arc eruptives
and range in composition
from basalt
to rhyodacite.
Basaltic
andesite
and andesite
are the most
abundant extrusive
rock types (Dunkley, 1983).
In addition
to the extrusive
centers,
a series
of irregularly
shaped
stocks
occur along the main arc in the Shortland
Islands,
New Georgia and
Guadalcanal.
The calc-alkaline
rocks
that
form these
stocks
range
in
composition
from gabbro to granodiorite
and quartz monzonite; quartz dio-rite
coulson,
Vedder:
Island
Geology
52
and tonalite
predominate,
(Dunkley, 1983).
Pliocene
to several of these stocks.
to
and all
exhibit
late-stage
Pleistocene
radiometric
ages
sodium enrichment
have been assigned
Gua da Lcarra Ls e-e-Orr Guadalcanal,
late cenozoic igneous activity
was concentrated
in the northwestern
part of the island where it reached a climax near
the end of Pliocene
time.
Plugs and dissected
volcanic
cones are composed
predominantly
of hornblende
andesite
flows
(Gallego Lavas of Thompson and
Pudsey-uawson,
1958).
These flow sequences are as much as 900 m thick at·the
type section.
Aprons of pyroclastic
breccia and coarse volcaniclastic
strata
flank the extrusive
centers.
A single K/Ar age of 6.39 ± 1.95 Ma (Snelling
in
Hackman 1980) indicates
that
much of this
flow sequence may be older
than
Pleistocene,
although
geothermal
areas
occur
nearby
and
intermittent
vulcanicity
continues
on nearby Savo Island.
The hornblende andesite
flows are blanketed by as much as 300 m of volcanic
agglomerate
(Tiaro
Tuff
Breccia
of Hackman, 1979)
which probably
represents
volcanic
mudflows.
The
largely
unstratified
tuff
breccia
interfingers
with poorly stratified
lithic
tuff near the volcanic
centers
and
grades laterally
into
stratified
volcanic
wacke and rudite
(Lungga Beds of
Wright,
1968).
These bedded strata,
which are predominantly
sandstone
and
siltstone,
were deposited
in narrow channels and basins between volcanoes and
contain air-fall
ash as well as material
derived from the underlying
andesite
flows.
Farther
east,
in the Gold Ridge area of central
Guadalcanal,
a thick
sequence of extrusive
andesite,
now eroded and buried by late Pliocene strata,
probably represents
an isolated
calc-alkaline
volcanic center.
Dioritic
rocks
(Koloula
Diorite
of
Hackman,
1980),
which
outcrop
in
south-central
Guadalcana1,
yield
K/Ar apparent
ages that range from 4.47 :t- 0.19 to 1.55 :t0.05 Ma and indicate
polyphase emplacement (Chivas and McDougall, 1978).
Savo. --Lying about halfway between the northwestern
tip
of Guadalcanal
and the Florida
Islands,
the island of Savo is a quiescent
Pelean volcano that
is the easternmost
in the chain of volcanoes along the southwest-facing
arc.
Lavas are composed mainly of hornblende
andesite,
and the volcanic
edifice
consists
largely
of agglomeratic
and tuffaceous
deposits
(Proctor
and Turner,
1977).
According to Coleman (1965) the most recent sequence of eruptions
was
in the period
1830 to 1840.
Active steam vents and solfataras
occur on the
southern and eastern
slopes of the volcano (Taylor, 1965).
Russell
Islands.--The
Russell
Islands,
which lie about midway between
Guadalcanal and eastern
New Georgia,
are the remnants of an emergent island
volcano composed of basaltic
andesite
breccia overlain by basalt
lavas (Pavuvu
Breccias and Banika Lavas of Danitofea and Turner, 1981).
The volcanic
core
of the islands
is encircled
by uplifted
siltstone
and reef
limestone
of
Pleistocene
age.
Mborokua.--Mborokua,
a small conical
island situated
between the Russell
Islands
and New Georgia,
is an extinct
volcano composed of massive basaltic
lava and volcanic
breccia
(Turner,
1975).
The south side of the crater
has
been breached by the sea.
Coulson,
Vedder:
Island
Geology
S3
cnc.Leeuf, --Near the center of the island of Choiseul,
Mount Maetarnbe is
composed predominantly
of andesi tic pyroclastic
deposits.
Chemically, these
are
calc-alkaline
rccke that
presumably are related
to northeast-directed
subduction
along the
southwest
side of the arc.
The pyroclastic
strata
(Maetambe volcanics
of Coleman, 1960a), consist
predominantly of water-laid
andesitic
tuff,
ash and breccia
and have an estimated minimumthickness
of 500
m, Flows have not been recognized.
These volcanogenic rocks generally unconformably overlie
basement rocks and upper Oligocene to upper Miocene turbi';'
dites
(Mole Formation).
At places,
the pyroclastic
rocks are interbedded
in
the upper part of the turbidite
sequence indicating
contemporaneous volcanism
and rapid sedimentation
on steep submarine slopes during the early stages of
this eruptive
episode.
Volcanism possibly
began as early as middle Miocene
time and may have continued
spasmodically
into
the Pleistocene
(Hughes,
1981).
Although no crater
is preserved,
geothermal springs on Mount Maetamhe
suggest the waning of relatively
recent volcanic activity.
In the area of Xomboro Peak and Laena Island in southeastern
Choiseul,
the
volcanic
rocks
consist
of a sequence of andesitic
breccia
and tuff
(XomboroVolcanics of Coleman, 1960a).
Schist and ultramafic
detritus
is pre.ent; at places in t;he breccia
beds. to direct
evidence for the age of these
volcanic rocks is available,
but ash-t'all
deposits
derived from Komboro Peak
occur in nearby strata
of early
Pliocene age.
The well-preserved
cone and
large
amount of andesitic
material
in adjacent
Pleistocene
deposits
suggest
that volcanic activity
continued into late Quaternary time (Strange,
1981a).
NewGeorqia.--The
islands
of NewGeorgia represent
the most extensive
and
voluminous development
of
the
late
Miocene to Holocene episode
of
arc
volcanism.
Virtually
the entire
island
group is
formed by a complex of
emergent and coalescing
volcanoes
that are encircled
by fringing
reefs
and
lagoons.
The volcanoes,
however, seem to be anomalously near the NewBritainSan Cristobal
Trench,
and many lavas are chemically
atypical
of the calcalkaline
suites
that occur elsewhere in the arc.
These apparent anomalies are
attributed
to the subduction of the active Woodlark spreading axis beneath New
Georgia (Taylor and Exon, 1984; Dunkley, 1983).
The volcanic rocks consist
of large volumes of highly porphyritic
olivine
basalt and pic rite
basalt
lava and breccia.
Hornblende basaltic
andesite
and
-naes Lee flows are subordinate.
Less commonbut widespread are fine-grained
.phyric magnesium-rich basalt
flows.
The basalt
and picrite
are hypersthene
normative and show a trend to magnesium enrichment, whereas the minor andesite
with which they are associated
shows a broad calc-alkaline
trend akin to the
so-called
normal volcanic
rocks on other islands
in the arc (Dunkley, 1983).
The petrogenetic
nature of the NewGeorgia suite is difficult
to define.
The
rocks are subalkaline
and have a low Ti02 content characteristic
of arc volcanics, but the suite is high in potash and the basalt contains as much as 2.6
percent ~O.
Two-pyroxene andesite
that is relatively
rich in MgOoccurs at
Simbo.
Intrusive
rocks include gabbro to tonalite
stocks and andesite
plugs
in the central
part of the island group.
Typically,
the volcano
flanks
display
lateral
facies
that
range from
massive flows to volcaniclastic
strata,
the
latter
showing an increasing
degree of reworking away from the eruptive
centers.
Slopewash detritus
is a
significant
facies within the volcanic edifices
(Dunkley, oral commun., 1982).
Coulson, Vedder:
Island
Geology
54
Shortland Islands.--Calc-alkaline
suites of igneous rocks include both
intrusive and extrusive rocks in the Shortland Islands.
In the Fauro Island
group, two intrusive bodies (Tauna Microdiorite, Fauro Dacite of Turner, 1978)
invade the basement lavas, and farther north a shallow intrusion of hornblende
andesite forms CemaIsland.
The horseshoe-shaped bay of north Fauro is believed to have been created by caldera collapse of a large volcano which produced reworked crystal tuff and lava flows at least 1,500 m thick as well as
the overlying 600 m of volcanic breccia,
tuff
and minor flows (Karia
Sandstones and Toqha Pyroclastics of Turner, 1978).
The age of the pyroclastic rocks is uncertain but may be Pliocene and Pleistocene based on comparison with similar rocks elsewhere in the SolomonIslands (Turner, 1978).
In the Alu Island area, the pre-Miocene (?) basaltic
lavas are intruded
by a microdiorite co~lex,
whose possible extrusive equivalent is pyroxene
andesite
(H1$iai Complex and Kamaleai Pyroxene Andesite of Turner, 1978).
Because the pyroxene andesite is overlain by middle and late Pliocene sedimentary rocks (Hughes, 1981"
the dioritic and andesitic rocks possibly range
in age from Mioceneto early Pliocene.
Sedimentary Rocks
General features.--ouring
the episode of volcanism that produced "ebe
islands along the southwest flank of the are, sedimentation continued in a
variety of environments throughout the Solomonsregion.
Uplift of the frontal
part of the are, coupled with active volcanism, resulted in high rates of
deposition in the newly developed intra-arc
basins.
Abrupt facies changes
were caused by deposition around a complex of emergent volcanoes, local
basins, and in inter-island
channels. Reefs did not flourish except on narrow
shelves that were protected from intense alluviation.
Elsewhere, low-energy
conditions prevailed,
and fine-grained calcareous deposits aceurnulated with
few incursions of primary volcanic material.
However, increasing amounts of
terrestrial
detritus
in depositional
sequences throughout the eastern and
northeastern parts of the region indicate uplift that ultimately resulted in
shoaling and island errergence.
The distribution
of late cenozoic strata is
shownon Figure 6.
Guadalcanal.--oetailed
mapping by Hackman.(19~0) has delineated a number
If sedimentary rock units that span late Miocene to Pleistocene time.
In
east-central
Guadalcanal, a sequence composedlargely of poorly sorted arenite
forms a lithosome as much as 4,000 m thick (Mbokokimbo
Formation of Hackman,
1980).
Complexlithofacies
relations and large .eecurre.eof volcanic detritus
typify the unit.
Although muddyfine-grained sandstone and siltstone
beds
predominate, claystone and coarse-grained volcaniclastic
sandstone form parts
of
the
section;
and sporadic
channel-fill
conglomerate lenses occur
throughout.
Carbonized wood and other plant remains are common,and dark
siltstone
zones conta.in pyritic
concretions indicative
of local anaerobic
conditions.
Sedimentary structures
including chaotic slump features suggest
steep gradients and rapid accumulation in envirorunents that ranged from
flUVial to outer shelf and that were periodically
disturbed by seismic
events.
Hughes (1982) assigned a late Miocene to late Pliocene or early
Pleistocene age to the litho some.
Coulson, Vedder:
Island Geology
55
Farther west, in the north-central
part of the island, a sequence of
volcaniclastic
rudite and arenite (TOni Formation of Hackman,1980) ranges in
age from middle (?) Miocene to late Pliocene.
Subsidiary strata consist of
pyroclastic
and extrusive rocks and limestone.
Facies changes characterize
the
sequence, which may be partly
nOMaarine a.nd which is
composed
predominantly of conglomeratic material derived largely from volcanic rocks of
the Gold Ridge area.
Beds of paraconglomerate and orthoconglomerate, as well
as volcanic and lithic arenite are included; these are locally calcareous and
grade into micrite and marl.
Hackman(1980) divided the formation into 10
mappable subformational units.
Lenses of massive coralgal limestone and
bedded calcarenite
(KombusoeLimestone of Hackman,1980) occur at places but
nowhere are they more than 100 m thick.
MJst of the coarse clastic strata
were deposited by slumping and turbidity flows in tectonically unstable shelf
and slope environments, although someof the beds maybe fluvial.
The intense
alluviation
may have derived its
energy from pliocene volcanism in the
vicinity of Gold Ridge.
In western Guadalcanal, strata that are laterally equivalent and partly
correlative with those in the north-central part of the island form a repetitive succession of arenite and wacke together with subsidiary conglomerate and
mudstone (Lungga Beds of Wright, 1968).
The clastic fragments were derived
principally
from volcanic rocks. ~ An age range of late (1) Miocene to late
Pliocene or early Pleistocene is assigned to these strata (Hughes, 1982).
In contrast to the high-energy terrigenous environments reflected by the
coarse clastic
strata in the central and western parts of the island, more
protected
conditions in northeastern
Guadalcanal permitted the growth of
fringing reefs (Valasi Limestone of Hackman,1968c). The reef limestone is
succeeded by a sequence of conglomerate and subordinate mudstone and siltstone
beds that has a maximumthickness of 700 m (Vatumbulu Beds of Hackman,
1968). These upper Pliocene to Pleistocene alluvial and shallow-shelf
deposits were derived mainly from pre-Miocene volcanic rocks.
Pleistocene
deposits along the northern fringe of the island consist
largely of reef complexes and shallow-marine and estuarine sediments (Honiara
Beds of Coleman, 1957).
Included in this sequence are coralgal limestone,
calcarenite,
and epiclastic
conglomerate.
The section is as muchas 800 m
thick but generally is muchthinner.
Gentle folds of probable Pliocene age affect upper Miocene and lower
Pliocene strata; only minor drag folds adjacent to faults have been recognized
in Quaternary strata.
North-northwest and north-northeast trending high-angle
faults cut Pliocene rocks in central Guadalcanal.
Oblique-slip step faults
affect Quaternary rocks along the southern part of the island (Hackman,1980).
Choiseul.--Pliocene strata on western Choiseul are conformable on Miocene
beds and consist largely of calcareous deposits approximately 400 m thick
(Pemba Formation [Pemba Siltstone]
of Coleman, 1960a).
In general, the
section is cceeosed of calcarenite
overlain by marl. The lower 200-250 m of
section,
consists
of lower to middle Pliocene calcarenite
beds (Sui
Calcarenite Memberof Strange, 1981b) and includes zones of non-calcareous
mudstone, calcisiltite
and paraconglomerate.
Common
slump and graded bedding
in the calcarenite
possibly represent seismically generated turbidites
from
the western slopes of MaetambeVolcano. The bedded calcarenite section grades
upward into upper Pliocene calcisiltite
beds (Mbani calcisiltite
Memberof
Strange, 1981) that are as much as 120 m thick.
There are no sedimentary
Coulson, Vedder:
Island Geology
56
structures
in the uniformly bedded calcisiltite
section, which grades upward
into massive calcilutite.
Increasing fineness upsection mayindicate a transition to deeper water conditions or reduction of source-area relief in middle
to late Pliocene time.
Quiet, shallow-water conditions prevailed in southeastern Cnoiseul, where
lower Pliocene clastic
strata
(Vaghena Formation of Hughes,1981) were
deposited unconformably upon the basement. These strata are at least 40 m
thick in the type area and have an estimated minimumthickness of 110-150 m
farther west.
They consist of thin-bedded calcareous arenite, siltstone
and
mudstone that contain varying amounts of tuffaceous debris possibly derived
from the nearby volcano at KomboroPeak. Deposition probably was in a lowenergy shelf environment less than 200 m deep (Hughes, 1981).
Following uplift,
tilting
and erosion, Pleistocene coralgal reefs (Nukiki
Limestone of Coleman, 1960a) formed above the Mioceneand Pliocene formations
and overlapped them onto basement rocks.
These largely recrystallized
limestone beds are preserved along the southwestern side of the island as
isolated uplifted and tilted
blocks as nuch as 150 m thick.
They include
fore-reef talus and reef-wall deposits as well as locally developed back-reef
and estuarine deposits (Hughes, 1981; Strange, 1981 a, b).
On northwestern
Choiseul, correlative limestone occurs as high as 440 m above sea level and is
~ilted, warped, and .faulted (Str~nge, 1~81a). Andesite. Breccia (Laena Breccia
of Strange, 1981a) containing a calcarenite matrix is present on Laena Island
east of KornboroPeak.
"
NewGeorgia Island Group.--Sedimentary rocks are limited in distribution
along the frontal part of the active are, particularly
in NewGeorgia.
On
Giza, limestone and arenite beds contain foraminiferal assemblages of early
Pliocene age {Hughes, 1981}. Late Pliocene shelf sed1Illents occur on Vella
Lavella,
but most of the section consists
of Pleistocene reef platform
deposits (Hughes, 1982). pleistocene reef limestone occurs on Ranonggaand on
western NewGeorgia Island.
On the uplifted islands of Tetepare and Rendova,
sequences of Pleistocene deep-water siltstone
and sandstone contain chaotic
masses of rudaceous material consisting of volcanic, plutonic, and coral-reef
debris that presumably slumped from high-standing parts of the arc to the
north. Someof the reefal deposits on Tetepare are uplifted as muchas 800 m
above sea level, and benthiC foraminiferal assemblages indicate deposition at
depths of 1.5 to 2.0 kIn, implying uplift
of 2-3 kIn during the Quaternary
(Dunkley, 1983).
Shortland
Islands.--In
the
Shortland
Islands,
Pliocene
and (or)
Pleistocene sediments are represented on the islands of Alu and Monoand
record a general transition
from hemipelagic deposition of fine volcanic
detritus to shallow-water reef construction.
On Alu, about 300 m of Pliocene
siltstone,
claystone and fine-grained
sandstone (Kulitanai Siltstones
of
Turner, 1978) represent open-marine shelf environments that received detritus
from Pliocene volcanoes on Bougainville and possibly Fauro. Late Pliocene and
Pleistocene calcareous sandstone, chalky limestone and recrystallized
reef
limestone (Lafang Limestone of Turner, 1978) form an intertonguing sequence of
shallow marine deposits as muchas 150 m thick.
Late Pleistocene deposits
(Toanapina Conglomerates of Turner, 1978) are composedof locally derived
alluvial and littoral
gravel.
Coulson, Vedder:
Island Geology
57
On MonO, about 250 m of pliocene
siltstone
(Mono Siltstones
of 'rurner ,
1978) is largely
an outer shelf deposit probably representing
ash falls
from
Bougainville
volcanism (Hughes, 1981).
Reef limestone
(Kolehe Limestones of
Turner, 1977) constitutes
a late Pliocene carbonate-platform
facies within t.he
ash-fall
siltstone
unit.
Early
Pleistocene
reef
limestone
(Bare Reef
Limestone and Toloko Reef Limestone of Turner,
1978) were deposited
in
association
with lagoonal siltstone
(Soanatalu Siltstone
of Tllrner, 1978).
Pliocene
(?) tuffaceous
sandstone beds on Fauro and neighboring islands
(Koria Sandstones
of Turner,
1978) probably represent
rapid deposition
of
locally
derived
volcaniclastic
sediment on a shallow shelf
(Hughes, 1981).
The sequence 1s as much as 1,400 m thick.
Minor limestone occurs near the
base, and primary tuff beds are present higher in the section.
According to Turner (1978), folds are essentially
absent and faults
are
small on the Shortland
Islands.
The northern
part of Fauro probably is a
large caldera-collapse
structure.
Florida
Islands.--Pliocene
strata
on the eastern
Florida
Islands
are
largely
residual
patches
of limestone
(Florida
Limestone of Thompson, 1958)
and generally
consist
of massive to thick-bedded calcarenite
(Thompson, 19581
Neef,
1979).
A sandy siltstone
marker bed is present
at places.
The
limestone is at least
250 m thick on Small .Nggela where 11: rests unconformably
upon folded lower Miocene strata.
Outcrops along the east
coast represent
patch-reef
accumulations
and
subordinate
lagoonal
deposits
and contain
foraminiferal
assemblages of early and middle Pliocene age.
Pliocene strata
are gently folded and cut by high-angle
faults.
Southward tilt
of the island
platform
and submergence occurred during Quaternary
time (Coleman, 1965).
San Cristobal. -w<:alcisiltite,
calcilutite,
siltstone
and muddy sandstone
beds (Uki Beds of Jeffery,
1975b) of late Miocene to middle Pliocene age occur
on Uki Ni Masi and are unconformably overlain
by Pleistocene
reef limestone
(Hughes,
1982).
On the
main part
of
san Cristobal,
Pliocene(?)
and
Pleistocene
deposits
are confined chiefly
to the northern and western coastal
fringes.
Pleistocene
reef facies
(Arosi Beds of Jeffery,
1975b) include forereef coralgal
limestone
and back-reef
calcarenite.
The island probably was
emergent during the Pliocene,
and Pleistocene
sea-level
fluctuations
led to
the establishment
of fringing
reefs
on wave-cut benches along the northern
coast (Jeffery,
1975b).
Malaita.--In
northern
Malaita,
a pelagic
chalk sequence (aeube Chalk of
Rickwood, 1957), the upper part
of which may be Pliocene,
is conformably
overlain
by about 250 m of siltstone
and fine-grained
sandstone (Tomba Silts
of Rickwood, 1957).
These clastic
deposits
consist
mainly of foraminiferal
and tuffaceous
debris;
they are overlain by Pleistocene
reef limestone.
Along the west coast in the southern part of the island,
upper Miocene to
Pliocene limestone beds ('Are'are
Limestones, upper part of Hughes and Turner,
1976) are unconformably overlain
by 180 m of interbedded
conglomerate,
sandstone and subordinate
siltstone
(Hauhui Conglomerates of Hughes and Turner,
1976).
These poorly
sorted,
locally
cross-bedded
coarse
clastic
strata
probably
accumulated
in
a shallow-marine,
possibly
fan-delta
environment
Coulson,
Vedder:
Island
Geology
58
during
early
Pleistocene
time
(Hughes and Turner,
1976).
Clasts
in the
conglomerate include subrounded pebbles and cobbles of limestone,
basalt,
and
chert.
On small
Malaita,
an upper Miocene to Pliocene
sequence
(Hada
Galcisiltites
of Hughes and Turner, 1976) consists
of well-bedded calcisiltite
and minor laminated
calcarenite
about
350 m thick.
This
sequence
is
unconformably overlain
by uplifted
coralgal
reef limestone of Pleistocene
age
(Rokera Limestone of Hughes and Turner,
1976).
The reef limestone generally
is 20 to 30 m thick and massive, vugular and recrystallized.
Large, relatively
tight
north •••.
est-trending
folds form an echelon pattern
in northern
Malaita and deform Pliocene
and older
rocks
(Rickwood, 1957).
Folding,
faulting
and island
emergence took place
during Pliocene
time' in
southern
Malaita followed by minor Quaternary faulting
and regional
tilting
(Hughes and Turner, 1976).
The dominant structures
trend north •••.
est.
Ulawa.--As much as 330 m of Pliocene
calcareous
conglomerate beds (the Holohau Mudstones of Danitofea,
They are
overlain
by Pleistocene
reef
limestone
as
(Ngorangora Limestone of Danitofea,
1978).
mudstone and slumped
1978) occur on Ula•••.
a.
much as 80 m thick
SUMMARY
Massive to pillOW'ed tholeiitic
basalt
and' its metamorphosed cOUl1.terparts
form most of the basement rocks of the Solomon Islands.
Volcaniclastic
strata
are a minor constituent
in the basement complexes.
These cretaceous
and
Paleocene ocean-floor
rocks are exposed on all
of the large
islands
except
those in the Ne•••.Georgia groupi they also occur on the smaller Florida Islands
and U1a•••.
a.
Isotopic
studies
of basalt
and alnoite
from Malaita suggest that
these rocks were derived
from an undepleted mantle source and not from old,
depleted oceanic mantle or subducted old continental
crust
(Bielski-Zyskind
et
aI, 1984).
other bodies included in the basement are intrusions
of gabbro and
diabase.
An altered
gabbro that intrudes
metamorphosed basalt
on Guadalcanal
yielded
an age of 92 .• 20 Ma (cencmanaant ) ,
On Malaita,
the upper part of
the
flow sequence
is
inter layered
with pelagic
limestone
that
contains
foraminiferal
assemblages
as old as early late Cretaceous
(early Cenomanian)
and possibly
as old as late
Early Cretaceous
(Albian).
Else •••.
here,
these
igneous
rocks
ShCM varying
degrees
of metamorphism,
largely
zeolite
to
greenschist
and amphibolite
facies.
A 66 Ma radiometric
age has been assigned
to
a flCM in the basement
sequence
on Santa Isabel,
where intercalated
pelagites
near the top are as old as late Paleocene.
K/Ar
apparent ages of
metamorphism on Choiseul
range from 32 to 51 Ma and on the Florida
Is lands
from 35 to 44 Ma.
Elongate belts,
pods and slab-like
bodies of ultramafic
rocks, chiefly
serpentinized
harzburgite,
are present
on Choiseul,
Santa Isabel,
Florida
Islands,
Guadalcanal,
and San cristobal.
It
seems likely
that
they are
protruded
masses and thrust
sheets
that
represent
either
dismembered and
remobilized
ophiolite
or fragmented roots of an island
arc.
A gabbroic rock
from the largest
belt
on Small Nggela in the Florida
island
group gave K/Ar
apparent
ages of 36.7
and 38.4
Mai elsewhere
these
rocks have not been
dated.
Detritus
shed from these
bodies first
appears
in strata
of early
Miocene age.
Coulson,
Vedder:
Island
Geology
59
Island-arc tholeiites
and related intrusive complexes dominate the late
Eocene(?) to early Miocene subduction-generated volcanic sequences on the
Florida Islands and Qladalcanal.
The volcanic rocks are chiefly basalt and
basaltic andesite and subordinate andesite.
Intrusions of dioritic
rocks are
ccemcnpf.ace
, and mantle-derived alnoite breccia was ert;)laced in diatreme-like
structures on Malaita.
Interlayered with and overlying the flow sequences are
volcaniclastic
strata and limestone.
Volcanic flows diminish in volume in the
early Miocene sequences, and redeposited volcaniclastic
strata
increase in
thickness and abundance.
Miocene carbonate beds, inclUding hemipelagic4
shallow-shelf
and reef
deposits,
are interbedded in the volcaniclastic
sequences and generally are increasingly cOIl'lmon
upsection.
These Oligocene to
middle Miocene igneous and sedimentary rocks may extend beneath the younger
volcanogenic rocks of the NewGeorgia island group.
Late Miocene to Holocene calc-alkaline
volcanic rocks and cogenetic
epiclastic
deposits form most of the NewGeorgia island group as well as Savo,
the Russell Islands, Mborokua,and parts of the Shortland fs Lande, Choiseul,
and Guadalcanal.
Rock types include pyroxene andesite,
olivine basalt,
picrite
(New Georgia), dacite
(Fauro), and intrusive
diorite
and gabbro
(Guadalcanal). Pyroclastic
deposits are common,particUlarly
in NewGeorgia
rnd Guadalcanal. Nearshore clastic
strata
ranging from clayey mudstone to
conglomerate are present on many of the islands but are especially
well
developed on Guadalcanal, where they are more than 5,000 m thick.
Alluvial
sand and gravel underlie most of the coastal plain of northern Guadalcanal but
are very limited on the other islands.
Cort;)ositional, distributional
and deformational patterns of exposed rock
sequences reflect
three
well-defined
episodes of
Eocene and younger
tectonism.
After ocean-floor generation and accumulation of Cretaceous and
lower Tertiary basalt and pelagic limestone, an episode of Eocene and (or)
early Oligocene metamorphismaltered the oceanic basement rocks to greenschist
and amphibolite at manyplaces and resulted in a regional unconformity. The
metamorphismmay have been a response to incipient southward subduction of the
Pacific plate beneath the Australia-India
plate.
The second episode of
tectonism both accompanied and followed the late Eocene to early Miocene
subduction-related
Oligocene magmatic event and created an influx of large
amounts of volcaniclastic
detritus onto upwarped shelves and slopes.
During
the latest
Oligocene and early Miocene, relatively
extensive deposition of
limestone, calcarenite,
and marl signified cessation of volcanism.
Uplift,
tilting,
and thrusting
probably occurred throughout the Miocene and are
anifested
in rocks on Guadalcanal, Choiseul, and Santa IsabeL
A
contributing factor to regional tectonism late in the second episode may have
been oblique impingement of the Ontong Java Plateau on the old forearc area of
the Solomons, a- circumstance that possibly led to transform structures
and
initial
deformation of the Malaita anticlinorium.
The third episode was
concurrent with the postulated reversal of arc polarity and a new phase of
subduction-related
magmatism that
began near the end of Miocene time.
Emergenceof the southwestern row of islands, extension, and developnent of
the Central Solomons Trough were direct results of this third episode of
tectonism, which seems to have been most active in Pliocene and Pleistocene
time.
Thick aprons of volcaniclastic
sediment accumulated on the back
(northeast)
side of the ne.•••arc and were dammedby remnants of the older
northward-facing arc to form the upper part of the depositional sequence in
the intra-arc
basin.
Post-Miocene faulting
and minor folding probably
continued on Guadalcanal, Choiseul, Santa Isabel, and San Cristobal,
and
Coulson, Vedder:
Island Geology
60
relatively
intense folding developed on Malaita during the Pliocene.
The
building of volcanic edifices
on the Shortland Islands,
Choiseul, and
Guadalcanal and the complete burial by the volcanoes of the Ne••••
Georgia group
obscured the middle Miocene and older arc rocks throughout the southwestern
flank of the intra-arc
basin.
Late Miocene and younger fringing reefs were
built
around inactive
volcanic islands,
and associated shelf and slope
carbonate deposits ••••
ere laid downat places in environments a••••
ay from active
volcanic centers.
Detailed mapping of santa Isabel and san Cristobal
and additional
paleontologic and radiometric dates are required on all of the islands before
many of the geologic enigmas of the SOlomonIslands can be resolved.
Among
the most perpleXing problems are the poorly understood relations
along the
K.1a-Korigole-Kaipito fault system of santa Isabel.
This fault system almost
certainly reflects
a major suture between two tectonostratigraphic
terranes
represented by rocks on Ulawa, Malaita and northeastern santa Isabel on the
northeast and san Cristobal,
Guadalcanal, southwestern Santa Isabel,
and
Choiseul on the southwest.
Whether it is a compressional or transform
feature, or both, is uncertain, and the timing of movementis unresolved.
"nother problem is the imprecisely knownage of emplacementof the ultramafic
bodies, ••••
hose parent terranes remain unidentified.
The axis of the northeastfacing Oligocene arc, and the location of its forearc and backarc basins are
not yet established.
Nor are the amountsof rotation and translation,
if any,
of various parts of the arc. The offset or superposition of the southwestfacing late
Miocene-Holocene arc ••••
ith
respect
to the older
arc are
conjectural,
and the reasons for gaps in seismicity and volcanism are in
question.
Effects of the subduction of the active Woodlarkspreading axis are
not entirely resolved.
In short, nuch more needs to be learned about the
geology of SolomonIslands before a plethora of questions can be answered.
Coulson, Vedder:
Island Geology
61
REFERENCES
Arthurs,
J.W.,
1981,
The geology of the Mbarnbatana area,
chct seut ,
An
explanation
of
1:50,000
scale
geological
map sheet
CH 5:
British
Technical Cooperation western Solomons Mapping Project,
Report no. 5, 99
p.
Bielski-Zyskind,
M., G.J. Wasserburg, and P.R. Nixon, 1984, sm-Nd and Rb-Sr
systematics
in volcanics
and ultramafic
xenoliths
from Malaita,
Solomon
Islands,
and the
nature
of
the
Ontong Java
Plateu:
Journal
of
Geophysical Research, v. 89, no. B4, p. 2415-2424.
Blake, D,H., and Y. Miezitis,
1967, Geology of Bougainville
and Buka Islands,
New Guinea:
Bureau of Mineral
Resources,
Geology,
and Geophysics,
Australia,
Bulletin
no. 93, PNG 1, 56 p.
Carey, S.W., 1958, The tectonic
approach to continental
drift,
in Continental
drift--a
symposium:
Geology Department, University
of Tasmania, Hobart,
p , 177-355.
Chivas, A.R., and I. McDougall, 1978, Geochronology of the Koloula porphyry
copper deposit,
Guadalcanal,
Solomon Islands:
Economic Geology, v, 73,
P 678-689.
"oLeman, P.J.,
1957, Geology of Western Guadalcanal, .ia Marshall,
C.E., et aI,
ede ; , Geological
reconnaissance
of parts
of the central
islands
of the
British
Solomon Is-Iands
Protectorate:
Colonial
Geology and Mineral
Resources, v. 6, no. 3, p. 267-307.
1960a, An introduction
to the geology of Choiseul
in the western
Solomons, 1957;
British
Solomon Islands
Geological
Record (1957-1958),
v , " p. 16-26.
196Gb, North-central
Guadalcanal an interim geological
report:
British
Solomon Islands Geological Record {1957-19581, v. 1, p. 4-13.
1962, An outline
of the geology of Choiseul,
British
Solomon Islands:
Journal of the Geological
Society of Australia,
v. B p. 135-158.
1965, Stratigraphical
and structural
notes on the British
Solomon
Islands
with reference
to the first
geological
map:
British
Solomon
Islands Geological Record (1959-1962), v, 2, p. 17-31.
1966, The Solomon Islands as an island arc:
Nature, v. 211, p. 12491251.
1970, Geology of the Solomon and New Hebrides Islands,
as part of the
Melanesian Re-entrant,
Southwest Pacific:
Pacific
Science,
v, 24, p ,
289-314.
1975, The Solomons as a non-arc:
Bulletin
of the Australian
Society of
Exploration
Geophysicists,
v. 6, no. 213, p. 60-61.
1976, A re-evaluation
of the Solomon Islands
as an Arc System, l.£. G.P.
Glasby and H.R. Katz,
eds ; , Marine geological
investigations
in the
Southwest Pacific
and adjacent
area: CCOP!SOPAC
Technical
Bulletin,
no.
2, p. 134-140.
and L.W. Kroenke, 1981, Subduction without volcanism in the Solomon
Islands arc:
Geo-Marine letters,
v, 1, p , 129-134.
and G.H. Packham, 1976, The Melanesian Borderlands
and India-Pacific
Plates'
boundary:
Earth-Science
Reviews, v. 12, p. 197-233.
---:;-;:0 et aL, 1965, A first
geological
map of the British
Solomon Islands,
1962,
2:!!.. Reports
on
the
geology,
mineral
resources,
petroleum
possibilities,
volcanoes,
and seismiscity
in the Solomon Islands:
Record
of the Geological
Survey of the British
Solomon :;:slands (1959-1962), v,
2, Report no. 28, p , 16-17.
Coulson,
Vedder:
Island
Geology
62
___
.,.._ B. McGowran, and W.R.H. Ramsay, 1978, New, early
Tertiary, ages for
basal pelagites,
northeastern
Santa Isabel,
Solornon Islands (central
southwest
flank,
Ontong
Java Plateau):
Bulletin
of the Australian
Society of Exploration Geophysicists,
v. 9, no. 3, p. 110-114.
:urtis,
J. W., 1973, Plate
tectonics
and the Papua Ne••••Guinea-Solomon
Islands
region:
Journal of the Geological Society of Australia, v. 20, pt. 1, p.
21-36.
Danitofea,
5.,
1978, The Geology of Ulawa Island:
solomon Islands Geological
Survey Bulletin No.4
(unpublished).
and C.C. Turner,
1981, The geology of the Russell
Islands
and
Mborokua:
Solomon
Islands
Geological
survey
Bulletin
No.
1"2
(unpublished) •
Dunkley,
P.M.,
1983, Volcanism and the evolution
of the ensimatic
Solomon
Islands
Are, in o. Shi.mozuru and I. Yokoyama,
eds ,, Arc volcanism:
physics and tectonics:
Terrapub, TOkyo, p. 225-241.
Falvey,
O.A., 1975, Arc reversals,
and a tectonic model for the North Fiji
Basin:
Bulletin of the Australian Society of Exploration
Geophysicists,
-e, 6, no. 213, p. 47-49.
Hackman,
B.D.,
1968,
Observation
on
folding
in
the
Oligocene-Miocene
limestones
of central
Kwara'ae,
Malaita:
British
Solomon
Islands
Geological Record (1963-1967), v. 3, Report no. 76, p. 47-50.
1973, The Solomon Islands fractured are, .!!L P.J. Coleman, ed,, The
Weste.rn Pacific:
Island arcs, marginal seas, geochemistry:
University
of Western Australia Press, p. 179-191.
1979, The geology of the Honiara area, Guadalcanal:
Solomon Islands
Geological Survey Bulletin, no. 3, 40 p.
1980, The geology of Guadalcanal,
Solomon Islands:
Overseas Memoir of
the Institute of Geological
Sciences, no. 6, Her Majesty's
Stationery
Office, London, 115 p.
Hill, J.H., 1960, Further exploration
in the Betilonga area of Guadalcanal:
British Solomon Islands Geological Record (1957-1958), v. 1, p. 81-94.
Hughes,
G.W.,
1977, The
geology
of the Lungga
Basin
area, Guadalcanal:
Solomon Islands Geological
Survey Bulletin No.6
(unpublished).
1981, The geology of the Ririo area, ChoiseUl, an explantation of the
1:50,000
Geological
Map
Sheet CH 4:
British
Technical
Cooperation
Western Solomons Mapping Project, Report No.5,
55 p.
1982, Stratigraphic correlation
between sedimentary basins of the ESCAP
region, Solomon Islands, in UN ESCAP Atlas of Stratigraphy
III, Mineral
Resources Development
Series:
United Nations, New York, p. 115-130.
and C.C. Turner,
1976, Geology of South Malaita:
Solomon Islands
Geological Survey Bulletin No.2,
80 p., 3 maps.
1977, Upraised Pacific Ocean floor, southern Malaita, Solomon
Islands:
Geological Society of America Bulletin, v. 88, p. 412-424.
Jeffery,
D.H., 1975a, Arosi,
San Cristobal
Sheet SC 1:
Geological
survey,
Honiara, Guadalcanal, Solomon Islands, 1:50,000.
1975b, Arosi--West Bauro, San Cristobal sheet SC 2: Geological Survey,
Honiara, Guadalcanal, Solomon Islands, 1:50,000.
Karig, D.E., and J. Mammerickx,
1972, Tectonic framework of the New Hebrides
island arc:
Marine Geology, v. 12, p. 187-205.
Kroenke, L.W., 1972, Geology of the Ontong Java Plateau:
Hawaii Institute of
Geophysics, Report no. HIG-72-5, University of Hawaii, 119 p.
Coulson,
Vedder:
Island Geology
63
1984, Solomon Islands:
San Cristobal
to Bougainville
and Buka, in LeW.
Kroenke, ede,
Cenozoic tectonic
developnent
of the southwest
Pacific:
CCOP/SOPAC
Technical Bulletin,
nOe 6, che 4, 22 p.
Neef, G.,
1978, A convergent
subduction
model for
the
Solomon Islands:
Bulletin
of the Australian
Society of Exploration
Geophysicists,
v, 9,
no. 3, p. 99-103.
1979, cenozoic stratigraphy
of Slnall Nggela Island,
Solomon Islands-early Miocene deposition
in a forearc
basin followed by Pliocene
patch
reef deposition:
New zealand Journal
of Geology and Geophysics, -r, 22,
no. 1, p , 53-70
and r , McDougall, I.,
1976, Potassium-argon
ages on rocks from small
Nggela Island,
British
Solomon Islands,
s. W. Pacific:
Pacific
Geology,
v» 11, p. 81-86
and I.R. Plimer,
1979, Ophiolite
complexes on small Nggela Island,
Solomon Islands;
summary: Geological Society of America Bulletin,
pt.
1,
v. 90, p.136-138.
Nixon, P.H.,
1980, Kimberlites
in the southwest Pacific:
Nature, v- 287, p;
718-720.
1978, Garnet-bearing
lherzolites
and discrete
nodule suites
from the
Malaita alnoite,
so tceon Islands,
and their
bearing
on the nature
and
origin of the Ontong Java Plateau:
Bulletin
of the Australian
Society of
Exploration
Geophysicists,
v. 9, nO. 3, pe 103-107.
Packham, G.B.,
1973, A speculative
Phanerozoic
history
of the Southwest
Pacific,
in
P.J.
Coleman, ed.,
The western
Pacific:
island
arcs,
marginal
se;s,
geochemistry:
University
of Western Australia
Press,
p,
369-388.
Plimer,
I.R.,
and G. Neef,
1980, Early
Miocene extrusives
and shallow
intrusives
from Small Nggela, Solomon Islands:
Geological
Magazine, v ,
117, no. 6, p , 565-578.
Proctor,
W.D., and C.C. Turner,
1977, The geology of save Island:
Solomon
Islands Geological Survey Bulletin
No. 11, 44 p.
Ramsay, W.R.H., 1978, Field,
mineralogical
and structural
observations
on some
basement rocks,
southeast
Choiseul,
Solomon Islands:
Bulletin
of the
Australian
Society of Exploration
Geophysicists,
v. 9, p. 107-110.
Ravenne, C., C.!.
de aroin,
and F. Aubertin,
1977, Structure
and history
of
the Solomon-New Ireland
region, ~
International
symposium on geodynamics
in the
SW Pacific,
New Caledonia,
August-september
1976:
Editions
Technip, Paris,
p. 37-50.
Rickwood, F.K.,
1957, Geology of
the
island
of Malaita,
.!E.. Geological
reconnaissance
of pair
of the central
islands
of the British
Solomon
Islands
Protectorate:
Colonial
Geology and Mineral gescurces , v, 10,
no. 2, p. 112-145.
Richards,
J.R.,
A.W. webb, J.A. Cooper, and P.J.
Coleman, 1966, Potassiumargon measurements of the age of basal
schists
in the British
Solomon
Islands:
Nature, v, 211, p , 1251-1252.
Smith, A., 1980, The geology of the Nuatambu area, Choiseul: an explanation
of
1: 50, 000 Geological
Map Sheet
CH 8:
British
Technical
Cooperation
western Solomons Mapping Project,
Report no. 8, 83 p.
Solomon Islands
Department of Geological
Surveys, 1969, Geological map of the
British
Solomon Islands:
Department of Geological
Surveys,
Honiara,
Guadalcanal,
Solomon Islands,
1: 1,000,000.
COUlson, Vedder:
Island
Geology
64
Stanton,
R.L.,
1961, Explanatory
notes to accompany a first
geological
map of
Santa Isabel,
British
Solomon Islands Protectorate:
Overseas Geology and
Mineral Resources,
v. 8, no. 2, p. 127-149.
and W.R.H. Ramsay, 1975, Ophiolite
basement complex in a fractured
island
chain,
santa
Isabel,
British
Solomon Islands:
Bulletin
of the
Australian
Society of Exploration
Geophysicists,
v, 6, no. 2/3,
p. 61-
64.
strange,
P.J.,
1981a, The geology of the Xomboro and Rob Roy Island
area,
Choiseul: an explanation
of the 1:50,000 Map Sheet CH 11:
British
Technical Cooperation Western Solomons Mapping Project,
Report no. 11, 72 p.
______
~ 1981b, The geology of the Katurasele
area, Choiseul; an explanation
of
the
1: 50,000 Map Sheet CH 6:
British
Technical
Cooperation
Western
Solomons Mapping project,
Report no. 6, 57 p.
Taylor, B.R., and N.F. Exon, 1984, An investigation
of ridge subduction in the
Woodlark--Solomons region:
introduction
and background, .!!!.. N.F. Exon and
B.R. Taylor,
compilers,
Seafloor
spreading,
ridge subduction,
volcanism
and sedimentation
in the offshore Woodlark-Solomons region and Tripartite
cruise report
for Kana Keoki cruise 82-03-16, Leg 4, CCOP/SOPAC
Technical
Report no. 34, p , 1-42.
Taylor,
G.R.,
1965, The Paraso
thermal
area,
Vella
teve Lfa , preliminary
report:
Solomon Islands GeOlogical Survey Bulletin,
no. 1, 12 p.
1976, Styles
of mineralizationin the Solomon Islands--a
review,
in
G.P. Glasby and H.R. Katz, eds •., Marine geological
investigations
in the
Southwest Pacific
and adjacent
areas:
UN ESCAPTechnical Bulletin
2, p.
83-91.
1977, Florida
Islands
Geological Map Sheet FL 1:
Geological
Survey,
Honiara, Guadalcanal,
Solomon Islands,
1:50,000.
Thompson, R.B.M.,
1958, The geology of the Florida
Group, .!!:!. The Solomon
Islands-geological
exploration
and research,
1953-1956:
Memoir of the
Geological Survey of the British
Solomon Islands,
no. 2, p. 97-101.
1960, The geology of the ultrabasic
rocks of the British
Solomon
Islands:
Unpublished Ph.D. thesis,
University
of Sydney, Australia.
1968, Southwest Guadalcanal--the
Itina
River Basin:
British
Solomon
Islands Geological
Record (1963-1967), v. 3, p. 9-14.
and P.A. PUdsey-Dawson, 1958, The geology of eastern
san Cristobal:
Memoir of the Geological
Survey of the British
Solomon Islands
1955-1956,
v, 2, p. 90-95.
Turner,
C.C.,
1975, The geology of Mborokua:
Solomon Islands
Geological
Survey Bulletin
No.7,
15 p.
1978, Shortland
Islands,
Shortland
Islands
Geological
Map Sheet SH
1A: Geological
Survey, Honiara, Guadalcanal,
Solomon Islands,
1:50,000.
and B.D. Hackman, 1977, The geology of the Beaufort
Bay area,
Guadalcanal:
Solomon Islands
Geological
Survey
Bulletin
No.
9
(unpublished) •
and J. Ridgway, 1982, Tholeiitic,
calc-alkaline,
and (1) alkaline
igneous rocks of the Shortland Islands,
Solomon Islands:
Tectonophysics,
v. 87, p. 335-354.
van Oeventer,
J.,
and J.A. Postuma, 1973, Early Cenomanian to Pliocene
deep
marine sediments from North Malaita,
Solomon Islands:
Geological society
of Australia
Journal,
v. 20, part 2, p. 145-152.
Coulson,
Vedder:
Island
Geology
65
weissel, J. K., B. Taylor, and G. D. Karner, 1982, The opening of the Wbodlark
Basin, subduction of the Wbodlark spreading system, and the evolution of
northern Melanesia since mid-Pliocene time:
Tectonophysics v, B7, p.
253.
Wright, P.C., 1968, Western Guad,~lcanal--the geology of the Lungga and Tenaru
River systems: British Solomon Islands Geological Record (1963-1967), v.
3, p. 25-40.
Coulson, Vedder:
Island Geology
66
CORRELATIONOF ROClt UNITS IN THS SOLOfI)N ISLANDS
U.S. Geological
L S. Pound
Survey, Menlo Park,
california
94025
INTRODUCTION
The stratigraphic
columns shown in this
paper were compiled from ene
available
literature
on the Solomon Islands
(See map packet).
Each colwnn
represents
the rock succession on parts of islands or on entire
island
groups,
as designated
on Figure
1.
Rock type,
thickness,
contact
relations,
and
chronology are indicated
on the columns. The stratigraphic
names used are from
the most recent
literature,
except where otherwise
indicated
in the text.
Where previous work has resulted
in conflicting
stratigraphic
nomenclature and
ages
(Ls e ,
san Cristobal),
the
available
data
are tabulated
(Table
1).
Interpretations
of regional
tectonic
activity
are given in the right-hand
column and are discussed
at the end of this paper.
The columns presented
in
this paper differ
from those of Coulson and Vedder (this volume) both in their
format, and in the correlation
of some of the rock units.
Some of the most critical
problems of stratigraphic
correlation
are
described
belo,., on an island-by-island
cae re , particularly
at points
where
they differ
from other
correlations
(Hughes, 1982; Coulson and Vedder, this
voLume
L,
For a detailed
description
of the on-land geology, see Coulson and
Vedder (this
volume).
In some regions
stratigraphic
data are sparse because
of the limited
amount of geologic exploration,
particularly
in such key areas
as Santa Isabel.
Recent work (Turner and Hughes, 1982; Turner and Ridgway,
1982; Dunkley, 1983) has focused on regional
syntheses of the sedimentary and
igneous rocks,
but earlier
studies
focused on local formational
units
rather
then regional
cor re.Lat.Lcns,
Where rocks of similar
lithologies
are now in
fault
juxtaposition,
such as the Miocene strata
(the Tanakau Group and the
Longuhutu and Bero Beds) or the Eocene~ligocene
volcanic
rocks (the Sigana
Volcanics) on santa I$abel,
correlations
have, in the past,
been made without
reservation
across
major
faults.
These correlations
probably
should
be
reevaluated.
Age control
is limited
to Cenozoic and late Mesozoic dates based upon
foraminiferal
assemblages in the sed1mentary rocks and radiometric
dates on a
few
volcanic
and
intrusive
rocks.
Hughes
(1981a)
summarized
age
determinations
for Solomon Islands rocks.
The tectonic
structure
of the Solomon Islands
region is complex and not
fully understood
(Vedder and Coulson, this volume). Development of a definite
model for the tectonic
evolution
of the region is hampered by the shortage of
detailed
petrogenic
data,
the structural
compleXity of the region,
and a
tendency to keep the model eImpLe,
Pound:
Correlation
67
SHORTLAND
ISLANDS
The geochemical
data obtained
from the volcanic
and intrusive
bodies of
the Shortla.nd Islands
remain problematic;
for details
refer
to Turner (1978),
Turner and Ridgway (1982) and Dunkley (1983).
other
workers have coxre Je'ee d
the basal volcanics
on Mono and Alu with basement volcanics
(Voza Lavas) on
Choiseul (Coulson and Vedder, this volume), althoUgh no radiometric
ages have
been obtained
for the basement rocks on Mono and Alu.
NEWGEORGIA
GROUP
Recently
published
geologic
maps of
Vella
Lavella,
Kolombangara,
Kohinggo, Parara,
Ranongga, Simbo, and Ghizo Islands
in the New Georgia Group
show many new formation
names for
the
highly
variable
volcanogenic
and
sedimentary sequences comprising the variable
island
successions
(Smith et aI,
1982: Abraham et aI,
1984( Abr.aham, Smith, and Hughes,
1982).
These new
formation names are not shown on the correlation
chart,
which- is based on the
reconnaissance
work of Stanton and Bell (1969).
The recent mapping confirms
the highly
variable
and complex nature
of the
volcanogenic
rocks,
which
include
large
volumes of olivine-rich
basalts
and picrites
as well as minor
amounts of hornblende
andesite
and two-pyroxene andesite
(Dunkley, 1983).
CHOISEUL
The Choiseul
Schists
and the Voza Lavas were treated
as two separate
units
by Coleman (1965).
Ramsay (1978), however, showed that in Ologholata
Harbour in southeast
Choiseul
the Voza Lavas may be the unmetamorphosed
equivalent
of the Choiseul
Schists
- an interpretation
which recent
mapping
elsewhere on Choiseul
seems to confirm (Arthurs,
1981; Hughes, 1981b, 1981c,
1981d. 1981el Philip,
1981; Smith,
1981: Strange,
1981a, 1981b, 1981c).
In
southeast
Choiseul,
a 44 ms y , +/- 18 m.y. radiometric
age is reported
for the
Choiseul
Schists
(Richards
et; aI,
1966).
Schists
in central
and northwest
Choiseul
are
interpreted
in this
report
as t¥ne' equivalent
to those
in
southeast
CnoLaeuL,
some:w-orkers"(Arthurs;
1981; Smith,
1981) have included
amphibolites
and greenschists
within the Voza Lavas.
However, in this report
metamorphic rocks are considered
as Choiseul Schist.
The contact
between the Choiseul
Schist
and the Voza Lavas has been
described
as both gradational
(Strange,
1981a) and faulted
(Hughes, 1981b).
Arthurs
(1981 l used
the
term
Upper Kolombangara
Tectonic
Unit
for
an
amphibolitized
zone of faulted
Choiseul Schists
and Voza Lavas that marks the
contact.
In southeast
and central
Choiseul,
the Voza Lavas are intruded
by
gabbroic
rock
(the
Oaka Metamicrogabbrol
that
has been ae cemorpnceed to
amphibollte
facies.
Gabbroic and diabasic
dikes intruding
the Voza Lavas in
northwest
Choiseul
are assumed to be coeval with the Qaka intrusive
body
(Strange,
1981a) of southeast
Choiseul.
In southeast
Choiseul
the Siruka
Ultramafics
overlie
the
Choiseul
Schists
and Voza Lavas
and have
been
interpreted
as a nearly
horizontal
thrust-emplaced
slab
(Hughes, 1981d).
The
timing for the emplacement of the Siruka Ultramafics
is unclear.
In Southeast
Choiseul,
the Voza Lavas,
Choiseul
Schists,
and Siruka
Pound:
Correlation
68
Ultramafics
are either
unconformably overlain
by, or in fault
contact
with,
the early Pliocene and younger KomboroVolcanics or the Vaghena Formation. The
xomboro
Volcanics
and
Vaghena Formation
are
discrete
time-equivalent
sequences.
Hughes (1981e) suggested that
the Vaghena Formation is in part
volcanic.
Northward, in central
Choiseul,
the Vaghena thins and contains midMiocene to
Pliocene
foraminifers.
The relationship
between
the
Vaghena
Formation with the underlying
Mole Formation in central
Choiseul is unclear
(Smith, 1981; Philip,
1981).
In central
and northwest Choiseul,
the Choiseul Schist and the Voza Lavas
are
unconformably
overlain
by the
Mole Formation.
There are conflicting
interpretations
regarding
a pyroclastic
unit that
Coleman (1965) and Hughes
(1981f) described as the basal Koloe Breccia member of the Mole Formation; the
unit
described
by Coleman (1965) and Hughes (1981f) contains
late Oligocene
foraminifers
(Hughes, 1981f).
Philip
(1981) described
the upper Voza Lavas as
containing
pyroclastic
material.
Smith (19B1) postulated
that
the Maetarnbe
Volcanics
lie
on the voaa Lava basement and are interbedded
throughout
the
Mole
Formation.
The xo Loe Breccia is here shown as the basal eember of the
Mole Formation,
and the Maetambe Volcanics
are shown to overlie
the Mole
Formation gradationally.
In northern northwest Choiseul,
the Mole Formation grades upward into the
Pemba Formation. The relationship
of the Pemba to the time-equivalent
Maetambe
Volcanics
of central
Choiseul has not yet been established.
The Pleistocene
Nukiki LiJnestone is
at the top of the sedimentary
sequence throughout
the
island.
SANTAISABEL
Relatively
little
work has been done on Santa
Isabel.
Most workers
(Stanton,
1961; Coleman, 1965) have assumed that
the volcanic
piles
to the
north and south of the Kaipito-Korigole-Kia
faults
are related,
as is implied
by their
commonname, Sigana Volcanics.
Furthermore,
it has been assumed that
the sedimentary
sequences
overlying
the Sigana Volcanics
to the north and
south
of the Kaipito-Korigole-Kia
faults
have a common origin.
However,
Coulson
and Vedder
(this
volume)
suggest
that
because
these
rocks
are
separated
by a maJor tectonic
structure
(the
Kaipito-Korigole-Kia
fault
system) they are unrelated.
The sedimentary
rocks northeast
of the Kaipito-Korigole-Kia
faults
have
collectively
been called
~he Tanakau Group (stanton,
1961; Coleman, 1965). The
coeval sedimentary
rocks to the south .••.
est of the faults
have been termed the
Longuhutu beds on San Jorge or the Sero and Rob Roy beds on Santa Isabel.
The
age of the Kia-Kaipito-Korigole
faults
is not known, and their
extent
and
nature beyond the northwest end of the island is uncertain.
Likewise, the age
and time of eIlFlacement of the ultramafiC
bodies is unknown.
The original
spatial
relationship
of the Sigana Volcanics to the basement complex and the
Vi toria
Microgabbros
is not evident
from the rock descriptions
or from the
geologic
map. The location
of the volcanic axis for the Sigana VolcaniCS is
not eva denu,
FLORIDAISLANDS
Mapping
significantly
Pound:
in northwestern
Nggela Sule shows that
the
geology differs
from that of Nggela Pile.
No attempt is made here to correlate
Correlation
69
the geology of Nggela Sule and Nggela Pile.
On Nggela Pile the east-trending
Hanuvaivine ultramafic
belt
separates
the northern
and southern
sedimentary
sequences.
The Siota beds to the north of the Hanuvaivine ultramafic
belt are
cut by the north-trending,
possibly
diapiric,
Siota Ultramafics.
The timing
and nature
of the ultramafite
eltq)lacement is not clear.
To the south of the
Hanuvauvine, the Ghumba beds in the eastern
part of the island
were folded
during middle to late Miocene time.
GUADALCANAL
Hackman (1980) provided the most up-to-date
synthesis
of the geology of
Guadalcanal,
as well as a review of the previous
geologic
investigations.
Hackman (1980) divided the pre-Miocene basement into (1) The Mbirao Group and
(2) The Guadalcanal Ultrabasics.
Nomenclature within the pre-Miocene basement
rocks
is
confusing.
The Guadalcanal
Ultrabasics
are termed the Ghausava,
Itina,
Suta, or Marau Ultrabasics,
depending on their
geographic location
and
structural
position.
Likewise, the Mbirao Group has been subdivided
(Hackman,
1980) into
(1) The Mbirao Metabas'ics;
(2) The Guadalcanal
Gabbro, (3) The
Mbirao Dolerites,
(4) The Tetekanji
Limestones,
an~ (-S). The Mbirao Volcanics.
Some of these units are further
subdivided
locally
(Hackman, 1980).
Limi ted
field
data
hinder
both
correlations
and
structural
interpretations
within the pre-Miocene basement.
Geochemical analyses of the
basement rocks are not adequate to determine their affinities
or deformational
history.
The 92 +/- 20 ms y , K/Ar age (Snelling,
1970, as cited in Hackman,
1980) for
the Guadalcanal
Gabbro is the only age control
within
the preMiocene basement.
The place the basement units
initially
formed in relation
to their
present
position,
or the position
of the Ontong-Java Plateau cannot
be determined from the on-land geology of Guadalcanal.
The Miocene and Pliocene sedimentary
sequences show complex intertonguing
relationships,
and their
correlation
across the island is uncertain.
Pliocene
and Pleistocene
volcanic and intrusive
rocks on the western part of the island
have complex and varied
stratigraphic
and intrusive
relationships
with the
associated
sedimentary units.
SANCRISTOBAL,UKI NI MASIANDPIO
Stratigraphic
nomenclature
for San Cristobal
is here based on that
of
Jeffery
(1975 a,b).
For regions
not covered by Jeffery
(1975 a,b),
the
stratigraphic
nomenclature
of Turner and Hughes (1982),
Hughes (1982), and
Coleman (1965) is integrated
with Jeffery's
(1975 a,b) system.
The changes in
stratigraphic
nomenclature
and age assignments
(Table 1) on San Cristobal
reflect
the lack of extensive
rgional
geologic data for San Cristobal.
Rapid
facies
changes
probably
also
account
for
the
variability
in
ages
and
lithologies
reported
for the late
Oligocene through the Pliocene;
otherwise,
the stratigraphic
succession varies
only slightly
across the island.
The basement
is
described
as essentially
a flat-lying
sequence of
volcanics
haVing undergone low-grade
rretamorphism (Coleman, 1965; Jeffery,
1975a,b).
The amount of internal
stacking
or deformation
is unclear.
The
contact
between the basement and the overlying
sedimentary
sequence has been
variously
interpreted
(Jeffery,
1975a,bl
Turner and Hughes, 1982).
Data
available
do not document in detail
changes or hiatuses
in the sedimentary
Pound:
Correlation
70
column, or the nature and extent of the lateral
facies changes in the MiocenePliocene
sedimentary
sequence.
Extensive
block faulting
of the basement
forned grabens in which portions
of the Mi.ocene-pliocene sediments
are now
preserved.
T~ng
of the faulting
is not well constrained.
MALAITA
Formation names and descriptions
of the rocks in northwest Malaita are
from Rickwood (1957).
In
Small Malaita
(Maramasike)
the
sedimentary
succession
is generally
similar
to that 1n northwest Malaita,
but different
formation
names are used (Hughes and Turner,
1976, 1977). In SInall Malaita
lateral
variation
in the lithologies
and poorly defined gradational
boundaries
are characteristic.
The succession
in south-central
Malaita is intermediate
between that
of northwest
Malaita and Small Malaita,
and Rickwood's (1957)
nonenclature
is
used.
Thus the
Are1are
Limestone
of
Hughes (1982)
is
equLvaLent; to Rickwoods' (1957) Alite Limestone. The term Alite Limestone is
here retained
for south-central
Malaita.
The alnoitic
breccia
within
the
Alite
Limestone of northwest -.Malaita is interpreted
as having a diapiric
origin,
but the nature
of "its contact
is unclear
(Hackman, 1968; Allen and
Deans, 1965).
Zircons from the alnoitic
breccia provide a pb/ur date of 33.9
- 34.1 ms y ,
The peperite
zones of south-central
Malaita and small Malaita
have been grouped with the Malaita Volcanics
by Hughes and Turner (1976).
Recently cotrq:liled maps of Malaita (Turner,
1979; Hughes, Proctor and Hackman,
1975; Turner,
1977, Turner,
1976) provide more detailed
information
on the
geology than is shown on the correlation
chart.
DISCUSSION
Tectonic
setting
Three broad phases of development can be recognized in the rocks of the
Solomon Islands:
1. Construction
of the Cretaceous Ontong Java Plateau,
and deposition
of
the overlying pelagic
sedimentary cover, probably far from its present
site.
Formation of the oceanic basement rocks which now form the basement for most
of the islands;
mOst of these rocks are now metamorphosed.
2. Development of a northward-propagating
Oligocene arc on the oceanic
basement, an event associated
with faulting,
diapiric
ultramafic
intrusions,
and followed ~ deposition
of both forearc and backarc wedges of sediment.
3. Generation of Pliocene and Holocene arc volcanism, an event associated
with rapid regional
uplift.
The timing
and recognition
of these
three
broad phases
prOVides a
background into which the complex stratigraphic
and structural
relationships
of the Solomon Islands
can be placed, and the problems identified.
Basement Rocks
The Malaita Volcanics
and their
sedimentary
cover have been correlated
with the Ontong Java Plateau
sequence,
on the basis of age and rock type
(Hughes and Turner,
1977).
Likewise the Sigana Volcanics and the overlying
basal
pelagites
on northeast
Santa Isabel
have been correlated
with the
Pound:
Correlation
71
sequences on Malaita and the Ontong Java Plateau.
It is unclear whether the
Warahito Lavas and the associated
sediments on san Cristobal
represent
a
younger, nearer-ridge
(?)
portion
of the Ontong Java Plateau,
or a remnant. of
oceanic crust derived from the west.
The basement rocks of Choiseul,
southwest santa Isabel
and Guadalcanal
are mostly metamorphosed to greenschist
and amphibolite facies
(Coleman, 1962;
Stanton,
1961; Hackman, 1980); the age of netamorphism appears to vary. Some
basement rocks, however, are unmetamorphosed (e.g. the Voza Lavas of Choiseul,
the Sigana Volcanics
of southwest
santa
Isabel,
and the Mbirao Group of
Guadalcanal).
The relationship
of the unmetamorphosed basement rocks to the
metamorphic rocks has been variously
interpreted
(e.g.
Stanton and Ramsay,
1975).
It is not known whether the basement rocks and their
unmetamorphosed
counterparts
shared a commonorigin or were juxtaposed during the formation of
the Oligocene arc.
Baserrent rretamorphism could have resulted
from (1) near-ridge
alteration
of oceanic crust associated
with a high thermal gradient
and low pressures,
or
(2)
from tectonic
burial
and deformation
associated
with high confining
pressures
and frictional
heating
on thrust
planes.
The metamorphic mineral
assemblages developed in each of the two cases outlined
above are destinctive
(Apted
and
Liou,
1983)1
the
low-pressure
mineral
assemblage
(calcicplagioclase-actinolite-chlorite)
is
the
mineral
assemblage
reported
from
Guadalcanal
(Hackman, 1980).
The high-pressure
assemblage
(albite-epidotehornblende),
however, is the characteristic
assemblage reported
from Choiseul
and Santa Isabel
(Hackman, 19801 coleman, 1965).
Clearly,
the relationship
between the R'etamorphic basement rocks and the development of the Oligocene
arc is complex.
Oligocene
to Middle Miocene Events
The transition
from an oceanic pelagic environment to an arc environment
is
indicated
in
the
deep-water
sediments
by the
increasing
amountS of
volcanogenic material,
as recorded on the rocks of Malaita and San Cristobal.
On land, Oligocene arc development is recorded by the Suta Volcanics and the
Poha and Lungga diorites
of Guadalcanal,
the older volcanics
of the Shortland
Islands,
and the
vcee Lavas of Chci.eeuL, Arc volcanism
was followed
by
extensive
erosion of regions of high relief,
followed by basin development and
rapid sedimentation,
as is indicated
by thick accumulations
of clastic
strata
on Guadalcanal and Choiseul.
Diapiric
intrusion
of ultramafic
bodies such as
those on the Florida
Islands,
Santa Isabel,
and .Malaita probably began during
this
time.
This phase of Arc magmatism and southwest-directed
subduction was
shut down by the collision
of the Ontong Java Plateau,
which subsequently
created the reversal
of arc polarity.
Pliocene
to Holocene Events
The geochemically
anomalous volcanic rocks on NewGeorgia, together
with
the
volcanic
centers
on Choiseul
and Guadalcanal
attest
to
recent
arc
volcanism
(Dunkley, 1983; Coleman and Kroenke, 1981).
These volcanics
were
generated
by northeast-directed
subduction of the active
Woodlark spreading
ridge,
which commenced during latest
Miocene.
Rapid uplift
is indicated
by
uplifted
limestone reefs
throughout the Solomons region.
Pound:
Correlation
72
REFERENCES
Abraham, 0., A. Smith, and G.W. Hughes, 1982, Ranongga, Simbo, and Ghizo
Islands, New Georgia Geological Map Sheet NG 2:
Geological Survey,
Honiara, Guadalcanul, Solo~n Islands, 1:100,000.
P.N. Dunkley, G.W. Hughes, and A. Smith, 1984, Kolombangara, Kohinggo,
and Parara Islands, New Georgia Geological Map Sheet NG 3: Geological
Survey, Honiara, Guadalcanal, Solomon Islands, 1:100,000.
Allen, J.B., and T. Deans, 1965, An alnoite breccia associated with the
ilrnenite-pyrope gravels of Malaita, 1962: The British Solomon Islands
Geological Record (1959-1962), v. 2, Report no. 49A, p. 136-138.
Apted, M.J., and J.G. Liou, 1983, Phase relations among greenschist, epidote
amphibolite, and amphibolite in a basaltic system, in H.J. Greenwood,
e d.; , Orville volume, Studies in metamorphism and metasomatism: American
Journal of Science, 283-A, p. 238-354.
Arthurs, J.W., 1981, The geology of the Mbambatana area, Choiseul, an
explanation of the 1:50,000 Geological Map Sheet CH 5: British Technical
Cooperation Western Solomons Mapping ProJect, Report no. 5, 99 p.
Coleman, P.J., 1962, An outline of the geology of Choiseul:
Journal of the
Geological Society of Australia, v. 8, pp. 135-58.
1965, Stratigraphical and structural notes on the British Solomon
Islands with reference to the first geological map:
British Solomon
Islands Geological Record (1959-1962), v. 2, Report no. 29, p. 17-31.
B. McGowran, and W.R.H. Ramsay, 1978, New, early Tertiary, ages for
basal pelagites, northeastern Santa Isabel, Solomon Islands (central
southwest flank, Ontong Java Plateau):
Bulletin of the Australian
Society of Exploration Geophysisists, v. 9, no. 3, p. 110-114.
and L.W. Kroenke, 1981, Subduction .••.
ithout volcanism in the Solomon
Islands arc: Geo-Marine Letters, v. 1, p. 129-134.
Dunkley, P.N., 1983, Volcanism and the evolution of the ensimatic Solomon
Islands Are, in D. Shinozura and I. Yokoyama, eds., Arc volcanism:
physics and tect;nics: Terrapub, Tokyo, p. 225-241.
Hackman, B.D., 1968, Observations on folding in the Oligocene-Miocene
limestones of central KWara'ae, Malaita:
British Solomon Islands
Geological Record (1963-1967), v. 3, Report no. 76, p. 47-50.
1980, The geology of Guadalcanal, Solomon Islands: OVerseas Memoirs of
the Institute of Geological Science, Her Majesty's Stationery Office,
London, no. 6, 115 p.
Hughes, G.w., 1981a, catalogue of age determinations of Solomon Islands
rocks:
Occasional Paper, Geological Survey, Honiara, Guadalcanal,
Solomon Islands, unpaginated.
1981b, The geology of the Ririo area, Choiseul: an explanation of the
1:50,000 Geological Map Sheet CH 4:
British Technical Cooperation
Western Solomons Mapping Project, Report no. 5, 55 p.
1981c, The geology of the Panggoe area, Choiseul; an explanation of the
1:50,000 Geological Map Sheet CH 7:
British Technical Cooperation
western Solomons Mapping Project, Report no. 7, 50 p.
1981d, The geology of the Claka and Siruka Bay area, Choiseul; an
explanation of the 1:50,000 Geological Map Sheet CH 10:
British
Technical Cooperation Western Solomons Mapping project, Report no. 10, 92
p.
1981e, The geology of Vaghena Island, Choiseul; an explanation of the
1:50,000 Geological Map Sheet CH 12:
British Technical Cooperation
Western Solomons Mapping Project, Report No. 12, 59 p.
Pound:
Correlation
73
1981f, The micropaleontology
of the Shortland Islands
and Choiseul:
British
Technical
Cooperation Western Solomons Mapping project,
Report
no. 17, 42 p.
1982, Stratigraphic
correlation
bet~een sedimentary basins of the ESCAP
region,
Volume VIII,
Solomon Islands,
ESCAPAtlas of Stratigraphy
III:
Mineral Resources Development series,
United Nations, New York, no. 48,
p. 115-130.
____~~W.D. Procter,
and B.D. Hackman, 1975, North 'Are'are,
Malaita Map Sheet
ML 12:
Geological
Survey,
Honiara,
Guadalcanal,
Solomon Islands,
1:50,000.
and C.C. Turner,
1976, Geology of southern Malaita:
Solomon Islands
Geological
Survey Bulletin,
no. 2, 80 p.
1977, Upraised Pacific
Ocean floor,
southern Malaita,
Solomon
Islands:
Geological Society of America Bulletin,
v. 88, p. 412-424.
Jeffery,
D.H., 1975a, Arosi,
san Cristobal
Sheet SC 1:
Geological
Survey,
Honiara, Guadalcanal,
Solomon Islands,
1:50,000.
1975b, Arosi-West Bauro, san Cristobal
Sheet SC 2:
Geological Survey,
Honiara, Guadalcanal,
Solomon Islands,
1:50,000.
Neef, G., 1979, Cenozoic stratigraphy
of Small Nggela Island,
Solomon ~s~~~dsearly
Miocene deposition
in a forearc
baa In followed by Pliocene patch
reef
deposition:
New zealand Journal
of Geology and Geophysics, v, 22,
no. 1, p. 53-70.
____~cand
I. McDougall, 1976, Potassium-argon ages on rocks from Small Nggela
Island,
British
Solomon Islands,
S.W. Pacific:
Pacific
Geology, v, 11,
p. 81-96.
and loR.
Plimer,
1979, Ophiolite
complexes on small Nggela Island,
Solomon Islands;
summary: Geological
Society of America Bulletin,
part
1, v, 90, p. 136-138.
Nixon, P.H.,
1980, Kimberlites
in the southwest Pacific:
Nature, v, 287, p ,
718-720.
_______ and P.J. Coleman, 1978, Garnet-bearing
lherzolites
and discrete
nodule
suites
from the Malaita alnoite,
Solomon Islands,
and their
bearing on
'the nature
and origin
of the Ontong-Java Plateau:
Bulletin
of the
Australian
Society of Exploration Geophysicists,
v. 9, no. 3, p. 103-107.
Philip,
P.R.,
1981, The geology of
the
Mt. Sambe area,
Choiseul;
an
explanation
of
the
1:50,000
Map Sheet
CH 9:
British
'recnn.rcaI
Cooperation Western Solomons Mapping Project,
Report no. 9, 38 p.
Ramsay, W.R.H., 1978, Field,
mineralogical,
and structural
observations
on
sorre basement rocks,
S.E. Choiseul,
Solomon Islands:
Bulletin
of the
Australian
Society of Exploration Geophysicists,
v. 9, no. 3, p. 107-110.
Richards,
J.R.,
A.W. Webb, J.A. Cooper, and P.J.
Coleman, 1966, Potassiumargon measurements of the ages of basal schists
in the British
Solomon
Islands:
Nature, v, 211, p. 1251-1252.
Rickwood, F.K.,
1957, Geology of
the
Island
of
Malaita
in
Geological
Reconnaissance
of parts
of the central
islands
of the British
Solomon
Islands
Protectorate:
Colonial Geology and Mineral Resources, v. 6, no.
3, p. 300-306.
Smith, A., 1981, The geology of the Nuatambu area, Choiseul, an explanation
of
the
1: 50 ,000 Map Sheet CH 8:
British
Technical
Cooperation
western
Solomons Mapping Project,
Report no. 8, 83 p.
___ .,,-_
P.R. Philip,
P.J. Strange, and G.W. Hughes, 1982, Vella Lavella Island,
New Georgia Geological
Map Sheet NG 1:
Geological
Survey,
Honiara,
Guadalcanal,
Solomon Islands,
1:100,000.
Pound:
Correlation
74
Stanton,
R.L•• 1961, Explanatory
notes to accompany a first
geological
map of
santa Isabel, British Solomon Islands Protectorate:
Overseas Geology and
Mineral Resources, London, v. 8, no. 2, p. 127-149.
_
__ -=::-:-;~, and J.D. Eell,
1965, New Georgia Group, a preliminary
geological
statement:
British
Solomon Islands Geological ~cord (1959-1962), v, 2,
Report no. 3', p. 35-36.
, 969, Volcanic and associated
rocks of the NewGeorgia group,
British
Solomon Islands
Protectorate:
Overseas
Geology and Mineral
Resources,
v, 10, no. 2, po 113-145.
and W.R.H. Ramsay, 1975, Ophiolite
basement complex in a fractured
island
chain,
santa
Isabel,
British
Solomon Islands:
Bulletin of the
Australian Society of Exploration Geophysicists,
v. 6, no. 2/3, p. 61-65.
Strange,
P.J.,
1981a, The geology of the O1.oiseul Bay and O1.orovanga area,
Choiseul,
an explanation
of the
1:50,000
Map Sheets CH 1 and CH 2:
British
Technical
Cooperation
western
Solomons Mapping Project,
Feport
no. 1-2, 95 p ,
1981b, The geology of the Katurasele
area, Choiseul;
an explanation
of
the
1:50,000
map sheet
CH 6:
British
Technical
Cooperation
Western
Solomons Mapping Project,
Report no. 6, 57 p.
1981C, The geology of the Komboro and Rob Roy Island area, Choise~l;
an
explanation
of
the
1: 50, 000 Map Sheet
CM 11:
British
Technical
Cooperation Western Solomons Mapping Project,
Report no. 11, 72 p.
Taylor,
G.R.,
1977, Florida
Islands
geological
map sheet FL 1:
Geological
Survey, Honiara, Guadalcanal,
Solomon Islands,
1:50,000.
Turner,
C.C., 1976, South Small Malaita,
Malaita Geological
Map Sheet ML 17:
Geological
Survey, Honiara, Guadalcanal,
Solomon Islands,
1:50,000.
1977, Central
Small Malaita,
Malaita
Geological
Map Sheet ML 16:
Geological
Survey, Honiara, Guadalcanal,
Solomon Islands,
1:50,000.
1978, Shortland
Islands
Geological
Map Sheet,
5H 1A:
Geological
Survey, Honiara, Guadalcanal,
Solomon Islands,
1:100,000.
1979, South 'Are' are, Malaita Geological
Map Sheet ML 15:
Geological
Survey, Honiara, Guadalcanal,
Solomon Islands,
1:50,000.
and G.W. Hughes, 1982, Distribution
and tectonic
illlplications
of
cre eeceoua-cue te rne ry
sedimentary
facies
in
Solomon
Islands:
Tectonophysics,
v, 87, p. 127-146.
_______ and J. Ridgway, 1982, Tholeiitic,
calc-alkaline
and (?}alkaline
igneous
rocks of the Short land Islands,
Solomon Islands:
Tectonophysics,
v. 87,
p. 335-354.
van Deventer, J.,
and Postuma, J.A.,
1973, Early Cenomanian to Pliocene
deep
marine sediments from North Malaita,
Solomon Islands:
Geological Society
of Australia
Journal,
v. 20, part 2, p. 145-152.
Pound:
Correlation
75
REGIONAL
OFFSHORE GEOLOGY OF THX SOLOMON ISLANDS
J.
U.S. Geological
Institute
Survey,
of Geological
G. Vedder
Menlo Park,
california
P. I. CoulSOD
Sciences,
Nicker Hill,
94025
Keyworth,
England
INTRODUCTION
Marine- surveys- to investigate
geologic problems and resource potent±al
in
the Solomon Islands
began about 20 years ago (Vedder and Coulson, this
volume).
Reconnaissance
geophysics
cruises
and DSDP drilling
have provided
a
great deal of information
on regional tectonics
and stratigraphy,
yet detailed
knowledge of offshore
geology remains relatively
meager.
This paper
is
intended not only as a synopsis of findings
from previous
work, but also as a
summary of new information
from the 1982 CCOP!SOPAC
R/V s.P. Lee Leg 3 cruise.
In order to facilitate
discription,
the Solomon Islands
segment of the
Melanesian Borderland
is divided into three physiographic-structural
elements
(Figures
1 and 2):
1.
2.
3.
Central
Solomons Trough and adjacent
intra-arc
Northeastern
Solomons sea-Coral
sea trench
troughs and rises.
Ontong Java Plateau and adjoining
structures.
basins.
systems
and
related
CENTRAL
SOLOMONS
TROUGH
Regional
setting
and Nomenclature
In the central
and western Solomon Islands,
the composite sedimentary and
structural
basin
that
separates
the two main island
chains
was called
the
Central
Solomons Trough by KatZ (1980).
This structural
trough supposedly is
part of a larger
feature
that was de signated
the Solomon Basin by Ravenne et
al
(1977),
a dismembered regional
structure
that
purportedly
extends northwestward along the southwest
side of Bougainville
into the New Ireland
area.
Individual
parts
of the composite
intra-arc
basin
that
forms the central
Solomons Trough and an adjoining,
separate
basin were named later
as a means
of easy reference
(Tiffin
et aL, 1983).
From northwest
to southeast,
these
structural
basins are:
Short land basin,
Russell basin,
Iron Bottom basin,
and
Indispensable
basin,
which lies
east of the Florida
Islands
platform
(Fig.
"1).
The same names were used by Maung and Coulson (1983) with slight
modification.
An additional
name, New Georgia wedge, is applied by A. Cooper et
Vedder,
Coulson:
Offshore
Geology
76
a L, (this
volume) to a large
lenticular
body of high-velocity
material
that
underlies
the southwest side of the central
Solomons Trough (Fig. 1).
As discribed
by de Brion et; al
(1977) and Ravenne et al (1977), the
Solomon Basin extends from the New Ireland
region
1,600 >em southeastward
to
the Florida Islands;
whereas the Central Solomons Trough of Katz (1980) is restricted
to the basin that underlies
NewGeorgia Sound.
In his study of offshore basins in the Solomon Islands,
Katz (1980) divided the region Lnt.c. two
geologic provinces
separated
by a major zone of tectonism.
He applied the
informal
names oceanic-pelagic
Malaita province
and main Solomon pecvance ,
which contains
the
Gentral
Solomons Trough.
Katz
recognized
internal
structural
complexities
in the trough, including possible
fragmentation
by a
fault
zone
beneath
Manning Strait
which he
believed
caused
echelon
segmentation
of the basin.
He stated
that
sediment
thickness
increases
correspondingly
with water depth and that the axial
part
of the trough is
underlain
by at least
2.5 to 4.5 kIn of relatively
undeformed strata.
He
described Pliocene and younger folds and faults that locally
are present along
the basin margins.
In a more recent report,
Maung and Coulson (19B3) proposed
the term central
Solomons Basin for the entire
semienclosed sea that stretches
from Bougainville
850 km southeastward
to its termination
in the area between
Ma-laiee- and San· Cristobal.
Kroenke
(198'4),' in
his
regional
tect.bliiic
synthesis,
attributed
the development of the central
Solomons Trough to the
reversal
of arc polarity
which resulted
in the transformation
of the former
back-arc basin into the present
intra-arc
basin.
Stratigraphic
and Structural
Intra-arc
Features
Shortland
basin. --This
basin,
which is approximately
140 Jan long and 60
km wide, is bounded by the Shortland Islands on the northwest,
Choiseul on the
northeast
and Vella Lavella on the southeast
(Fig. 2).
seismic-reflection
and
refraction
data show that the basin contains as much as 5 kIn of strata
above
acoustic
basement (A. Cooper, et aI, this volume; Bruns et aI, this volume).
Three large,
lenticular
sedimentary
bodies,
are
delimited
by conspicuous
unconformities.
The upper two bodies,
each as much as 2 km thick,
probably
were derived mainly from the late
Miocene to Holocene volcanic
arc to the
south
and west and presumably
consist
largely
of Pliocene
and younger
volcaniclastic
strata.
The lowermost body, which at places is as much as 3 km
thick,
was derived from the late Eocene (?) to early Miocene arc to the north
and northeast
and is likely
to be composed mainly of detritus
eroded from
basaltic
flows and schistose
basement.
This oldest
sedimentary
body is
interpreted
to be of late
Oligocene
and Miocene age.
Faults
and folds
generally
trend northwest in the Short land basin.
In the central part of the
basin,
folds
deform only the lower part of the section,
whereas along the
northeast
flank,
folding
affects
all but the youngest Quaternary strata.
On
the southern margin of the basin the upper two sedimentary bodies are arched
over the ridge,
possibly
in response to Q.laternary tectonism along the inner
wall of the NewBritain
Trench.
New Georgia wedge. --A lenticular
mass of relatively
high-velocity
material
occurs at shallow depth beneath the northeast
margin of the New Georgia
island
group (A. Cooper et aI, this
volume).
This high-velocity
mass may be
more than 4 kIn thick directly
east of the main island.
The wedge probably is
Vedder,
Coulson:
Offshore
Geology
77
compc ae d
largely
of
Pliocene
and Pleistocene
(1)
effusive
rocks
and
volcaniclastic
strata
that
were generated
by volcanoes
to the south
and
west.
A horst-like
feature
(Fatu 0 Moana of Taylor and Exon, 1983) overlies
the nor-ehse see rn end of the wedge midway between Choiseul and Kolombangara.
Until
1982, the highest
part of this
uplifted
feature
was interpreted
as a
submarine volcanic
center based upon its morpholoqy (Katz, 1980; Coleman and
Kroenke,
1981).
However, the surveys by R/V Kana Keoki (Exon and Taylor,
1984) and R/V S.P. Lee (Bruns et aI, this volume) showed conclusively
that the
specious
volcanic
center
consists
of warped and upfaulted
strata
of late
Pliocene
(1) and younger age.
Russell
basin. -~his
basin
is
centered
between santa
Isabel
and the
Russell
Islands
and coincides
with the
southeastern
half
of the
central
Solomons Trough (Fig. 2).
It
is the largest
single
structure
beneath New
Georgia
Sound where
it
forms
a
flat-floored
bathymetric
depression
apprOXimately 300 km long and as much as 80 km wide.
\di~
seismic reflection
profiles
show that
three well-defined
unconformities
disrupt
the 5-km thick sedimentary section
in the Russell basin (Bruns et al,
~t,l:J,is- YQl,.WlIet~
This
stratigraphic
sequence
presumably
consists
_of
volcaniclastic
rudstone
and sandstone,
largely
turbidites,
and subordinate
carbonate deposits
derived from fringing
reefs.
A segment of the NewGeorgia
wedge is included on the southwestern
flank of the basin.
At times, sediment
transport
may have been down the trough axis from sources to the north,
south
and east.
The Russell
basin
is
a downwarped structure
that
is
bordered
by
northwest-trending
step faults
on its
northern
and southern edges.
Several
small{1) northeast-trending
faults
transect
the northwestern part of the basin
and have orientations
similar
to faults
on the islands
of the New Georgia
group.
Many of the faults
affect
sea floor topography and apparently
have
been recently
active
(Colwell and Tiffin,
this volume).
Katz (1980) and Maung and Coulson (1983) suggested that folds are late
Pliocene
in age and that
the unconformable cover is Pleistocene.
However,
low-amplitude,
northwest-trending
folds and faults
south of san Jorge deform
strata
as young as Holocene.
Larger folds of unknown orientation
deform all
but the youngest beds in the ~aternary
section
in the southeastern
part of
nd
Russell basin (Bruns et a L, this volume).
Iron Bottom basin. -r-r ne expanse of water known as Iron Bottom Sound
between Guadalcanal,
Savo Island and the Florida islands
group is underlain by
the smallest
of the bathymetric
basins
within
the central
Solomons Trough
(Fig. 2).
This small feature
is approximately
4S km long and 30 km wide.
Strata
beneath
this
bathymetric
basin
are
continuous
with those
in
Russell
basin.
seismic-reflection
profiles
show well-defined
and continuous
sedimentary
reflectors
throughout
most of the section
(Vedder et aL, 1983) I
which has a maximumthickness
of about 4.5 km directly
north of Honiara (Bruns
et al,
this volume).
Most of the strata
probably were derived from igneous
and metamorphiC rocks on Guadalcanal,
mainly as turbidites
with secondary
contributions
from similar
rocks on the Florida
islands
group.
Quaternary
volcaniclastic
material
from Savo, detrital
carbonate,
and reef
limestone
probably are subordinate
constituents
in the sequence.
Minor, low-amplit1lde
folds
and faults
of unknown orientation
affect
the strata
in Iron Bottom
basin.
Vedder,
CcuLecm
Offshore
Geology
78
.~"-
less
than
1. a Jan thick
probably consist
of ~aternary
reef
detritus
and
volcaniclastic
beds.
The platform
under Bougainville
Strait
is floored
by Quaternary reefs,
beneat.h which pliocene and olde!~ rocks similar to Miocene and Pliocene clastic
strata
on Choiseul and Fauro are believed to be present.
Dredge samples (Colwell and Vedder, this volume) from the subsea ridge
west of Vella Lavella
are predominantly Q.laternary volcaniclastic
sandstone
that
represents
the
upper part
of
a
2.0-to
2.S-Jan thick
sequence
of
intercalated
flOW', pyroclastic,
and volcaniclastic
rocks of probable
late
Miocene and younger age.
These strata
appear to be gently arched over the
ridge and probably are faulted
along the southwestern flank where the slope
begins to steepen
into the NewBritain Trench.
The same beds dip northward
and thicken into the Short land basin.
Int.ra-arc
Depositional
Environments
From the foregoing,
it is concluded that the Central Solomons Trough is a
largely extensional
intra-arc
basin that began to trap sediment as early as
the late Miocene and that continued to enlarge since that time.
The islands
and
submergent
ridges
that
enclose
the
trough,
together
with
the
unconformities
in it attest
to rapid uplift
and deep erosion accompanied by
volcanism.
The sediments that formed coalescing aprons behind (southwest of)
the old,
north-facing
arc presumably consist
of detritus
that
was derived
directly
from Oligocene and older rocks on Choiseul, Santa Isabel,
the Florida
islands
group, and Guadalcanall
and substantial
lithofacies
variation
within
them is e xpect.abLe,
By analogy with island successions
(Vedder and coulson,
this volumef
Pound, this volume) Miocene reef limestone and shelf carbonate
deposits are likely
to be interspersed
with coarse clastic
strata
in offshore
areas close to the islands.
Bathyal turbidites
and slump deposits are likely
to typify
sedimentary
sequences in the deeper parts
of the trough
(Katz,
1980).
Deposits
similar
to those
of the
late
Miocene to Pleistocene
lithosornes on Guadalcanal
(Hackman, 1980) probably occur at places
in the
trough,
particularly
along
the
southwest
flank
where active
volcanism
prevailed at the time.
Katz (1980) suggested that the floor of the trough may
have subsided
as mioh
as 2,000 m during the last
1.0 m.y.,
a rate
that
corresponds to some of those implied by analysis
of dredge samples (Colwell
and Vedder, this volume;
Fasig, this volume).
The compleXity of the central
Solomons Trough and its
flanking
ridges
defies
classification
in standard
island-arc
terminology
and reflects
the
changes in pattern
of tectonism in the region.
The thickness
of sedimentary
fill
and lack of a throughgoing central
magnetic anomaly seem to disqualify
the Central
Solomons Trough as a typical
back-arc basin.
If a northeastfacing ancestral
arc is represented by the remnants of a volcanic axis through
San
Cristobal,
Guadalcanal,
southwestern
Santa
Isabel,
Choiseul
and
Bougainville,
then it seems that the northeastern
flank of this belt must have
constituted
the fringe of a fore-arc basin in Oligocene and early Miocene time
as suggested by Neef (1979).
Furthermore,
if reversal
in arc polarity
and
upl.i.ft and extension
related
to late Miocene to Holocene northeast-directed
subduction are accepted,
then the thick sedimentary fill
is attributable
to
rapid erosion of the new as well as the old island chains.
Vedder, Coulson:
Offshore
Geology
80
SOLOMON
5EA-cORAL
SEAREGION
General
setting
An irregular
tract
of deep-sea floor characterized
b¥ a rami form pattern
of ridges,
basins,
troughs
and plateaus
lies
south of the New Britain-San
Cristobal
Trench system (Fig.
1).
This segment of the Oligocene to HoLeeene
Outer Melanesian Arc (carey,
1958) may include remnants of the older (Eocene?)
Inner Melanesian Arc (Glaessner,
1950;
Hackman, 1980).
Although the crustal
structure
and tectonic
history
of this complicated plate-boundary
region have
not been completely
resolved,
Kroenke's
(1984) summary provided a reasonable
interpretion
of the early
evolution
of the various
arc components.
In his
reconstruction,
he postulated
early Tertiary
convergence and formation of the
Rennell are, early
Eocene termination
of seafloor
spreading
in the Coral sea
Basin and initiation
of subduction
along the Rennell Trench segment of the
Papuan-New caledonia
subduction
Zone.
Be further
proposed
that subduction
ended in late Eocene time when the Louisiade Plateau obstructed
the trench.
Tectonic
Elements
Woodlark Basin. --This
basin lies
southwest of the Solomons arc and borders the southern
side of the New Britain-san
Cristobal
Trench system.
The
bathymetric
basin is 3 to 4 Jan deep and is bounded on the northwest by the
Woodlark Rise
and on the
southeast
by the Pocklington
Rise.
Sediment
thickness
varies
from less than 50 m in the center of the basin to more than
500 m in local ponds near the margins (Taylor and Exon, 1983).
The area is
characterized
by high heat flow, at least
3 to 4 H.F.U., reflecting
the young
age of the oceanic crust
(Halunen and von Herzen, 1973; Taylor and Exon, 1983)
and is floored
by basaltic
pillow
lavas that appear to be typical
mid-ocean
ridge type (Taylor and Exon, 1983).
Following Carey's
(1958) suggestion
that the Woodlark Basin originated
as
an extensional
feature,
Luyendyk et al
(1973) and Curtis
(1973) delineated
plate
boundaries
on the basis of a few magnetic seafloor-spreading
anomalies
and seismicity.
Weissel et al.
(1982) further
defined
the spreading
axis,
which
extends
southwestward
from the
New Georgia
island
group to
the
D'Entrecasteaux
Islands.
Recent investigations
of the Woodlark Basin during the R/V ~
Keoki
cruise
82-03-16,
Leg 4 have shown that the spreading
center
is marked by an
axial
rift
valley
about
10 km wide, with 500 to 1,OOO.m of relief.
The
pattern
of seafloor
spreading
is characterized
by east-west
trending magnetic
lineations
sinistrally
offset
by north-south
fracture
zones.
Simbo Ridge,
which marks a major fracture
zone, offsets
the anomalies
by about 65 km.
(Taylor and Exon, 1983).
Exon and Taylor (1984) concluded that initial
separation
of the woodlark
and Pocklington
Rises
occurred
approximately
4 Ma and that
since
then,
seafloor
spreading
has averaged a total
opening rate
of 6 to 7 em/yr.
They
found,
however,
that
this
spreading
has been asymmetric,
as the rate
is
approximately
twice as fast
on the northern
limb.
During the 4-m.y. opening
history
of the Woodlark Basin, the spreading ridge and newly formed crust have
been subducted at the New Britain-San
Cristobal
Trench during convergence of
the Pacific
and Australia-India
Plates.
There is no apparent
flexure
of
Woodlark Basin crust into the subduction zone, as the basin crust simply abuts
Vedder,
Coulson:
Offshore
Geology
81
the fore-arc
lower slope;
furthermore,
there is no recognizable
bathymetric
trench opposite
New Georgia, and seismicity
is low (Exon and Taylor,
1984).
This active
spreading
system is entering
an oceanic subduction zone at a high
angle, a relation
that may be unique at present.
The influence
of this nearly
orthogonal
convergence on the petrochemistry
of the volcanic rocks as well as
on regional
tectonism
is not entirely
clear but may be the cause of some of
the enigmatic features
of the arc.
Pocklington
Trough.--Lying
to the north of the Louisiade
Plateau
the
Pocklington
Trough stretches
in a sigmoid pattern
from the Louisiade
Islands
almost to Q1adalcanal
(Fig. 1).
The trough contains a thick sedimentary secion and is marked by a large negative
gravity
anomaly (Coleman and Packham,
1976).
seismically
inactive,
it has been interpreted
as a relict
subduction
zone by Karig (1972).
Recy et al (1977) suggested that the gross morphology
of the trough supports the contention
that it is a fossil
north-directed
subduction zone.
If correct,
this feature
may constitute
another fragment of the
Eocene (1) Inner Melanesian Arc.
Rennell Arc.--South
of Guadalcanal,
the islands
of Rennell and Bellona,
Indispensable
Reef and adjoining
Rennell Trench form a northwest-trending
trench-ridge
pair about 300 km long (Fig. 1).
The Rennell Trench generally
is
less than 4,500-m deep and is flat
floored and partly
filled
with undeformed
sediments
of Oligocene
(?)
and younger age.
On the northeast,
the trench
abuts a 1,300-m deep submarine plateau
that is crowned by the uplifted
atolls
of Rennell and Bellona and barely
emergent Indispensable
Reef.
Landmesser
(1974) proposed that the Rennell Trench and adjoining ridge are vestiges
of a
former northeast-directed
subduction zone along an intermediate
plate boundary
that
once was contiguous
with New caledonia.
Asymmetric trench morphology
suggestive
of a former subduction
zone is evident
in reflection
profiles
across
the trench
(Recy et a L, 1977), but correlation
of the undisturbed
ponded sediments
with DSDP data is uncertain.
Kroenke (1984) favored an
Eocene subduction
event in the Rennell Trench, coeval with that in NewGuinea
and eew caledonia;
if correct,
then the Rennell Arc represents
the earliest
subduction period in the Solomon Islands region.
South Rennell Trough.--This
northeast-trending
trough (Fig. 1) lies south
of the Rennell Arc and is about 700 km long, as much as 30 km wide and as deep
as 5,000 m,
On the basis at magnetic spreading-- anomalies,
graVity data, and
bottom samples, Larue et al (1977) postulated
that this trough is a remnant of
a spreading ridge, possibly of Oligocene Age.
Santa Cruz Basin (Torres Basin) .--Little
is known about the Santa cruz
Basin, which lies southwest of the Juncture of the san Cristobal
and North New
Hebrides Trenches between San cristobal
and the Santa Cruz island group (Fig.
1).
Luyendyk et al (1974) and Ravenne et al (1977) described a subhorizontal,
well-bedded sedimentary section less than 1,000 m thick.
Ravenne et al (1977)
alluded to steep slopes in the eastern
part of the basin and indicated
that it
might represent
a swell along the outer edge of the subduction zone.
Klein et
a L, (1975), reporting
on the Glomar Challenger Leg 30 DSDPresults,
described
Vedder,
Coulson:
Offshore
Geology
82
a 650-m thick
southern santa
succession are
middle sccene to Pleistocene
Cruz Basin.
Probable turbidites
overlain
by pelagic sediments.
sedimentary
succession
in the lower two-thirds
in the
of the
ONTONG
JAVAPLATEAU
ANDADJOINING
STRUCTURES
General setting
The North Solomon Trench, a relatively
shallow, poorly defined and partly
filled
trench,
separates
the Solomon Islands
segment of the OUter Melanesian
Arc from the Ontong Java Plateau and Pacific
OCean floor
(Fig.
1).
To the
northwest,
the North Solomon Trench connects with the Kilinailau
Trench; and
to the southeast
apparently
intersects
the cape Johnson Trench.
According to
Kroenke (1984), subduction
probably began along the North Solomon Trench in
the late Eocene and was accompanied by uplift,
metamorphism, and magmatism.
Volcanism along the northeast-facing
arc probably culminated in the Oligocene,
and subduction
apparently
ended in the early
Miocene when the Ontong Java
Plateau entered the trench.
Ontong Java Plateau.-~he
oceanic Onton-g Ja"va Plateau
lies north of the
main group of the Solornon Islands
(Fig. 1).
The plateau is more than 1,600 kIn
long and 800 kIn wide and its long dimension is nearly parallel
to the island
chain.
Water depths average less than 2,000 m over the central
portion,
which
culminates in several
atolls.
The crustal
structure
was described by Furumoto
et al (1976), Murauchi et al (1973), and Hussong et al (1979).
The crust is
anomalously thick
(43 kIn); layer velocities
are similar
to normal oceanic
crust,
but each layer is abnormally thick.
Kroenke (1972) demonstrated that a
uniformly
thick
(1,000+
m), stratified
sedimentary
sequence blankets
the
plateau.
Where it
adjoins
the Solomons along its
southwestern
edge, the
entire
plateau
sequence
plus
the
underlying
basement
rocks
are
highly
deformed.
Drilling
at DSDPSites 64, 288 and 289 disclosed
the plateau stratigraphy
(Winterer,
Reider et aI,
1971;
Andrews et aI, 1975).
Strata at Site 64 are
correlative
with laterally
continuous seismic reflectors
that are traceable
to
Malaita (Kroenke, 1972;
this volume) where they match the Lower Cretaceous to
Holocene stratigraphic
succession
on north Malaita.
In all
three
holes,
cherty limestone occurs near the base of the section;
and at Site 289, these
overlie
Lower Cretaceous
basalt.
The younger
strata
are
primarily
nannofossil-foraminiferal
limestone,
chalk and ooze.
Although basaltic
flows
were not drilled
in the older limestone sequence in the three holes,
Kroenke
(1972) interpreted
seismic reflectors
south of the plateau
as lava flows or
sills
that could be lateral
equivalents
of the younger basalts
and peperites
on southern Malaita
(Hughes and Turner,
1977).
Nixon (1980) suggested that
some of the seismic
discontinuities
may represent
alnoite
pipes similar
to
those exposed in north Malaita.
The southwestern
flank of the plateau,
adjacent
to the Solomon Islands,
is dominated by the Roncador homocline-Stewart
arch,
a lithospheric
flexure
believed
to have resulted
from severe bending during unsuccessful
subduction
of the plateau as part of the Pacific Plate at the North Solomons Trench prior
to late Miocene arc reversal
(Kroenke, 1984; this volume).
The DSDPdrilling
results
indicate
the presence
of a fracture
zone that
offsets
the plateau
Vedder, Coulson:
Offshore
GeolOgy
83
between DSDP Sites
288 and 289.
This dislocation
was projected
southward
between Santa Isabel
and Malaita by Coleman and Kroenke (1981) but is not
shown by Kroenke (1984;
this
volume).
Kroenke (1972) suggested that
the
plateau arose about an exceptionally
slow spreading axis and that the maeuave
outpourings
of basalt
(6 million
kIn3)
may mark an early
stage of continent
generation.
The highly
deformed nature
of the southwestern
margin of the
Ontong Java Plateau and the apparent correlation
of the stratigraphic
sequence
on the plateau with that of Malaita led Kroenke (1972;
1984; this volume). to
postulate
that Malaita and the northwestern
margin of Santa Isabel
together
constitute
an obducted
slice
of the plateau,
tectonically
welded to the
Solomon Islands
during collision
in late Miocene time.
Bielski-Zyskind
et al
(1984) suggest that the Ontong Java Plateau is composed of undepleted mantle
overlain
by older depleted upper mantle.
North Solomon Trench.--Kroenke
(1984, Fig 4.3) depicts
the North Solomon
Trench
as a structural
hinge line
or regional
syncline.
Presumably it
represents
the site of a late Eocene to early Miocene subduction zone that was
oppilated
by the impingement of the Ontong Java Plateau
during late Miocene
time (Coleman and Kroenke, 1981).
sediments in the narrow part of the trench
directly
north of ;the northwest
end of Malaita are 1. Q to 1.5 kIn thic:k'·and,
essentially
undeformed.
Uniform acoustic
reflectors
suggest that the sequence
consists
largely of turbidites.
Malaita Anticlinoriurn.--The
North Solomon Trench is bordered on the south
by the Malaita anticlinorium
of Kroenke (1972), a northwest-trending
set of
structures
that is the offshore
counterpart
of the post-Miocene echelon fold
belts on the island of Malaita.
On both the northern and southern ends of the
island,
some of the folds
are overturned
and verge southwestward (Rickwood,
1957; Hughes, 1975; Turner, 1976, 1977).
Offshore, the folds appear to more
symmetrical northwest of the island
(Kroenke, this
volume).
Steep submarine
scarps reflect
the presence
of large faults.
Farther
northwest,
the folds
decrease
in number and amplitude and tend to die out north of santa IsabeL
Only a single northwest-trending
trough occurs north of Choiseul.
Vedder,
Coulson:
Offshore
Geology
84
REFERENCES
Andrews, J.E.,
G.H. Packham, et aI,
1975, Initial
Reports of the Deep sea
drilling
Project,
30: U.S. Government Prining Office, Washington, D.C.,
753 p.
Bielski-Zyskind,
M., G.J. Wasserburg, and P.H. Nixon, 1984, sm-Nd and Rb-Sr
systematics
in volcanics
and ultramafic
xenoliths
from. Malaita,
Solomon
Islands,
and the
nature
of
the
Ontong Java
Plateu:
Journal,
of
Geophysical Research, v. 89, no. 54, p. 2415-2424.
Carey, S.w.,
1958, The tectonic
approach to continental
drift,
in Continental
drift--a
symposium:
Geology Department, University
of Tasmania, Hobart,
p. 177-355.
Coleman, P. J.,
and A.A. Day, 1965, Petroleum possibilities
and marked gravity
anomalies
1n
north-central
Guadacanal:
British
Solomon Islands
Geological Record (1959-1962),
v. 2, p. 112-119.
-----and L.W. Kroenke, 1981, Subduction without volcanism in the Solomon
Islands
arc: Geo-Marine Letters,
v. 1, p. 129-134.
-----and G.H. Packham, 1976, The Melanesian Borderlands
and India-Pacific
plates'
boundary:
Earth-Science
Reviews, v. 12, p. 197-233.
Curtis,
J.w.,
1973, Plate tectonics
and the Papua New Guinea-Solomon Islands
req-:i.on:; Journal of the Geological
Society- of Australia,
-e, 20, pt. 1, p.
21-36.
de Brion, C.E., F. Aubertin,
and C. Ravenne, 1977, Structure
and history
of
the Solomons-New Ireland
region:
International
Symposium on Geodynamics
1n Southwest Pacific,
Editions
Technip, Paris,
p. 37-50.
Exon, N.F.,
and B.R. Taylor,
1984, seafloor
spreading,
ridge
subduction,
volcanism,
and sedimentation
in the offshore Woodlark-Solomons region and
Tripartite
cruise
report
for
Kana Keoki cruise
82-03-16,
leg
4:
CCOP!sOPAC
Technical Report no. 34, p. 1-42.
Furumoto, A.s.,
J.P.
webb, M.E. Odegard, and D.M. Hussong, 1976, seismic
studies
on the Ontong Java Plateau,
1970: Tectonophysics,
v. 34, p. 4190.
Glaessner,
M.F., 1950, Geotectonic
position
of NewGuinea:
A.A.P.G. Bulletin,
v, 34, p. 856-881.
Hackman, B.D.,
1980, The geology of Guadalcanal,
Solomon Islands:
Overseas
Memoirs of the Institute
of Geological
Science, Her Majesty's
Stationery
office,
London, no. 6, 115 p.
Ha Lunen, A.J.,
and von Herzen, R.P.,
1973, Heat; flow in the western equatorial
Pacific
Ocean:
Journal
of Geophysical Research, v. 78, p. 5195-5208.
Hughes, G.W., 1975, Dorio, Malaita
~ological
Map Sheet ML 11:
Geological
survey, Honiara, Guadalcanal,
Solomon Islands,
1:50,000.
-----and C.C. Turner,
1977, Upraised Pacific Ocean floor,
southern Malaita,
Solomon Islands:
Geological
Society of America Bulletin,
v; 88, p. 412424.
Hussong, D.J.,
L.K. Wipperman, and L.W. Kroenke, 1979, The crustal
structure
of the Ontong Java and Manihiki oceanic plateaus:
Journal of Geophysical
Research,
v. 84, p. 6003-6010.
Karig, D.E.,
1972, Remnant arcs:
Geological
Society of America Bulletin,
v,
83, p. 1057-1068.
Katz,
H. R.,
1980, Basin
development
in the Solomon Islands
and their
petroleum potential;
CCOP/SOPAC
Technical Bulletin
No.3,
p. 59-75.
Klein,
G.D., 1975, Sedimentary tectonics
in southwest Pacific
Marginal basins
based on Leg 30 Deep sea Drilling
Project
cores from the South Fiji,
Vedder,
Coulson:
Offshore
Geology
85
Hebrides,
and Coral Sea Basins:
Geological
Society of America Bulletin,
v, 86, p. 1012-1018.
Kroenke, L.W., 1972, Geology of the Ontong Java Plateau:
Hawaii Institute
of
Geophysics,
Report no. HIG-72-5, University
of Hawaii, 119 p.
-----1984, Solomon Islands;
san Cristobal
to Bougainville
and Buka, ~
L.W.
Kroenke,
e d; , cenozoic
tectonic
developnent
of the southwest
Pacific:
CCOP/SOPAC
Technical
Bulletin,
no. 6, ch. 4, 22 p.
Landmesser,
C.W., 1974, Submarine geology of the eastern
Coral Sea Basin,
Southwest Pacific:
M.S. thesis,
University
of Hawaii, Honolulu, Hawaii,
64 p.
-----1977, Evaluation
of potential
hydrocarbon occurrence
in the so Lomon
Islands:
CCOP/SOPAC,Technical Report, V. 1, no. 5, p. 47-53.
Larue,
B.M., J. Daniel,
C. Jouannic,
and J. se cy , 1977, The South Rennell
trough;
evidence for a fossil
spreading
zone:
International
symposium on
Geodynamics in Southwest Pacific,
Editions Technip, Paris,
p. 51-62.
Luyendyk, B.P.,
K.C. MacDonald, and W.B. Bryan, 1973, Rifting
history
of the
Woodlark Basin in the Southwest Pacific:
Bulletin
of the Geological
Society of America, v. B4, p. 1125-1134.
-----W.B. Bryan, and P.A. Jezek, 1974, Shallow structure
of the New Hebrides
arc:
Geological
Society of America Bulletin,
v. 85, p. 1287-1300.
Maung, T~ -v., and F.I. Coulson, 1983, Assessment of petroleum potential
of the
central
Solomons Basin;
CCOP/SOPAC
Technical Report No. 26, 68 p.
Murauchi,
S.,
W.J. Ludwig, N. Den, H. Hotta,
T. Asanuma, T. Yoshii,
A.
Kubotera,
and K. Hagiwara, 1973, seismic refraction
measurements Ontong
Java Plateau
northeast
of New Ireland:
Journal of Geophysical
Research,
v. 78, p. 8653-8663.
Neef, G., 1979, cenozoic stratigraphy
of Small Nggela Island,
SOlomon Islandsearly
Miocene deposition
in a forearc
basin followed by Pliocene
patch
reef
deposition:
New Zealand Journal
of Geology and Geophysics,
v, 22,
no. 1, p , 53-70.
Nixon, P.R.,
1980, Kimberlites
in the southwest Pacific:
Nature,
v, 287, p.
718-720.
Ravenne, C., C.E. de Broin,
and F. Aubertin,
1977, Structure
and history
of
the Solomon-New Ireland
region,
in International
symposium on geodynamics
in
the
SW Pacific,
lew ca Led"'Onia
, August-September
1976:
Editions
Technip, Paris,
p. 37-50.
Recy , J.,
J.
Dubois,
J.
Can.LeL, J.
Dupont, and J. reunay , 1977, Fossil
subduction
zones:
examples in the
south-west
Pacific:
International
Symposium on Geodynamics in Southwest Pacific,
Technip,
Paris,
p. 345356.
Rickwood,
F.K.,
1957,
Geology of
the
Island
of Malaita
in Geological
Reconnaissance
of parts
of the central
islands
of the British
Solomon
Islands
Protectorate;
Colonial Geology and Mineral Resources,
v. 6, no.
3, p. 300-306.
Taylor,
B.R•• and N.F. Exon, 1983, 1982 R/V Kana Keoki cruise
in the WoodlarkSolomons region
(abs.):
Basic geo-scientific
marine research
required
for
assessment
of minerals
and hydrocarbonsa
in the South Pacific,
A
workshop, Suva, Fiji,
October, 1983.
----------1984, An investigation
of ridge
subduction
in the Woodlark-Solomons region:
introduction
and background,
in N.F. Exon and B.R.
Taylor,
compilers,
Seafloor
spreading,
ridge
subduction,
volcanism
and
sedimentation
in the offshQre
Woodlark-Solomons region
and Tripartite
cruise
report
for ~
Keoki cruise
82-03-16, Leg 4, CCOP/SOPAC
Technical
Report no. 34, p. 1-42.
Vedder,
Coulson:
Offshore
Geology
86
Tiffin,
D.L., J.G. Vedder, A. Cooper, and Shipboard SCientific
Party,
1983,
Multichannel
seismi.c and geophysical
survey of "The Slot" and adjacent
areas
in the Solomon Islands,
19 May - 11 June 1982:
CCOP/SOPAC
Work
Program, Cruise Report No. 71, 16 p.
Turner,
C.C.,
1976, South small Malaita,
Malaita Geological
Map Sheet ML 17:
Geological
survey,
Honiara, Guadalcanal, Solomon Islands,
1:50,000.
-----1977, central
small
Malaita,
Malaita
Geological
Map Sheet ML 16:
Geological
survey, Honiara, Guadalcanal,
Solomon Islands,
1:50,000.
Vedder, J.G. et aI,
1983, leg 3, central
Solomons Trough, .l:.!! H.G. Greene and
F.L. WOng, eda ; , Hydrocarbon resource studies
in the southwest Pacific:
u.s. Geological
Survey Open-File Report 32-293, p. 11-24.
Weissel,
J.L,
B. Taylor,
and G.D. Karner, 1982, The opening of the Woodlark
Basin, subduction
of the WOodlark spreading system, and evolution
of the
northern
Melanesia since the mid-Pliocene time:
Tectonophysics,
v. 87,
p. 253-277.
Winterer,
E.L., W.R. Reidel,
and others,
1971, Initial
reports
of the Deep Sea
Drilling
Project,
v.7.,
U.S. Government Printing
Office,
Washington, D.C.
Vedder,
Coulson:
Offshore
Geology
87
TBCTORICS OF THE SOOTBEASTERN SOLOI«)N ISLAHDS:
FORMATION OF TIlE MALAITA ANTICLI!I)RIUM*
L •. W•. lCroenke, J •. M~ Resiq, Po. A•. Cooper
Hawaii Institute
of Geophysics, University
of Hawaii,
2525 Correa Road, Honolulu, Hawaii 96822
INTRODUCTION
The Solomon Islands,
first
deac r-Lbe d by cc Leman (1966) as an isl,an(L.a:r;c,
form a northwest-trending
double
chain
of
islands
(Fig.
1) along
the
convergent
boundary between the Pacific
and Indo-Australian
plates.
To the
northeast,
on the Pacific
Plate,
lies
the Ontong Java plateau
(OJP), an
extensive
area of shallO'W sea floor
underlain
by anomalously thick
oceanic
crust
(Hussong, Wipperman and Kroenke, 1979).
To the southwest,
deep trenches
are the surface manifestation
of the currently
active
subduction zone which
dips northeast
beneath
the volcanic
arc
(Cooper, Kroenke and Resig,
this
volume).
Island arc volcanic
activity,
however, is absent at the southeastern
end of the group, northeast
of the San cristobal
trench
(Coleman and Kroenke,
1981).
The tectonic
setting
is also complicated by the current subduction of
an active
spreading
ridge
and the newly formed lithosphere
contained in the
adjoining Woodlark Basin (Taylor,
1984).
The Solomon Islands
were divided by Coleman (1965) into three
geologic
prcvi ncee on the basis
of their
lithology
and structure
(Fig. 2), i.e.:
the
Pacific
Province,
embracing tn.awe, Malaita,
Small Malaita,
and the northeast
flank
of santa
Isabel;
the
central
Province,
including
san Cristobal,
Guadalcanal,
the Florida
Islands,
the southwest flank. of Santa Isabel,
and
Choiseul; and the Volcanic Province,
containing westernmost Guadalcanal,
Savo,
the
Russell
Islands,
the
New Georgia
Group, and the
Shortland
Islands.
Bougainville~
although
politically
part
of Papua New Guinea,
was later
included in both the central
and Volcanic prOVinces (Coleman, 1970).
An en echelon series
of submarine ridges
and troughs northeast
of the
group (Fig.
3) constitute
a large
fold
belt,
the
Malaita
Anticlinorium
(Kroenke, 1972).
These folds crest
subaerially
on the Island of Malaita and
the northeastern
flank of Santa Isabel.
The basement rocks and sedimentary
section
of the anticlinorium
are correlated
with those of the OJP (Kroenke,
1972; Hughes and Turner,
1977; Kroenke, this volume).
Two autochthonous
island
arcs
of opposing polarities,
each bounded by
trenches,
and an allochthonous
accreted
terrane
of open ocean origin form the
regional
framework of the Solomon Islands
(Kroenke, 1984, cenozoic Tectonic
*Hawaii Institute
Kroenke,
of Geophysics
ReSig, P. Cooper:
Contribution
Malaita
No. 1548
Anticlinorium
88
Development).
The island
arcs (Fig. 3) include:
the northeast-facing
North
Solomon Arc (Melanesian Arc) active
during Oligocene and early
Miocene time
(-38-22
Mal and the southwest-facing
South Solomon Arc, active
from late
Miocene to the present
(.....
10-0 Ma).
Except for a brief
resurgence
of arc
volcanism in the mid~le Miocene (-15 Ma) along the adjoining
Manus and Vitiaz
arc segments,
the intervening
middle to late
Miocene time was relatively
quiescent.
The allochthon,
comprising
both the Pacific
Province
of the
Solomon Islands and the Malaita Anticlinorium,
is believed
to be a segment of
the southwestern
margin of the OJP which collided
with and overthrust
~e
former North Solomon Arc.
This Chapter summarizes arguments leading to these
conclusions
and presents
a revised
chronology for the sequence of tectonic
events.
THEONTONG
JAVAPLATEAU
The geology of Santa Isabel includes
two principal
units
(Stanton 1957,
1961):
an undifferentiated
basement complex, overlain
by m1ldy folded
mudstones, shales,
and tuffs
of the Tanakau Group, and an intensely
folded
succession
of basaltic
pillOlof lavas (Sigana Volcanics),
conformably overlain
by pelagic
limestones
of 1;he Tanakau G:(oup. At least .one major thrust
fault
is. prest;lnt"7_-t;.he
.Ka"ipito furigole
Fault (Stanton,~j)-!~lJ.::-,..alongwh~ch \ll:t:r.~fip,:.
rocks occur as alpine-type
lenses.
These ultramafic
rocks have also been
described
as ophiolites
(Neef,
1978) •
Stanton
(1961l noted simi larities
between the Sigana pillow
lavas and overlying
Tanakau limestones
of Santa
Isabel
and the
lithostratigraphic
succession
found on Malaita
(Rickwood,
1957).
Coleman et al
(1965) noted that the Kaipito Koregole thrust
divides
the
island
chain
into
two broad structural
zones:
a northeastern
one,
embracing Malaita and southern
Santa Isabel and dominated by folding,
and a
southwestern
one, inclUding Guadalcanal and San Cristobal
and characterized
by
block faulting.
Geological
reconnaissance
mapping of the island
of Malaita
(Rickwood,
1957) provided an initial
insight
into the stratigraphic
succession
and the
structural
relationships
present
there.
Seismic reflection
profiling
across
the OJP northeast
of the Solomon Islands
(Kroenke, 1972) revealed the presence
of the large anticlinorium
adjoining
Malaita and santa Isabel and established
the continuity
of the acoustic
stratigraphy
from DSDPSite 64 on the plateau
into
the Anticlinorium
(Kroenke,
1972).
Similarities
between the geology
mapped onshore and that
investigated
offshore
led to the correlation
of the
Malaita
and the OJp sedimentary
sequences and to the conclusion
that all of
the
Malaita
Anticlinorium,
inclUding
the
island
of
Malaita
and
the
northeastern
flank of Santa Isabel,
are a part of the OJP.
Refinement of the stratigraphy
of Malaita by Hughes and Turner (1977)
reinforced
this conclusion,
as did their
recognition
of two phases of igneous
activity
on Malaita:
a widespread
"older"
tholeiitic
phase,
assumed to be
part
of the lower Cretaceous
OJP volcanism that formed the basement, and a
restricted
"younger" alkalic
phase that intruded the overlying
Cretaceous and
lower Tertiary
pelagic
sediments.
Of 14 "older" basalts
analyzed,
11 strongly
resemble
mid-ocean
ridge
(MORl tholeiites,
particularly
in
TiOZ and KZO
contents,
whereas 3 are primitive
alkalic
basalts,
moderately
high in ~O,
similar
to basalts
from mid-ocean islands,
and extremely low in Ti02, similar
to basalts
from mid-ocean plateaus
(Tokuyama and Bitaza,
1981).
Kroenke
(197Z)
suggested
that
the OJP originated
by emplacement of
submarine flood basalts
and compar-ed it to other oceanic plateaus
including
Kroenke,
Resig,
P. Cooper:
Malaita
Anticlinorium
89
F'_
Iceland
(Kroenke, 1974).
Major element constituents
of the basalts
from Deep
Sea Drilling
Project
(DSDP) Site 289 on the northern OJP are similar
to those
of MORtholeiites
as are the minor and trace element concentrations
(Stoeser,
1975).
Like Icelandic
basalts,
however, the Site 289 basalts
have more K20
and Sr than normal MORbas aI t s,
Moreover, the older Malaitan
basalts
are
massive and pillowed,
without major dike formation
(Hughes and Turner,
1977),
which also suggests
a flood basalt origin.
Drilling
at
DSDP Sites
288 and 289 produced
evidence
for
an age
progression
across
the
OJP, (Summary and Conclusions,
Site
288, Andrews,
Packham et
a L, 1975).
Coleman, acccvren
and Ramsay (1978),
and Hussong,
Wipperman and Kroenke (1979),
using
the Aptian age (110 Ma) obtained
for
basement at Site 289, extended the progression
to Santa Isabel
where ages for
basement were derived
from fossils
in the sediments overlying
basement (-58
Ma) and directly
from a radiometrically
dated basement dolerite
(66 Ma). They
concurred
on a spreading
ridge origin
for the Plateau
as did Coleman and
Kroenke (1981).
In addition,
Hussong, Wipperman and Kroenke (1979) concluded
that
a simple thickening
of all oceanic crustal
layers by a factor
of 5 was
compatible with known seismic crustal
structure
of the plateau.
Bielski-Zyskind,
Wasserburg and Nixon (1984), in presenting
results
of a
Sm-Nd and as-sstudy on samples of alnoite,
ultramafic
xenoliths,
and the
basalt
country rock from Malaita,
report that isotopic
data from one sample of
the Malaita- basalt.
falls'
to the right
of the mantle array for ~-$F'-;-:1'"" NdJ~
close to the ocean island
field.
More important,
however, they emphasize that
there is no hint in the available
data from Malaita of the isotopic
signature
of old continental
crust.
Indeed, Butler
(in prep.)
concludes that Walker's
(1977) observations
of high-frequency
Po phases across the OJP indicate
that
the underlying
lithosphere
is oceanic
in character
and not of continental
origin.
Thus, the available
evidence suggests that the OJP originated
along a
mid-ocean ridge,
perhaps in similar
manner to the Icelandic
Plateau.
THEMALAITA
ANTICLINORIUM
Multichannel
seismic
data, acquired
during a 1982 R/V S. P. LEE cruise,
permit a direct
correlation
of strata
between the southern margin of the OJP
and the Malaita
Anticlinorium
just
west of Malaita
(Kroenke, this
volume).
These data substantiate
the correlation
proposed earlier
by Kroenke (1972).
The deep troughs
northeast
of both the anticlinorium
and the Solomon
Islands
are part
of the fossil
North Solomon Trench (Kroenke, 1972).
The
presence of deformed Miocene strata
overlain
by ponded sediment in the troughs
northeast
of the anticlinorium
and the occurrence
of terrigeneous
debris
in
the deep water sediments
of Miocene-Pliocene
age on Malaita
suggest that the
southwestern
margin of the plateau
collided
with and overthrust
the Solomon
Arc from the northeast
in late Miocene time (Kroenke, 1972).
Upper Oligocene-lower
Miocene Suta and Marasa Volcanics
on Guadalcanal
(Hackman, 1980), the latter
intruded
by the Poha Diorite
dated at 24.4 :: 0.3
Ma (Chivas and McDougall, 1978), and the Oligocene Kieta vc Icaru.ce , intruded
by the upper Oligocene
Umumand Kunai Hills
plutons
(Blake and Miezitis,
1967),
attest
to
the
presence
of Oligocene-early
Miocene arc
volcanism.
Furthermore,
aspects
of
late
Oligocene-early
Miocene sedimentation
and
geochemistry
of contemporaneous
lavas on Small Nggela also indicate
that
a
northeast-facing
arc
was present
during
late
Oligocene-early
Miocene time
(Neef and Plimer,
1979).
Kroenke,
Resig,
P. Cooper:
Malaita
Anticlinorium
90
,.'>0.
Hughes and Turner
(1977) supported
the concept of a pre-late
Miocene
trench
and subduction
zone of the northeast
side of the Solomon Islands and
agreed that a shallowing
of the Malaita area occurred during late Miocene to
early
Pliocene
time when a greater
amount of
terrigenous
detritus
was
introduced
into
the
section.
They aLe.
noted that
folding
and faulting
occured
on Malaita
during
Pliocene
time when the
island
first
emerged.
Additionally,
extensive
slumping within
deposits
of Oligocene to early
and
middle Miocene age indicate
unstable
depositional
conditions
on an inclined
substratus,
and Oligocene
distal
turbidites,
later
described
as tuffaceous
beds (Turner and Hughes, 1982), suggested a fixed source location
relative
to
the area of deposition.
Piercement
structures
with lateral
reflectors
interpreted
as sills
were
recognized
in seismic reflection
profiles
on the outer trench slope north of
Malaita
and along
the
crest
of the Roncador Homocline and Stewart
Arch
(Kroenke,
1972).
These structures
might be equivalent
to the intrusive
alkalic
basalts
on Malaita
(Kroenke, 1972), or some might also be kimberlite
pipes,
emplaced in similar
fashion
to the alnoitic
intrusions
of Malaita
(Nixon, 1980).
The Malaita alnoites
apparently
were derived in Oligocene time
(.....
34 Ma) as a partial
melt from the base of the
lithosphere
(Nixon and
Coleman, 1978).
Coleman and Kroenke (1981) proposed that intrusions
of the
same type occurred
prior
to the late
Miocene arc reversal
along tensional
ruptures
formed, during severe bending of the OJF lithosphere
over the ere-noh
outer-rise.
This
deformation
and igneous
activity
would create
slope
instability
and a provenance
for the distal
turbidites
(tuffaceous
beds)
described by Hughes and Turner (1977).
Paleomagnetic
analysis
of sediment and basalt
samples from DSDPSite 289
on the OJF indicate
a latitude
of formation
for the site
of about 33°S in
Aptian time (..•110 Ma) (Hammondet a L, 1975).
Therefore,
since formation,
the
OJP and its marginal areas,
inclUding the Pacific
Province Islands,
must have
undergone a substantial
northward displacement.
However, because of past
convergence
azimuths
between
the
Pacific
and
Indo-Australian
plates,
convergence vectors
were almost congruent with latitude
(Fig. 8.4, Kroenke,
Cenozoic Tectonic
Developnent,
1984) and thus any change in paleolatitude
between Malaita-QJP
and the
remainder
of the
Solomon Islands
should be
negligible.
Apparently
the North Solomon Arc overlay a southwest-dipping
subduction
zone that
was active
during
Oligocene and early
Miocene time (-38-22 Ma)
{Kroenke, 1984bl, an appropriate
time for the emplacement of alnoitic
and
alkalic
intrusive
rock suites
seaward of the subduction
zone.
A late Miocene
("'10 Mal arc reversal
followed
a quiescent
period
spanning early
to late
Miocene time (Kroenke, this
volume).
A brief
resurgence
of igneous activity
punctuated
that quiet along the adjoining
west Melanesian
(Manus) and Vitiaz
arc segments in middle Miocene time ("'15 Ma), implying reactivation
of the
southwest-dipping
subduction zone.
Reactivation
of a segment of the southwest-dipping
subduction zone may be
occurring
tic day ,
Cooper, Kroenke and Resig,
this
volume, report
that
a
southwest-dipping
zone of low-level
earthquake
activity,
associated
with the
North Solomon Trench, extends
to 200 km in depth.
They believe
that
the
feature
is part of a relict
W-Bzone and conclude that the area over which the
zone is observed represents
a preViously subducted segment of Pacific
crust.
Kroenke,
Resig,
P. Cooper:
Malaita
Anticlinorium
91
CONCLUSIONS
That part of the southwestern
margin of the OJP now forming the Malaita
Anticlinorium
encountered the Nbrth Solomon subduction zone in late Oligocene
to early
Miocene time.
Bending of the thick crust and lithosphere
over the
trench outer rise
facilitated
intrusion
of alkalic
basalts
such as are found
on Malaita as well as the sills
and dikes (piercement structures)
observed in
reflection
records across the Roncador homocline and Stewart arch.
Subduction
apparently
ended when the thickest
part of the plateau
crust and lithosph""re
encountered
the trench in early Miocene time (-22 Ma), causing the convergent
boundary to shift
elsewhere in the region.
Follol.Jing
the
initial
collision
(22-20 Ma) a period
of
relative
quiescence
ensued along the North Solomon Arc.
Except for a brief resurgence
of activity
along the West Melanesian
(Manus trench)
and Vitiaz
segments of
the subduction
zone in middle Miocene time, the quiescent
period lasted about
10 Ma, after
which convergence recurred
on the south side of the old North
Solomon Arc.
When convergence shifted
back to the Solomon Islands
in late
Miocene time
(10-8
Ma),
it
formed the
northeast-dipping
South Solomon
subduction zone and created the New Britain
- san Cristobal
trenches.
During the formation
of the South Solomon subduction
zone, the Solomon
Island platform was pushed northeastward
into the southwestern
margin of the
OJP, which began to overthrust
the fore-arc
of the old North Solomon Arc.
The
ensuing overthrusting
resulted
in uplift
and folding of the oceanic crust of
the plateau
to form the Malaita
Anticlinorium
and Pacific
Province of the
Solomon Islands
and caused the emplacement of ophiolites
along the KaipitoKorigole
fault
system.
Contact
between northeast-dipping
Indo-Australian
lithosphere
and southwest-dipping
Pacific
lithosphere
and the absence of a
wedge of asthenosphere
above the W-B zone may have inhibited
island-arc
volcanism
at the southeastern
end of the Solomon Islands.
In fact,
the
process
of overthrusting,
folding,
and ophiolite
emplacement may still
be
occurring
today.
ACKNOWLEDGMENTS
We are grateful
for the constructive
criticism
of W. T. Coulbourn of BIG
and D. L. Tiffin
of CCOP/SOPAC
Technical
secretariat.
We appreciate
the
secretarial
and graphics
support provided by RIG.
In particular
•..••
e thank E.
Norris,
R. Rhodes, and M. Prins.
Kroenke,
Resig,
P. Cooper:
Malaita
Anticlinorium
92
REFERENCES
Andre.•••
s,
J.
E.,
Packham,
G.,
et
e L,
1975,
Initial
Reports
of
DSDP:
Washington, U.S. Government Printing
Office,
v. 30, p. 175-230.
Bielski-Zyskind.
M•• wasserburg,
G. J.,
and Nixon, P. H., 1984, Sm-Nd and RbSr
systematics
in
volcanics
and ultramafic
xenoliths
from Malaita,
Solomon Islands,
and the nature
of the Ontong Java Plateau:
Journal
of
Geophysical
Research,
v. 89, p. 2414-2424.
Blake,
D. H. , and Miezitis,
Y. ,
1967, Geology of Bougainville
and Buke
Islands,
tew Guinea, Commonwealth of Australia:
Department of National
Develofll\ent,
Bureau
of
Mineral
gesourcee ,
Geology
and Geophysics,
Bulletin
No. 93, Bulletin
No. PNG1.
Butler,
R., in preparation,
Regional seismic
observations
of the Ontong Java
Plateau
and East Mariana Basin.
Chivas,
A. R., and McDougall, I.,
1978, Geochronology of the Koloula porphyry
copper prospect,
Guadalcanal,
Solomon Islands:
Economic Geology, v. 73,
p. 678-689.
Coleman, P. J.,
1965, Stratigraphical
and structural
notes
on the British
Solomon Islands
with reference
to the first
geological
map, 1962, in The
British
Solomon
Islands
Geological
Record,
Volume
II,
1959-62:
Department of Geological
surveys,
HOniara, p. 17-31.
"~
Coleman, P• .1., 1966, The Solomon Islands
as an island
arc:
Nature,
v, 211,
p. 1249-1251.
Coleman, P• .1., 1970, Geology of the Solomon and te .•••Hebrides Islands,
as part
of the Melanesian re-entrant,
Southwest Pacific:
Pacific
Science,
v. 24,
p.289-314.
Coleman, P• .1., Grover,
J. C't Stanton,
R. L't
and Thompson, R. B., 1965, A
first
geological
map of the British
Solomon Islands,
1962 in Grover, J.
C. et aI, The British
Solomon Islands
Geological
Record, Volume II,
195962:
Department of Geological
Surveys,
Honiara, p. 16-17.
Coleman, P• .1., McGowran, B., and Ramsay, W. R. H., 1978, New, early Tertiary,
ages
for
basal
pelagites,
northwest
Santa
Isabel,
Solomon Islands
(central
southwest
flank,
Ontong Java Plateau):
Australian
Society
of
Exploration
and Geophysics Bulletin
no. 9, p. 110-114.
Coleman, P• .1., and Kroenke, L. W., 1981, Subduction without volcanism in the
Solomon Islands
arc:
Geo-Marine Letters,
v. 1, p. 129-134.
Hackman, B. D., 1980, The geology of Guadalcanal,
Solomon Islands:
Institute
of Geological-Sciences,
Natural Environment Research, Overseas Memoir, v.
6, 115 p ,
Hammond, S. R., Kroenke, L. w., Theyer,
F.,
and keling,
D. L.,
1975, Late
Cretaceous
and Palaeogene
palaeolatitudes
of the Ontong Java Plateau:
Nature,
v. 255, p. 46-47.
Hughes, G. W'i and Turner,
C. C., 1977, Upraised Pacific
OCean floor,
southern
Malaita,
Solomon Islands:
Geological
Society of America Bulletin,
v. 88,
p. 412-424.
Hussong,
D. M., Wipperman, L. K.,
and Kroenke,
L. W., '979,
The crustal
structure
of the Ontong Java and Manihiki Oceanic Plateaus:
Journal
of
Geophysical
Research,
v. 84, p. 6003-6010.
Kroenke, L. W., 1972, Geology of the Ontong Java Plateau:
Ph. D. thesis,
119
p., University
of Hawaii, HIG-72-5.
Kroenke, L. w., 1974, Origin of continents
through development and coalescence
of oceanic
flood basalt
plateau:
EQS Transactions,
American Geophysical
Union, v. 55, p. 443.
Kroenke,
L. W., 1984a,
Interpretation
of a multichannel
seismic
profile
Kroenke,
Resig,
P. Cooper:
Malaita
Anticlinorium
93
northeast
of the Solomon Islands,
from the southern flank of the Ontong
Java Plateau across the Malaita Anticlinorium,
this volume.
Kroenke,
L. w.,
1984b,
cenozoic
tectonic
development
of
the
Southwest
Pacific:
U.N. ESCAP, CCOPjSOPAC
Technical Bulletin
6, in press.
Neef, G.,
1978, A convergent
subduction
model for
the
Solomon Islands:
Australian
Society of Exploration and Geophysics, v. 9, p. 99-103.
Neet, G. and Plimer,
I. R., 1979, Ophiolite
complexes on small Nggela Island,
Solomon Islands:
Geological
Society of America Bulletin,
part
II, .p.
313-348.
Nixon, P. H.,
and Coleman, P. J.,
1978, Garnet-bearing
lherzolites
and
discrete
jodule
suites
from the Malaita Alnoite,
Solomon Islands,
and
their
bearing
on the
nature
and origin
of the Ontong Java Plateau:
Australian
Society of Exploration
and Geophysics Bulletin,
v, 9, p. 103107.
Nixon, P. H., 1980, Kimberlites
in the southwest Pacific:
Nature, v. 287, p.
718-720.
Rickwood, F. K., 1957, Geology of the island of Malaita,
in Marshall,
C. E.,
et; a 1, Geological
Reconnaissance of parts of the central
Islands of the
British
Solomon Islands
protectorate:
Colonial
Geology and Mineral
Resources, v. 6, p. 300-305.
Stanton,
R~ L.,- 1957, Geology of southeastern
Santa Ysabel and5an.
.Jorge
Island in Marshall,
C. E. et e L, Geological
Reconnaissance of Parts of
the
central
Islands
of
the
British
Solomon Islands
Protectorate:
Colonial Geology and Mineral Resources, v. 6, p. 269-286.
Stanton, R. L., 1961, Explanatory notes to accompany a first
geological
map of
Santa Ysabel, British
Solomon Islands protectorate:
Oversea Geology and
Mineral Resources,
v. 8, p. 127-149.
Stoeser,
D. B.,
1975, Igneous rocks from Leg 30 of the Deep sea Drilling
Project,
in Andrews, J.
E.,
Packham, G., et aL, Inltial
Reports
of
DSDP: Washington, U.S. Govt. Printing
Office, v. 30, p. 401-414.
Taylor,
B.,
1984, A geophysical
survey of the Woodlark Solomons region
in
Taylor,
B.,
Exon, N. F.,
eda s r
American Association
of Petroleum
Geologists
Earth Science Series,
in preparation.
Tokuyama, H. and Batiza,
R., 1981, Chemical composition of igneous rocks and
origin
of the sill
and pillO\ri-basalt
cOIt\'lex of Nauru Basin,
southwest
Pacific
in Larson,
R. L., Schlanger,
S. 0., et aI,
Initial
Reports of
DSDP: Washington, u.S. Goverment Printing
Office,
v. 61, p. 673-687.
Turner, C. C. and Hughes, G. w., 1982, Distribution
and tectonic
implication
of
Cretaceous
Quaternary
sedimentary
facies
in
Solomon Islands:
Tectonophysics,
v, 82, p , 127-146.
Walker, D. A., 1977, High-frequency
Pn and Sn phases recorded in the Western
Pacific:
Journal of Geophysical Research, v. 82, p. 3350-3360.
Kroenke, Resig,
P. Cooper:
Malaita
Anticlinorium
94
TBC'l'OHIC IMPLICATIONS OF SEISMICITY
TBB SQLC.K)N ISLANDS*
II.lRTREAST OF
P. A. Cooper, L. W. Kroenke, J. M. Resig
Hawaii Institute
of Geophysics, University
of Hawaii,
2525 Correa Road, Honolulu, Hawaii 96822
INTRODUCTION
The Solomon Islands
lie
along a NW-SEconvergent
boundary between the
Pacific
and Indo-Australian
plates
(Fig. 1). The structural
complexity of the
area results
from at least
two tectonic
events:
The introduction
of ma:s'sive
quasi-continental
lithosperic
units
into
the
subduction
zone, and an arc
reversal.
About 22-20 Ma the Ontong-Java Plateau
(OJP) converged with the
southern North Solomon Trench in an older,
southwest-directed
subduction zone
(Kroenke, 1984, cenozoic Tectonic Development).
After a relatively
quiescent
period,
convergence ze s umed in a northeast-directed
subduction zone about 10
Ma (Kroenke, P.esig and Cooper, this
volume).
Further
complication
resulted
from subsequent
entry
of the
Woodlark spreading
system into
the
latest
subduction zone (Weissel, Taylor and Karner, 1982).
At least two small plates
are located between the large Pacific
and IndoAustralian
plates
(Fig.
1):
The Solomon Sea Plate,
which is bounded by the
New Britain
arc-trench
system to the northwest,
by the diffuse
seismicity
of
the northern
New Guinea coast
to the west, and by the Woodlark spreading
system, part of which extends into the Papuan peninsula
to the south and east;
and the Bismarck Sea Plate,
bounded by the New Britain
trench to the south,
New Ireland to the east,
the Bismare-k Sea seismic lineation
(Denham, 1969) to
the
north,
and the New Guinea Highlands to the west.
Whereas the IndoAustralian
Plate
is
currently
underthrusting
the
Pacific
Plate
in
a
northeasterly
direction
at about
11 cm/yr (Luyendyk, Macdonald and Bryan,
1973), the Solomon Sea Plate
is now underthrusting
the BismarCk Sea Plate in
a more northerly
direction
at about 14 cm/yr (Taylor,
1984).
The Bismarck sea
Plate,
whose relative
movement is
not as well
constrained
by data,
is
apparently
moving at about 13 cm/yr in an average direction
of N60W(Taylor,
1979) •
The main seismic
features
of the region are as follows:
A zone of
shallow and intermediate
seismicity
parallels
northern
New Guinea.
A second
zone of shallow seismicity
the
'Bismarck sea seismic
lineation'
of Denham
(1969), extends due west from New Ireland
to New Guinea.
A third
zone of
shallow
seismicity
1s associated
with the Woodlark spreading
system and
extends
from the
west
Woodlark Basin
near
New Georgia,
south
through
D'Entrecasteaux
Islands
and into
the
Papuan Peninsula.
Both northerly
-Hawaii Institute
of Geophysics
P. Cooper, Kroenke,
Resig:
Contribution
Seismicity
No. 1545
95
extension and northwest left-lateral
strike-slip
focal mechanisms characterize
this
zone
(Rupper,
1980).
A zone of
shallow,
intermediate
and deep
earthquakes
curves along the Solomon Islands arc-trench
system and continues
to the southwest,
paralleling
the New Britain Arc.
Last, a diffuse
zone of
shallow
and intermediate
seismicity
which apparently
defines
a separate
tectonic
regime, extends north of the Solomon Islands beneath the OJP.
of the many studies
of the
seismicity
(Denham, 1969; Hilsom,
1970;
CUrtis,
1973, seismicity)
and seismotectonics
of the Solomon Islands
region
(Johnson and Molnar,
1972;
Krause,
1973; Curtis,
1973, plate
tectonics;
Taylor,
1975; Pascal,
1979;
Weisse1, Taylor and Karner,
1982),
few have
focused on the southeastern
end of the region and none have addressed the
implications
of the diffuse
zone of seismicity
extending northeast
beneath the
Ontong-Java Plateau.
This report discusses the relationship
between regional
structure
and seismicity
and speculates
within the framework of plate tectonic
theory on the cause of the earthquakes occurring northeast
of the main Solomon
Islands arc-trench
seismicity.
SEISMICITY
The intensity
and distribution
of hypocenters along the Solomon Islands
arc-trench
system vary
dramatically
at
all
depths
(Fig.
2).
Shallow
seismicity
is most intense
in the northwestern Solomon Islands
and decreases
abruptly
just
southeast
of Bougainville,
where an active
section
of the
Woodlark spreading
system enters
the trench
(Weisse 1, Taylor and Karner,
1982).
In the vicinity
of the New Georgia Group in the central
Solomons,
shallow activity
is much less intense
and earthquakes
are typically
low-tomoderate in magnitude.
Generally,
earthquake activity
associated
with the
northeast-directed
subduction in the area of the Woodlark Basin portion of the
Indo-Australian
plate
is
poorly
developed.
Shallow seismicity
is fairly
intense
in the southeastern
Solomon Islands.
Several moderate events located
far
into the OJP are suitable
for focal mechanism analysis.
Intermediate
seismic activity
in the northwestern
Solomons ceases at about 200-kIn depth,
whereas in the central
and southeastern
Solomons the Wadati-Benioff
(W-B) zone
extends no deeper than 100 km,
The W-B zone is contoured in Fig. 2.
Deep
seismic activity
is observed throughout the Solomons.
All deep earthquakes
occur
in
'pod-shaped'
groupings
at
depths between 380-550
km and are
apparently
related
to subducting slabs detached from and acting independently
of the presently
subducting
lithosphere
(Cooper and Taylor,
in prep.).
The
main
seismic
activity
can
be
attributed
to
the
Solomon Sea
Plate
underthrusting
the northwest Solomon Islands near Bougainville.
Beneath the arc the distribution
of shallow hypocenters with respect
to
depth (Fig. 2) indicates
a W-B zone dipping to the northeast;
the attitude
of
the
deeper portions
of the W-S zone varies
from near vertical
north of
sougainville,
to horizontal
in the central
Solomons and back to vertical
in
the southern Solomons.
Near the New Georgia Group in the central
Solomons
where the Woodlark Basin is being subducted, the W-B zone is ill
defined,
there are no intermediate
earthquakes,
and a roughly inverted V-shaped gap in
seismicity
is apparent when the W-S zone is viewed as a section parallel
to
the strike
of the trench
(Fig. 3e in Cooper and Taylor, in prep.).
This gap
is offset
to the south in the direction
of convergence of the Pacific-Indo
Australian
plates.
According to the model of active ridge subduction proposed
by Marshak and Karig (1977),
as ridge sections are consumed at the trench,
a
gap forms in the subducting
lithosphere
and grows in width with continued
P. Cooper, Kroenke, Resig:
Seismicity
96
subduction.
The gap noted above may be explained by this model.
High heat
flow values reported
by Halunen and Von Herzen (1973) for the Woodlark Basin
are indicative
of the young, hot lithosphere
of that area now being subducted
in the central
Solomons near NewGeorgia.
The positive
buoyancy of this newly
generated
lithosphere
may explain
the
shoaling
of
the
New Britain-San
cristobal
trench south of NewGeorgia Islands
(Delong and Fox, 1977), and the
continued
uplift
of adjacent
islands
(Hughes, Varol and Dunkley, 1984) and
contiguous
seafloor
(Resig,
1984).
The lack of a well-developed
w-s zone
(Cooper and Taylor,
in prep.)
and the
presence
of abnormal
island-arc
volcanism
(Weissel,
Taylor and Karner,
1982) may also be explained
by the
subduction of hot, young, and thin lithosphere.
The diffuse
zone of seismicity
around and extending north of Santa Isabel
(Fig.
2) is more difficult
to explain.
Figure
3 is a cross
section
of
seismicity
along approximately
the same line as the multichannel
reflection
profile
described
by Kroenke (lmJltichannel
interpretation,
this
volume).
A
northeast-dipping
zone of earthquakes extending to about 100 km in depth marks
the modern W-S zone associated
with the san Cristobal
Trench.
A southwestdipping zone of earthquakes
extending to 200 km in depth locates
another W-B
zone (Cooper and Taylor,
in prep.).
This second feature,
which can be
associated
with
the
North Solomon Trench,
is
most evident
on profiles
orthogonal
to the
island
of Santa- Isabel.
It
is- believed
to represent
vestigal
motion or a resurgence
of motion along the relict
North Solomon
subduction
zone.
Scattered
hypocenters
located elsewhere
along the North
Solomon trench
are insufficient
to define this
zone.
The low incidence
of
earthquakes
with focal nechanisms showing thrusting,
however, indicates
that
the southwest-dipping
slab is not vigorously
subducting.
The seismicity
may
be due to stress
adjustment
of the relict
slab (Toksoz, 1975), or it may
result
from
the
modern,
northeast-subducting
slab
pushing
against
and
depressing
the
old
slab,
causing
it
to
settle
deeper
into
the
aesthenosphere.
we conclude, as did Cooper and Taylor (in prep.),
that this
southeast-dipping
seismicity
represents
a previously
subducted segment of the
Pacific
plate.
One earthquake
focal
nechanisrn near Malaita,
and possibly
another near
Santa Isabel,
indicates
thrusting
(Fig.
2).
In view of seismic reflection
profiles
that
show continuous
stratification
between the Ontong Java Plateau
and the Solomon Islands PaGific Province (Kroenke, this volumel Kroenke, Resig
and Cooper, this volume), and paleontological
evidence of thousands of meters
of uplift
of the Pacific
Province in late
Neogene time (Resig,
Cooper and
Kroenke, this volume), the mechanisms can be interpreted
in terms of OJP crust
overthrusting
the Solomon Arc.
In this
interpretation,
the crust along the
edge of the OJP has detached from the underlying lithosphere
and has overriden
the
North Solomon Arc, whereas the
lithosphere,
stripped
of crust,
has
descended beneath the old arc along the nOWrelict
W-B zone.
The presence of
a decollement
may be indicated
by the increase
in depth of hypocenters
from
20-25 kIn near the Florida Group to 40-50 km north of Malaita,
a depth close to
that estimated
for the crustal
thickness
of the OJP (Furumoto et a L, 1970).
In contrast,
the sparse hypocenters
located farther
north beneath the OJP are
typically
50-100 krn deep.
CONCLUSIONS
Determination
of the present
active
subduction of the Indo-Australian
lithosphere
beneath the southeastern
Solomon Islands region is complicated by
P. Cooper,
Kroenke,
Resig:
Seismicity
97
uplift
associated
with the subduction of the active Woodlark spreading system,
contact
of
OJP
lithosphere
with
actively
subducting
Indo-Australian
lithosphere,
movement or adjustment of the older,
southwest-dipping
OntongJava lithosphere,
and finally,
by possible
continued underthrusting
of the
southern margin of the OJP by the Solomon Arc.
The seismic activity
concentrated
printarily
at the base of the OJP and
evidence of recent
episodes of uplift
and deformation strongly
suggest that
obduction of the Pacific
plate in the vicinity
of the OJP is presently
taking
place.
We believe
that
part
of the relative
motion between the
IndoAustralian
and Pacific
plates
has been, and may still
be, accommodated by
obduction and shortening
of Ontong-Java crust.
ACKNOWLEDGtENTS
We acknoc Ledqe the generosity of the seismology Division,
Lamont Doherty
Geological Observatory in allowing us access to their
film-chip
library.
We
appreciate
the assistance
there of Margie Yamasaki.
We thank Jim Dewey for
copies of his earthquake
location programs and Selena Billington
for helpful
discussion
regarding
their
use.
We are grateful
to D. L. Tiffin
and W. T.
Coulbourn for critically
reviewing the manuscript.
P. Cooper, Kroenke,
Resig:
seismicity
98
REFERENCES
Cooper,
P. and Taylor,
B., in press, Earthquake
seismology
of the Solomon
Islands.
Curtis,
J. w.
1973a, The spacial
seismicity
of Papua-New
Guinea-Solomon
Islands region:
Journal of the Geological Society of Australia, v, 20,
p. 1-19.
Curtis, J. w., 1973b, Plate tectonics of the Papua-New Guinea-Solomon
Islands
region:
Journal of the Geological Society of Australia, v. 20, p. 21-36.
Delong,
S. E. and Fox,
P. J.,
1977, Geological
consequences
of ridge
subduction .f.£. Talwani, M.,
and Pitman, W. E., III, eds., Island Arcs,
Deep sea Trenches
and Back Arc Basins:
American
Geophysical
Union,
Maurice Ewing Series, v. 1, p. 221-228.
Denham,
D.,
1969, Distribution
of earthquakes
in the New Guinea-Solomon
Islands region:
Journal of Geophysical Research, v. 74, p. 4290-4299.
Furumoto, A. S., Hussong, D. M., Campbell, J. F., Sutton, G. H., Malahoff, A.,
Rose, J. C. and Woollard, G. P., 1970, Crustal and upper mantle structure
of the Solomon
islands
as revealed
by seismic
refraction
survey
of
November-December
1966:
Pacific Science, v. 24, p. 315-332.
Halunen,
A. J. and Von
Herzen,
R. P.,
1973, Heat flow in the Western
Equat~rial
Pacific:
Journal of Geophysical
Research,
v, 78, p. 10251083.
Hughes,
G. W., Varol,
0.,
and Dunkley,
P. N.,
1984, Foraminiferal
and
calcareous nannofossil
evidence for Quaternary vertical tectonics in the
Solomon
Islands
Iebe , J,
International
Symposium
on
recent
crustal
movements of the Pacific Region:
Royal Society of New Zealand and InterUnion Comm. on the Lithosphere,
Wellington, New zealand, Feb. 1984.
Johnson, T. and Molnar, P., 1972, Focal mechanisms and plate tectonics of the
Southwest Pacific:
Journal of Geophysical Research, v. 77, p. 5000-5032.
Krause, D. C., 1973, Crustal plates of the Bismarck and Solomon Seas:
In
Frazer, R., ed., Oceanography
of the South Pacific 1972, p. 271-280
Kroenke,
L. W.,
1984,
Cenozoic
tectonic
developnent
of
the
Southwest
Pacific:
U.N. ESCAP CCOP/SOPAC Technical Bulletin No.6,
in press.
Luyendyk, B. P., Macdonald,
K. C., and Bryan, W. B., 1973, Rifting history of
the Woodlark
Basin
in the Southwest
Pacific:
Geological
Society
of
America Bul~etin, v. 84, p. 1125-1134.
Marshak,
J. S. and Karig,
D. E., 1977, Triple
junctions
as a cause for
anomalously
near-trench
igneous activity between the trench and volcaniC
arc:
Geology, v» 5, p. 233-236.
Milsom, J. S., 1970, Woodlark Basin, a minor center of sea-floor spreading in
Melanesia:
Journal of Geophysical Research, v. 75, p. 7335-7339.
Pascal,
G'I
1979, Seismotectonics
of the Papua New Guinea-Solomon
Islands
region:
Tectonophysics,
v. 57, p. 7-34.
Ripper, I., 1980, Seismicity,
earthquake focal mechanisms and tectonics of th~_
Indo-Australian/Solomon
sea plate boundary:
Department
of Mineral and
Energy Report 8015, Geological Society of Papua New Guinea, 55 p.
8m.! th,
I. E., 1976, Peralkaline
rhyolites
from the D' Entrecasteaux
Islands
Papua New Guinea.l!!..Johnson, R. W., ec,.; Volcanism
in Australia, p. 275285.
Taylor,
B., 1975, The tectonics
of the Bismarck Sea region ta.sc , Honours
thesis):
University of Sydney, New South Wales, 103 p.
Taylor, B., 1979, Bismarck
Sea:
Evolution of a back-arc basin:
Geology, v.
7, p. 171-174.
P. Cooper,
Kroenke,
Resig:
Seismicity
99
Taylor,
B.,
1984, A geophysical
survey of the Woodlark-Solomons region l£.
Exon, N. F., and Taylor, B., eds.,
Seafloor spreading,
ridge subduction,
volcanism and sedimentation
in the offshore woodlark-Solomons region,
and
Tripartite
cruise
-repor-c
for
Kana Keoki Cruise
82-03-16
Leg 4:
CCOP!SOPAC
Technical
Report No. 34, p. 13-42.
'rckeoe , N., 1975, The subduction of the lithosphere:
Scientific
American, v ,
233, p. 88-98.
Weissel,
J.
K., Taylor,
B. and :Karner, G. D., 1982, The opening of the
Woodlark Basin,
subduction
of the Woodlark spreading
system and the
evolution
of northern
Melanesia since mid-Pliocene time:
Tectonophysics,
v. 87, p. 253-277.
P. Cooper,
Kroenke,
Resig:
Seismicity
100