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Geological Society, London, Special Publications
The geology and tectonic evolution of the Bacan region, east
Indonesia
Jeffrey F. A. Malaihollo and Robert Hall
Geological Society, London, Special Publications 1996; v. 106; p. 483-497
doi:10.1144/GSL.SP.1996.106.01.30
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Robert Hall on 26 July 2010
© 1996 Geological Society of
London
The geology and tectonic evolution of the Bacan region,
east Indonesia
JEFFREY
E A. M A L A I H O L L O
& ROBERT
HALL
SE Asia Research Group, Department of Geological Sciences,
University College London, Gower Street, London WCIE 6BT, UK
Abstract: Bacan is located in the zone of convergence between the Eurasian, Philippine Sea
and Australian plates. The oldest rocks in Bacan belong to the Sibela Continental Suite and are
probably of Precambrian age. The complex includes continental phyllites, schists and gneisses of
upper amphibolite facies. Isotopic dating yielded extremely young ages due to interaction with
hydrothermal fluids. Juxtaposed against the continental rocks is the mostly unmetamorphosed,
arc-related Sibela ophiolite, probably derived from the Philippine Sea plate. Isotopic dating
yielded a Cretaceous age with an Oligocene-Miocene overprint. In north Bacan, the oldest
formation is the Upper Eocene Bacan Formation which comprises interbedded arc volcanic and
turbiditic volcaniclastic rocks, metamorphosed under conditions between the prehnitepumpellyite and greenschist facies. A similar Lower Miocene sequence, assigned to the same
formation, is exposed in south Bacan. The Oligocene Tawali Formation on Kasiruta, NW of
Bacan, consists of arc basalts and volcaniclastic turbidites, metamorphosed to zeolite facies. The
Bacan and Tawali Formations represent different parts of an arc, active from Late Eocene until
Early Miocene, resulting from northward subduction of the Australian plate under the Philippine
Sea plate. There is a major Lower Miocene unconformity, representing collision of the Australian
continent with the Philippine Sea plate, above which shallow marine limestones of the LowerMiddle Miocene Ruta Formation were deposited. This deposition was interrupted by sudden
influxes of volcaniclastic sands, forming the Amasing Formation. The Upper MiocenePleistocene Kaputusan Formation, rests locally unconformably on older rocks, and includes three
members. The Goro-goro Member consists of arc andesites, originating from four eruption
centres, which erupted from Late Miocene to Pleistocene, with the oldest in south and the
youngest in north Bacan. The Pacitak Member consists of shallow marine pyroclastic rocks. The
Mandioli Member formed fringing coastal reef limestones. The volcanic rocks of this formation
were produced by eastward subduction of the Molucca Sea plate. Quaternary basalts are related
to movement along the Sorong fault. It is concluded that most of the Bacan region has been part
of the Philippine Sea plate since the Cretaceous; volcanic rocks of different ages all have an arc
character and chemistry, and lithological variations reflect different positions within the volcanic
arc; and there is evidence for continental crust of Australian origin in the Bacan area by the Early
Miocene.
Three major plates, the Eurasian, Philippine Sea
and Australian plates, converge in the Bacan region
(Fig. 1). The Eurasian plate has a complex eastern
margin which includes continental and volcanic arc
crusts accreted during collision with the Philippine
Sea plate (e.g. Rangin 1991). The Philippine Sea
plate has a long arc history (Hall et al. 1995a),
extending back at least to the Cretaceous, and is
currently converging with Eurasia in the
Philippines, and overriding the Molucca Sea plate
in the south. The Australian plate has been moving
northwards since the Cretaceous. During much of
the Palaeogene, oceanic crust of the Australian
plate was subducted under the southern edge of
the Philippine Sea plate (Hall et al. 1995b). A
strike-slip fault system, the Sorong fault, subsequently developed at this plate boundary.
Bacan contains rocks of continental, ophiolitic
and arc affinities (Fig. 2), and therefore an under-
standing of the tectonic evolution of Bacan should
greatly enhance our knowledge of the development
of this area of complex plate boundaries. This
paper briefly describes and summarizes the
geology of each of the formations in the Bacan
region, discusses them in the light of regional
tectonic events, and proposes a synthesis of the
tectonic evolution of this region. A detailed
description of the lithostratigraphy of Bacan
(Fig. 3), with chemical and palaeoenvironmental
+analyses can be found in Malaihollo (1993).
Geology and tectonic evolution of Bacan
Sibela continental suite
The oldest rocks in the Bacan region form part of
the Sibela Continental Suite. The complex includes
continental phyllites, schists and gneisses of upper
From Hall, R. & Blundell, D. (eds), 1996, TectonicEvolution of Southeast Asia,
Geological Society Special Publication No. 106, pp. 483-497.
483
484
J.F.A. MALAIHOLLO 8z R. HALL
PHILIPPINE
SEA
500 krn
-4°N
I
CELEBESSEA
PACIFIC
OCEAN
f~
2
Morotai
~
GULF OF TOMINI
o~..
MOLUCCA
SEA
I
I BACAN
~ano'kwari 1"to,.
LMAHERA
-.,-... o, Gag ~
Waigeo
~
"
~$orang Fau, "~0.~(
~
Bird's
Head
IRIAN JAYA
%
BANDA SEA
FLORES SEA
ARAFURA SEA
D
o
Fig. 1. Present-day tectonic setting of east Indonesia based on Hamilton (1979).
amphibolite facies (c. 600°C, c. 5 kbar), with strong
penetrative fabrics indicative of a polyphase
deformation and recrystallization history, typical
of Barrovian type dynamo-thermal metamorphism
(Brouwer 1923; Hall et al. 1988). There is also
evidence for retrograde metamorphism associated
with post peak-metamorphism deformation. The
protoliths were pelites with minor amounts of
sandy and carbonate/marly horizons. Whole rock
chemistry of the pelites suggests that they were
derived from a cratonic area and were deposited on
an active margin underlain by continental crust.
Based on regional stratigraphical arguments
(Hamilton 1979) and the Pb isotopic signatures
of Quatemary volcanic rocks (Vroon 1992), these
metamorphic rocks are postulated to be of
Precambrian or Palaeozoic age. This suggestion
has not been confirmed isotopically, as K-Ar and
Ar-Ar step heating results yielded extremely young
ages (< 0.21 Ma; Malaihollo 1993), interpreted to
be a result of interaction with a hydrothermal fluid
carrying fractionated argon which were subsequently concentrated between the sheet silicate
layers, causing an apparent young age. This mechanism could account for other unusually young
ages from high-grade metamorphic rocks in the
region (cf. Linthout et al. 1991).
There are highly foliated metasedimentary rocks
in the south Saleh Islands and the SE comer of
north Bacan. These rocks have a similar character
to the Sibela Continental Suite, particularly in
having brown metamorphic biotite which throughout the whole region is found only in the Sibela
Continental Suite, but are of lower metamorphic
grade.
This paper interprets the Sibela Continental Suite
to be derived from the Australian plate, where these
rocks formed either part of the Mesozoic-Tertiary
passive margin of north Australia or fragments
rifted from Australia during Gondwana break-up.
Many authors have suggested or implied that the
continental rocks in Bacan were introduced into the
region via strike-slip motion of the Sorong fault
system from the Bird's Head region of Irian Jaya
(e.g. Hamilton 1979; Silver et al. 1985) or central
Papua New Guinea (Pigram & Panggabean 1984).
87Sr/86Sr isotopic compositions of volcanic rocks
(E. Forde, pers.comm. 1993) suggest that the Upper
Miocene Kaputusan Formation on south Bacan was
erupted through continental crust, indicating the
presence of continental material under the Bacan
region by the Late Miocene. The presence of a
Lower Miocene post-collisional intrusion (the Nusa
Babi Monzodiorite) in Bacan suggests the presence
of continental crust by the Early Miocene (as
discussed below).
7
"•
.I~"
.
, ~
"
a
k
~
:.Sibela Mts::
~
v.'
~,,%-
Goro-goro
Isles~,
Saleh
1127"45'E
~
~
®
L)
Continental
Suite
Sibela Ophiolite
I'7~_-~'~ Bacan Fm
Tawali Fm
Intrusions
Ruta Fm
NusaBobi
~
Volcanics
Kaputusan Fm
Mandioli Mb
Kaputusan Fm
Pacitak Mb
Kaputusan Fm
Goro-Goro Mb
Saleh
Intrusions
Amasing Fm
~
~
~
~
o~
o
o
8
i
-
~_
®
o~
(~
•
Alluvium&
Limestones
LEGEND
[--1
Fig. 2. New simplified geological map of the Bacan region, based on aerial photographic interpretation and field mapping (Malaihollo 1993).
20
t
i
km
i
I ~27°3°'E
Nusa Babi
30
10
~
K,apuTusan ~,
,
~
=~-~.
!
I
,,'
I
i
-
J127°15'E
0
Kasiruta
~e--(TawaliBesar)
,,s
t
s'
0"45'S
m
0,30,'~
0"~5"~
4~
Oo
C~h
>
Z
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z
©
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486
J.F.A. MALAIHOLLO 8Z R. HALL
Ma
0-
EPOCH
Kasiruta
Alluvium
Reef
Bacan
~
Pleistocene
Pliocene
5.2
Possible Tectonic Events
Quaternary volcanism related to movements on Sarong Fault.Upliftof Sibela Block.
Thrustingin Halmahera, movement on splaysof Sarong Fault.
aputusan m
Kapulusan FB
Goro-goro Mbr
Goro-goro Mbr
Pacitak Mbr
Pacltak Mbr
Mandioli Mbr
Mandioli Mbr
10,4
Associated subaerial and shallow marine pyroclasticdeposits,and fringingreefs,
Four eruptive centres which migrated northwards with time.
Start of volcanism in Bacan (-7 Ma), Halmahera and Obi (- 12 Ma).
Initiationof eastward subducfion of Molucca Sea Plate under Halmahera,
Miocene
Ruta
Formation
16.3 m
Shallow marine carbonate platform [open platform, patch reef,tidalbar and foreslope facies).
Local shallow marine volcaniclasticand littoraldeposit,
Collisionof Australiaand arc on the PhilippineSea plate.
23.3
Tawali Fm
Marikapal Mbr
Oligocene
Tawali Fm
Jojok Mbr
Bacon
Formation
Arc volcanic and turbidites,
Repeated, high concentration, proximal volcaniclasticturbidites, i Metamorphosed to PrehniteInner and middle fan facies,
Pumpellylte to Lower
Greenschist facies.
I
Volcanism related to the
northward movement of
Pillowlavas with volcanic arc chemistry.
Australiaunder the
Zeolitefaciesmetamorphism.
PhilippineSea plate,
I
35.4
Eocene
I Similarsequence in Bird's
Head and southern
Philippines,
Fig. 3. Cenozoic stratigraphy of the Bacan region with related local tectonic events. Timescale after Harland et al.
(1990).
Sibela ophiolite
Juxtaposed against the continental suite is the
Sibela ophiolite (Hall et al. 1988). There are two
types of material recognized in this suite: ophiolitic
rocks, most of which are of lower crustal plutonic
and mantle origin (microgabbros, gabbros and
serpentinised harzburgites) with few volcanic
rocks, and spatially-related rocks including unusual
amphibole cumulates and many rocks with
magmatic-tectonic fabrics. This dismembered,
incomplete ophiolitic complex is mostly unmetamorphosed, although locally there are
mylonitic rocks which have been affected by upper
amphibolite-lower granulite facies metamorphism
(c. 1000oc, c. 5 kbar) related to ductile deformation
of hot rocks at or near their place of formation, and
local recrystallization in shear zones (Malaihollo
1993).
Chrome spinel compositions suggest that the
peridotites may have formed in an arc setting. This
is supported by geochemical evidence from
cogenetic metagabbros. Petrographic study reveals
that most of the cumulate rocks consist mainly of
cumulus amphibole with minor pyroxene and intercumulate plagioclase, indicating crystallization
under hydrous conditions, implying an arc-related
setting. The cumulate rocks in the Sibela ophiolite
are very different from those of the Halmahera
ophiolite, which consist of olivine, orthopyroxene
and clinopyroxene (Ballantyne 1992). The
Halmahera cumulates are genetically related to the
ophiolite which is interpreted to have formed in a
supra-subduction zone, normally associated with
the initiation of a subduction zone, from the intraPacific subduction (Ballantyne & Hall 1990;
Ballantyne 1991). Preliminary Nd-Sm ages on the
Halmahera cumulates are Jurassic (M. E Thirlwall,
pers. comm. 1992). Ar-Ar and K-Ar dating of
the Sibela cumulates and metagabbro (Malaihollo
1993) yielded Cretaceous (97-94 Ma) and
Oligocene-Miocene (37-25 Ma) ages, although
amphiboles analysed contain excess argon and
therefore plateaux ages will be slightly older than
closure ages (Lanphere & Dalrymple 1976). The
Cretaceous age may be linked to volcanic activity
recorded in the east Halmahera ophiolite (9480Ma; Ballantyne 1992), suggesting a link
between the Sibela ophiolite and the east
Halmahera ophiolite. The Oligocene-Miocene ages
are related to the Oligocene volcanism (discussed
below).
GEOLOGY • TECTONICS OF BACAN, E INDONESIA
There are two possible interpretations for the
association of ophiolitic and cumulate rocks. The
first possibility is that the ophiolitic rocks represent
older crust of possibly Jurassic age (cf. east
Halmahera-Gag ophiolite) intruded by Cretaceous
arc cumulates related to intra-oceanic subduction.
This interpretation is supported by chemical
similarities between the cumulates and zoned
ultramafic complexes, normally associated with arc
magmatism. Alternatively, both the ophiolitic and
arc cumulate rocks may be related to the same
intra-oceanic subduction during the Cretaceous.
The age and nature of the original crust upon which
the arc was built is unknown.
Juxtaposition of continental and
ophiolitic rocks
The continental and ophiolitic rocks are juxtaposed
in the Sibela Mountains. Differences in metamorphic character indicate that metamorphism
occurred before amalgamation of continental and
ophiolitic rocks. The continental suite is interpreted
to be part of the Australian continental basement,
whereas the ophiolitic rocks represent the
Philippine Sea plate. Regional mapping shows that
since the Early Miocene, rocks from these two
plates have the same geological history, indicating
an Early Miocene collision (Hall et al. 1995a).
There are three possibilities for the mode of juxtaposition of the continental and ophiolitic rocks:
(1) the Sibela Mountains represent a suture zone
resulting from collision between the Australian and
Philippine Sea plates; (2) the continental rocks
collided with the Philippine Sea plate and were
subsequently translated by strike-slip faulting as
part of a composite fragment to their present
position; (3) the continental rocks remained part of
the north Australian Margin until the late Neogene
and were then translated by strike-slip faulting on
the Strong fault into the region. These possibilities
are discussed below.
487
erupted in an arc setting. Turbiditic rocks were
deposited by dilute, low concentration, possibly
distal or low energy currents. The Bacan Formation
was affected by a thermal event at c. 15 Ma.
In south Bacan the oldest rocks dated are a
very similar sequence of Early Miocene age.
These interbedded volcanic and volcaniclastic
turbidite rocks were metamorphosed to prehnitepumpellyite facies (c. 240-330°C, c. 2 kbar) due
to burial metamorphism, with local hydrothermal
alteration. Whole rock and mineral chemistry
indicates an arc origin for the volcanic rocks.
Graded volcaniclastic arenites indicate deposition
by turbidity currents which dissipated their energy
as they travelled down slope. Isotopic dating shows
that this formation was affected by a thermal event
atc. 8Ma.
Undated metabasites in the Saleh Islands were
metamorphosed at conditions between upper
prehnite-pumpellyite and lower greenschist facies
(c. 250-360°C, c. 4 kbar). The metamorphic
character of these rocks suggests static, regional
metamorphism. Geochemical and lithological
evidence from the metabasites indicates that they
represent an arc-related calc-alkaline sequence,
possibly formed in a backarc, and they have similar
chemical characteristics to the Upper Eocene and
Lower Miocene sequences.
The Upper Eocene, Lower Miocene and the
Saleh Island sequences are similar in all aspects,
except age. More dating is needed, although this
may prove difficult, since despite examination of
> 200 thin sections, only four samples were found
to be suitable for biostratigraphic or isotopic dating.
This is attributed to lack of fossils in the original
sediments, metamorphism which destroyed fossils,
and alteration by thermal overprints of the volcanic
rocks. The rocks assigned to the Bacan Formation
could represent either different parts of a single,
continuously active arc, or separate arcs active at
different times between the Late Eocene and Early
Miocene.
The Tawali Formation
The Bacan Formation
In north Bacan the oldest rocks exposed belong
to the Upper Eocene Bacan Formation. This
comprises interbedded basic-intermediate volcanic
and volcaniclastic turbiditic rocks. The Bacan
Formation was folded and subsequently metamorphosed under conditions transitional between
the prehnite-pumpellyite and lower greenschist
facies (250-330°C, c. 2 kbar upwards), with
characteristics of burial metamorphism. Hydrothermal alteration may be related to the Neogene
Kaputusan volcanism. Mineral and whole rock
geochemistry indicates that the volcanic rocks were
The oldest rocks on Kasiruta belong to the newly
defined Tawali Formation, named after the island
of Kasiruta (also known as Tawali Besar). Rocks
forming part of this formation were originally
assigned to the Bacan Formation (Yasin 1980), but
here they are assigned to a new formation because
of differences in lithology and the character of
metamorphism and deformation. The lower part of
the Tawali Formation consists of basaltic pillow
lavas and interbedded sediments (Jojok Member),
and it is overlain by fossiliferous volcaniclastic
turbidites (Marikapal Member). This formation
is affected by burial metamorphism transitional
488
J.F.A. MALAIHOLLO & R. HALL
between low and high temperature zeolite facies
(c. 180°C, < 2 kbar). Whole rock chemistry indicates that the Lower Oligocene basalts are highly
differentiated arc lavas, erupted in a deep, open
marine environment above the CCD. The Upper
Oligocene volcaniclastic rocks are products of
repeated, high concentration, proximal turbidity
currents with associated slumped deposits.
The Tawali Formation is interpreted as an
equivalent of the Bacan Formation although there
are several differences between them. There is an
apparent lack of Oligocene rocks on Bacan,
although as noted above, only a few samples of
the Bacan Formation have proved dateable. There
are also differences in lithology and metamorphic
character (particularly the lower grade of metamorphism suffered by the Tawali Formation relative to the Bacan Formation), and trace element
differences between the Tawali and the Bacan
Formations. Rocks of similar lithology and age
to the Tawali Formation have now been identified
throughout the Halmahera-Waigeo region (the
Tawali Formation in Halmahera, the Anggai River
Formation in Obi and the Rumai Formation in
Waigeo; Hall et al. 1991). The Oha Formation of
Halmahera is probably not of Upper CretaceousEocene age as previously suggested (Hakim & Hall
1991), but is interpreted as part of the same arc as
the Bacan Formation. Despite differences, all these
formations range in age from Upper Eocene to
Upper Miocene and they all pre-date the c. 22 Ma
unconformity (discussed below) interpreted to be
related to the arrival of the continental crust in the
region. Differences in lithology could be explained
if the different formations represent different parts
of the arc (e.g. arc slopes, backarc basin, forearc
slopes). Differences in the grades of metamorphism
~
TawaliFm,,MarikapalMb.&
RumaiFm.(turbidites)
~
could be a function of different structural positions
during collision with the Australian continent or
different thermal environments due to different
positions in the pre-collision arc.
Thus, the simplest explanation for all these
Upper Eocene, Oligocene and Lower Miocene
volcanic arc formations is that they represent arc
volcanism at the edge of the Philippine Sea plate
due to the subduction of the Indo-Australian plate
under the Philippine Sea plate (Fig.4). This is
supported by the fact that all of these formations
record similar palaeomagnetic declinations and
inclinations which are characteristic of the
Philippine Sea plate (Hall et al. 1995a; Ali & Hall
1995).
Early Miocene unconformity
Above the Bacan and Tawali Formations, there is a
major regional unconformity. This is characterized
by a major change in lithological, metamorphic and
structural character. Rocks below the unconformity
are folded arc volcanic rocks and turbidites, metamorphosed up to greenschist facies, whereas the
rocks immediately above it are mostly unfolded,
unmetamorphosed carbonates. The age of the
unconformity is estimated to be c. 22 Ma (Early
Miocene) throughout the Halmahera region (Hall
et al. 1995a).
Nusa Babi intrusive rocks
The Nusa Babi Monzodiorite (NBM) includes
quartz-monzodiorite dykes and plugs with associated aplite dykes of monzogranite composition.
These intrude the Bacan Formation and have been
reported to intrude the Sibela Complex (Yasin
BacanFm.arc BacanFm.&AnggaiRiverFm,
(forearcturbidites
(back-arcpillowbasalts)
SibelaContinentalSuite
.
Philippine Sea plate
X~4
×
×X
XX
,
Australian plate
No Scale Intended
Fig. 4. Different positions, in the pre-collisional stack, of the volcanic formations in the Bacan-Halmahera-Waigeo
region which were part of the Late Eocene-Early Miocene arc.
GEOLOGY & TECTONICS OF BACAN, E INDONESIA
1980), but do not intrude the Ruta and Amasing
Formations. The NBM is locally altered, possibly
due to auto-metasmorphism. Magma evolution was
primarily controlled by fractionation and the addition of water. Whole rock geochemistry indicates
that the NBM is of plutonic arc or collisional
magmatism origin. K-Ar dating yields an Early
Miocene age (Baker & Malaihollo 1996). The geochemistry, isotopic age and the fact that the NBM
intrudes the Bacan Formation indicate a postcollisional origin, probably related to the arrival
of the Sibela Continental Suite in the region.
489
The Saleh Diorite
The Saleh Diorite intrudes the Sibela Continental
Suite in the Saleh Islands and is juxtaposed against
it in central Bacan. This intrusion consists of
amphibole-bearing diorite and micro-diorite.
Locally these rocks have suffered low temperature
alteration, probably due to auto-metamorphism,
and subsequent hydrothermal alteration. Whole
rock geochemistry indicates that the Saleh Diorite
is different from the NBM and is of arc or postcollisional origin. K-Ar dates suggest the c. 15 Ma
age is related to the initiation of the Halmahera arc.
The Ruta and A m a s i n g Formations
The Lower-Middle Miocene Ruta Limestone lies
unconformably above the Bacan and Tawali
Formations. There are four microfacies recognized
in the Ruta Limestone: skeletal wackestonespackstones, algal boundstones, foraminiferal
packstones and bioclastic-lithoclastic packstones.
These represent deposition on an open platform,
a platform margin build-up (patch reef), a tidal bar
downslope from a patch reef (open platform) and
foreslope talus respectively, all forming part of a
shallow marine carbonate platform. The deposition
of the Ruta Formation began in the Early Miocene,
predominantly as open platform, platform margin
build-up and foreslope talus facies• Between the
Early~ Miocene and Middle Miocene, deposition
was locally interrupted by sudden and high
influxes of volcaniclastic material forming sandstones of the Amasing Formation. Three facies in
the Amasing Formation have been recognized:
shallow marine with storm horizons, shoal or
estuarine, and beach deposits. These facies
represent shallowing upwards from an open
carbonate platform to a beach environment.
Deposition of the Ruta Formation continued until
the Middle Miocene and its upper part is dominated by the tidal bar facies. There was widespread
development of carbonate platforms at this time
throughout the Halmahera-Waigeo region (Hall
et al. 1995a), in north Irian Jaya (Pigram & Davies
1987; Pigram et al. 1990) and in the Philippines
(Mitchell et al. 1986). Widespread carbonate
platform development suggests that most of the
Bacan-Halmahera-Waigeo region was in a quiet
tectonic setting, in an equatorial position and was
part of an extensive shallow marine area.
In north Irian Jaya, the Middle Miocene Moon
Volcanics (Pieters et al. 1989) may be linked with
the Maramuni arc of Papua New Guinea which
Dow (1977) suggested represented post-collision
subduction reversal. However, the character and
distribution of this volcanism is still poorly known
and if an arc existed, it may have been limited to
the eastern part of New Guinea (Ali & Hall 1995).
The Kaputusan Formation
The Kaputusan Formation was deposited above
the Ruta and Amasing Formations. It consists of
three members: the Goro-goro Volcanic, the Pacitak
Volcaniclastic, and the Mandioli Limestone
Members• Field mapping and aerial photographic
interpretation indicates that the Kaputusan
Formation rests directly upon the Bacan Formation
in some areas implying an unconformable contact
at its base.
The Goro-goro Member consists of twopyroxene andesites (TPAN), hornblende-pyroxene
andesites (HPAN), hornblende andesites (HBAN),
and hornblende-biotite andesites (HBIAN).
Associated with the volcanic rocks are subaqueous
pyroclastic flows with related base surge deposits.
The petrography, mineral chemistry, whole rock
major and trace element chemistry of these rocks
are typical of island-arc volcanic rocks. They are
mostly fresh, akhough locally they are metamorphosed to zeolite facies, interpreted as due to
hydrothermal activity. Magma diversification of
the Goro-goro Member was achieved mainly by
fractionation of plagioclase, pyroxene, Fe-Ti oxide,
amphibole and biotite, with evidence for replenishment, immiscibility, resorption and assimilation.
Magmatic conditions were < 8 kbar and 2-10 wt.%
H20 with PH2U
'-' < Ptotal" The Goro-goro Member
includes at least four eruption centres (as defined
by bulk trace element ratios): in south Bacan
(mostly HBIAN), the Goro-goro area (mostly
TPAN), and the north Mandioli and the Kaputusan
areas (largely HPAN and HBAN). The Mandioli
group are shoshonitic rocks, which may indicate
eruption in an extensional setting within a dominantly convergent zone, such as a strike-slip zone
where similar rocks commonly occur (e.g. Ellam
et al. 1988). K-Ar ages indicate that these centres ~
were erupting from the Late Miocene (c. 7.5 Ma)
to the Pliocene (c. 2.2 Ma). Each of these centres
appears to have been active for c. 2 Ma, with a
history of multiple eruptions. The Upper MioceneUpper Pliocene Pacitak Member consists of
•
490
J.F.A. MALAIHOLLO ,~ R. HALL
Discussion
reworked pyroclastic and volcaniclastic material
from the Goro-goro Member, deposited in an
oxygen-rich, nearshore, shallow marine environment. The Upper Miocene-Lower Pliocene
Mandioli Member consists of wackestones and
packstones which formed fringing coastal reefs.
The Kaputusan Formation is interpreted to be the
product of the eastward subduction of the Molucca
Sea plate under Halmahera, and can be correlated
with the Weda Group of Halmahera and the Woi
Formation of Obi (Hall et al. 1995a; A l i & Hall
1995).
The history and motion of the Philippine Sea plate
has been complex, although recent palaeomagnetic
results are contributing to a clearer picture. The
work of Hall et al. (1995a, b) suggests that between
c. 50-40 Ma the plate experienced a c. 50 ° clockwise rotation with a southward translation; no
rotation between 40 to 25 Ma, and from 25 to 0 Ma
the plate rotated clockwise 35 ° with northward
translation. The first rotation may be related to the
change of motion in the Pacific plate (Clague &
Jarrard 1973) at about 42 Ma. Australia was
moving north throughout the Tertiary and we
interpret the regional unconformity at c. 22 Ma as
the result of collision of Australia with a volcanic
arc at the southern edge of the Philippine Sea
plate (discussed below). This may have caused or
contributed to renewed clockwise rotation of the
Philippine Sea plate.
Q u a t e r n a r y deposits
Although currently there are no active volcanoes
on Bacan, Quaternary volcanic rocks are present.
These are fresh olivine-, pyroxene- and plagioclase-phyric basalts. Mineral and whole rock geochemistry indicate that these are arc basalts which
erupted through a quartz-rich basement (Sibela
Continental Suite). There is evidence of a withinplate basalt component from whole rock and
mineral chemistry, possibly indicating magmatism
related to movements along the sinistral Sorong
fault. This is supported by the linear distribution
of the volcanic centres, and in accordance with
the view of Silitonga et al. (1981). Detritus from
the Sibela Continental Suite can be found solely
in the Quaternary deposits, indicating that it
has become available for erosion only recently,
suggesting extremely fast rates of uplift (c.
Pre-Tertiary and early Tertiary
evolution of Bacan
There are two types of pre-Tertiary rocks in Bacan:
continental crust and ophiolitic/arc material. The
continental crust is presumably of Australian
margin or micro-continental origin; north of this
margin was oceanic crust. Ophiolite/arc material in
Bacan records Cretaceous and Oligo-Miocene ages,
both of which can be correlated with ages recorded
in Halmahera which now forms part of the
1 mm a-l).
I / / I / I / I / / / / / / / I ~
,~/, ,~//I Philippine [~ /, // /, /, /, 1
[Se 0 .Plgte b / / / / / ]
/I/I/////~
i/i/ill/i/i/////
// JPhilippi ne I ii i,i i,i ii i i i ,
/
e a Plate / / / / / / /
4 / / ~ P h i l i p p i n e i~,
/ 4 Sea Plate I /
/ L./ / / /
eurasia I = , i # / ~
///
/ / / /
IIIIIIIIIII/I//
~ J l l l l l ~
i i i / / / / / / 1 1 1 / / i
-
IIIIIIIIIIlilll
.~111111~-~1
~ ~
/ / /
:::.::::.
::::::'*::::,.
i:iSi:i:!:!:!:!..
Late Eocene
, ~
i
'Iregion
[
. . . . .
i
"
I
II
i
/
I
.
::'.::::
Oligocene
Early Miocene
Fig. 5. Simplified tectonic setting of the Bacan region during the Late Palaeogene-Early Miocene. The oceanic crust
of the Australian plate was being subducted under the Philippine Sea plate. The Bacan region moved from arc-forearc
(Late Eocene) to a backarc (Oligocene) setting and returned to an arc-forearc position (Early Miocene).
GEOLOGY ~ TECTONICS OF BACAN, E INDONESIA
Philippine Sea plate. This implies that the ophiolitic
rocks of Bacan were part of the Philippine Sea plate
since at least the Cretaceous. Plate tectonic reconstructions show that almost all the oceanic floor
of the Mesozoic west Pacific has long since been
subducted. The ophiolites of Halmahera and
Waigeo, which represent the oldest parts of the
Philippine Sea plate, were formed in an intraoceanic arc setting (Ballantyne 1991, 1992) and the
Cretaceous Halmahera volcanic arc was situated at
sub-equatorial latitudes (Hall et al. 1995a). There
is no lithostratigraphic evidence on Bacan for the
history of the region between the Cretaceous and
Late Eocene.
45-22 Ma
Between the Late Eocene and Early Miocene there
was extensive arc activity throughout the BacanHalmahera region. Sukamto et al. (1981) suggested
that Oligocene volcanism in the Bacan-Halmahera
region was the result of west-dipping subduction
from the Pacific side, implying that the region lay
on the Eurasian plate. Recent palaeomagnetic
evidence (Hall et al. 1995a) from the Tawali
Formation shows that the region was part of the
Philippine Sea plate. Oceanic crust of the
Australian plate was subducted northwards,
forming an arc at the southern margin of the
Philippine Sea plate and fragments of this arc
extend from northern New Guinea, through the
Bacan-Halmahera-Waigeo region, into the east
Philippines (Rangin et al. 1990; Hall 1995; Hall
et al. 1995a). The Oligocene Tawali Formation is
interpreted as part of this volcanic arc.
The Upper Eocene to Lower Miocene Bacan
Formation is genetically related to the Tawali
Formation and represents temporal and spatial
variations of the same arc (Fig. 5). There are two
possible correlatable volcanic arcs in the east
Indonesia-west Pacific area: the OligoceneMiocene volcanic rocks of Irian Jaya and Papua
New Guinea (e.g. Pigram & Davies 1987) or the
Eocene-Oligocene Palau-Kyushu remnant arc
(Karig 1975; Sutter & Snee 1980). The PalauKyushu and the West Mariana ridges are associated
with opening of the Parece Vela Basin (at 3017 Ma) and these remnant arcs are attributed to
subduction of Pacific ocean (e.g. Uyeda & BenAvraham 1972; Seno & Maruyama 1984) and are
therefore different from the arc represented by the
Tawali Formation. Correlation of volcanic rocks
in Bacan and the Bird's Head was suggested by
Van Bemmelen (1949) and Verstappen (1960). The
arc volcanic rocks of north Irian Jaya, for example
the Arfak (Ratman & Robinson 1981; Pieters et al.
1982), Batanta (Sanyoto et al. 1985), Yapen
(Atmawinata et al. 1989) and Mandi Formations
491
(Pieters et al. 1989) and Papua New Guinea, for
example the Bismarck Volcanic Province (Dow
1977), are interpreted to be related to subduction
between the Australian and Pacific plates (Dow
1977; Pigram & Davies 1987) and may be a
continuation of the Bacan-Tawali Formation
volcanic arc.
22 Ma unconformity
A collision between the Australian margin and an
island-arc has been implied or suggested by many
authors (e.g. Dow 1977; Jaques & Robinson 1977;
Pieters et al. 1983; Pigram & Davies 1987),
although suggestions of the age of this collision
vary. Pigram & Symonds (1991) review estimates
of its age ranging from Eocene to Late Miocene; for
east New Guinea they argue for a collision unconformity at c. 30Ma. Charlton et al. (1991)
tentatively suggested a mid-Oligocene age for
collision on Waigeo, based on a poorly defined
Upper Oligocene age of the Mayalibit Formation.
However, new isotopic dating of volcanic rocks
from this formation indicates a narrow age range,
of uppermost Oligocene-lowermost Miocene (our
unpublished results). At the south end of Mayalibit
Bay on Waigeo these volcanic rocks dip at up to
40 ° beneath Miocene limestones, suggesting they
are below the unconformity.
The presence of the Australian continental rocks
on Bacan could be due to: (1) Early Miocene
collision; (2) Early Miocene collision followed by
Neogene strike-slip translation; or (3) Pliocene or
younger pure strike-slip translation. Although there
is no indisputable evidence, the proximity of the
Sibela Continental Suite to the arc, interpreted to be
the result of subduction of the Indo-Australian plate
under the Philippine Sea plate, and the juxtaposition of the continental rocks with the ophiolite
interpreted to represent Philippine Sea plate
material favours a collisional interpretation (1 or 2).
The character of the Nusa Babi Monzodiorite,
suggesting post-collisional melting of a continental
source, and the evidence of a continental crustal
contribution to Late Miocene volcanic rocks also
suggest the presence of a continental basement
beneath central and south Bacan by the early
Neogene.
On Bacan there is a regional unconformity of
Early Miocene age which is interpreted as resulting
from the collision of Australian and Philippine
Sea plates (Fig. 6). There is a change from folded,
metamorphosed arc rocks to unmetamorphosed
carbonates and a possible stitching intrusion of
Philippine Sea plate rocks (the Bacan and Tawali
Formations) and Australian plate rocks (Sibela
Continental Suite) by the Nusa Babi Monzodiorite
which is of Early Miocene age. This interpretation
492
J.F.A. MALAIHOLLO • R. HALL
////////////////////
/~///~/ /~/~/ /~l ~P~h'i'l' ip i e ] p n
/ / / / /, ,
// / // / // /// /// /// /// /// /~Sea
//// Platel/////
,/
~-.i~/Iregion
," !
.:.:....:.:.:.:.::..:.
+
1~ : ! : ! $t . - '.-.mi!~!ii!!~iiiiiiiiir,~+
: i~: ! : ! : +! : ! : ! :i
-:
,...................
,:-:-.*.:o:-.'.;..'-,'..'.."
~r ................
':':':':':':':':':':':';'.
.-'.:.:.:.;-'.:.-'.'..'.'..:.:.:.:.;..
+
===================== ,',.'.
x
"1";'.":
+
;.;.:.:.,.;
Fig. 6. Simplified tectonic setting of the Bacan region during the Early Miocene. Continuation of subduction
culminated in collision and thrusting of the continental crust, under the arc-forearc region of the Philippine Sea plate.
Following collision, the Philippine Sea plate rotated westward.
is supported by a similar lithostratigraphy throughout the Bacan-Halmahera region (Hall et al. 1988
1991), where most of the Lower to Middle Miocene
is represented by carbonate rocks deposited above
Upper Eocene-Lower Miocene arc rocks of
Philippine Sea plate affinity and rocks of Australian
continental affinity. It is therefore concluded that
the collision in east Indonesia occurred in the Early
Miocene and the authors agree with Ali & Hall
(1995) who suggest that much of the evidence from
east Indonesia and New Guinea can be understood
in terms of pre-Miocene intra-oceanic arc tectonics,
an arc-continent collision at about 22 Ma, and
Neogene strike-slip tectonics.
19-15 Ma
After the Early Miocene collision, the region was
uplifted and deposition of shallow marine
carbonates followed. A tectonically quiescent
period is envisaged at this time. Localized uplift
contributed influxes of volcaniclastic material onto
the carbonate platform (Fig. 7), probably derived
from the pre-Miocene formations.
Initiation of Molucca Sea subduction and
s u b s e q u e n t arc v o l c a n i s m
Northward movement of Australia during the
Neogene occurred without subduction at the
boundary of the Australian and Philippine Sea
plates, which was the left-lateral strike-slip Sorong
fault system. However, arc volcanism did result
from the eastward subduction of the Molucca Sea
plate under Halmahera (the Philippine Sea plate) at
the Halmahera trench (Fig. 8). On Bacan there is
a local unconformity of Late Miocene age, and in
places the Upper Miocene Kaputusan Formation
rests directly on the Bacan Formation. The
Kaputusan Formation is the equivalent of other
Upper Miocene arc sequences on Halmahera and
Obi. Although the oldest isotopic age obtained
from the Kaputusan Formation is c. 7.5 Ma, there
was a thermal event at c. 15 Ma recorded by widespread resetting of isotopic ages in the Bacan
Formation and rocks on Obi. The Saleh Diorite,
interpreted as a precursor to the Kaputusan
volcanics, was intruded at c. 15 Ma. The oldest
isotopic ages obtained from volcanic sequences on
Obi are c. 12 Ma (Baker & Malaihollo 1996). Thus,
initiation of subduction possibly started at c. 15 Ma
and the first volcanic products were erupted at
c. 12Ma.
Late Neogene-Recent
At c. 3 Ma there was more than 60 km shortening
between east and west Halmahera, attributed to
movement along the Sorong fault (Nichols & Hall
1991). Splays of the Sorong fault running through
the Bacan region may have contributed to the
ending of Kaputusan volcanism. Quaternary
volcanism was later reactivated along the fault
splays (Fig. 9). These faults are also responsible for
the shaping of Bacan coastlines and the creation of
deep basins surrounding Bacan. The culmination
of collision processes in the Molucca Sea will
eventually result in the transfer of the Bacan-
GEOLOGY & TECTONICS OF BACAN, E INDONESIA
493
~ / / / / / / / / / / /
I;~:::::::::::::::~////l
Philippine 1/
/ / / / ] S e a Plate[ /
I-.----------4////)
......
/
1::74=. . . . . ~ , . , I : = ~ / / / / " I I . Z _ ~ / / /
#,!
r~
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~"q,',',1~,,~ I
~
I
I
I
I~
¢-
~
~
~
" ~
~.
~
~
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~, , ......
2,',',',',',',"
t
I
I
I
I
~'~I.~L~ ° "
~
-
I
1
i
i
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-,,a,~
1 : ~ ~ I I / / / / / / / / / / / / /
l---~--'---J/..,~../////////
i
I
•
i,
,
volcaniclastic
I
,
i
LOCQI ~
Ie
I
,
i
~
i
I
,
~
i
I ~'~-~.#
~
,
~
,_.t
I
I
~
,
,
.%1
I
,
~
I
,
i
~
i
I
,
~
i
[',
~
~'
{
I
'
~
_
t
~
+
~
% +~J~l-~,~_,_%~_~o?
I
~
"
~
"
~-",ae,-".,. ~':':.:.:,
i : ~ . . , a k T d ] ~ i ' % ~ : ; : ; : ; :
..if
.%:.:
~
~
I
ioput - "
~
~
~
+
~
+
~
'
!
~
i
::iiii
Fig. 7. Simplified tectonic setting of the Bacan region during the Middle Miocene. Northward movement of Australia
was accommodated by a transform boundary, which moved north with Australia (Hall e t al. 1995a, b). The Philippine
Sea plate was rotating clockwise due to subduction on all sides. There was widespread development of platform
carbonates in the Bacan region, away from the plate boundary.
H a l m a h e r a region f r o m the edge of the Philippine
Sea plate to the Eurasian margin (Hall & Nichols
1990).
Implications for regional tectonics
This study has p r o v i d e d n e w geological data and
elucidated the tectonic d e v e l o p m e n t of the Bacan
region. In a regional context, it has contributed to
an u n d e r s t a n d i n g o f the tectonic history o f the
convergent zone b e t w e e n the Australian, Philippine
Sea and Eurasian plates in east Indonesia. S o m e o f
the m o s t important implications of this w o r k are:
(1) M o s t of Bacan is interpreted as part o f the
Philippine Sea plate. Cretaceous links are indicated by the presence of the Sibela ophiolite,
'~
./
!
~lZl
~.
~,
,&:~!~F~i~."i!~!##!~7
"~;~i~i;ii;;i;!!;!!!;/'
I " VolcQniccentres
I~ ~g,%%o~,oMb
irn~in
~
F / ]Philippine I
I¢ " 4seQ PIQtel
E / / "" " "" "
F
/
/
/
/
/
/
/
/
A:~:i:i&. . . . . . . .
.:
!!i!i.:i!i.:i::"..i!i!i!i!i!i!i!i!i!:====================================================
!:!:!:~:!:..':!:!:..':!:!:..':!:!:!:!:!:
~iiii~iiiiiiiiiiiiiiiii~-
iiiiiii i!ii!!!iiiiiii
::;:::::;St::::::;:::::;::::::::::,
:,:,:,:,:,:,;,:°:,:,:°:,:,:,:,:,:°:,:,,
+ : :::::::::::::::::"
~ i:'.i~i!i!iiiii~ii~i~ii~i~i:..
~!~!~!~!i
+ +
~x?-~~' +' 3~
Fig. 8. Simplified tectonic setting of the Bacan region during the Late Miocene. Westward subduction of the Molucca
Sea plate under the Philippine Sea plate started at c. 15 Ma, probably from the south and moved northward.
Volcanicity started in Bacan at c. 7.5 Ma. Shallow marine sequences and fringing reefs formed around volcanic
centres.
494
J.F.A. MALAIHOLLO ~; R. HALL
Fig. 9. Simplified tectonic setting of the Bacan region during the Late Pliocene-Quaternary. The Sorong fault system
transform boundary cut through the Bacan region, contributing to the cessation of Miocene-Pliocene volcanism.
Quaternary volcanism erupted through continental crust along strands of the Sorong fault. Faults controlled coast
lines and basins developed between strands of the fault.
(2)
(3)
(4)
(5)
and younger connections are indicated by the
similarity of Eocene-Oligocene arc sequences
which also record a similar palaeomagnetic
history (Hall et al. 1995a).
The Bacan and Tawali Formations are most
likely to be arc-related sequences produced as
a result of subduction of oceanic crust of the
Australian plate, probably directly linked to the
Indian Ocean, under the Philippine Sea plate.
The Sibela Continental Suite arrived in the
Bacan region after collision of Australia or a
rifted fragment of Australia with the Philippine
Sea plate.
The collision of Australia with the Philippine
Sea plate can be dated at c. 22 Ma in the Bacan
region.
Younger Neogene-Recent movements on
strands of the Sorong fault have modified the
geology of the region and permitted emplacement of younger volcanic rocks.
Several different models for the tectonic development of NE Indonesia have been proposed, e.g. the
indentor model of Charlton (1986); the terrane
model of Silver et al. (1985); and the marginal
basin model of Ben-Avraham (1978), and all were
based upon limited geological data. The BenAvraham model considers Bacan-Halmahera as
part of the Australian continent, which is wrong
as almost all of the region has been part of
the Philippine Sea plate since Cretaceous. Both the
Charlton and Silver et al. models imply that the
Sibela continental block was translated by postMiocene strike-slip motion to the Bacan region,
which has been disputed by results of this study. Ali
& Hall (1995) and Hall (1995) offer alternative
interpretations of the region which incorporate the
results of this and other recent geological and geophysical studies.
In a wider context, if the terrane definition of
Howell et al. (1985) is followed, the Sibela
Continental Suite, the Sibela ophiolite, the Bacan,
Tawali and Kaputusan Formations could be considered as five different tectono-stratigraphic
terranes. This may lead to interpretations such as
those of Struckmeyer et al. 1993, who apparently
assign separate histories to each of these 'terranes'.
Karig et al. (1986) argued that rocks in the north
Philippines, which have a similar character to the
Sibela Continental Suite, Sibela ophiolite, Bacan
and Kaputusan Formations, were separate allochthonous terranes juxtaposed by strike-slip faulting. Whilst not arguing against their interpretation,
this study has shown that the juxtaposition of rocks
of different character does not necessitate the
notion of allochthonous terranes. In the Bacan
region, the only fragment that may genuinely be an
allochthonous terrane is represented by the Sibela
Continental Suite. For the most part, the region has
always been at the edge of the Philippine Sea
plate, and different 'tectono-stratigraphic terranes'
reflect different tectonic regimes at the edge of the
plate.
The use of stratigraphical similarities is often
used in the literature to correlate tectono-stratigraphic terranes and ultimately tectonic history
(e.g. Hamilton 1979; Pigram & Panggabean 1984).
Using this technique, the Bacan region could be
GEOLOGY • TECTONICS OF BACAN, E INDONESIA
correlated with parts of Papua N e w G u i n e a
(Oligocene pillow lavas against continental basement; Dow 1977), the Zamboanga Peninsula of
Mindanao (high grade continental rocks juxtaposed
against meta-ophiolite unconformably overlain by
Middle Miocene volcanic rocks; Rangin 1991) and
north Philippines (see above; Karig et al. 1986).
This type of correlation may lead to erroneous
interpretations since each of these regions has
different tectonic affinities (Bacan was and is still
part of the Philippine Sea plate; North Papua New
Guinea was part of the Philippine Sea plate, but
now is part of the Australian plate; Z a m b o a n g a and
north Philippines were and are still part of the
Eurasian plate). Similarities in the stratigraphy are
consequences of similar tectonic histories (areas
recording the collision of continental with oceanic
plate), and one should be cautious in correlation
and interpretation of m o v e m e n t of terranes along
strike-slip fault. All too often tectonic reconstruc-
495
tion of a region is based on meagre geological data,
such as in the Bacan region, resulting in a diversity
of interpretations. Although this study has provided
the most comprehensive geological dataset from
the Bacan region, the history of the region in
the Early M i o c e n e and before the E o c e n e is
still unclear. Bearing this in mind, one should be
cautious in interpreting the tectonic evolution of
other, similarly complex, older regions.
This work was supported by NERC award GR3/7149,
grants from the Royal Society and the University of
London SE Asia Geological Research Group, and
financial assistance from Amoco Production Company.
We thank S. J. Baker, P. D. Ballantyne, E T. Banner, T. R.
Charlton, E. M. Finch, G. J. Nichols and S. J. Roberts for
their contributions and discussion, and C. C. Rundle and
D. C. Rex for guidance and help with isotopic dating.
Logistical assistance was provided by GRDC, Bandung
and the Director, R. Sukamto with field support by D. A.
Agustiyanto, A. Haryono, and S. Pandjaitan.
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