Download Newly discovered occurrences of the Paney Rhyolite and

Newly discovered occurrences of the
Paney Rhyolite and associated volcaniclastic
deposits, Mount Friday & Mount Double areas,
Gawler Range Volcanic Province
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1
1
Werner, Mario , Krapf, Carmen , McAvaney, Stacey and Fabris, Adrian
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1
Geological Survey of South Australia, Department of State Development, Adelaide, Australia
Introduction
The Mesoproterozoic Upper Gawler Range Volcanics succession is dominated by felsic coherent facies interpreted as silicic lava flows hundreds of
meters thick. Rare volcaniclastic facies locally overlie the coherent lava facies of the Paney Rhyolite Member and thus provide potential
stratigraphic markers. The Geological Survey of South Australia (GSSA) recently undertook geological field mapping along the southern margin of
the Gawler Range Volcanic Province (Fig. 1) complimenting a large regional drilling program - the Mineral Systems Drilling Program (MSDP) - run by
the GSSA.
Ceduna
Hiltaba
Mt Friday
Yardea
Narlaby Well
Mt Double
EY
Great
Australian
Bight
Nonning
RE
During field work, previously unrecognised occurrences of the Paney Rhyolite were discovered on Hiltaba Station in the vicinity of Mount Friday
and Eurilla Hill, Narlaby Well area, in the northwestern part of the Gawler Ranges (Figs. 2 & 3). These newly discovered occurrences currently
represent the northwesternmost known outcrops of the Paney Rhyolite, extending its lateral extent to ~200 km. The newly discovered
volcaniclastic deposits are of regional significance as the only other known volcaniclastic rocks within the Upper Gawler Range Volcanics, the
Mount Double Ignimbrite near Paney and silicified sandstones near Nonning, occur at the same stratigraphic level forming an important marker
horizon. Recently drilled holes in the Narlaby Well area, MSDP 5 & 7, intersected volcaniclastic rocks from the same stratigraphic position and
provide a valuable comparison to field exposures (Fig. 4). Drill cores enable the study of clast shapes and textures, bedding structures, contact
relationships and depositional trends in great detail and provide more continuous and better quality records of these volcaniclastic successions
than natural outcrops. The combined information from drill core and outcrop enhances characterisation and interpretation of these deposits and
improves the picture of their lateral distribution and variation.
Myall Creek
Siam
Paney
Hiltaba Suite Granitoids
33°S
Upper Gawler Range Volcanics:
AY
HW
G
Moonaree Dacite Member, Yardea Dacite
HI
Wudinna "
Pondanna Dacite Member, Yardea Dacite
Whyalla "
33°S
Meso- & Neoproterozoic" sedimentary
rocks
Streaky Bay
Kimba "
Eucarro Rhyolite, incl. Paney Rhyolite Member
Undifferentiated
134°ELower Gawler Range Volcanics
135°E
0
50
100 km
136°E
Spencer
Gulf
137°E
During a recent visit of the Paney area we also mapped and sampled the Mount Double Ignimbrite in detail. We present here some preliminary
results of this outcrop study in order to compare the facies of the Mount Double Ignimbrite with those of the volcaniclastic deposits of the Mount
Friday area (Figs. 5 & 6).
Fig. 1: Generalised geology of the southern margin of the Gawler Range Volcanic Province, South Australia, showing the locations of
the Mount Friday and Mount Double study areas.
-32.21°
135.17°
135.18°
135.19°
135.20°
135.2°
-32.2°
"
Lake
Gairdner
32°S
32°S
Lake
Acraman
135.3°
135.4°
!
!
!
!
!
!
!
!
!!
U
D
0
1 km
Mt Friday
Cooria Hill
!
¦
! !
!
0.5
!
!
60
!!
!
!
Hil
Yard taba ea T
rack
!
-32.3°
-32.22°
¦
80
!!
Hilt
aba
GRV Granit
e
con
tact -
!
!
Eurilla Hill
Moonaree Dacite
Narlaby Well
Moonaree Dacite:
black basal facies
!
Pondanna Dacite
autobreccias &
!
volcanogenic sediments
75
Paney Rhyolite:
U
!
!
D
!
!
!
volcanic flow-banding
!
sedimentary bedding
(in siltstones)
!
!
Peltaninna Hill
!
!
55
Mt Centre
!!!
!
!! !
!
80
¦
80
35
field observation point
!
MSDP 7 A
!
o
¦
o
35
Hiltaba Granite
!
!
!
Bittali Rhyolite
!
!
flow-banded coherent facies
Eucarro Rhyolite
75!
¦
60
Paney Rhyolite:
!
-32.4°
65
¦
¦¦
-32.23°
¦
Pondanna Dacite:
black basal facies
0
2.5
5 km
!
MSDP 5 A
Fig. 2: Right: Overview geology of the Narlaby Well area showing the distribution of newly discovered Paney Rhyolite outcrops and the locations of MSDP drill holes 5 & 7.
Left: Detailed geology of the Paney Rhyolite and associated volcaniclastic deposits in the Mount Friday area.
Mount Friday Area
Fig. 3 (above): Sample photos of the Paney Rhyolite and associated volcaniclastic rocks in the Mount
Friday area: (a) outcrop of coherent, flow-banded lava facies; (b) outcrop of clast-supported autobreccia
facies; (c) & (d) outcrops of matrix-supported, very poorly sorted, diamictic, volcanogenic brecciasconglomerates with angular to subrounded clasts of porphyritic, partly flow-banded rhyolite (debris flow
deposits); (e) to (g) cut slabs of matrix-supported, very poorly sorted, diamictic, volcanogenic brecciasconglomerates with angular to subrounded clasts of porphyritic, partly flow-banded rhyolite (debris flow
deposits) and white kaolinitic, green sericitic or brown ferruginous-siliceous matrix; (h) cut slab of poorly
sorted, matrix-supported volcanogenic granule breccia with dark green chloritic-sericitic matrix; (I) cut
slab of rhyolite clast-poor and crystal-poor, mud-rich, volcanogenic diamictite (mud flow deposit); (j) cut
slab of moderately sorted, clast-supported, volcanogenic rhyolite granule conglomerate with brown
ferruginous-siliceous matrix; (k) cut slab of relatively well sorted, coarse-grained, massive, volcanogenic
sandstone with green sericitic matrix; (l) outcrop of finely laminated siltstone layer overlying clastsupported, volcanogenic rhyolite granule conglomerate.
135.65°
!
CONTACT
!
!
alternating layers of crystal-poor/-rich ash tuffs,
lithic clast-free, massive to laminated
¹
¹
¹
!
!
!
!
pumice- and lithic clast-rich rhyolite,
eutaxitic, variably welded
!
0.25
0.5 km
!
!
!
!
!
!
!
75
Mount Double
Ignimbrite
135.65°
volcanic flow-banding
Yannabie
Rhyolite
80
-32.62°
-32.62°
50
!
80
¹
lower lithic clast-free rhyolite,
flow-banded, columnar jointed
!
field observation point
!
upper lithic clast-poor rhyolite,
very finely flow-banded
!
!
!
0
35
!
! !!
! !
50
!
!
!
!
!
!
!
!
¹
In the northwest of the Mount Double Ignimbrite outcrop, alternating layers of crystal-rich
and crystal-poor rhyolitic rocks directly underlie the Pondanna Dacite. The crystal-poor
layers either display a very fine lamination or are massive with randomly dispersed
feldspar crystals (Figs. 6m-o). These rocks are interpreted as ash fall and ash flow tuffs
deposited on top of the densely welded and strongly rheomorphic portion of the Mount
Double Ignimbrite.
Mt Double
Pondanna Dacite
!
The Mount Double Ignimbrite occurs as a small exposure between flattish outcrops of the
Paney Rhyolite and the range-forming outcrops of the Pondanna Dacite on the southwestern slope of Mount Double (Fig. 5). In-situ outcrops are rare but detailed mapping has
shown that the Mount Double Ignimbrite can be subdivided into a number of easttrending facies zones.
The lower part is formed by a red-brown, strongly flow-banded, columnar jointed, mainly
feldspar-phyric rhyolite free of lithic clasts and with an apparent coherent texture (Fig. 6c).
In contrast, the middle part of the Mount Double Ignimbrite is characterised by a clast-rich
facies. Some collected samples contain mainly pumice clasts set in a fine-grained matrix
free of or poor in feldspar crystals and rhyolitic lithic clasts (Fig. 6d). The pumice clasts are
often plastically deformed, compacted and attenuated to lenticular fiamme by welding
processes but some show ragged edges and remnants of tubular vesicle texture (Figs. 6ef ). The majority of samples from this facies zone are rich in reddish-brown, mm- to cmsized, angular lithic clasts composed of porphyritic rhyolite and the matrix of these clastrich samples commonly display a eutaxitic texture (Figs. 6g-h). At the base of this clast-rich
zone also pyroclastic rocks with no apparent welding signs can be found (Fig. 6i). In
upward direction, the eutaxitic texture of these clast-rich rocks grades into a more
continuous welding lamination and clast rotation indicates rheomorphic flow. The upper
part of the Mount Double Ignimbrite is characterised by a clast-poor/free, typically very
finely flow-laminated facies with dispersed mm-sized feldspar crystals and rare quartz
crystals (Figs. 6j-l).
135.66°
-32.61°
In the Mount Double area the Paney Rhyolite is an orange-red, typically flow-banded and
flow-folded, quartz- and feldspar-phyric rhyolite (Fig. 6a). Its coherent nature shows that it
was emplaced as a lava flow. However, no autoclastic facies have been found yet on top of
this lava flow. Locally, the lower part of the Paney Rhyolite is very transitional to the
underlying reddish, feldspar-phyric Eucarro Rhyolite and has a fiamme-like texture (Fig.
6b). This lenticular, pseudo-fiamme texture is not interpreted as pyroclastic in origin but is
probably the result of mingling processes of the separate Paney and Eucarro magma
batches.
Dr Mario Werner
Geological Survey of South Australia
Level 4, 101 Grenfell Street
SA 5000 Adelaide
Phone: +61 (8) 8463 3065
Email: [email protected]
Fig. 4 (right): Cores of drill holes MSDP 5 & 7 with (a) autobreccia top; (b) diamictic volcanogenic
breccia to sandstone; (c) volcanogenic breccia-sandstone sharply overlain by laminated siltstone;
(d) laminated siltstone erosively overlain by volcanogenic granule conglomerate; (e) volcanogenic
sandstone grading into partly laminated mudstone and erosively overlain by volcanogenic
diamictic breccia.
-32.61°
Mount Double Area
On Hiltaba Station, the quartz- and feldspar-phyric Paney Rhyolite occurs as two NW-trending series
of outcrops and is represented by flow-banded coherent facies occurring at the northeastern
margin of the plagioclase-phyric Eucarro Rhyolite lava flow. Of particular interest are the outcrops
near Mount Friday as here the coherent facies of the Paney Rhyolite is locally overlain by volcaniclastic deposits (Fig. 2 left). The Paney Rhyolite lava flow is here typically flow-banded (Fig. 3a). Its
upper part is locally strongly veined by cryptocrystalline chalcedonic silica. This coherent but
strongly fractured volcanic facies grades upward into coarse breccias characterised by highly
angular rhyolite clasts and clast-supported texture (Fig. 3b). The latter facies is interpreted as the
autoclastic carapace of the underlying lava flow. These autobreccias are overlain by matrix-rich, very
poorly sorted, coarse diamictic breccias with angular to subrounded rhyolite clasts and kaolinitic,
sericitic-chloritic or ferrugineous-siliceous matrix (Figs. 3c-g). These rocks are succeeded by finer
grained and better sorted breccias, muddy diamictites and pebble-granule conglomerates, interbedded with massive, medium- to coarse-grained sandstones and laminated finegrained sandstones to siltstones (Figs. 3h-l). This fining-upward succession is interpreted to represent fluvially
reworked volcanogenic sediments sourced from the underlying autoclastic rhyolite. This reworked
volcaniclastic facies records surface water runoff on top or along the margin of the Eucarro-Paney
lava flow with local deposition and preservation in shallow palaeo-depressions. Comparable
volcaniclastic and volcanogenic deposits were found at the stratigraphic level of the Paney Rhyolite
in both of the nearby MSDP 5 & 7 drill holes (Fig. 4a-e).
Paney Rhyolite
135.66°
Fig. 5 (above): Geological map of the Mount Double area showing the volcanic
facies zones within the Mount Double Ignimbrite outcrop.
Fig. 6 (right): Outcrop and sample photos of the Paney Rhyolite and the overlying
Mount Double Ignimbrite: (a) flow-banded and flow-folded Paney Rhyolite; (b)
fiamme-like mingling texture at the Eucarro-Paney rhyolite transition; (c) flowbanded, clast-free, apparently coherent facies of the Mount Double Ignimbrite
(lower part); (d) moderately welded Mount Double Ignimbrite with flattened
pumice clasts; (e) flattened pumice clast-rich ignimbrite; (f) detail of pumice clast
with ragged left edge and relictic tubular vesicle texture; (g) slightly welded, clastrich ignimbrite with eutaxitic texture (clasts are mainly formed by porphyritic
rhyolite and rare tube pumice); (h) slightly welded, eutaxitic ignimbrite rich in
angular clasts of porphyritic rhyolite; (i) non-welded, moderately crystal-rich tuff
with large volcanic lithic clast (base of clast-rich zone Mount Double Ignimbrite);
(j) finely laminated, moderately crystal-rich, densely welded and rheomorphic
ignimbrite (upper part Mount Double Ignimbrite); (k) flow-banded, crystal-poor,
densely welded and rheomorphic ignimbrite with rare rhyolite clasts; (l) cut
section of crystal-poor, flow-banded and rheomorphic ignimbrite with rare
rhyolite clasts; (m) crystal-rich tuff with interlayer of laminated crystal-free ash tuff;
(n) crystal-rich rhyolite tuff overlain by massive ash tuff with finely dispersed
crystals; (o) alternating layers of crystal-rich and crystal-poor rhyolitic ash tuffs
from the northwestern top of the Mount Double Ignimbrite.
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