Geological Note
Yorke Peninsula basement drilling and
contribution to regional geology
Wenlong Zang and Steve Hore (Geologists, Geological Survey Branch, MER)
supported by seismic interpretation
(Zang and Tucker, 2000). Thick
Cambrian carbonate has both petroleum
and base metal potential.
Introduction
Drillhole TER 1, funded by the TEISA
(Targeted Exploration Initiative South
Australia) Program, was drilled to test
crystalline basement and stratigraphy to
the west of Minlaton, Yorke Peninsula.
The drilling project used newly acquired
TEISA aeromagnetic data on the
peninsula, which delineated several TMI
(total magnetic intensity) anomalies in
the southwestern coastal region between
Minlaton and Warooka townships. The
drillhole was designed to test one of
these anomalies and provide geological
information, particularly on basement
rocks, needed for completion of the
1: 250 000 MAITLAND geological map.
Palaeoproterozoic and Mesoproterozoic basement rocks are not known in
either outcrop or drillcore in the area
west of Minlaton. Several oil exploration
wells were drilled near Minlaton, but
none penetrated basement rocks and
no cores were collected. Some 20 km
south, near the township of Warooka,
Peninsula Oil 1 was drilled to total depth
of 345 m during 1931–33 and intersected
Precambrian mica schist and gneiss at a
depth of 201.5 m (Ward, 1944). Current
geological knowledge suggests that late
Neoproterozoic sediments are probably
absent in the region and basement rocks
may belong to the Lincoln Complex,
Wallaroo Group, or Arthurton Granite.
TER 1 was completed in six days
(23–28 June 2000) and bottomed in the
Donington Suite at a depth of 199 m.
Drilling summary
Layered adamellite of the Donington Suite,
with a granite vein. Depth 191.8 m. (Photo
48344)
(Conor, 1995). No outcrops of basement
rocks are known in the Minlaton area.
Neoproterozoic sediments are not
known in the southern portion of Yorke
Peninsula, but are present and thicken
towards the north. Palaeozoic rocks in
the Minlaton area comprise Cambrian
sandstone and conglomerate of the
Winulta Formation and carbonate of the
Kulpara Formation. To the east of the
peninsula, Cambrian Parara Limestone,
Minlaton Formation and younger strata
also occur. Most Cambrian sediments
were deposited in rifted graben or halfgraben structures, an interpretation
Geophysical surveys
Regional geology
Yorke Peninsula is located on the
southeastern margin of the Gawler
Craton. Pre-Neoproterozoic basement
rocks were described by many earlier
workers, and assigned as Archaean rocks
by Crawford (1965). Until recently,
basement rocks have been described
as Palaeoproterozoic Lincoln Complex
(Thomson, 1969; Parker, 1993) to the
south, and Wallaroo Group to the north
The initial aim of this project was to test
a distinct positive TMI anomaly along
the western coast of Yorke Peninsula. A
reconnaissance field survey revealed that
the source was beneath farmland which
was under crop preparation at the time and
the other accessible sites over the anomaly
were too near the coastal protection zone.
Alternative localities were therefore
chosen in the area, following a ground
magnetic survey and interpretation of the
TEISA TMI data (Fig. 1), which indicated
that depth to basement in the area ranged
from 100 to 500 m. The final locality was
chosen on the basis of interpreted minimum
depth to basement (calculated from the
modelling of gravity and TMI data to be
~200 m), and also drill-site suitability. The
site was within a fallow cereal and grazing
paddock. TER 1 was drilled by rotary mud
with diamond tailing. The hole intersected
Quaternary calcrete, Permian diamictite,
Cambrian dolomite and sandstone, and
Palaeoproterozoic basement rocks, with a
final depth of 189.5 m (Fig. 2). The drilling
of TER 1 has provided valuable testing of
actual basement depth versus geophysical
modelling.
Unconformity between the Cambrian
sandstone and Palaeoproterozoic basement.
The basement is carbonate-rich and intruded
by pegmatite. Depth 189.5 m. (Photo 48340)
Three ground magnetic profiles were
surveyed: two traverses included the
location of TER 1, and the other extended
from Hardwicke Bay to Stansbury.
Ground magnetic and gravity data have
been projected to seismic line Y82-A3 in
the Hardwicke Bay to Stansbury section
(Fig. 3). Seismic data indicate that, in
the west, basement is overlain by a thin
sediment cover, mainly Permian tillite
with thin Cambrian strata. To the east,
thick (>1500 m) Cambrian sequences
were intersected in petroleum well
Stansbury West 1. A gently dipping
MESA Journal 28
January 2003
53
Geological Note
eastern sedimentary profile from Minlaton
to west of Stansbury is well recognised in
seismic interpretation, and matches with
both TMI and gravity data.
The gravity values for the east–west
traverse from Hardwicke Bay to
Stansbury can be divided into three parts
(Fig. 3a). In the west, moderate values
are probably a result of thick Permian
claystone cover and a granitic basement
high. Lower values in the mid-sector
suggest thick Permian and Cambrian
siliciclastic sediments. High values to the
east are probably derived from uplifted
metamorphic basement and Early
Cambrian carbonate. The shift of values
is coincident with major fault zones.
Contribution to geology
Fig. 1 Geophysical features of Yorke Peninsula, and drillhole location. (a) Gravity image (b)
TMI image.
54
MESA Journal 28
January 2003
Little was known of basement rocks to
the west of Minlaton prior to this drilling
program. TER 1 revealed, for the first
time, the presence of Donington Suite
equivalents and probable carbonaterich Wallaroo Group (Purvis, 2000),
providing valuable information on
basement lithologies of central Yorke
Peninsula. The drilling provided useful
evidence that granite gneisses of the
Donington Suite equivalent are the
basement rocks to the Wallaroo Group
on Yorke Peninsula.
Cambrian sediments in TER 1 are
relatively thin. Basal conglomerate and
sandstone of the Winulta Formation
(7.4 m thick) are the first intersected
in the area, and the sandstone grades
into silty dolomite, passing upwards
into the dolomite unit. Full-core
intersection of the Winulta–Kulpara
transition is revealed for the first time
on southern Yorke Peninsula. The
dolomite (171–182.1 m) is quite porous
and further study may determine its
suitability and potential as a petroleum
reservoir. Some sulphide layers, mainly
pyrite, are present in the dolomite. The
occurrence of Cambrian sediments may
indicate that the Early Cambrian marine
transgression was beyond the western
coast of Yorke Peninsula.
Intercepted
Permian
diamictite
contains mainly erratics, claystone and
minor sandstone. Unexpectedly, six
layers of sandy limestone or calcareous
sandstone are present in TER 1, ranging
from 60 mm to 1.80 m thick. Sandy
limestone consists mainly of quartz
grains cemented by calcite. Sedimentary
Geological Note
0
3 m Quaternary sand (Qhe)
9 m Calcrete (Qpca)
TER 1 (Terrible 1),
Yorke Peninsula
evidence suggests that
Permian sediments in
the area might have
been deposited from
restricted or marginal
marine to glacio-deltaic
and glacio-fluvial settings.
Upper Cape Jervis Formation (Cp-j)
Silty mudstone with sandy limestone
interbeds. Few erratics.
?Glaciomarine deposit.
Depth (metres)
50
Fig. 2 Lithological column, TER 1.
Conclusions
91 m
100
Lower Cape Jervis Formation (Cp-j)
Sandy siltstone with abundant erratics, typical glacial
diamictite. Lower part with rounded pebbles or
breccia. ?Non-marine (glaciolacustrine) deposit.
150
171 m
200
Kulpara Formation (Eok): Stylolitic dolomite, upper
part with 3 m thick dolomite conglomerate or
breccia.
182.1 m Winulta Formation (Eoi): Sandstone, siltstone,
dolomite and conglomerate at base. Bioturbation.
189.5 m Metasomatic rocks, possible WALLAROO GROUP.
190.5 m
Foliated granite gneiss (Donington Suite)
intruded by pegmatite dykes.
TD 199 m
200458_002
TER 1 is the first drillhole in the area west of
Minlaton to penetrate
Palaeoproterozoic
crystalline basement.
The hole intersected
granite gneiss of the
Donington Suite, carbonate-flooded gneiss
of probable Wallaroo
Group
equivalents,
Cambrian
Winulta
and Kulpara Formations, and diamictite
and sandy limestone
of the Permian Cape
Jervis Formation. The
presence of the granite
gneiss provides strong
evidence
that
the
(a)
(b)
Stansbury West 1
(c)
Stansbury Town 1
0.0
1.0
Ba
2.0
s
?
ne
zo
h
ts
en
em
r
ea
T
P
3
2
1
0.0
1.0
Palaeoproterozoic basement complex
~5 km
2.0
200458_014
Fig. 3 (a) Bouguer gravity, (b) ground magnetic, and (c) seismic data and interpretation in the
Hardwicke Bay – Stansbury section (Y82-A3).
Donington Suite is the basement to the
Wallaroo Group. The sandy limestone
or calcareous sandstone beds in the
Permian Cape Jervis Formation suggest
that a marginal marine environment was
probably present in the southern Yorke
Peninsula region in the late stage of glacial deposition.
Acknowledgments
The authors acknowledge the support
of Sue Daly, Domenic Calandro and
Gary Reed during the data processing,
and particular thanks are extended to
Michael Schwarz who helped with the
project from inception. John Nankivell is
thanked for his kind consent to allow the
drillhole to be located on his property.
References
Conor, C.H.H., 1995. Moonta–Wallaroo
region, an interpretation of the
geology of the Maitland and Wallaroo
1:100 000 sheet areas. South
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Industries and Resources. Open file
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Crawford, A.R., 1965. The geology of
Yorke Peninsula. South Australia.
Geological Survey. Bulletin, 39.
Parker,
A.J.
(Compiler),
1993.
Palaeoproterozoic. In: Drexel, J.F.,
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The geology of South Australia. Vol.
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Geological Survey. Bulletin, 54:51105.
Purvis, A.C., 2000. Mineralogical report
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Pty
Ltd,
Petrographic
report
(unpublished).
Thomson, B.P., 1969. Precambrian
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January 2003
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