Wilcolo Geology Walk
The rolling hills of the Bunyeroo Valley present a gentler face of the Flinders Ranges. Even though
the walk does not scale the heights of Wilpena Pound, it provides rewarding views of the peaks,
making it an enjoyable outing for the whole family.
The walk traverses three Formations in the lower part of the Wilpena Group the Brachina
Formation, the ABC Range quartzite and the uppermost Bunyeroo Formation. Between the last two
is a narrow band of Wilcolo Sandstone, which marks an important change in the sedimentary pile, as
explained below.
The stratigraphic section shows the sedimentary pile relevant to this geology. The Brachina
Formation and the underlying Nuccaleena Formation together make up the lower of two
transgressive-regressive cycles (where ancient seas encroach over, and later retreat from,
neighbouring shores) of the Wilpena Group. Following a glaciation (the Elatina) sea levels rose and
fine-grained silts which formed the Brachina Formation and later coarser sands of the ABC Range
Quartzite, were laid down in the deltas of huge rivers flowing out of land masses to the west (about
700 million years ago).
As these sediments accumulated the sea retreated to its former position. The Wilcolo Sandstone
is a thin layer of dark red sandstone deposited by meandering streams on a land surface. It marks
the end of this cycle. The next cycle began when Bunyeroo Formation shales were deposited in deep
water as sea levels rose again and the sea moved landward. The sea was finally filled with the sands
of Rawnsley Quartzite (forming the Heysen Range) ending the second cycle.
A short diversion described later shows you material thrown widely out of the Lake Acraman
meteorite crater on Eyre Peninsula some 280 km distant.
Distance and time: This is a 9.2 km circuit walk, including the short spur track to the viewpoint; allow 5 hours.
Bunyeroo Viewpoint: from 290 metres at Checkpoint B1 to 490 metres at Checkpoint B5 (the hilltop viewpoint).
Access: The trailhead (Checkpoint B9) is at the Bunyeroo car park in Flinders Ranges National Park – 18 km
north of Wilpena along the Bunyeroo Gorge Road.
Track: Easy grade on formed walking trail. Some sections of service track and creek bed (Bunyeroo Road).
Distances in km decrease from Checkpoint 1, as you proceed around the circuit.
Notes read as for an anticlockwise circuit.
Trailhead – Bunyeroo Car Park – Checkpoint 9.
Follow the Wilcolo sign to cross creek at . . .
9.2 Checkpoint B1 – next to boom gate.
The map plate indicates the route beyond the boom gate
Heading south from the trailhead along Wilcolo Valley, the walk follows Bunyeroo Formation of red
to purple shale, which breaks down into pencil shaped fragments visible on the track surface.
The rock colour results from abundant haematite (iron oxide) which stains all the grains in the
rock. These fine grains were laid down in deep, quiet water.
The western wall of the valley is the rugged Heysen Range which can be best appreciated from the
lookout at Checkpoint 5. Thick sandstones and quartzite form the cliffs, the most rugged at the
top composed of Rawnsley Quartzite. The lower almost treeless slopes are pale limy Wonoka
Formation.
The valley floor trail passes through a woodland of Northern Cypress-pine (Callitris glaucophylla).
8.3 Cross Wilcolo Creek. It has a large catchment including small gorges that drain the ABC Range
and the Heysen Range which includes the slopes below St Mary Peak and almost as far as
Wilpena. Trees frame dramatic views of St Mary Peak and Wilpena Pound.
7.1 Checkpoint B2 – track junction.
Leave the service track and follow the foot track to the east. The track descends the red shales to a
flood terrace of Wilcolo Creek.
6.7 Checkpoint B3 – track junction with Heysen Trail.
Cross creek and continue east.
Just over the creek are outcrops of deep purple coarse-grained Wilcolo Sandstone. This was
deposited by rivers on the exposed surface of the ABC Range Quartzite sand flats. It is only 2 m
thick here, but up to 30 m thick further north.
A narrow gorge takes you through the ABC Range. It has huge River Red Gums which tap into
underground water.
6.4 Pale green to grey flaggy Brachina Formation siltstones underly the quartzite. Many surfaces
display ripple marks indicative of shallow water conditions when the sediments were
deposited.
5.8 The grey-green siltstones gradually change to more blocky yellow-brown to red siltstones, still
part of the Brachina Formation.
4.8 Checkpoint B4 – saddle.
Take a short spur to the lookout.
Checkpoint B5 – Bunyeroo Viewpoint.
From this point can be seen a number of high points indicated on the topogram plate. The rim of
Wilpena Pound is marked by St Mary Peak and the Heysen Range displays a suite of peaks
(Mounts Rupert, Sinnett and Abrupt) and these peaks, with Patawarta to the north, are
significant Rawnsley Quartzite features of the Flinders Ranges. To the north-east is
Mt Sunderland, a bare hill of limy sediments of the Umberatana Group folded into a dome with
the beds dipping towards the lookout. Return to B4 and continue east downhill.
4.4 Checkpoint B6 – join maintenance track.
Track goes along the strike of the Brachina Formation. In this lower part of Brachina Formation
there are more fine grained, softer and more easily eroded sediments which were deposited in
deep water.
2.4 Checkpoint B7 – under a magnificent Red Gum; Heysen Trail goes down to cross the creek.
Geology trail runs NW along a creek terrace of Bunyeroo Creek, which cut a new deeper channel
when faulting caused land to rise.
1.6 Checkpoint B8 – Bunyeroo Gorge Road.
Brachina Formation siltstones continue down to Bunyeroo Creek which has cut through the
siltstones and ABC Range Quartzite. This section of the walk may be wet and cars a hazard;
stick to the marked trail which stays clear of the road and traffic. Along here ripple marks can
be seen in both siltstone and quartzite exposures.
0.1 km Just before Checkpoint B1 a small exposure of purple Wilcolo Sandstone can be seen.
0.0 km Checkpoint B1.
Acraman ejecta layer Before you return to your car walk about 50 m north up the road. Where it
starts to bend, cut across to large gums trees at the side of the creek. Sandwiched in the
Bunyeroo Formation shales in the creek bed there is a thin gritty band (about 6 cm thick) which
has been shown to be material thrown out of the ground on Eyre Peninsula by a massive
meteorite. It is estimated as having been 5 kilometres in diameter and the lingering impression
of this impact, after 580 million years of erosion, is the location of Lake Acraman. The gritty
deposits in the Flinders Ranges are 280 kilometres east of the Lake; they have also been found
in drill holes in the Officer Basin hundreds of kilometres west of Lake Acraman.
The vast amount of dust thrown into the atmosphere would have reduced sunlight reaching the
earth to zero for perhaps several centuries. Super hurricanes and tsunamis would have been
generated and early life floating in the sea (phytoplankton) would have become largely extinct.
As sunlight returned, new life in the sea developed
Text prepared from field notes supplied by Nick Langsford
GLOSSARY OF GEOLOGICAL TERMS
For components of different rock Formations and Groups, see Stratigraphic Column.
Acraman impact ejecta layer (AIEL) Layer of shattered volcanic rock blasted from the Gawler Ranges about 580 million
years ago by the impact of a meteorite about 5 km across. The layer, generally about 5 cm thick, was deposited over
an area at least 500 km across. Where the material fell into the sea it was preserved as the Acraman Impact Ejecta
Layer (AIEL).
delta An alluvial deposit radiating out from the mouth of a river, where sediment is dropped into oceans or lakes; usually
triangular in shape.
flaggy Breaks readily into flat plates comparable in thickness to paving stones.
folded Rock layers which have been bent by earth movements.
Formation A sedimentary or volcanic rock unit which can be physically mapped and defined by features observed in the
field.
Group A unit which includes two or more Formations of usually sedimentary rocks.
haematite Dark grey iron oxide; when scratched it shows a red streak. Not magnetic. Haematite in the Flinders Ranges
commonly has a shiny mica-like habit and is called specularite.
iron A metallic mineral, Fe; an important industrial commodity.
limy Derived from, or affected by, limestone, CaCO3
meteorite A mass of matter that has fallen from outer space on to the earth’s surface.
outcrop Exposure of rock layers protruding through the overlying soil and debris.
phytoplankton Microscopic floating plants. Numerous and diverse in modern waters.
quartzite A rock which was originally quartz sandstone which has been heated or compressed sufficiently to fuse the
grains together, producing a tough resistant rock.
ripple marks Sedimentary structures formed on sandy surfaces by wave or current action. Very common in sandstones
and on present day beaches. Has the appearance of corrugated iron.
sandstone A rock made of sand – generally quartz sand.
shale A sedimentary rock largely made up of very fine grains – generally quartz.
siltstone A rock composed silt sized mineral grains (less than 0.1 mm) usually mainly quartz.
strike The orientation on a horizontal surface of the trace of any linear geological structure.
transgressive – regressive cycles Events during which sea-level rises, advancing (transgressing) onto the land surface;
followed by a withdrawal (regression) of the sea by sea level fall or the extension of the land by filling of the sea with
sediments, outwards from the coastline.