Oppaminda-Nudlamutana Geology Walk
The pile of sediments which accumulated in the Adelaide Geosyncline and comprises the current
Flinders/My Lofty Ranges system is estimated as being 15 km thick in the Arkaroola area. Over
300 million years, erosion has occurred and earth movements have considerably modified the rock
layers we see today.
In the Northern Flinders Ranges older sediments in the great pile are revealed. While younger
rocks of the Wilpena Group can be seen from high points, this walk crosses very old rocks in the
Umberatana and Burra Groups (see stratigraphic column). They were all sediments which varied in
composition according to their origins in the ancient oceans and early land masses.
A significant feature in the Northern Flinders is the Paralana Fault. This originated as the earth’s
crust stretched and split when the Geosyncline formed. Layers to the west subsided along a vertical
plane and thicker layers of sediment accumulated there. Later earth movements brought the west
side up again and compression changed the vertical fault into a west dipping fault plane, so that
rocks to the west were pushed up and over rocks to the east.
Of the sediments deposited here the Skillogalee dolomite is most striking because of the easily
recognised vegetation it supports. It is divided into an upper and a lower part; the lower is mainly a
pale dolomite which supports sparse vegetation, while the upper has much darker dolomites
containing fine grained carbon derived from algal mat floating on muddy water. The pale coloured
soils carry Curly Mallee. In clearer water stromatolites formed on the floor of shallow seas.
Features of the upper Skillogalee Dolomite are the great extent of shallow seas and lagoons and
the concentration of magnesium in the pure dolomite and magnesite precipitated from the sea and
deposited as sediment — imagine an ephemeral sea over 100 km wide sometimes shallowing to grey
mud flats and sometimes blindingly white with a magnesite mud crust. Quite a sight from an
ancestral Mt Painter!
From the summit of modern day Mt Warren Hastings, there is a different view of the landscape
after millenia of change.
Distance and time: 15.2 km linear walk; allow 7 hours one way.
Altitude Range: 250 m (Munyallina Trailhead) to 590 m (Mt Warren Hastings).
Access: The walk starts either at the Arkaroola Village Trailhead or Munyallina Trailhead adjacent to the southern entrance
to Arkaroola Wilderness Sanctuary.
Track: Easy grades on formed walking track; one short rock scramble, some rough vehicle tracks.
Distances marked in kilometres decrease as you approach Arkaroola.
Notes read as for a start at the Munyallina Trailhead (entrance to the Arkaroola Sanctuary). You
could arrange to be dropped there, in order to start the walk back to Arkaroola. Residents at
Nudlamutana Hut can start the walk at the hut, but need to organise transport back from Arkaroola.
15.2 Munyallina Trailhead
Track passes over alluvial deposits of this minor creek with Nardlamathanha Hill to the left. It is
deeply weathered and iron stained Angepena Formation of red siltstones and dolomite.
Beyond the hill we cross an eastern strand of the Paralana Fault and pass into dolomite of the
Balcanoona Formation.
14.6 a track leads up a small gully to the Wooltana Bat Cave. This is a 60 m deep vertical solution
pipe dissolved in closely jointed Balcanoona Formation dolomite (Umberatana Group). It was
an ideal bat habitat and was occupied during a warm wet climate by the Ghost Bat
(Macroderma gigas) now confined to northern Australia. It is possible that the last few died as
a result of the drought in 1864.
Over 1000 tonnes of bat droppings (guano) accumulated. A few hundred tonnes were mined
between 1924 and 1931 and trucked to Copley. Shovelling guano on the floor of the cave must
have been a memorable experience!
14.0 Shortly after the dolomite outcrops, the way crosses the western strand of the Paralana fault.
This major fault brings lower Burra Group rocks into contact with much younger rocks of the
middle of the Umberatana Group.
13.8 At outcrop of weathered coarse grained sandstone of the Blue Mine Conglomerate (Burra
Group) is well exposed in the creek bed, where cross bedding is readily seen in the fresher pink
sandstone.
13.6 At little further on from this point well preserved halite (salt) casts are present in a bed of fine
grained silty sandstone, about 10 m to the left of the track. This shows that the Blue Mine
Conglomerate was deposited in shallow seawater, with dry periods allowing large salt crystals
to form on the sediment surface. Mud pellets are also present here.
13.4 Checkpoint 7 – track junction; old Arkaroola Road.
There are outcrops of typical coarse grained Blue Mine Conglomerate, with abundant well rounded
white quartz pebbles derived from eroded quartz veins.
The conglomerate is overlain by dark grey scapolite-spotted siltstone, and phyllite with a 'knotted'
texture, caused by clots of andalusite – a mineral formed by metamorphism. There are thin
bands of dark green soft talcy siltstone, which can be easily scratched with a finger nail.
13.2 Dolomitic siltstone and brown dolomites are common at the gate. The track past Nudlamutana
(Nardlamathanha) Hut and tanks is over scree of Wortupa Quartzite which forms the ranges on
each side of the track.
The hut, like Grindell Hut, was an outstation of Balcanoona and like other buildings of the station it is
a solid stone structure; a large tank of good well water is available if you need supplies.
From the hut, banded dolomites of the Skillogalee Dolomite are clearly visible ahead. The track
turns right after the second gate and passes along the strike of the dolomite; several boulders
of jasper are on the left.
13.0 to 12.8 The track passes over stromatolitic dolomite, oolitic dolomite and dolomitic siltstones.
Just before the creek good examples of polygonal desiccation cracks (fossil mud cracks) can be seen
in slabs torn up on the left of the track. There are beds of dolomite with coarse sand grains
interbedded with dolomitic siltstones. Mud cracks and mud pellets can be seen on many
bedding surfaces where slabs have been tilted up by the bulldozing of the track, especially on
the right. The well banded appearance of the bare lower slopes is due to darker beds of sandy
dolomite about a metre thick with pale dolomitic siltstone between.
12.3 Here is an abrupt change in vegetation from bare slopes to dense stands of Curly Mallee. This
reflects soils (and hence bedrock) with high magnesium carbonate content. Most of the rocks
are very dark, almost pure dolomite or dolomitic siltstone. Curly Mallee (Eucalyptus gillii) is a
character plant of upper part of Skillogalee Dolomite.
12.2 A band of rock consisting of white pea sized grains in a dark matrix – this is a conglomerate of
magnesite pellets. These are quite common in this part of the Skillogalee Dolomite and formed
as thin layers of magnesite were broken up in sheet wash and redeposited into conglomerate.
Frequently associated with the magnesite rich bands are beds rich in rounded lumps of black
chert – a very fine grained quartz.
12.1 The vegetation change to Mulga reflects a bed of grey weakly dolomitic siltstone.
11.8 The Curly Mallee and dark pure dolomite return. From this ridge there are good views to the
left of the Skillogalee Dolomite hills showing how continuous the individual beds are.
Well preserved small (5 cm across) stromatolites can be seen on the left of the track.
11.45 The track runs parallel to a ripple marked sandstone bed to the right of the track; just behind
it is a bed of laminated dark dolomite with layers of black chert lumps.
11.2 Just after this turn is a good example of magnesite conglomerate and ripple marked bedding
planes on sandstone beds can be seen over the next 100 m.
The magnesite conglomerates, beds with black chert and very dark dolomites are characteristic of
the upper part of the Skillogalee Dolomite and occur widely over the northern and western
Flinders ranges.
10.15 The dark dolomites, magnesite beds and attendant pale soils and Curly Mallee of the
Skillogalee Dolomite are overlain here by the Myrtle Springs Formation, where sparse
vegetation occurs on grey and green siltstones, sandstones and sandy dolomites with cross
bedding of the Myrtle Springs Formation, overlying the upper Skillogalee Dolomite. At . . .
9.8 . . . Myrtle Springs Formation is overlain by the Bolla Bollana Tillite. The contact between the
Myrtle Springs Formation and the tillite is abrupt, and the tillite contains numerous fragments
of locally derived dolomite and has a dolomite rich matrix. There is a major unconformity here.
The massive tillite passes to laminated dolomite, cross bedded quartzite, and siltstone with
sparse erratics and a few bands of massive tillite.
9.2 to 9.0 Some facetted and striated quartzite boulders of glacial origin can be seen on left of the
track, where numerous erratics are concentrated on the surface as the soft matrix has
weathered away.
9.0 to 8.8 Tillite with green mudstone matrix occurs.
8.7 Check point 6 – Junction with Nudlamutana – Oodnaminta track.
Take the uphill route to Mt Warren Hastings.
The track now passes over pebbly siltstones and sandstones, tillite with mainly small erratics although there are a few very large ones. Beds of laminated and cross-bedded quartzite occur
within the tillite.
8.3 There are good views east to Lake Frome, over layered hills of Burra Group. Close to the summit
the pebbly rocks are iron stained, particularly along fractures.
7.5 Checkpoint 5 – Summit of Mt Warren Hastings (590 m).
This hill was named by surveyor Samuel Parry, after an official of the East India Company who
became governor of Bengal. He was impeached for corruption between 1788 and 1795 before
being found innocent. Parry named several places in the Flinders Ranges after British officials
who had nothing to do with Australia.
There are good views in all directions – refer to the topogram. To the north, the jumbled peaks of
the basement rocks include Mt Painter; and east to south east, the dark tillite range includes
Mt Jacob which is a repeat, across the Paralana Fault, of the unit on which you stand. To the
south are Nepouie Peak and Mt McCallum on a range of Balcanoona Formation.
The present lateral separation seen from here gives a sense of the scale of the movement on the
Paralana Fault; the west block has been pushed up about 5 km along the west dipping fault.
The Balcanoona Formation of the Oodnaminta Reef can be seen to the southwest, with the Gammon
Range peaks of Rawnsley Quartzite behind.
The brown sheer faces to the west are very continuous iron stained planar surfaces of joints in the
tillite; the beds actually dip back into the hill. To the south the rugged dark hills show quartzite
and siltstone interbedded with the tillite. Layers of quartzite and siltstone may reflect warmer
periods.
7.4 Checkpoint 4 – below summit.
Goats often camp at this point. The hilltop has been a haunt for goats for many years and the
condition of grass trees is a sad reflection of their attention. In 1968 Warren Bonython
observed a mob of 30-40. From the summit the track passes back down through the tillites,
pebbly siltstones and quartzite.
6.4 Large scale traces of the layering in the tillite can be seen on the opposite side of the valley, to
the right. Coarse tillite with a dolomitic matrix occurs at this point, at the base of the Bolla
Bollana Tillite.
The Myrtle Springs Formation, grey-green siltstones, laminated quartzite and dolomite forms the
lower slopes before the old fence line is reached
5.9 Checkpoint 3 – old fence and old Oppaminda track
Magnesite pebble conglomerate provides suitable habitat for Curly Mallee and also the very wispy
Balcanoona Wattle.
5.4 From here the dolomite is strongly recrystallized to a coarse grained marble which sparkles on
the road.
4.8 Fossil mud cracks can be seen on roadside slabs. Sparkly dolomite marble is visible on the road
surface. In a creek bank on the right of the track at ~3.65 km is an excellent example of a
magnesite pellet conglomerate bed, containing rip-up fragments of grey dolomite.
3.5 Checkpoint 2 – end of Oppaminda Trail.
Spectacular cliff exposures of Skillogalee Dolomite; interbedded dolomites and dolomitic siltstones
can be seen on the right-hand side of the bend in Wywhyana Creek and these continue to the
sharp right-hand bend where the track passes over an alluvial terrace.
2.2 – 1.8 The ridge to the south is flaggy, scapolitic and tremolitic marble, with thin mud-cracked
siltstone.
1.0 Quarry with green talcy siltstones and marble with bands of coarse fibrous tremolite.
0.8 This is the contact with the Wortupa Quartzite, below the dolomite unit. The road then passes
the ridge of dark Wortupa Quartzite (Mawson-Spriggina Geology Walk) on the left.
0.6 Checkpoint 1 – to Arkaroola Village Trailhead in front of the Arkaroola reception. A very good
view of the Wortupa Quartzite and underlying banded Opaminda Formation can be seen on the
flank of Griselda Hill to the east.
GLOSSARY OF GEOLOGICAL TERMS
For composition of different rock Formations and Groups, see Stratigraphic Column.
Adelaide Geosyncline A deep, subsiding trough into which, over about 350 million years, material eroded
from adjacent land masses was deposited.
andalusite High temperature alumino-silicate mineral, common in metamorphic rocks.
banding Parallel stripes of different colours, generally but not always, representing bedding in sedimentary
rocks.
Bolla Bollana Tillite product of the Sturtian glaciation; occurs west of Paralana Fault (see Merinjina Tillite)
chert Very fine grained quartz.
conglomerate Sedimentary rock with a major component of coarse usually rounded rock fragments.
bedding, layers, interbedding Bedding is the original surface on which successive layers of sediments were
deposited. Originally these were essentially horizontal (see cross bedding).
cross bedding Where sediment layers are at an angle to the main stratification, often outlined by dark heavy
minerals.
crystallised, recrystallised When rocks are heated, they undergo physical and eventually chemical changes.
The most common is recrystallisation of low temperature sedimentary rocks where the original fine
grains are enlarged by grain growth, often as individual crystals. Common examples are the formation of
marble from limestone, or quartzite from quartz sandstone.
erratic In glacial sediments, large rocks which have been transported by ice a notable distance from their
point of origin.
dolomite Rock that is largely composed of the mineral dolomite, CaMg(CO 3)2
facetted Rock which has been abraded to flattish surface e.g. by glaciations.
fault, faultline, fault plane A fault is a physical break in a body of rock. Faults may be of huge displacement,
many kilometres (e.g. Paralana Fault), or only a few centimetres.
flaggy Breaks readily into flat plates comparable in thickness to paving stones.
Formation A sedimentary or volcanic rock unit which can be physically mapped and defined by features
observed in the field.
glacial Pertaining to formation by processes involving lots of ice.
Group A unit which includes two or more Formations of usually sedimentary rocks.
halite Common salt in mineral form.
iron A metallic element, Fe. An important industrial commodity.
jasper Essentially chert, red to brown due to very fine grained iron oxides. May be attractively banded.
joint Fracture in rock more or less vertical or horizontal to bedding.
laminated, laminations Thinly, regularly banded layers of sediment; almost always reflects bedding.
magnesite Heavy mineral, magnesium carbonate MgCO 3; source of magnesium and used as a refractory in
steelworks.
magnesium A metal found only in other compounds, such as dolomite, CaMg(CO 3)2
marble Recrystallised carbonate sedimentary rock.
matrix A rock in which other material is embedded.
Merinjina Tillite product of the Sturtian glaciation; occurs east of Paralana Fault (see Bolla Bollana Tillite).
metamorphism Change produced in a rock due to dramatic changes in temperature and pressure. While the
overall chemical composition may not change much, the minerals present and the ‘fabric’ of the rock can
alter dramatically.
mud cracks The wedge shaped cracks bounding polygonal plates formed when thin layers of mud or silt are
dried by the sun. The cracks are filled by overlying sand layers and are preserved when the mud flakes
are weathered away.
mudstones Rocks made up largely of very fine grains, mainly clays – very similar to shales. A rock which was
once mud.
oolitic Composed of mm scale rounded carbonate grains made up of numerous concentric layers. Formed by
the gentle rolling to and fro of grains in warm shallow marine environments.
outcrop Exposure of rock layers protruding through the overlying soil and debris.
phyllite A somewhat metamorphosed slate, with abundant mica on splitting planes giving a distinct ‘sheen’.
polygonal desiccation cracks – see mud cracks.
quartz The most common crustal mineral – silicon dioxide, SiO2
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 largely quartz sand.
scapolite, scapolitic A complex calcium-sodium alumino-silicate, which can accept large amounts of
carbonate, chloride or sulphate in its crystal structure. Often occurs as matchstick shaped crystals in
altered limestones.
siltstones A rock composed of fine mineral grains – usually mainly quartz.
striated Scratched parallel markings, such as those resulting from sharp stones in glaciers.
strike The orientation on a horizontal surface of the trace of any linear geological structure.
stromatolites Domal or columnar layered structures built up by microbial organisms. Among the earliest life
forms and some survive to the present.
talc Complex mineral with soapy feel, Mg3Si4O10(OH)2
tillite Rock formed by glacial activity, containing a very wide range of grain sizes, usually including some large
fragments obviously not locally-derived, known as erratics.
tremolite, tremolitic A magnesium rich alumino-silicate, commonly with a fibrous appearance.
unconformity A surface separating rocks of distinctly different ages. The upper at least must be sedimentary
or volcanic.