Mawson-Spriggina Geology Walk
This circuit walk extends the experience of the Acacia Ridge Walk. Mawson Valley is a
continuation of the Devils Pass which carries the main road in to Arkaroola Village. The valley is
named for Sir Douglas Mawson, Antarctic explorer and geologist who spent 25 years from the 1920s
describing the geology of the Flinders Ranges. As is Devils Pass, the valley is floored by pale shaly
sediments of the Opaminda Formation. The main rocks here are pale green dolomitic siltstone,
talcose green shale and dolomitic marble deposited in very shallow water. The valley is bounded to
the north by a ridge of Blue Mine Conglomerate and to the south by a ridge of Wortupa Quartzite.
The sediments date from about 860 million years ago, but as folding began to occur about
500 million years ago igneous granitic material intruded from below gave rise to a breccia (broken
rock) consisting of Opaminda Formation and Wortupa Quartzite fragments. These tough weathering
mounds of breccia around granite and pegmatite cores are exposed as peaks in the valley – Sitting
Bull, The Pinnacles and elsewhere in the neighbouring national park, The Needles.
With the granite intrusions came a rush of very hot water and gases which altered the host rocks
and introduced a variety of unusual minerals such as tourmaline (a boron rich silicate) and ‘rare’
metals such as niobium.
The return walk down Spriggina Ridge looks down a landscape model of a fossil flatworm and
reveals extended views of distant hills.
Distance and time: 8.0 km circuit from the Village: allow 4 hours.
Altitude Range: 330 m (Arkaroola Village) to 496 m (Spriggina Ridge).
Access: Start at the Arkaroola Village trailhead at the road junction.
Track: Generally grades are gradual, rough and steep on the road between 5.8 to 6.0 km
Distances marked in km increase as you return to Arkaroola village.
Notes read as for an anticlockwise circuit.
0.0 Checkpoint A1 - Arkaroola Village Trailhead.
Walk past the motel units, following the markers through the village into the Mawson Valley.
The valley is cut into soft Opaminda Formation flanked by ranges of more resistant Wortupa
Quartzite (left) and Blue Mine Conglomerate.
0.2 The track passes a bed to the left. The dolomite beds in the Opaminda Formation have been
recrystallised to form dolomite marble; here the marble contains large needles of green
actinolite, a calcium-iron-magnesium silicate mineral. The green siltstones are abundantly
spotted by scapolite. Along the track the Opaminda Formation appears as windows of outcrop
between scree of Wortupa Quartzite. Strongly scapolite spotted laminated talcy green siltstones
is exposed 0.4-0.8 km; the marble bands have white needle like tremolite crystals.
The Wortupa Quartzite on the left makes a craggy skyline. The soils developed on the Opaminda
Formation are very pale and the growth of curly mallee reflects their high magnesium content.
1.4 Some notably talc-rich very soft dark layers are present in the laminated siltstones.
1.6 the outcrops of Sitting Bull can be seen ahead. The Opaminda Formation here is broken and
contorted, and blocks of dolomite breccia can be seen. The brecciation is caused by rush of hot
water and gases, associated with the intrusions.
Sitting Bull was described and named in 1945 by Sir Douglas Mawson, when Reg Sprigg was an
assistant geologist on the trip. Sitting Bull is a large bulbous mass of intrusive pegmatite made
up of very large crystals of quartz, feldspar and tourmaline, up to 20 cm across, and some minor
outcrops of finer grained granite. The intrusives are surrounded by breccia which has been
metamorphosed by the heat of the intrusion and the hot water associated with them. At the last
large granite boulder by the side of the track there are abundant scatterings of black magnetite
(magnetic iron oxide) crystals with octahedral shapes.
2.2 Close by is unaltered Opaminda Formation – very good exposures of dark green blocky talcy
siltstones (with poorly preserved fossil mud cracks) and laminated sandy dolomite marble can be
seen in a small gully on the right.
The track follows close to the base of the Opaminda Formation at 2.8 km and Blue Mine
Conglomerate can be seen just across the creek to the right.
2.8 Just past this point well preserved fossil mud cracks can be seen on the left.
3.2 The pegmatite and granite plugs of The Pinnacles are visible ahead. These were also named by
Mawson in 1945; he whimsically named the smaller bodies the Eastern Addendum and Western
Addendum. Like Sitting Bull, the igneous plugs are surrounded by breccia which has been
metamorphosed and altered.
3.4 to 3.6 Some of the resulting minerals can be seen displayed around track-side boulders, near the
main outcrop; black biotite mica is fairly common; long fibres of white tremolite can be seen;
coarse grained magnetite is common around the boulders; black fine-grained rock is hornfels,
formed when siltstones are strongly baked by igneous intrusions.
The high ridge on the right, past the Eastern Addendum, is a crag of Wortupa Quartzite. The breccia
around the Pinnacles intrusions continues as far as 4.0 km
3.8 Checkpoint M1 – Pinnacles car park.
From the car park the track passes over spotted green siltstones, which although very similar to the
Opaminda Formation are probably part of the lower Skillogalee Dolomite, which forms the well
banded hills to the south. At 4.7 km the track passes a thick laminated dolomite which has been
metamorphosed to marble amid the green spotted siltstones. The dolomitic marble beds
continue from here as far as 5.4 km and 6.0 km are pale marbles with a ‘foliated’ structure in
spotted green siltstone.
5.8 Old track junction.
6.0 A large model layout of a fossil of the Ediacaran period, Spriggina floundersi, can be seen in the
valley floor. This fossil is named in honour of Reg Sprigg the discoverer of the suite of flatworms
known as the Ediacaran Fauna (and, of course, founder of the Arkaroola Sanctuary).
Well banded Skillogalee Dolomite is on the hills behind the model; Wortupa Quartzite forms Griselda
Hill to the northeast.
The track passes along the flank of a bed of grey fine grained quartzite which is part of the lower
Skillogallee Dolomite; Wortupa Quartzite is the backbone of the ridge to the left.
Mt Oliphant and Mt Elva are visible in the direction of Arkaroola and, in the opposite direction,
Sprigg Knob.
6.2 The vegetation of the low ridge you are on carries mulga woodland, with a variety of shrubs—
sennas with bright yellow flowers, lobe-leaved hopbushes with bright red 'hops' in winter and
spring and the rock fuchsia with mauve flowers in spring and early summer.
6.6 The silver dome of Arkaroola's Dodwell observatory is clearly visible and, to the south in a low
range on the horizon, is Mt Warren Hastings, with a flat-cap quartzite layer which is interbedded
with glacial tillite (see Oppaminda – Nudlamutana walk).
7.0 An interesting plant grows hereabouts; called Camel Poison, this small willowy tree is short-lived
and never occurs very prolifically; male flowers are borne at the top of the stem with the female
flowers (which give rise to the ribbed yellow-green bell-shaped fruit) below them. A few
individuals are known to grow near here, so look have a look around.
7.2 Km Continue downhill and cross the campground road to reach . . .
7.3 km Checkpoint M3 - road junction.
Follow the main road back to the village (about 0.5 km).
Text prepared from field notes supplied by Nick Langsford
GLOSSARY OF GEOLOGICAL TERMS
For composition of different rock Formations and Groups, see Stratigraphic Column
actinolite A green alumino-silicate mineral with a characteristic fibrous structure.
biotite Black mica.
breccia Broken-up rock; can be formed by a wide range of processes.
calcium Greyish white metal, Ca, the basis of limestone, calcium carbonate (CaCO3)
conglomerate Sedimentary rock with a major component of coarse usually rounded rock fragments.
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.
dolomite Rock that is largely composed of the dolomite mineral, CaMg(CO3)2.
Ediacaran Period, system; Ediacaran Golden Spike The Ediacaran Period (1000 million years ago to 543
million years ago) contains evidence of the first multicellular animals to occur on Earth about 560 million
years ago; their fossils are impressions of soft-bodied organisms. The base of the Period is physically
defined by a bronze spike at the base of the Nuccaleena Formation in Enorama Creek, in the Flinders
Ranges National Park, where there is the Golden Spike marker.
feldspar A complex set of alumino-silicate minerals, with varying amounts of sodium, potassium and calcium.
In important constituent of many rocks, especially igneous e.g. granite
foliated layered arrangement of platy mineral grains in metamorphosed rock e.g. gneiss
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.
granite An igneous rock, formed at considerable depth, made up of millimetre to centimetre sized grains of
quartz, feldspar and mica.
Group A unit which includes two or more Formations of usually sedimentary rocks.
hornfels A hard, fine grained metamorphic rock produced by the baking of sedimentary rock by an intrusive
igneous rock.
igneous Description of rocks formed by solidification from molten or semi-molten state
iron A metallic element, Fe. An important industrial commodity.
magnesium A metal found only in other compounds, such as dolomite, CaMg(CO3)2.
magnetite Magnetic iron oxide.
marble Recrystallised carbonate sedimentary rock
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.
niobium A metallic element, a component of the complex ore of tantalum – this last, once used as metallic
filament in electric light globes.
outcrop Exposure of rock layers protruding through the overlying soil and debris.
pegmatite Essentially very coarse grained granite. Individual ‘grains’ can be up to metres long. As they
represent the final phase of crystallisation of a granite body, they often contain exotic minerals.
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.
recrystallised see crystallised.
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.
shale A sedimentary rock largely made up of very fine grained material which includes a large proportion of
clay minerals, micas and quartz.
silicate A salt containing an oxide of silica, SiO4
siltstone A rock composed of silt-sized (less than 0.1 mm) mineral grains – usually mainly quartz.
Spriggina floundersi A flatworm of the Ediacaran fauna, named for Reg Sprigg and Ben Flounders.
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.
tourmaline A complex boro-silicate; black schorl is the most common but gem varieties can be green, red or
blue.
tremolite A magnesium rich alumino-silicate commonly with a fibrous structure.
vulcanism Natural process resulting in volcanoes, lava flows.