Terrapinna Tors and Waterhole, South Australia
Terrapinna is located in the north east corner of the Flinders Ranges in South Australia. It is a very remote location and is accessed by a three hour, spine jarring and suspension killing four wheel drive track from the North Gammon Ranges and Arkaroola.
These Rocks are Very Old
The rocks that comprise Terrapinna, Mt. Nwill, Yerila and Wattleowie form the basement on which were deposited sediments of the Adelaide Geosyncline between 800 ma and 500 ma. The basement rocks have been geochemically analysed using Pb/Pb dating of magmatic zircons at ca 1576 Ma which makes them much older than the rocks that formed the adjacent Arkaroola and North Gammon Ranges.
To give you an indication to the immense age of these rocks - the dinosaurs became extinct at the end of the Cretaceous period 144 million years ago, vertebrates evolved in the Cambrian period 509 million years ago, and land plants first evolved in the Silurian period 420 million years ago. Homo sapiens evolved around 2 million years ago (genus Homo), and you were born probably between 20 and 60 years ago! The rocks at Terrapinna are 1576 million years old. The earth was formed around ~4500 million years ago.
Quartz-feldspar augen gneisses, quartz augen schists and trondhjemites outcrop at Terrapinna and form part of what is known as the Proterozoic Mt. Painter Inlier of the northern Flinders.
The landscape comprises granitic tors which are the exposed and eroded remnants of a laccolith intruded during the Mesoproterozoic. The term "tor" is a Cornish word meaning isolated granite boulders or masses of blocks on hilltops. The term has been loosely used to describe the geomorphic weathering characteristic exhibited by some rock types.
The Terrapinna Tors have been formed by the erosion of a porphyry granite (the Terrapinna Granite) which is been described as massive with a fine to medium- grained groundmass. The rock is composed of: feldspars (microline, K-feldspar and small amounts of oligoclase), recrystallised quartz and biotite mica; the later abundant and occurring as aggregates. Accessory minerals are rutile, ilmenite, tourmaline, euhedral zircon, sphene, apatite and hornblende. Microline ovoids are occasionally mantled with clear albite, while some ovoids include concentric shells of biotite and plagioclase.
The clear white mineral is quartz, the grey opaque mineral are plagioclase feldspars, the pink and red crystals are K-feldspars, and the thin platty minerals are biotite and muscovite mica (black and silver coloured). The composition of the above minerals precipitate out of the magma according to certain pressure and temperature gradients (Bowen's Reaction Series). The individual crystal size, mineral composition and their relationship to each other, provide information for geologists to determine the host rocks temperature and pressure at time of formation.
The crystals (commonly called augens) at Terrapinna are large, have well developed cleavages and are oval in shape. Large well developed crystals comprising K-Feldspars and quartz are indicative of formation under relatively low pressure at low temperatures. The oval shape of the crystals is caused by post depositional metamorphism as suggests that the crystal grew as ovoids. Since a sphere has the least surface energy of any geometric shape the ovoidal form may be due to the effect of the strong confining pressures which might be expected at great depth on crystal growth.
The Terrapinna Granite was emplaced as a magma during the Terrapinnian Phase of metamorphism and igneous activity and intruded as a large mushroom-shaped body known as a laccolith. The intrusion cooled at a depth beneath the earth's surface that was favourable to large crystal growth (temperature and pressure).
After cooling, post deformation, intrusion, and faulting have occurred during a number of regional folding events.
Where granite is below the ground’s surface, water percolates through fractures in the rock, bringing about chemical erosion which causes the granite to separate into its constituent crystals. This is further enhanced by the actual rock decompressing and expanding as regolith above the rock is slowly removed. When the rock is finally exposed to the atmosphere, physical erosion occurs separating the loose crystals which fall away leaving rounded boulders or tors. Fluvial and aeolian activity then sort the loose material, referred to as grus, according to size and density.
Further evidence of physical and chemical weathering at Terrapinna is evident where the rock has been hollowed out to form small caverns; this is a characteristic of mylonitic/granitic terrains. The caverns form by the natural salt damp processes where repeated salt crystallisation between winter and summer gradually break up the rock. In some tors case-hardened rinds are resistant to such attack and may also result in cavern formation.
Rocks when subjected to immense stress behave in a characteristic manner depending on the style of metamorphism. Metamorphism is a rock altering processes that involves the transformation and /or replacement of pre-existing rocks by heat, pressure, and chemical active fluids. It can be regional in extent, as occurs along tectonic convergent boundaries, localised, for example along fault zones, or be contact orientated, such as when hot rock intrudes cool rock.
High level metamorphism called blue schist facies cause rocks to become strongly foliated and exhibit a layered appearance. This foliation is a result of intense heat and pressure which causes rocks to behave plastically and begin to flow. These rocks may also exhibit shearing similar to the slippage that occurs between individual playing cards when a deck is held between your hands and the top of the deck is moved relative to the bottom.
Less intense metamorphism called greenschist facies does not cause rock to melt and flow, however, deformation stress and strain cause the rock to become brittle and fracture. This property is called rock cleavage or slaty cleavage and, depending on rock type, is often associated with the realignment of minerals so that their flat surfaces are nearly parallel to the direction of stress. Thermal metamorphism (heat) can occur when magma rises up and intrudes the country rock causing mineral alteration; the degree of alteration being a factor of heat and the distance from the source.
Terrapinna Rocks Tell A Story
Rock types, textures and shapes provide evidence of metamorphic and other igneous processes since deposition. At Terrapinna, outcrops of smooth greyish coloured quartz have been deformed so that it looks layered in appearance. When quartz is altered by metamorphic processes it becomes a quartz mylonite. Mylonite is only derived from granite which has been deformed under great strain, causing the rock to become plastic and flow in the direction of the overall direction of the strain. This is what causes the thinly layered appearance of the rock.
Other areas of Terrapinna exhibit quartz that is very blocky and bright white in colour. The quartz appears in veins a few centimetres thick and was precipitated after the mylonite was formed. The quartz was injected into the cooled mylonite and preferentially followed fractures within the rock, in some cases expanding the fracture before precipitating and solidifying.
History Recorded in Terrapinna Rocks
The geological history has been recorded in the rocks throughout the Terrapinna landscape and it’s possible to determine geological events that have occurred millions of year before present.
A geologist can determine the timing of a geological event by the principle of cross cutting relationships and the law of superposition. For example a greenish coloured amphibole dyke has intruded the Terrapinna granite. The original intrusive rock was dolerite which was intruded along a fault which was the catalyst for the formation of a fault-controlled valley to the east. The dyke intrusion occurred after the granite was formed, but the alteration of the dyke rock from dolerite to amphibolite occurred after fault formation during the last major folding of the ranges.
A similar assumption can be made for the blocky quartz mentioned earlier in this document.
This brief discussion is not meant to be an in-depth account of the geology of the region. For those interested in more thorough and scientific explanation of the Terrapinna Tors I recommend you read the article: Age and Metasomatic Alteration of the Mt. Neill Granite, Australian Journal of Earth Science (search for it on google).
Waterhole is a Vital Link for Fauna
Terrapinna waterhole is located at the edge of the Terrapinna Tors and is the only reliable water resource for many animals. Historically the water resource had been utilised by graziers for watering domestic stock cattle, however, pressure from conservation groups and State Government has caused the waterhole and the surrounding area to be allocated conservation park status. Cattle have been removed from the surrounds of the waterhole and the area allowed to regenerate to its native state.
Terrapinna Tors is home to a now thriving population of yellow footed rock wallabies Petrogale xanthopus, which until recently were arguably one of South Australia's most endangered mammals. The wallabies live high in the tors and can be observed at dusk and dawn hopping from rock to rock. These animals are very shy and secretive and during the day shelter from the intense sun under rocky outcrops and in caverns, only approaching the waterhole in the evening to drink.
The waterhole is also habitat for the freshwater bony bream which congregate in deep water holes such a Terrapinna during protracted drought periods.
The waterhole plays a vital survival link for animals living in the region as well as for migrating species. During my visit I observed: pelicans, cormorants, galahs, peregrine falcons, corellas, Spinifex pigeons, euros, yellow foot wallabies and dingoes.
Images taken with Canon G5. They are to be replaced soon with images taken on my recent visits.