Brachina Gorge Geological Trail

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Brachina Gorge Geological Trail: A snapshot of geological time in the Flinders Ranges National Park in South Australia.

Published by the Geological Society of Australia, South Australian Division.


One of the best records in the world of sedimentary deposition in the period of geological time between about 800 million and 500 million years ago is exposed in the Flinders Ranges, Mount Lofty Ranges and the Olary region in South Australia. Sandy and silty sediments derived from erosion of older rocks of the Gawler Craton in the hinterland to the west, and island masses of this basement rock rising from an undersea ridge over 200 km to the east, were deposited into an extensive marine basin called the Adelaide Geosyncline in which the seafloor was slowly subsiding along a series of elongated north-south step or graben faults.

During the 300 million years of continuing but intermittent subsidence of the basin floor, a thick pile of sediment accumulated in the geosyncline. This sequence was then compressed and hardened by deep burial and later folded into a high mountain range by a new regime of earth movements.

Subsequent erosion has reduced these highlands to their present form and deposited huge amounts of sediment to the east into younger sedimentary basins formed by later crustal down warping. Continue reading “Brachina Gorge Geological Trail”

Silcrete of the Strzelecki Track

The quartzose silcrete of the Strzelecki Track formed in the Tertiary Era, as a duricrust formed through silica rich mineral deposition from groundwater at the low lying fringes of the Flinders Ranges.  It appears to be cryptocrystalline in nature, made of a minute crystal structure also typical of chert and chalcedony.

Highly resistant to weathering, the silcrete formation reveals some very finely detailed fossil casts of vegetation.  The fossils in this formation contain casts of plant detritus of varying species, one of which appears to be Casuarina.

This formation has intriguing layering of well preserved crushed quartz, silcrete and sand calcite balls as seen in this vertically upturned rock.  Continue reading “Silcrete of the Strzelecki Track”

Stromatolite Seam in Magnesite

An old mine exploration site offered an interesting opportunity to walk through the layers of magnesite rich dolomite near Copley.

Between layers of white magnesite, and light and dark dolomites is a gnarly seam of fossilised stromatolites.