Ajax Limestone Archaeocyath fossils

The Ajax Limestone in the Northern Flinders Ranges includes a significant array of Lower Cambrian Archaeocyath fossils, replaced by quartz, exposed in ground-level limestones.

This fossil site on a low gibber-covered rise to the north of Beltana represents the most species-diverse archaeocyath locality worldwide, with a count of 82 species. One of seven sites in the Flinders Ranges currently under consideration for UNESCO World Heritage status, it provides a significant record of an extinct group of marine sponges (Phylum Porifera) that lived around 525 million years ago and played a dominant role in constructing the first reefs on Earth.

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Skillogalee Dolomite Stromatolites

The Skillogalee Dolomite is a 750 million-year-old formation that is host to stromatolite fossils – the only evidence of complex living organisms at that time. Stromatolites are fossilised biochemical accretionary structures created by Cyanobacteria (‘blue-green algae’) colonies which form microbial mats and laminated mounds in shallow saline waters. Living stromatolites are found at the tip of the Yorke Peninsula (SA) and Shark Bay in (WA).

Width approx. 340mm
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Triassic plant fossils in concretions

Triassic plant fossils within concretions formed in the sedimentary layers of the Leigh Creek coal formation. The concretions appear to have been formed in concentric layers within the sediment, and within these are enclosed Triassic plant fossils, approximately 230 million years old.

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.

Excerpt:

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”