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.

Excellent exposures of rocks deposited in the Adelaide Geosyncline between about 650 and 500 million years ago can be seen in Brachina Gorge, particularly on the southern wall.

Here the sequence dips at moderate angles to the west except at the western end of the trail where the rocks have been buckled by faulting.

Much younger flat-lying sediments deposited in the Pleistocene Epoch between about 35 000 and 18 000 years ago can be seen midway along the trail.

A plaque describing a site of international geological significance, marking the base of the Ediacaran Period of geological time, was placed in 2005 on the southern bank of Enorama Creek, 500 m east of the Enorama Campsite. This records a major change, which began about 635 million years ago, in the evolution from simple single-celled life forms to more complex multi-cellular organisms.

Evidence of other significant events in the geological history of the rocks in the Brachina Gorge includes;

• depositional structures in the sandy and silty sediments eroded from adjacent hinterlands, such as cross bedding and mud cracks, resulting from varying conditions of water depth and temperature during deposition.

• dumping of glacial till in the melt water of a world wide period of continental glaciation.

• precipitation of ironstone, limestone and dolomite in the sea by chemical and biological action.

• debris from the impact of a large meteorite which fell near Lake Acraman in the Gawler Ranges about 300 km to the west.

• some of the very earliest life forms which existed before the world renowned Ediacaran fauna.

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