Brachina Gorge trail in Flinders Ranges, South Australia: 100 million years' geology with a special golden spike

The many geological eras (listed at right), visible along the Brachina Ge\orge trail in South Australia's Ikara-Flinders Ranges, included the dividing line – marked with a golden spike – between the Cambrian and pre-Cambrian (including Ediacaran) eras.
Images courtesy Royal Geographical Society of South Australia

Brachina Gorge in South Australia’s Ikara-Flinders Ranges offered a 20-kilometre self-guided geological trail passing through 130 million years of Earth history – from 640 million to around 540 million years ago.

With the Cambrian geological era starting around 540 million years ago, the entire east-west sequence of the trail (from the Brachina Gorge/Blinman Road junction) is pre-Cambrian. This slice of the pre-Cambrian era was explored along the Brachina Gorge trail by an Adelaide University student of geologist and explorer Douglas Mawson, Reg Sprigg, who discovered the Ediacaran fossil imprints that gave their name to a whole new geological era.

A golden spike – global boundary stratotype section and point (GSSP) – on the Brachina Goreg trail marks the boundary, 542 million years ago, between the Cambrian and pre-Cambrian (Ediacaran) era. This records the major change, starting about 635 million years ago, in the evolution from simple single-celled life forms to more complex multi-cellular organisms.

The roacks long the  Branchina geological trail were laid down in the 100 million (640-540) years aligning closely with the Ediacaran. The rocks were sedimentary: all laid down as flat layers under water. Over time, the layers built up and were compressed by the weight of layers above. The different layers of rocks representing different geological conditions, were uplifted and tilted before being weathered away to exposing the edges of all the layers buried over the 100 million years to bury. These rocks were deposited in the larger geological Adelaide Superbasin (formerly Adelaide Geosyncline). Much younger flat-lying sediments deposited in the Pleistocene Epoch between about 35,000 and 18,000 years ago could be seen midway along the trail.

In 2022, signage upgrades to the Brachina Gorge trail, to aid the nomination pf the Flinders Ranges for UNESCO world heritage listing, included  the setting and access to the golden spike – the only one in the southern hemisphere.