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Uncovering the Acraman ejecta layer

The Acraman Story is a fascinating interactive presentation outlining the discovery of the Acraman ejecta layer by geologist Dr Victor Gostin and his colleagues.

The Acraman layer is a narrow pebbly deposit of volcanic material set within the purple siltstone of the Flinders Ranges Bunyeroo Formation. While the oldest rocks of the Flinders Ranges were 900my, the Acraman samples were dated at 1600my. Early speculation pointed toward a volcanic bomb coming from the near west but this and other suggestions of deposition by glacial rafting, cliff debris or river movement were not viable.

After years of investigation, attention turned to the possibility of asteroid impact. A large asteroid impact scar (30-90km diameter) was found in the 1600my granite rich Gawler Ranges to the west of the Flinders Ranges – a perfect date match for the Acraman volcanic deposit.

The team pieced together an amazing story. The 4.8km wide Acraman asteroid hit the Gawler Ranges at around 22km per second around 680my, the age of the Bunyeroo silts. The asteroid itself was vaporised on impact and the impact into the ancient volcanic terrain displaced granitic rock (ejecta) layer which landed as far as the Flinders Ranges to the east.

Enter the The Acraman Story interactive.

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The Legacy of Time: Story of the Flinders Ranges

Excerpt: The legacy of time

First published by The Royal Geographical Society of South Australia Inc. in 1995.

The Flinders Ranges have been shaped by geological processes that, over hundreds of millions of years, have built the stage upon which the drama of biological interactions is being performed.

The Flinders Ranges form part of a highland chain extending from Kangaroo Island in the south through the Mount Lofty Ranges and Flinders Ranges to Marree and beyond in the north, to Olary in the east, and to Spencer Gulf and Lake Torrens in the west. Like many other mountain chains, they began their history as a subsiding sedimentary basin.

During the late Precambrian era, the Earth’s crust in South Australia consisted of granitic, metamorphic, sedimentary and volcanic rocks formed between 2600 and 1400 million years ago. In the present Flinders, the only rocks from that time are exposed near Arkaroola, including what are now known as the Freeling Heights Quartzite, Terrapinna Granite (Terrapinna Tors Walk) and Mt Neill Granite. Rocks that now constitute the eastern states of Australia were yet to form and the ancestral Pacific Ocean may have had its shores in South Australia. To the south, Australia was still joined to Antarctica.

The main area in South Australia then lay west of an approximate line from Adelaide to Oodnadatta; a smaller one existed in the vicinity of Lake Frome. The intervening area, where the Flinders and Mount Lofty Ranges now stand, became a long-lived basin known as the Adelaide Geosyncline, in which sediments accumulated 800 to 500 million years ago.

Continue reading – download PDF The Flinders Ranges – Legacy of Time