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The Acraman Story Flinders Ranges

Our story begins in 1983.  Associate Professor Victor Gostin and colleagues from University of Adelaide conducted field studies with students in the Flinders Ranges. 

'The FootBall'

Acraman Asteroid Ejecta by David Salomon on Sketchfab

 SHRIMP U-Pb Geochronology

The sensitive high mass-resolution ion microprobe or SHRIMP, is a spectrometer which measures lead/uranium ratios in single zircon crystals. Knowing the radioactive half-life of uranium and the lead/uranium ratio enables the age of rock strata to be calculated.

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Searching for samples Lecturers, researchers and students from University of Adelaide Geology Department search for volcanic material for dating. Samples were discovered in the Stirrup Iron Range and at Pichi Richi. The red and maroon samples were noticeably different in colour to typical Flinders Ranges sandstones.

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Mounting evidence Discovering volcanic debris from widespread areas all with the same composition, from fine grains to chunks larger than a football, provide evidence of a major event in the distant past. The samples were sent to ANU for dating with the expectation of finding a date for a major volcanic event during the formation of the Bunyeroo siltstones.

Explaining the anomoly Perhaps samples from "volcanic bombs" of older material were dragged up and thrown out by a more recent volcano. How else could an older layer of volcanic material be sandwiched between two younger layers of siltstone?

Glacial Dropstone by David Salomon on Sketchfab

Glacial drop stone

Pitchi Richi "tuff"" by David Salomon on Sketchfab

Debris carried in ice flows is deposited as ice melts.

Ice rafting

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Siltstone

Volcanic Tuff

Ancient volcanics Sandwiched between younger siltstones

As glaciers melt they release debris to the sea floor. Over time the rocks and mud compact becoming tillite.

Tillite formed by ice rafting Rocks dropped into mud at the sea bed. 

Excitement is mounting Tuffs and volcanic rocks are discovered over a wider and wider area. They range from the size of an egg to chunks much larger than a football, spread out over more than 100 km of north-south distribution.

Locations of Volcanic Deposits

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Volcanic bombs

Older material is drawn up from the edges of a volcano.

Asteroid impact

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Material ejected from the impact site

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Mystery origins The Bunyeroo siltstones formed in gentle waters as mud and silt that settled to the bottom. They are up to 400 m thick. In the middle of this layer is a distinctly different layer of volcanic origin, ranging in size from fine grains to football-sized chunks. The distribution indicates it came in from the west. However, the acidic dacite samples are different from the basalts to the west. Again, no match there.

Out of the blue On recalling lectures from his student days on asteroid impacts on the moon, Vic wondered if this could be an explanation. He and his team looked for new evidence to the west of the Flinders Ranges.  The first indications were encouraging. Then new evidence started to flow in from people working in the field.

Finally a match Samples collected in the Flinders Ranges and the Gawler Ranges were a match both in composition and in dates. Furthermore, satellite images of South Australia showed what could be an ancient scar from an asteroid impact in the Gawler Ranges.

Assembling the evidence Vic and his team's discovery revealed a far more complex story than first realised.  It is supported by evidence from satelite images, paleomagnetic studies, alongside the original field research.

Searching for answers Over the next two years Vic looked for an explanation. Could the debris have fallen off a cliff, perhaps been brought in by rivers or rafted in by glaciers?  If it was a volcanic eruption, where was the volcano and if not a volcano then where had the tuffs come from?

Surprising dating results Eventually the dates came back, but they were contradictory. All the samples from north to south dated consistently at approximately 1,600 M years. This did not add up.  At 1,600 M years old, the volcanic material was more than twice the age of the layers they were sandwiched between. The tuffs must have been formed somewhere else, long before the Flinders Ranges existed.

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Pichi Richi Pass

Locations of volcanic deposits

Stirrup Iron Range

Asteroid impact site and ejecta

L to R: Kevin Hamdorf (Photography), David Salomon (Videography & Editor), Associate Professor Victor Gostin (Presenter), Andrei Gostin (Drone Photography)

All the pieces come together Not only does an asteroid impact explain the mystery layer, it stimulates research into related areas as well.

Asteroids 3640 Gostin and 3700 Geowilliams. Astronomers Carolyn and Eugene Shoemaker at the Palomar Observatory, California recognised the significance of the Acraman impact site with its distinctive widespread and distant ejecta into the Flinders Ranges, South Australia. This was the first time that a large asteroid impact structure was correlated with its distant ejecta. Dr George Williams [Fellow Australian.Acad.Sci.] and Associate Professor Victor Gostin at the University of Adelaide were honoured with the naming of these asteroids.

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New KEY REFERENCES - by date  Gostin, V.A., Haines, P.W., Jenkins, R.J.F., Compston, W., & Williams, I.S., 1986,  Impact ejecta horizon within late Precambrian shales, Adelaide Geosyncline, South Australia. Science, 233: 198-200. Williams, G.E., 1986. The Acraman impact structure: source of ejecta in late Precambrian shales, South Australia.  Science, 233: 200-203.  Compston, W., Williams, I. S., Jenkins, R. J. F., Gostin, V. A. & Haines, P. W. 1987.  Zircon age evidence for the Late Precambrian Acraman ejecta blanket.  Australian Journal of Earth Sciences 34: 435 – 445.  Williams, G.E. & Wallace, M.W. 2003. The Acraman impact, South Australia: magnitude and implications for the late Vendian environment. Jour. Geol. Soc., London 160: 545-554.  Grey, K., Walter, M.R. & Calver, C.R. 2003. Neoproterozoic biotic diversification: Snowball Earth or aftermath of the Acraman impact? Geology, 31: 459-462.  Williams, G.E. & Gostin, V.A. 2005. Acraman - Bunyeroo impact event (Ediacaran), South Australia,  and environmental consequences: twenty-five years on. Australian Journal of Earth Sciences, 52: 607-620.  Gostin, V.A., McKirdy, D.M., Webster, L.J. & Williams, G.E. 2010. Ediacaran ice-rafting and coeval asteroid impact,  South Australia: insights into the terminal Proterozoic environment. Australian Journal of Earth Sciences 57: 859-869. Williams, G.E. & Schmidt, P.W. 2021. Dating the Acraman asteroid impact, South Australia: the case for deep drilling  the ‘hot shock’ zone of the central uplift. Australian Journal of Earth Sciences, 68: 355-367.

KEY REFERENCES - by date    Gostin, V.A., Haines, P.W., Jenkins, R.J.F., Compston, W., & Williams, I.S., 1986, Impact ejecta horizon within late Precambrian shales, Adelaide Geosyncline, South Australia. Science, 233: 198-200. Williams, G.E., 1986. The Acraman impact structure: source of ejecta in late Precambrian shales, South Australia. Science, 233: 200-203.  Compston, W., Williams, I. S., Jenkins, R. J. F., Gostin, V. A. & Haines, P. W. 1987. Zircon age evidence for the Late Precambrian Acraman ejecta blanket. Australian Journal of Earth Sciences 34: 435 – 445. Williams, G.E. & Wallace, M.W. 2003. The Acraman impact, South Australia: magnitude and implications for the late Vendian environment. Jour. Geol. Soc., London 160: 545-554. Grey, K., Walter, M.R. & Calver, C.R. 2003. Neoproterozoic biotic diversification: Snowball Earth or aftermath of the Acraman impact? Geology, 31: 459-462. Williams, G.E. & Gostin, V.A. 2005. Acraman - Bunyeroo impact event (Ediacaran), South Australia, and environmental consequences: twenty-five years on. Australian Journal of Earth Sciences, 52: 607-620. Gostin, V.A., McKirdy, D.M., Webster, L.J. & Williams, G.E. 2010. Ediacaran ice-rafting and coeval asteroid impact, South Australia: insights into the terminal Proterozoic environment. Australian Journal of Earth Sciences 57: 859-869. Williams, G.E. & Schmidt, P.W. 2021. Dating the Acraman asteroid impact, South Australia: the case for deep drilling the ‘hot shock’ zone of the central uplift. Australian Journal of Earth Sciences, 68: 355-367.