Acraman impact structure is a deeply eroded

impact crater An impact crater is a depression (geology), depression in the surface of a solid astronomical body formed by the hypervelocity impact event, impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal c ...

in the

Gawler Ranges The Gawler Ranges are a range of stoney hills in South Australia to the north of the Eyre Peninsula. The Eyre Highway skirts the south of the ranges. The Gawler Ranges National Park is in the ranges north of Kimba, South Australia, Kimba and Wud ...

South Australia South Australia (commonly abbreviated as SA) is a States and territories of Australia, state in the southern central part of Australia. With a total land area of , it is the fourth-largest of Australia's states and territories by area, which in ...

. Its location is marked by Lake Acraman, a circular ephemeral

playa lake A dry lake bed, also known as a playa (), is a basin or Depression (geology), depression that formerly contained a standing surface water body, which disappears when evaporation processes exceed recharge. If the floor of a dry lake is covered b ...

about in diameter. The discovery of the impact structure and independent discovery of its

ejecta Ejecta (; ) are particles ejected from an area. In volcanology, in particular, the term refers to particles including pyroclastic rock, pyroclastic materials (tephra) that came out of a explosive eruption, volcanic explosion and magma eruption v ...

were first reported in the journal ''

Science Science is a systematic discipline that builds and organises knowledge in the form of testable hypotheses and predictions about the universe. Modern science is typically divided into twoor threemajor branches: the natural sciences, which stu ...

'' in 1986 by

George E. Williams George E. Williams may refer to: * George E. Williams (geologist), Australian geologist * George E. Williams (New Jersey politician), member of the New Jersey General Assembly * George E. Williams (New York politician) (1828–1914), New York as ...

and Victor A. Gostin. The evidence for impact includes the presence of

shatter cone Shatter cones are rare geological features that are only known to form in the bedrock beneath meteorite impact craters or underground nuclear explosions. They are evidence that the rock has been subjected to a shock with pressures in the ran ...

s and

shocked quartz Shocked quartz is a form of quartz that has a microscopic structure that is different from normal quartz. Under intense pressure (but limited temperature), the crystalline structure of quartz is deformed along planes inside the crystal. These pla ...

in shattered bedrock on islands within Lake Acraman. The impact structure is deeply eroded, and its original size must be inferred by indirect means. Some authors estimate an original diameter of up to , while other suggest a smaller size, perhaps only , closer to that of the depression in which Lake Acraman is centred. The larger size estimate would imply an energy release of 5.2 million

megatons of TNT TNT equivalent is a convention for expressing energy, typically used to describe the energy released in an explosion. A ton of TNT equivalent is a unit of energy defined by convention to be (). It is the approximate energy released in the det ...

. The impact event is estimated to have occurred about 590 million years ago during the

Ediacaran The Ediacaran ( ) is a geological period of the Neoproterozoic geologic era, Era that spans 96 million years from the end of the Cryogenian Period at 635 Million years ago, Mya to the beginning of the Cambrian Period at 538.8 Mya. It is the last ...

; this age is not derived from the crater itself but from the position of ejecta within nearby

sedimentary basin Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock They form when long-term subsidence ...

s. The Lake Acraman Impact Structure is listed on the

South Australian Heritage Register The South Australian Heritage Register, also known as the SA Heritage Register, is a statutory register of historic places in South Australia. It extends legal protection regarding demolition and development under the ''Heritage Places Act 1993'' ...

Ejecta layer

A widespread layer of ejecta, believed to be from the Acraman impact structure, is found within Ediacaran rocks of the

Flinders Ranges The Flinders Ranges are the largest mountain ranges in South Australia, which starts about north of Adelaide. The ranges stretch for over from Port Pirie to Lake Callabonna. The Adnyamathanha people are the Aboriginal group who have inhab ...

at least east of the crater, and in drill holes from the Officer Basin to the north. At the time these areas were shallow sea, and the ejecta settled into mud on the sea floor. The ejecta, containing shocked minerals and small shatter cones, is composed of rock similar in age and composition to that at the crater, and is associated with an

iridium anomaly The term iridium anomaly commonly refers to an unusual abundance of the chemical element iridium in a layer of rock strata at the Cretaceous–Paleogene (K–Pg) boundary. The unusually high concentration of a rare metal like iridium is often take ...

suggesting contamination with extraterrestrial material. An

evolutionary radiation An evolutionary radiation is an increase in taxonomic diversity that is caused by elevated rates of speciation, that may or may not be associated with an increase in morphological disparity. A significantly large and diverse radiation within ...

within marine microorganisms (

acritarch Acritarchs () are organic microfossils, known from the Precambrian to the present. The classification is a catch all term used to refer to any organic microfossils that cannot be assigned to other groups. Their diversity reflects major ecolog ...

s) occurs just above the level as the ejecta layer, and some authors believe there may be a connection. The proximity of the crater to the type area for the

Ediacara Biota The Ediacaran (; formerly Vendian) biota is a taxonomic period classification that consists of all life forms that were present on Earth during the Ediacaran Period (). These were enigmatic tubular and frond-shaped, mostly sessile, organis ...

is noted, though probably not significant given the likely global consequences of the impact.

John Acraman

The Acraman impact structure, Lake Acraman and the nearby Acraman Creek are named after South Australian colonial businessman

John Acraman John Acraman (1829 – 22 June 1907) was a prominent businessman in the colony of South Australia, and he has a place in the history of Australian football in that state. History John Acraman was born in England, a son of William Edward Acrama ...

. Manning, Geoffrey H. (2012)
A Glossary of South Australian Place Names - From Aaron Creek to Zion Hill
Gould Genealogy & History, 2010. Via State Library of South Australia. Retrieved 7 March 2025.

Gallery

File:Acraman.jpg, Landsat image of Lake Acraman; screen capture from

NASA World Wind NASA WorldWind is an open-source (released under the NOSA license and the Apache 2.0 license) virtual globe. According to the website, "WorldWind is an open source virtual globe API. WorldWind allows developers to quickly and easily c ...

File:Acraman x10.jpg, Oblique Landsat image of Lake Acraman draped over digital elevation model (10x vertical exaggeration), looking east towards the Flinders Ranges where ejecta has been found; screen capture from NASA World Wind. File:Acraman Impact Structure, South Australia.jpg, Orange and brown soils mix with off-white saltpans, including Lake Gairdner and Lake Everard in this true-colour image.

References

External links

Acraman at Earth Impact Database

Satellite image of the region
(from Google Maps) {{Impact cratering on Earth Impact craters of South Australia Proterozoic impact craters Precambrian Australia Far North (South Australia) South Australian Heritage Register Gawler bioregion