Acraman impact structure

Template:Short description {{#invoke:other uses|otheruses}} Template:Use Australian English Template:Use dmy dates Template:Infobox terrestrial impact site

Acraman impact structure is a deeply eroded impact crater in the Gawler Ranges of South Australia.<ref name=EIDBAcraman>Template:Cite Earth Impact DB</ref> Its location is marked by Lake Acraman, a circular ephemeral playa lake about Template:Convert in diameter. The discovery of the impact structure and independent discovery of its ejecta were first reported in the journal Science in 1986 by George E. Williams<ref name=Williams1986>Template:Cite journal</ref> and Victor A. Gostin.<ref name=Gostin1986>Template:Cite journal</ref> The evidence for impact includes the presence of shatter cones and shocked quartz 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 Template:Convert,<ref name=Williams2005>Template:Cite journal</ref> while other suggest a smaller size, perhaps only Template:Convert, closer to that of the depression in which Lake Acraman is centred.<ref name=Shoemaker1996>Template:Cite journal</ref> The larger size estimate would imply an energy release of 5.2 × 10⁶ megatons of TNT.<ref name="Williams2005" />

The impact event is estimated to have occurred about 590 million years ago during the Ediacaran; this age is not derived from the crater itself but from the position of ejecta within nearby sedimentary basins.<ref name="Williams2005" />

The Lake Acraman Impact Structure is listed on the South Australian Heritage Register.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Ejecta layer

A widespread layer of ejecta, believed to be from the Acraman impact structure, is found within Ediacaran rocks of the Flinders Ranges at least Template:Convert east of the crater,<ref name="Gostin1986" /> and in drill holes from the Officer Basin to the north.<ref name=Wallace1989>Template:Cite journal</ref> 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,<ref name="Gostin1986" /> and is associated with an iridium anomaly suggesting contamination with extraterrestrial material.<ref name=Gostin1989>Template:Cite journal</ref> An evolutionary radiation within marine microorganisms (acritarchs) occurs just above the level as the ejecta layer, and some authors believe there may be a connection.<ref name=Grey2003>Template:Cite journal</ref><ref name=Williams2003>Template:Cite journal</ref> The proximity of the crater to the type area for the Ediacara Biota 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.<ref> 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.</ref>

Gallery

• Acraman.jpg

  Landsat image of Lake Acraman; screen capture from NASA World Wind

• 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.

• 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.

External links

• Acraman at Earth Impact Database
• Satellite image of the region (from Google Maps)