| Acraman crater | |
|---|---|
 Acraman lake (circle), ringed byLake Gairdner and others, outlining the Acraman crater depression | |
| Impact crater/structure | |
| Confidence | Confirmed |
| Diameter | up to 90 km (56 mi) |
| Age | ~590 Ma Late Ediacaran |
| Exposed | Yes |
| Drilled | No |
| Bolide type | Chondrite |
| Location | |
| Location | Gawler Ranges |
| Coordinates | 32°1′S135°27′E / 32.017°S 135.450°E /-32.017; 135.450 |
| Country | Australia |
| State | South Australia |
  Location of the impact structure in South Australia | |
Acraman impact structure is a deeply erodedimpact crater in theGawler Ranges ofSouth Australia.[1] Its location is marked by Lake Acraman, a circular ephemeralplaya lake about 20 kilometres (12 mi) in diameter. The discovery of the impact structure and independent discovery of itsejecta were first reported in the journalScience in 1986 byGeorge E. Williams[2] andVictor A. Gostin.[3] The evidence for impact includes the presence ofshatter cones andshocked 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 85 to 90 kilometres (53 to 56 mi),[4] while other suggest a smaller size, perhaps only 35 to 40 kilometres (22 to 25 mi), closer to that of the depression in which Lake Acraman is centred.[5] The larger size estimate would imply an energy release of 5.2 millionmegatons of TNT.[4]
The impact event is estimated to have occurred about 590 million years ago during theEdiacaran; this age is not derived from the crater itself but from the position of ejecta within nearbysedimentary basins.[4]
The Lake Acraman Impact Structure is listed on theSouth Australian Heritage Register.[6]
A widespread layer of ejecta, believed to be from the Acraman impact structure, is found within Ediacaran rocks of theFlinders Ranges at least 300 kilometres (190 mi) east of the crater,[3] and indrill holes from theOfficer Basin to the north.[7] 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,[3] and is associated with aniridium anomaly suggesting contamination with extraterrestrial material.[8] Anevolutionary radiation within marine microorganisms (acritarchs) occurs just above the level as the ejecta layer, and some authors believe there may be a connection.[9][10] The proximity of the crater to the type area for theEdiacara Biota is noted, though probably not significant given the likely global consequences of the impact.
The Acraman impact structure, Lake Acraman and the nearbyAcraman Creek are named after South Australian colonial businessmanJohn Acraman.[11]
