Acraman crater

Acraman crater is a deeply eroded impact crater in the Gawler Ranges of South Australia. Its location is marked by Lake Acraman, a circular ephemeral playa lake about 20 km in diameter.

The discovery of the crater and independent discovery of its ejecta were first reported in the journal Science in 1986.cite journal | author = Williams, G.E. | year = 1986 | title = The Acraman Impact Structure: Source of Ejecta in Late Precambrian Shales, South Australia | journal = Science | volume = 233 | issue = 4760 | pages = 200 | doi = 10.1126/science.233.4760.200 | pmid = 17737291 ] cite journal | author = Gostin, V.A. | coauthors = Haines, P.W.; Jenkins, R.J.F.; Compston, W.; Williams, I.S. | year = 1986 | title = Impact Ejecta Horizon Within Late Precambrian Shales, Adelaide Geosyncline, South Australia | journal = Science | volume = 233 | issue = 4760 | pages = 198 | doi = 10.1126/science.233.4760.198 | pmid = 17737290 ] The evidence for impact includes the presence of shatter cones and shocked quartz in shattered bedrock on islands within Lake Acraman.

The crater is deeply eroded and its original size must be inferred by indirect means. Some authors estimate an original diameter of up to 85–90 km,cite journal | author = Williams, G.E. | coauthors = Gostin, V.A. | year = 2005 | title = Acraman-Bunyeroo impact event (Ediacaran), South Australia, and environmental consequences: twenty-five years on | journal = Australian Journal of Earth Sciences | volume = 52 | issue = 4-5 | pages = 607–620 | doi = 10.1080/08120090500181036 ] while other suggest a smaller size, perhaps only 35–40 km, closer to that of the depression in which Lake Acraman is centred.cite journal | author = Shoemaker, E.M. | coauthors = Shoemaker, C.S. | year = 1996 | title = The Proterozoic impact record of Australia | journal = AGSO Journal of Australian Geology and Geophysics | volume = 16 | pages = 379–398 ] The larger size estimate would imply an energy release of 5.2 × 10⁶ megatons of TNT.

The impact event is estimated to have occurred about 580 million years ago during the Ediacaran Period; this age is not derived from the crater itself but from the position of ejecta within nearby sedimentary basins.

Ejecta layer

A widespread layer of ejecta, believed to be from the Acraman crater, is found within Ediacaran rocks of the Flinders Ranges at least 300 km east of the crater, and in drill holes from the Officer Basin to the north.cite journal | author = Wallace, M.W. | coauthors = Gostin, V.A.; Keays, R.R. | year = 1989 | title = Geological Note: Discovery of the acraman impact ejecta blanket in the officer basin and its stratigraphic significance | journal = Australian Journal of Earth Sciences | volume = 36 | issue = 4 | pages = 585–587 | url = http://www.informaworld.com/index/781041341.pdf | doi = 10.1080/08120098908729511] At the time these areas were shallow sea, and the ejecta settled into mud on the sea floor. The ejecta contains 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 suggesting contamination with extraterrestrial material.cite journal | author = Gostin, V.A. | coauthors = Keays, R.R.; Wallace, M.W. | year = 1989 | title = Iridium anomaly from the Acraman impact ejecta horizon: impacts can produce sedimentary iridium peaks | journal = Nature | volume = 340 | issue = 6234 | pages = 542–544 | doi = 10.1038/340542a0 ] 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.cite journal | author = Grey, K. | coauthors = Walter, M.R.; Calver, C.R. | year = 2003 | title = Neoproterozoic biotic diversification: Snowball Earth or aftermath of the Acraman impact? | journal = Geology | volume = 31 | issue = 5 | pages = 459–462 | url = http://geology.geoscienceworld.org/cgi/content/abstract/31/5/459 | doi = 10.1130/0091-7613(2003)031<0459:NBDSEO>2.0.CO;2] cite journal | author = Williams & Wallace | year = 2003 | title = The Acraman asteroid impact, South Australia: magnitude and implications for the late Vendian environment | journal = Journal of the Geological Society of London | volume = 160 | issue = 4 | pages = 545–554 | doi = 10.1144/0016-764902-142] 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.

References

External links

* [http://www.unb.ca/passc/ImpactDatabase/images/acraman.htm Acraman at Earth Impact Database]
* [http://maps.google.com/maps?ll=-32.015533,135.441513&spn=1.146767,1.714279&t=k&hl=en Satellite image of the region] (from Google Maps)