Location of the impact structure in South Australia
Acraman impact structure is a deeply eroded
impact crater in the
Gawler Ranges of
South Australia.[1] Its location is marked by Lake Acraman, a circular ephemeral
playa lake about 20 kilometres (12 mi) in diameter. The discovery of the impact structure and independent discovery of its
ejecta were first reported in the journal
Science in 1986.[2][3] 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 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 × 106megatons of TNT.[4]
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.[4]
A widespread layer of ejecta, believed to be from the Acraman impact structure, is found within Ediacaran rocks of the
Flinders Ranges at least 300 kilometres (190 mi) east of the crater,[3] and in
drill holes from the
Officer 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 an
iridium anomaly suggesting contamination with extraterrestrial material.[8] 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.[9][10] 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.
Landsat image of Lake Acraman; screen capture from
NASA World Wind
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.
Orange and brown soils mix with off-white saltpans, including Lake Gairdner and Lake Everard in this true-colour image.
^
abcWilliams, 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 (4–5): 607–620.
Bibcode:
2005AuJES..52..607W.
doi:
10.1080/08120090500181036.
S2CID128751351.
^Shoemaker, E.M.; Shoemaker, C.S. (1996). "The Proterozoic impact record of Australia". AGSO Journal of Australian Geology and Geophysics. 16: 379–398.
Bibcode:
1990LPICo.746...47S.
^Williams, George E. & Wallace, Malcolm W. (2003). "The Acraman asteroid impact, South Australia: magnitude and implications for the late Vendian environment". Journal of the Geological Society of London. 160 (4): 545–554.
Bibcode:
2003JGSoc.160..545W.
doi:
10.1144/0016-764902-142.
S2CID129728586.