Lake Acraman is a deeply eroded impact crater in the
Gawler Ranges of South Australia, a circular 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. The evidence for impact includes the presence of bleep ter cones and shocked quartz in bleep tered bedrock on islands within Lake Acraman. bleep ter cones are found at all impact sites.
The crater is deeply eroded and its original size must be inferred by indirect means. Some estimate an original diameter of up to 85–90 km, while others suggest a smaller size, perhaps only 35–40 km, closer to that of the depression in which Lake Acraman is centred. The larger size estimate would imply an energy release of 5.2 × 106 megatons of TNT.
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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.
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 in Western Australia and South Australia that covers about 410,000 km2 and has a maximum sediment thickness of 10,000 metres.
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At the time these areas were shallow seas, and the ejecta settled into mud on the sea floor. The ejecta contains shocked minerals and small bleep ter 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. 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. 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.
At the
Bunyeroo Gorge walking trail, 50m north into a small
creek and the outcrops of the maroon coloured shales of the Bunyeroo Formation.
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The
creek cuts through a narrow but prominent band of green shale enclosing a 4 10 cm bed of reddish sand; this is the Acraman ejecta layer, which can be traced throughout the
Flinders Ranges.
On the west side of the ranges, it is some 80 m above the ABC Range Quartzite.
A basal lag of angular breccia, embedded in the underlying shale represents the fallout of large fragments of Mesoproterozoic
Gawler Range Porphyry, dated at 1590 Ma. Quartz crystals in the clasts show deformed laminae.
A few millimeters above, a swaley cross bedded, feldspar
sandstone represents the settling out of finer
debris from the ejecta and reworking by tsunami surge, following a meteorite impact on the
Gawler Craton at what is now Lake Acraman.
The geochemical signature and textural characteristics of the lithic fragments are identical to that of rock in the center of Lake Acraman, the remnant of a probable astrobleme 300 km WSW on the
Gawler Craton.
Although analysis of the melt rock failed to give a reset age of impact, the Bunyeroo Formation is likely to be of the order of 570 580 Million years ago.
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