Acraman crater
Encyclopedia
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. The evidence for impact includes the presence of shatter cone
s 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, while other 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
.
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 basin
s.
at least 300 km east of the crater, and in drill holes from the Officer Basin
to the north. 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. An evolutionary radiation
within marine microorganisms (acritarch
s) 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.
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
in the Gawler Ranges
Gawler Ranges
The Gawler Ranges are a range of stoney hills in South Australia to the north of Eyre Peninsula. The Eyre Highway skirts the south of the ranges. The Gawler Ranges National Park and Gawler Ranges Conservation Reserve are in the ranges north of Wudinna...
of South Australia
South Australia
South Australia is a state of Australia in the southern central part of the country. It covers some of the most arid parts of the continent; with a total land area of , it is the fourth largest of Australia's six states and two territories.South Australia shares borders with all of the mainland...
. 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
Ejecta blanket
An ejecta blanket is a generally symmetrical apron of ejecta that surrounds crater; it is layered thickly at the crater’s rim and thin to discontinuous at the blanket’s outer edge....
were first reported in the journal Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....
in 1986. The evidence for impact includes the presence of shatter cone
Shatter cone
Shatter cones are rare geological features that are only known to form in the bedrock beneath meteorite impact craters or underground nuclear explosions...
s and shocked quartz
Shocked quartz
Shocked quartz is a form of quartz that has a microscopic structure that is different from normal quartz. Under intense pressure , the crystalline structure of quartz will be deformed along planes inside the crystal...
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, while other 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
TNT equivalent
TNT equivalent is a method of quantifying the energy released in explosions. The ton of TNT is a unit of energy equal to 4.184 gigajoules, which is approximately the amount of energy released in the detonation of one ton 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 basin
Sedimentary basin
The term sedimentary basin is used to refer to any geographical feature exhibiting subsidence and consequent infilling by sedimentation. As the sediments are buried, they are subjected to increasing pressure and begin the process of lithification...
s.
Ejecta layer
A widespread layer of ejecta, believed to be from the Acraman crater, is found within Ediacaran rocks of the Flinders RangesFlinders Ranges
Flinders Ranges is the largest mountain range in South Australia, which starts approximately north west of Adelaide. The discontinuous ranges stretch for over from Port Pirie to Lake Callabonna...
at least 300 km east of the crater, and in drill holes from the Officer Basin
Officer Basin
The Officer Basin is a large intracratonic sedimentary basin in Australia, lying across the states of South Australia and Western Australia. It is named after Officer Creek which is a watercourse that drains a small part of the basin. Deposition of up to 10 km of marine and non-marine sedimentary...
to the north. 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
Iridium anomaly
The term iridium anomaly commonly refers to an unusual abundance of the chemical element iridium in a layer of rock strata, often taken as evidence of an extraterrestrial impact event because of the case of such an anomaly at the Cretaceous–Tertiary boundary...
suggesting contamination with extraterrestrial material. An evolutionary radiation
Adaptive radiation
In evolutionary biology, adaptive radiation is the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage. Starting with a recent single ancestor, this process results in the speciation and phenotypic adaptation of an array of species exhibiting different...
within marine microorganisms (acritarch
Acritarch
Acritarchs are small organic fossils, present from approximately to the present. Their diversity reflects major ecological events such as the appearance of predation and the Cambrian explosion.-Definition:In general, any small, non-acid soluble Acritarchs are small organic fossils, present from...
s) 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
Ediacara biota
The Ediacara biota consisted of enigmatic tubular and frond-shaped, mostly sessile organisms which lived during the Ediacaran Period . Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms.Simple multicellular organisms such as...
is noted, though probably not significant given the likely global consequences of the impact.
External links
- Acraman at Earth Impact Database
- Satellite image of the region (from Google Maps)