My OperaAcraman Impact Crater
Tuesday, March 2, 2010 7:35:01 PM
NASA Earth Observatory has published a nice satellite image of the Acraman impact structure in South Australia. The Acraman crater is a so-called complex crater, as opposed to simple craters - see drawing below:
The intermediate crater ring and the (possible) outer crater ring are indicated by dotted white lines.
The impact event is estimated to have occurred about 580-590 million years ago. This age is not derived from the crater itself but from the position of ejecta within nearby sedimentary basins. A remnant of the impact is Lake Acraman, seen in the middle, a small, circular, shallow salt lake about 20 km in diameter.
The evidence for impact includes the presence of shatter cones and shocked quartz in shattered bedrock on islands within Lake Acraman.
Geological studies have found that, over time, Acraman’s surface eroded several km below the original crater floor. Studies of the current land surface and debris ejected by the collision suggest that the impact produced an uplifted ring spanning roughly 40 km in the crater’s center. The rim surrounding the inner ring may have spanned 85 to 90 km, and the total area of disturbed rocks might have been as wide as 150 km.
Radiometric dating of the rocks affected by the Acraman impact indicates that the event occurred almost 600 million years ago, during a geologic period known as the Ediacaran (sometimes called the Vendian). Although our planet was more than 4 billion years old at that time, it was still before the time when life forms began leaving behind a rich fossil record. Nevertheless, fossils do occur in Ediacaran rock layers, and many of them are acritarchs—small (often microscopic) fossils that could result from a wide range of organisms.
A dramatic change in the types of acritarchs found in the Ediacaran rock record coincides with the estimated occurrence of the Acraman impact. Although some studies have linked this fossil turnover to a nearly worldwide glaciation (“snowball earth”), others have suggested the Acraman impact as a possible cause of the widespread disturbance to life on Earth at that time.
The discovery of the crater and independent discovery of its ejecta were first reported in the journal Science in 1986.
Location: S 32° 1' E 135° 27'
The intermediate crater ring and the (possible) outer crater ring are indicated by dotted white lines.
The impact event is estimated to have occurred about 580-590 million years ago. This age is not derived from the crater itself but from the position of ejecta within nearby sedimentary basins. A remnant of the impact is Lake Acraman, seen in the middle, a small, circular, shallow salt lake about 20 km in diameter.
The evidence for impact includes the presence of shatter cones and shocked quartz in shattered bedrock on islands within Lake Acraman.
Geological studies have found that, over time, Acraman’s surface eroded several km below the original crater floor. Studies of the current land surface and debris ejected by the collision suggest that the impact produced an uplifted ring spanning roughly 40 km in the crater’s center. The rim surrounding the inner ring may have spanned 85 to 90 km, and the total area of disturbed rocks might have been as wide as 150 km.
Radiometric dating of the rocks affected by the Acraman impact indicates that the event occurred almost 600 million years ago, during a geologic period known as the Ediacaran (sometimes called the Vendian). Although our planet was more than 4 billion years old at that time, it was still before the time when life forms began leaving behind a rich fossil record. Nevertheless, fossils do occur in Ediacaran rock layers, and many of them are acritarchs—small (often microscopic) fossils that could result from a wide range of organisms.
A dramatic change in the types of acritarchs found in the Ediacaran rock record coincides with the estimated occurrence of the Acraman impact. Although some studies have linked this fossil turnover to a nearly worldwide glaciation (“snowball earth”), others have suggested the Acraman impact as a possible cause of the widespread disturbance to life on Earth at that time.
The discovery of the crater and independent discovery of its ejecta were first reported in the journal Science in 1986.
Location: S 32° 1' E 135° 27'
- http://earthobservatory.nasa.gov/IOTD/view.php?id=42813
- http://www.unb.ca/passc/ImpactDatabase/images/acraman.htm
- http://www.redorbit.com/images/images-of-the-day/img/28440/acraman_impact_structure_south_australia/index.html?source=r_earthiod
- http://geology.geoscienceworld.org/cgi/content/abstract/31/5/459
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