Studies of drill core rock samples up to 2 km below the ground in north-eastern South Australia, Queensland and the Northern Territory suggest the largest asteroid impact zone yet found (Glikson et al. 2015).
The impact zone was found in the ~400,000 km2 Warburton Basin (that has East and West sub-basins) in two parts, each 200 km wide, potentially forming a total 400 km diameter impact area. The asteroid that created them may have been as large as 10 km in diameter, before splitting into two and then striking the Earth.
The evidence of the impact was discovered during drilling as part of geothermal research, in the Warburton Basin. The impact zones seem to extend down through the Earth’s crust, to the mantle. The surrounding rocks are between 300 and 450 million years old, suggesting a similar age for the impact event.Quartz grains have been found that have layers that are curved or bent which can be caused by (normal) tectonic deformation or by crustal rebound due to an impact. Such shocked quartz is considered a tell-tale sign of an asteroid impact. Both the Warburton East and Warburton West Basins also have large magnetic bodies at depths of 6-10 km, and gravity anomalies. Seismic studies suggest fracturing of the crust to 20 km depths or more.
One of the suggested scenarios that could explain the geology is a large asteroid impact that has caused central uplift in the area, causing the removal of Devonian (419-359 million years ago or 419-359 Ma) and Carboniferous (359-299 Ma) strata. Around this time period Australia, South America, Africa, India and Antarctica were joined into the Southern Hemisphere continent of Gondwana.
Part of the interest in the Warburton Basin findings is the relationship between large asteroid impacts and mass extinctions on Earth. When large (km-sized) asteroids hit Earth, usually at very high velocity, dust and debris rain down, disrupting the climate and causing extinction on a global, rather than local, scale.
Arguably the most famous impact event is the K-T (Cretaceous-Tertiary boundary) event. It is typically related to the disappearance of dinosaurs on Earth. At this time all large vertebrates on Earth suddenly became extinct about 65 Ma, the end of the Cretaceous Period. As well most plankton became extinct, and land plants were diminished. This event marks a defining moment in Earth’s history. The K-T extinctions were worldwide, affecting all the major continents and oceans. The K-T impact crater is a roughly oval geological structure called Chicxulub, deeply buried under the sediments of the Yucatán peninsula of Mexico. The structure is about 180 km across, and the igneous rock under Chicxulub contains high levels of iridium. Iridium is usually rare, yet in rocks near the boundary it is concentrated and abundant. Meteorites have a similar, high abundance in iridium.
There is evidence of mass extinctions around the inferred time of the Warburton Basin impact. One of the five major extinctions events that have affected Earth’s biota occurred ~360 Ma. Three quarters of all species died out in this Late Devonian mass extinction, though past research suggests a series of extinctions over several million years, with two main periods, rather than a single event.
In particular the Kellwasser Event (about 375–360 Ma) marks when 50% of all genera went extinct. Major victims included ammonites, brachiopods, corals, jawless fishes, sponges and trilobites. The primary Devonian reef-builders (tabulate corals and stromatoporoids) never truly recovered from the extinctions and the changes in reef ecology were profound for at least 100 million years.
The recent results of the Warburton Basin suggest more study and further exploration is required. “When we know more about the age of the impact, then we will know whether it correlates with one of the large mass extinctions”, says research team leader Dr Andrew Glikson.
For more information, see
- World’s third largest asteroid impact zone found in South Australia, The Conversation, 15 February 2015
- World’s largest asteroid impact zone found in central Australia, scientist says, The Age, 24 March 2015
- World’s largest asteroid impacts found in central Australia, ANU Newsroom, 23 March 2015
- Geophysical anomalies and quartz deformation of the Warburton West structure, central Australia, Glikson et al. (2015), Tectonophysics, 643, p55
- Giant Asteroid Impact Basin Found in Australia, Sci-News.com, 23 March 2015
- The K-T Extinction, Richard Cowen
- Late Devonian Mass Extinctions, Devonian Times