|Aeromagnetic image of Setlagole-Madibogo meteorite impact site showing the circular ring structure and cross-cutting dykes|
A spectacular megabreccia (a coarse rock assemblage composed of large angular-to-rounded fragments, some over 6m in length, held together by a mineral cement – in this particular case by melted rock in the form of fine crystalline glassy material) in the Kraaipan granite-greenstone terrane, located roughly midway between Mafikeng and Vryburg, has provided the first clues to the recognition of a new meteorite impact locality. The discovery adds a sixth impact site to the list previously recorded in southern Africa and is exceeded in size only by the Vredefort and Morokweng impact structures.
Geological mapping in the Archaean granite-greenstone terrane of the North West province has revealed the megabreccia which crops out sporadically in an otherwise poorly exposed part of the west-central region of the Kaapvaal Craton. Younger Kalahari sand and calcrete blankets much of the Archaean basement, which consists of remnants of the Kraaipan greenstone terrane and a variety of granitoid rocks.
The Kraaipan granite-greenstone basement in the Setlagole-Madibogo area of the North West province acted as a target environment for a meteorite impact event. The megabreccia, formed by an impacting bolide (an exploding or exploded meteor or meteorite), contains countless rock fragments and microscopic particles of the Archaean basement. The 300km diameter Vredefort Structure, 240km to the east, is known to be about 2020 million years old, whereas the 70-80km diameter of the Morokweng impact structure, about 135km to the west, has been dated at approximately 145 million years.
The exact dimensions of the Setlagole impact structure have not yet been accurately determined, but preliminary estimates put the diameter at about 25km. When exactly the event occurred also still remains to be determined. What is known at this stage is that the ring structure, defined aeromagnetically (a magnetic survey of the earths surface carried out with an airborne magnetometer) by Dr Edgar Stettler, is cut by one or possibly two dyke events. One of the dykes, which is not exposed is possibly of Karoo age. This may suggest that the impact structure is at least older than Karoo magmatism dated at about 180 million years. It is also older than the Morokweng impact structure.
There are other linear features transecting the Setlagole ring structure, several in the northwest and north and another in the southeast. These may represent either faults or dykes that are strongly remanently magnetized. It remains to be determined if some dykes may represent feeders to Ventersdorp volcanism present in the region. If they are linked with the Ventersdorp event, the impact structure could have a Neoarchaean age (about 2700 million years) making it possibly the oldest known impact structure on Earth.
The way forward
|Dyke of fine-grained presumably impact ‘melt rock’, intruded into the Setlagole Megabreccia|
It has taken a wide range of specialist Earth scientists many decades to unravel the history and evolution of impact structures present around the world, including the Vredefort structure. Even here there is not always consensus of views and ongoing debates still persist on various issues. The Setlagole impact structure will doubtless also engender divergent views and opinions as time passes. Work still in progress and being planned is aimed at diminishing the speculative aspect of the issues relating to the impact structure and a number of collaborative studies have been initiated.
Dr Edgar Stettler, one of the joint discoverers of the structure employing aeromagnetic techniques, has made available the geophysical data set to Prof Gordon Cooper (Geophysics, Wits University) for the enhancement and analysis of magnetic signatures using filtering techniques involving fractional derivative and circular-shaded relief algorithms. Sue Webb (Geophysics, Wits University) is committed to undertaking a gravity survey that may eventually define more accurately the true width of the structure and other internal features.
The relative age of the structure, the intruding dykes and the metamorphic overprinting is also receiving attention together with details of geochemistry and petrology of the megabreccia and associated basement granitoids. Finally, the potential for mineralisation associated with the impact warrant consideration particularly in the light of Ni-Cu (nickel-copper) and PGMs (platinum group metals) mineral deposits being directly linked to impact structures such as the famous Sudbury Structure in Canada.
Carl Anhaeusser is Professor Emeritus in the Economic Geology Research Institute in the Wits School of Geosciences. The text is an edited version of an article authored by Prof. Anhaeusser, which was first published in June 2007 in â€˜Geobulletinâ€™ Volume 50, No 2, a quarterly publication of the Geological Society of South Africa.
Prof. Anhaeusser will be delivering a Geotalk in the Geology Department on Thursday, 18 October at 16:30 in Room 101, 1st Floor Geosciences Building, East Campus on this topic. All welcome.