Harrow and Hillingdon Geological Society
Dr. Kieren Torres Howard
Department of Mineralogy
The Natural History Museum
Investigations in the Australasian tektite strewn field and new insights from Darwin Crater
Hypervelocity meteorite impacts result in the formation of impact craters and are now recognized as important agents of geologic change that have shaped the surface of Earth and lives of its inhabitants since the beginning of time. On Earth impact craters fall into two main morphological types - simple and complex. Simple craters are of the classical bowl shape. As impact energy (controlled by size of impactor) increases, the diameter of the resulting crater increases and the crater morphology changes. With increasing diameter, steepening of the inner walls leads to slumping and in response to the compressive impact pressures the crater floor rebounds producing terracing of the crater rims and raised central peaks (For an excellent review see French 2004; Traces of Catastrophe http://www.lpi.usra.edu/publications).
During the excavation of impact craters rocks may be brecciated, shocked, melted and completely vaporized (along with the meteorite) by pressures that may exceed 50GPa and temperatures >2000 °c. This produces a range of crater-fill lithologies including lithic and melt bearing (suevite) breccias and impact melt rocks that are described with examples from the 230m Darwin Crater drill core (Howard & Haines 2007; EPSL 260) and elsewhere. Darwin Crater is a buried simple crater located in a dense rain forested valley in western Tasmania and is the source of Darwin glass (Howard 2008; Meteorit. & Planet. Sci 43).
In some impacts melt ejection appears to be extremely efficient resulting in production of widely dispersed impact melt. Tektites are the best example of such melt and exist in 4 or 5 strewn fields that may extend many thousands of kilometers. The origin of tektites that Henry Faul famously remarked are probably the most frustrating stones ever found on earth (Faul 1966; Science 152) - remains poorly constrained. The most studied tektite field is the Australasian strewn field that stretches from on-land in south China to the Antarctic highlands. Despite being the youngest strewn field (ca. 800ka), no source crater has been identified in what has remained a significant scientific mystery. In a series of field seasons in NE Thailand we explored the distribution of tektites and placed constraints on the impact location by documentation and paleo-magnetic dating of flood deposits contemporaneous to the time of impact (Haines et al. 2004; EPSL 225). Containing a mixture of scorched and variably burnt fossil trees along with pristine tektites, these flood deposits may be a rare insight into the dynamic range of impact induced environmental effects.
At Darwin Crater, studies of the glass distribution reveal processes involved in impact glass and tektite formation (Howard 2009; Meteorit. & Planet. Sci. 44). Work at this crater also provides new insights into the role of volatiles in the impact process. We demonstrate that the presence of a surface swamp at the time of impact produced a volatile-charged target stratigraphy and increased the magnitude of the impact explosion resulting in wide dispersion of melt, with implications for tektite origins. Recently in Darwin glass unique inclusions have been discovered. These are revealing the effects of impact processing of organic carbon during impact, with implications for astrobiology. These inclusions also contain the first co-genetic crystalline minerals to be described in an ejected impact glass or tektite.