Monday, 9 January 2012

Possible impact crater in Palm Valley, Northern Territory, Australia.

Much of Central Australia is covered by ancient deserts, that have been continuously exposed since at least the Mesozoic, and in places considerably longer (exposed implying exposed to the air, not covered by sea; rocks under these conditions may be eroded, but will not be covered by new deposits to any extent). As such Central Australia presents an excellent area in which to look for ancient impact craters; to date five such craters have been detected, at Gosse's Bluff, Henbury, Boxhole, Kelly West and Amelia Creek.

On 5 January 2012 a team lead by Duane Hamacher of the Department of Indigenous Studies and the Research Centre for Astronomy, Astrophysics and Astrophotonics at Macquarie University published a paper on the arXiv online database at Cornell University Library, in which they discuss the possibility that a semi-circular depression in Palm Valley, Central Australia may be an impact crater.

The putative Palm Valley crater. Image originally from Google Maps (2009) and used in Hamacher et al. (2012).

The depression is approximately 280 m across, and is set into the northern rim of an ancient river valley. The valley is thought to have been cut by an ancient river which has flowed in the area intermittently for at least 300 million years, and possibly as much as 400 million years. Circular depressions can be caused in three ways, they may be volcanic (which can be ruled out in this case), they may be erosional, or they may be impact craters.

It is possible that this structure, since it is set in the side of a river valley, may be part of an older river system, the remains of a sharp bend or ox-bow lake on a meandering river. Hamacher et al. attempted to use gravimetric data, which can be used to determine rock density (rocks beneath impact craters tend to be compressed, and therefore more dense than the surrounding rocks), but their results were inconclusive.

The gold standard methods of determining impact craters are deemed to be detecting the presence of meteor fragments, discovering chemical traces of meteoric material, finding glasses and minerals formed at high pressures and temperatures, and finding 'shock structures' in quartz. However these are not easily applied to a structure as small as the Palm Valley depression, since such a small crater is likely to have been formed by a small meteor exploding in the air, and unlikely to have released enough energy to melt rocks and form new minerals, nor to cause shock structures in quartz. Hamacher et al. did find some quartz grains at the site that appeared to show shock structures, but they were set in an unshocked matrix, implying that they had been shocked before they became part of the rock.

Quartz crystals showing what appear to be shock deformations, from the Palm Valley depression. The deformations do not extend into the matrix (rock around the crystals) suggesting that they existed before the crystals were trapped in the matrix. Hamacher et al. do not state what sort of rock the crystals were found in, nor what the matrix is, but it is presumably of either marine or volcanic origin, therefore predating the formation of Palm Valley by some time.

In short, Hamacher et al. were unable to say that the Palm Valley structure definitely is or isn't an impact structure, but consider that the shape of the structure suggests an impact origin is more likely than an erosional one. They welcome further research into the site by other groups, and in particular ideas on how to better determine small impact craters from erosional structures.

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