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Geophysical techniques

We have already noted that geophysical techniques have been used to establish the existence of buried impact structures. There are, of course, a number of techniques which are available for such studies. However, we feel that a discussion of such techniques is outside the scope of this book, which has largely been based on experimental and geological field studies of impact events.

Australia has, for some years, been the happy hunting ground for those studying craters. The Australian Geological Survey Organisaton (AGSO) published an important journal (Themic Issue: Australian Impact Structures, Vol. 16, 4, 1996) which was fittingly dedicated to the honour of R.Dietz (1914-95) and Eugene and Carolyn Shoemaker for their major contribution to the study of impact structures.

Figure 5.23b Clear Water Lakes N Canada (courtesy of Canadian Geological Survey).
Figure 5.24a Shatter cones. (b) Stress configuration required to produce shatter cones, provided the differential stress exceeds 20 kb (2 GPa).

Naturally, several of the papers are dedicated to the study of Australia's most famous impact structure, Gosses Bluff (Figure 5.25a). These articles deal not only with the traditional geological mapping of the structure (Milton et al., 1995a), but also with seismic, magnetic and gravity studies (Milton et al., 1995b). A section of Gosses Bluff based on seismic data is diagrammatically represented in Figure 5.25b. It can be

Figure 5.25a Oblique view of Gosses Bluff.

Figure 5.25b Section through Gosses Bluff impact structure. The ringed-uplift feature is probably the result of differential erosion (after Milton et al.).

inferred that the circular wall formed by the bluff may be an artifact caused by differential erosion, though, of course, fundamentally controlled by the mechanics of impact.

Those readers who are interested in the application of geophysical techniques to the study of impact structures are directed to the papers cited above and that of Grieve and Pilkington (1996).

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