THE PETROLEUM POTENTIAL OF AUSTRALIAN PHANEROZOIC IMPACT STRUCTURES

Abstract

This paper examines proven, probable, possible and speculative impact structures in Australian Phanerozoic strata and their petroleum potential. There are two classes of crater: simple and complex. The former usually assumes a bowl shaped depression with a raised and overturned rim with a diameter rarely more than three kilometres, with complex structures generally occurring above diameters of two kilometres in sedimentary rocks and four kilometres in crystalline rocks. Complex craters are characterised by a central uplifted area and a classic 'sombrero' structure and can be very large and have diameters of over 800 km. Criteria for the identification of terrestrial impact structures include: (a) circular plan; (b) faulted rim structure; (c) flat floor with central uplift (may not always be present) or interior ring(s); (d) negative gravity anomaly; (e) magnetic low with subdued magnetic relief; (f) brecciated crater fill; (g) low seismic velocities in the crater fill; (h) shock metamorphism (coesite, multiple sets of planar shock lamellae in quartz grains, shatter cones); (i) meteoric material; (j) presence of melt rock; (k) distal ejecta; and occurrence of an annular trough between the central uplift and the outer faulted rim.

Proven impact features, like Gosses Bluff and Mt Toondina, are well exposed, contain indisputable evidence of shock metamorphism, and have had extensive geophysical surveys conducted over them: these structures provide models to interpret completely buried structures. Subsurface impact structures have been detected in areas where there has been intensive seismic surveying in the search for hydrocarbons. The Tookoonooka, Talundilly and Mulkarra structures all occur in the Cooper-Eromanga Basin, an area of high intensity exploration. The best known wholly subsurface impact feature is the 66 km diameter Tookoonooka Structure in southwestern Queensland, which exhibits several of the accepted criteria for an impact origin, including shock metamorphism, and is classified as a probable complex type impact structure. The Talundilly Structure, a possible impact feature, lies 300 km to the northeast of the Tookoonooka Structure and is of the same general age. The two structures could reflect the impact of fragments of the same bolide. The Yallallie Structure lies in a moderately explored hydrocarbon province in the central Perth Basin. It has a classic 'sombrero' shape in section view with a central uplift and evidence of shock metamorphism. Yallallie is a probable complex impact structure. The Mulkarra Structure, located in northeastern South Australia, has been classified as a simple type of impact crater lacking a central uplift, but recent geophysical work indicates a probable complex impact origin. Other possible and speculative impact related features described here owe their recognition to good quality seismic surveying.

'there are yet some geologists who, adhering to Lyellian dogma, devoutly refuse to accept that large objects fall out of the sky.' (Shoemaker, 1997).