Geophysical evaluation of the enigmatic Bedout basement high, offshore northwestern Australia

摘要

The Bedout High in the Roebuck Basin (formerly offshore Canning Basin) on the northwest shelf of Australia is an unusual structure, which has been controversially interpreted as an end-Permian impact structure similar in size to the K-T boundary Chicxulub Crater. We present a geophysical perspective of the associated debate, based on deep seismic reflection, refraction and well data. The basement and crust in the Roebuck Basin display a number of features that distinguish them from other basins along the northwest Australian margin, including major crustal thinning and the presence of a thick layer of interpreted magmatic underplating. The Bedout High consists of two separate highs separated by a Paleozoic fault, and is associated with a Moho uplift of 7-8 km, and is about 40-50 km wide. The normal fault separating the two highs trends NNW-SSE, roughly paralleling a Palcozoic fault system associated with rifting in the Canning Basin and terminating below the interpreted top-Permian reflection. There are no circular, symmetric fault zones bounding the proposed annular trough, and the distinct difference in seismic character normally associated with impact breccias versus layered sediments above are not expressed in deep multichannel seismic data. The end-Permian horizon exhibits little topography, with well-layered units both below and above. The area around the Bedout High stands out as an area of low velocity basement: 5400-5600 m/s compared to 5800-6000 m/s for other nearby basement areas located in a similar depth range, but known complex impact sites are not characterized by a unique seismic basement velocity signature. Both seismic velocity analysis, revealing a thick underplated. layer in the lower crust, and thermal modelling based on data from well La Grange-1 and basalts drilled on top of the Bedout High, are consistent with rifting above anomalously hot mantle. The available geophysical and geological data are compatible with an interpretation of the Bedout structure as a basement high formed by two consecutive Paleozoic and Mesozoic episodes of rifting roughly orthogonal to each other, associated with basin formation east and west of the Bedout High, but fail nearly all unequivocal criteria for impact crater recognition. (c) 2005 Elsevier B.V. All rights reserved.