Australia: The Land Where Time Began

A biography of the Australian continent 

Bedout High (Bedout Rise, Bedout Structure) see Marusek Hypothesis

Situated off the northwestern margin of the Australian continent, the Bedout High has been suggested to be an impact structure, and one that is being seen by some as a prime candidate for an end of Permian structure. Drill cores taken from the top of the structure have found melt rocks and impact breccias, as well as data from seismic and gravity studies, that are consistent with the presence of a buried impact crater. Silica glass, SIO2, that was almost pure, and plagioclases that were fractured and shock-melted, and spherulitic glass, have been found in the breccia. The presence of a melt sheet is indicated by the distribution over hundreds of metres of drill core of glass and shocked minerals. The Bedout High is suggested by the available gravity and seismic data to be the central uplift of a crater of similar size to that of Chicxulub. An Ar/Ar age of 250.1 ± 4.5 My has been determined for plagioclases separate from the Lagrange-1 exploration well. In Permian-Triassic boundary sediments worldwide, the reported presence of impact debris can be accounted for by the location, size and age of the Bedout Crater (Becker et al., Source 1).

Evidence has been presented by the authors, geochemical, biological and petrological, that genetically links the Bedout Structure to the worldwide impact deposits of end-Permian age (Fig.1, Source 1). According to the authors, the Bedout High is an example of old impact structures that have not retained the normal characteristics of younger impact craters such as Chicxulub (Hildebrand et al., 1991), the Bedout High being suggested to be a possible impact structure by the recognition of impact breccia. Only the upper portion of the Bedout High has been sampled, there ~22 m of intact drill core from Bedout-1 and 391 m of cuttings from Lagrange-1of the impact melt breccia, most of the material having been highly shocked. The shock pressures of 35-45 GPa recorded in the core from Bedout were high enough to produce maskelynite (French, 1998), and >45-65 GPa, to produce silica glass. The pressures are too high to preserve PDFs in quartz (<35 GPa), though high enough to form stishovite (15-40 GPa) and possibly hexagonal diamond (70-140 GPa) (Stöffler, 1972).

The authors suggest that the evidence from the Bedout High adds to the growing evidence for a link between impacts and flood basalt eruptions, based on the impact event being coincident, within the age uncertainty, with severe flood basalt volcanism. They also suggest that the question is raised of the relationship of such catastrophes with each other, as well as with mass extinction events (Richow et al., 2002). Speculation has been increasing that the impacts of large bolides have been responsible for processes such as eruptions of continental flood basalt and mantle plumes (Jones et al., 2002; Glickson, 1999).

Present models suggest that volcanic eruptions may be induced  by a pre-existing hot spot being struck by a bolide, though the probability of such an event is extremely remote (Melosh, 2000; Ivanov & Melosh, 2003). The craters at Chicxulub and Bedout, are opposite, not exactly antipodal (Fig.1, Source 1) to the volcanic provinces of Deccan and Siberia respectively. The amount of kinetic energy required to create the volume of the Deccan Traps has been calculated (Melosh, 2000), about 500,000 km3, about 5 x 1023 J, twice the amount of the kinetic energy generated by the Chicxulub impactor (10 km at 20 km/s). The authors suggest the impact may not be the direct cause for the volume of flood basalts, it may act as a trigger of the event. According to the authors, volcanic rocks with affinities to mantle plumes have been shown to predate the main pulse of flood basalt at both the Deccan Traps and Siberian Traps. This suggests that the catastrophic eruption of a pre-existing mantle plume might be enhanced by the impact(s).

Evidence of an impact dating to the Permian-Triassic boundary is most abundant in the Southern Hemisphere, in the continents that were part of Gondwana, such as Australia and Antarctica. This led the authors to concentrate their search for an impact crater in these continents. Gorter suggested that the Bedout ("Bedoo") High might be the central uplift of a large end-Permian impact crater, based on a single seismic line (Gorter, 1996; Gorter, 1998). The Bedout High is in the Roebuck Basin that forms the northwest margin of the Australia continent (Fig.2, Source 1). A number of studies have been carried out on the Bedout High. AGSO carried out 2 regional seismic surveys and the Japan Oil Company, the quality of the data from the Japan Oil Company surveys being from poor to moderate, the results were being reprocessed in an attempt to improve quality. On the top and flank of the Bedout High 2 exploratory wells, Bedout-1 and Lagrange 1, were drilled 9 km apart, extending to 3052 m (9986 ft) and 3273 m (10,738 ft) respectively (Fig. 2 and fig. S2, Source 1). Both wells reached a breccia dating to the Late Permian (Smith, 1999, fig. S2), after passing through about 3 km of marine and fluvial sediments, comprising carbonates, with some interbeded siltstones and mudstones, that have been dated to the Cretaceous to Tertiary, as well as sandstones interbeded with claystones, siltstones and coal, Triassic to Cretaceous. The AGSO crossed the Bedout High with 2 of their 14 seismic lines (Source 1, Fig. 2). There were also 4 wells offshore helping to identify seismic reflectors defining the structure and stratigraphy of the Bedout High, that penetrated Permian strata, 2 of which are shown in Fig. 2 (Source 1). Fluviatile and marine Keraudren sediments, from the Middle to Late Triassic, in cores and cuttings from Lagrange-1 and Bedoit-1,  show sediments that were deposited directly on top of the breccia from the Late Permian (Source 1, Fig. 4, figs. S2 & S3).

The Bedout High reaches several kilometres above the surrounding basement at present (Purcell & Purcell, 1994). The Bedout High is suggested to be underpinned by elevated middle and lower crust by deep crustal reflections and seismic refraction velocities. The core and cuttings catalogued as basalts, that have been referred to as "volcanic breccia", are from the top of the High. The "Bedout Movement" from the P/Tr boundary, is the regional volcanism associated with rifting of the Australian continental margin (Purcell & Purcell, Eds., 1994). At the top of the High there is a regional angular unconformity, that is consistent with uplift and erosion at the end-Permian, that is immediately after the Bedout High formation and termination of the Bedout Movement (Well reports Lagrange-1 & Bedout-1, 1971, 1983). The continental "Sibumasu Sliver" rifted off northeastern Gondwana, coincident with the formation of the Bedout High (Charlton, 2001). The complex crater morphology was deformed and the Bedout structure was overprinted by the tectonism, uplift, faulting and erosion that occurred in the Triassic and Jurassic as a result of the impact.

James Marusek proposed a mechanism that could link very large impacts with the Siberian Traps and Emeishan Traps, in China.

See Source 1 for more detailed information

Sources & Further reading

  1. Becker, L., R. J. Poreda, A. R. Basu, K. O. Pope, T. M. Harrison, C. Nicholson, and R. Iasky. "Bedout: A Possible End-Permian Impact Crater Offshore of Northwestern Australia." Science 304, no. 5676 (June 4, 2004 2004): 1469-76.
  2. Bedout: A Possible End-Permian Impact Crater Offshore of Northwestern Australia
Author: M. H. Monroe
Last Updated 10/05/2011 



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