Bedout: A Possible End-Permian Impact Crater Offshore of Northwestern Australia
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L. Becker C. Nicholson R. J. Poreda American Geophysical Union (AGU) Abstract December 12 to 17 2002 OS22C-0291 (2002).
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Similar metal grains are found in the boundary layer (at the base of bed 25) at Meishan China ( 5 ). The common occurrence of Fe-Ni-Si grains at the Graphite Peak Antarctica and Meishan China P-T boundary layers is evidence for their apparent relationship as pointed out in ( 8 ). These “eventmarker” magnetic grains occur only in the boundary layer and are absent in samples above and below both at Meishan and Graphite Peak. The unique chemical composition of the metal-rich grains (for example condensates) suggests formation in the vapor cloud as a result of the impact event ( 8 ).
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Well reports La Grange-1 and Bedout-1 exploration wells [Geological Survey of Western Australia (GSWA) Perth Australia 1971 and 1983].
The Bedout-1 impact melt breccia is similar to the Yucatan-6 (Fig. 4) melt breccia with centimeter-sized clasts of fine-grained to glassy typically altered melt rock in a fine- to medium-grained melt rock matrix composed mainly of feldspars chlorite and carbonate. If the Bedout impact melt breccia reflects the compositions of the target rocks then one can assume that the upper part of the Bedout basement was dominated by more feldspar-rich rocks or basaltic volcanics ( 24 ). The difference between the Bedout-1 and Yucatan-6 impact melt breccias is that most of the clasts and the matrix in Bedout have been pervasively altered to chlorite.
The fossil ooid fragments and carbonate clast lack shock features but are intimately associated with the glassy (silicate) matrix which is consistent with an impact origin. Similar observations have been made for the Haughton Ries and Chicxulub crater breccias. These textural features may be attributed to carbonate-silicate liquid immiscibility ( 27 ). The recognition of fossil ooids in the end-Permian–aged Bedout-1 impact melt breccia suggests that sedimentary (marine) target rocks were also present at the time of impact.
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We used the biotite standard GA1550 developed at the Australian National University Research School of Earth Sciences (RSES) in Canberra Australia the in-house standard for the past 35 years that is now widely recognized as one of the best primary (meaning fundamentally calibrated) standards in the world. For the purpose of this study the 98.5 My (biotite) standard age was used which is good to better than 0.3% for determining the J-value neutron flux parameter of irradiation. All materials were inspected under a binocular microscope before irradiation. Notable brown staining discolored most of the grains and is likely due to iron oxides. The concentrates were weighed and wrapped in aluminum foil. Samples were then sealed in an outer aluminum canister. The inner packaging components consisted of a pure silica glass tube with a cadmium liner (0.2 mm thick) between the glass and outer canister. The fluence monitor biotite GA1550 (K/Ar age of 98.5 ± 0.8 My) was packed in the canister at regular intervals. The canister was then irradiated for 4 days in the Heavy Ion Fusion Accelerator Reactor (HIFAR) reactor at Lucas Heights New South Wales. The canister was inverted three times during the irradiation to reduce the neutron fluence gradient across the container. After irradiation and a cooling-off period samples and standards were repacked in aluminum foil. The biotite standard and plagioclase unknowns were loaded onto an extraction line connected to a VG 3600 gas source mass spectrometer with a resolution of ∼600. Samples were heated in a series of steps with each sample subjected to approximately 15 steps for a duration of 14 min for each step. Data were reduced using the Macintosh program Noble developed at the RSES Canberra Australia. Correction factors to account for K- Cl- and Ca-derived Ar isotopes are ( 36 Ar/ 37 Ar)Ca = 3.5 × 10 –4 ( 39 Ar/ 37 Ar)Ca = 7.86 × 10 –4 ( 40 Ar/ 39 Ar)K = 2.2 × 10 –2 ( 38 Ar/ 39 Ar)K = 0.136 and ( 38 Ar)Cl/( 39 Ar)K = 8.0. Blanks and backgrounds were generally atmospheric and/or insignificant in terms of fraction of gas analyzed. Air standards were used to determine mass fractionation which is known within about 0.3% and was assumed not to vary on the time scale of sample analysis.
Sample cuttings from the Lagrange-1 well (latitude 18°16′37.4″S longitude 119°18′0.7.2″E) were provided by the British Petroleum Company (BP) to A. Webb of Amdel Petrology Australia. The results were published in the BP company report ( 20 ) and are currently available upon request from Geoscience Australia in Canberra or the GSWA. K/Ar dating was performed on plagioclases handpicked by Webb from cuttings sampled in the lowest (10 215 feet) section of the Lagrange-1 exploration well. This sample was described as suitable for age dating resulting in an age of 253 ± 5 My.
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Like Chicxulub the refraction data show that the Moho is distorted beneath the Bedout High ( 33 34 ). The rise of the Moho however is slightly offset from the central peak suggesting that the material beneath the transient crater (∼20 km of crust) was not just simply pushed down under the crater floor as observed for Chicxulub ( 34 ). The deeper crustal structure of Bedout is less well resolved ( 32 ); thus its relation to the Bedout High and subsequent continental rifting needs further investigation.
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Supported by NASA grants in Exobiology and by an NSF Continental Dynamics Workshop sponsored by L. Johnson. We thank P. Cronin and E. Resiak for assistance with the Bedout-1 and Lagrange-1 core sampling and A. Fleming for access to the AGSO regional seismic survey. We also thank the GSWA in Perth for hosting the NSF workshop and R. Emms and A. Mory for assistance with additional core sampling and access to well reports. Special thanks go to J. Hunt for assistance with the microprobe; AINSIE and ANSTO for the neutron irradiation of the Bedout core material and funding; A. Lockwood for the Bedout High gravity model; J. Dunlap for assistance with the argon dating; F. Tsikalas for use of the Mjølnir figure; A. Kritski and S. Smith for access to their thesis data; and G. Retallack A. Glikson J. Gorter and the Bedout Working Group for many helpful discussions and suggestions.