References and Notes
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9. Similar metal grains are found in the boundary layer (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/Tr boundary layers, is evidence for their apparent genetic relationship as pointed out in (8). These “event-marker” magnetic grains occur only in the boundary layer and are absent in samples above and below, both in Meishan and Graphite Peak. The unique chemical composition of the metal-rich grains (e.g. condensates) suggests formation in the vapor cloud as a result of the impact event (8).
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16. The JNOC data range from very poor to moderate in quality. Most sections are adversely affected by seafloor multiples due to shallow water depth. These lines are now being reprocessed by Seismic Australia to improve the quality of the data and will be incorporated in future studies.
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26. The Bedout-1 impact melt breccia is similar to the Yucatan-6 (Fig. 4) melt breccia with cm-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 (25). 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.
27. The fossil ooid fragments and carbonate clast lack shock features but are intimately associated with the glassy (silicate) matrix, 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 (28). 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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29. We used the biotite standard GA1550, developed at the Australian National University Research School of Earth Sciences (RSES), 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 Ma (biotite) standard age was used that is good to better than 0.3% for determining the J value of irradiation. All materials were inspected under a binocular microscope prior to 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 (0.2mm thick) between the glass and outer canister. The fluence monitor biotite GA1550 (K/Ar age of 98.5 ± 0.8 Ma) was packed in the canister at regular intervals. The canister was then irradiated for 4 days in the 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 tin 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 minutes for each step. Data was 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 (36Ar/37Ar)Ca = 3.5 x 10-4, (39Ar/37Ar)Ca = 7.86 x 10-4, (40Ar/39Ar)K = 2.2 x 10-2, (38Ar/39Ar)K = 0.136, (38Ar)Cl/(39Ar)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.
30. Sample cuttings from the Lagrange-1 (Lat: 18o16’ 37.4” S; Lon: 119o18’ 0.7.2” E) well were provided by British Petroleum Company (BP) to Dr. A. Webb of Amdel Petrology Co., Australia. The results were published in the British Petroleum company report (20) and are currently available upon request from Geoscience Australia in Canberra or the Geological Survey of Western Australia (GSWA). Potassium/Argon (K/Ar) dating was performed on plagioclases handpicked by Webb from cuttings sampled in the lowermost (10,215’) section of the Lagrange-1 exploration well. This sample was described as suitable for age dating resulting in an age of 253±5 Ma.
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53. Supported by NASA grants in Exobiology and by an NSF Continental Dynamics Workshop sponsored by L. Johnson. We thank P. Cronin and E. Resiak at Geoscience Australia (GA) in Canberra for assistance with the Bedout-1 and Lagrange-1 core sampling and Anne Fleming for access to the AGSO regional seismic survey. We also thank the Geological Survey of Western Australia in Perth for hosting the NSF Workshop and R. Emms and A. Mory (GSWA) for assistance with additional core sampling and access to well reports. Special thanks goes to J. Hunt at Cornell University for assistance with the microprobe, A. Lockwood (GSWA) for the Bedout High gravity model, J. Dunlap at Australian National University (ANU) in Canberra for assistance with the argon dating, F. Tsikalas for use of the Mjolnir figure, A. Kritski and S. Smith for access to their thesis data, and G. Retallack, A. Glikson (ANU), J. Gorter and the Bedout Working Group for many helpful discussions and suggestions.