Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.528k&link_type=abstract
Meteoritics, vol. 30, no. 5, page 528
Mathematics
Logic
3
Breccia, Exmore, Chesapeake Bay Crater, Impact Craters, Tektites, North American
Scientific paper
Poag et al. [1] identified a late Eocene boulder bed in drill cores from southeast Virginia, and interpreted it as an impact-generated tsunami deposit. Seismic studies and other geophysical evidence indicated the existence of a possible impact structure centered at Chesapeake Bay (37 degrees x 15' N and 76 degrees x 04' W), which may be 85-90 km in diameter [2]. Four drill cores have penetrated into the breccia, although none is available from the center of the structure, or reaches basement. A central peak-ring of crystalline rocks with about 25 km diameter is surrounded by a 30 km-wide annular trough and terrace terrane. The trough is filled with polymictic breccia composed mainly of autochthonous sedimentary clasts in a sandy matrix with some angular clasts of granitic and metasedimentary basement rocks [2]. The Chesapeake Bay crater is of special interest, because it is close to the region identified as the possible source region for the North American tektites, is of about the expected size, and has an age identical to that of the tektites [3]. While the source craters for the Central European and Ivory Coast tektite strewn fields are known, the source crater of the North American tektites has remained elusive. A variety of locations were suggested, including Popigai (Siberia), Wanapitei (Canada), Mistastin (Canada), and Bee Bluff (Texas), but all were later discounted. The distribution of the tektites and microtektites in the strewn field suggests that the North American tektite source crater is likely to be located at or near the eastern coast of the North American continent, maybe underwater [4,5]. The location of the Chesapeake Bay structure is in agreement with the area suggested before [4,5]. We have started a petrological and geochemical study of target rocks and breccias from the Chesapeake Bay structure. We analyzed the major and trace element composition of 17 mainly sedimentary samples, for comparison with North American tektite values. 14 of these samples, from 368 to 423 m depth, are from the Exmore core, which was drilled at the outer limit of the crater. The samples consist mainly of various siltstones, sandstones, with some shale and graywacke. The silica content of the samples studied ranged between 32 and 73 wt%. Some samples are very rich in carbonates. Recalculating the high-silica samples (some sandstones) to a carbonate- and water-free composition yields results that are similar to, but not identical with, those of the more silica-poor North American tektites. Petrographical thin section studies were done on about 40 sediment, metasediment, and crystalline rock samples from the Exmore and Kiptopeke cores. The samples included some crystalline rock clasts and millimeter-sized loose particulates. We found abundant evidence of shock metamorphic effects in numerous samples from the Exmore core, at depths from 372 to 414 m. Shock metamorphic effects were recorded as planar deformation features (PDFs) in quartz, K-feldspar, and plagioclase. In quartz and K- feldspar, up to three intersecting sets of PDFs with characteristic crystallographic orientations were found. A histogram of PDF orientations showed predominantly {10bar13} (omega), {10bar12} (pi), and {22bar41} orientations, with only very few unindexed planes. The PDF orientations suggest a peak pressure >16 GPa. Shock effects were recognized only in crystalline rock fragments or clasts, but not in sedimentary material. In addition, numerous clasts show evidence of partial or complete melting and annealing, and some small impact melt rock inclusions (probably part of impact melt breccias) were recognized. Our findings provide confirming evidence that the Chesapeake Bay structure is indeed an impact structure. Acknowledgments: Supported by FWF Project P-8794-GEO. References: [1] Poag C. W. et al. (1992) Geology, 20, 771-774. [2] Poag C. W. et al. (1994) Geology, 22, 691-694. [3] Poag C. W. and Aubry M.-P. (1995) Palaios, 10, 16-43. [4] Koeberl C. (1989) Proc. LPSC 19th, 745-751. [5] Glass B. P. and Wu J. (1993) Geology, 21, 435-438.
Brandt Daniel
Koeberl Christian
Poag Wylie C.
Reimold Wolf Uwe
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