Computer Science
Scientific paper
Mar 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998m%26ps...33..229g&link_type=abstract
Meteoritics & Planetary Science, vol. 33, no. 2, pages 229-241.
Computer Science
16
Scientific paper
During Leg 150 of the Ocean Drilling Project (ODP) two sites (903C and 904A) were cored which have sediments of the same biostratigraphic age as the upper Eocene tektite-bearing ejecta layer at Deep Sea Drilling Project (DSDP) Site 612. Core 45X from ODP Site 904A (~ 4 km north of Site 612) contains a 5-cm-thick tektite-bearing ejecta layer and Core 56 from Site 903C (~ 8 km north-northwest of Site 904) contains a 2-cm-thick layer of impact ejecta without any tektite or impact glass. Shocked quartz and feldspar grains, with multiple sets of planar deformation features, and abundant coesite-bearing grains are present at both sites. The major oxide contents, trace element compositions, and rare earth element patterns of the Site 904 tektites are similar to those of the Site 612 tektites and to North American tektites (especially bediasites). The Sr and Nd values for one composite tektite sample from Site 904 fall within the range previously obtained for the Site 612 tektites, which defines a linear trend that, if extrapolated, would intersect the values obtained for North American tektites. The water contents of eight tektite fragments from Site 904 range from 0.017 to 0.098 wt. %, and, thus, are somewhat higher than is typical for tektites. The heavy mineral assemblages of the 63 - 125 m size fractions from the ejecta layers at Sites 612, 903, and 904 are all similar. Therefore, we conclude that the ejecta layer at all three sites are from the same impact event and that the tektites at Sites 904 and 612 belong to the North American tektite strewn field. Clinopyroxene-bearing (cpx) spherules occur below, or in the lower part of, the main ejecta layer at all three sites. At all three sites the cpx spherules have been partly or completely replaced with pyrite which preserved the original crystalline textures. Site 612, 903, and 904 cpx spherules are similar to those found in the Caribbean Sea, Gulf of Mexico, central equatorial Pacific, western equatorial Pacific, and eastern Indian Ocean. The cpx event appears to have preceded the North American tektite event by 10 - 15 ka or less. The fining-upward sequence at all three sites and concentration of the denser, unmelted impact ejecta at the top of the tektite layer at Sites 612 and 904, suggest that the tektite-bearing ejecta layers are not the result of downslope redeposition and that the unmelted ejecta landed after the glass. Geographic variations in thickness of the tektite-bearing ejecta layer, the lack of carbonate clasts in the ejecta layer, and the low CaO content of the tektite glass, suggest that the ejecta (including the tektite glass) were derived from the Chesapeake Bay structure rather than from the Toms Canyon structure. A sharp decline in microfossil abundances suggests that local environmental changes caused by the impact may have had adverse effects on benthic foraminifera, radiolaria, sponges, and fish as well as the planktic foraminifera.
Blum Julie D.
Glass Billy P.
Koeberl Christian
McHugh Cecilia M. G.
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