Abstract:
An impact ejecta bed containing shocked quartz and diagenetically altered tektite spherules coincides exactly with biostratigraphic placement of the Cretaceous-Tertiary (K-T) boundary in three drill cores recovered from Ocean Drilling Program Site 1049 (located in the subtropical North Atlantic Ocean). Both the bracketing pelagic ooze and the ejecta bed are undisturbed at Site 1049, allowing detailed examination of the expression of the boundary event in an open ocean setting. The youngest Cretaceous sediments contain a diverse assemblage of well-preserved upper Maastrichtian Tethyan microfossils. The overlying ejecta bed varies laterally in thickness, has sharp lower and upper contacts, and contains features (e.g., presence of a foraminiferal grainstone layer at its base and large chalk clasts in its middle, and dominance of poorly sorted coarse grains throughout) that suggest it was deposited by one or several mass-flow events. The oldest Danian ooze contains abundant, tiny planktonic foraminifera characteristic of the early Danian Palpha Zone as well as common, large Cretaceous individuals. The lowermost Danian P0 Zone (assemblage dominated by Guembelitria cretacea) is apparently absent. This absence could reflect restriction of the P0 assemblage to shallower settings, slow sedimentation rates coupled with bioturbation mixing Tertiary forms into (and thus obscuring) the P0 Zone, or an interval of erosion or nondeposition. Cretaceous species decline and last occur in the first several meters of section above the ejecta bed. This pattern could be interpreted as evidence for gradual extinction above the impact bed, but thin-section observations, relative abundance counts, size-distribution analyses, and comparison with species extinctions at other K-T sections demonstrate sudden extinction, nearly all post-K-T occurrences of Cretaceous planktonic foraminiferal species being explained as the result of sediment reworking.