Synchroneity of the KT oceanic mass extinction and meteorite impact : Blake Nose , western North Atlantic

A 10-cm-thick layer of green spherules occurs precisely at the biostratigraphic boundary between the Cretaceous and Paleogene (K-T boundary) at Ocean Drilling Program Site 1049 (lat 30°08′N, long 76°06′W). The spherulitic layer contains abundant rock fragments (chalk, limestone, dolomite, chert, mica books, and schist) as well as shocked quartz, abundant large Cretaceous planktic foraminifera, and rounded clasts of clay as long as 4 mm interpreted as altered tektite glass probably derived from the Chicxulub impact structure. Most of the Cretaceous foraminifera present above the spherule layer are not survivors since small specimens are conspicuously rare compared to large individuals. Instead, the Cretaceous taxa in Paleocene sediments are thought to be reworked. The first Paleocene planktic foraminifera and calcareous nannofossil species are recorded immediately above the spherule bed, the upper part of which contains an iridium anomaly. Hence, deposition of the impact ejecta exactly coincided with the biostratigraphic K-T boundary and demonstrates that the impact event was synchronous with the evolutionary turnover in the oceans. These results are consistent with a reanalysis of the biostratigraphy of the K-T boundary stratotype, which argues that shallow-marine K-T boundary sections are not biostratigraphically more complete than deep-sea K-T boundary sites. foraminifer similar to Woodringina. Parvularugoglobigerina eugubinaranges 83 cm above the top of the spherule layer (defining Biozone P α). Planktic foraminifer Biozone P0 (defined by the absence of P. eugubina, e.g., Smit, 1982) was not recognized (except for the spherule bed), which raises the possibility of a hiatus between the spherule layer and the overlying ooze. Like the record of percentage coarse fraction, the abundance of Cretaceous planktic foraminifera and calcareous nannofossils decreases rapidly above the limonitic layer (Fig. 2,A and C). Of the 41 Cretaceous planktic foraminifera identified at the top of the Maastrichtian, 38 are present within 1 cm above the boundary, and 29 of these range at l ast 40 cm above the boundary. Planktic foraminifera typical of the Cretaceous are present as rare specimens into Eocene sediments, but they become extremely scarce more than 70 cm above the spherule layer. Likewise, of the 50 Cretaceous calcareous nannofossil species identified at the op of the Maastrichtian, 31 are present within 1 cm above the boundary, and 9 range at least 40 cm above the boundary. A rapid decrease in the abundance of survivor species ( Thoracosphaera spp.,Baarudosphaera bigelowii, Placozygus sigmoides, Markalius inversus, Octolithus multiplus, and Goniolithus fluckigeri ) occurs within 1 cm above the boundary. We believe nearly all the Cretaceous taxa found in the Paleocene are reworked from Maastrichtian sediments (see below). A very similar pattern of widespread occurrence of Cretaceous species in Paleocene sediments has been reported by Pospichal (1994) and Olsson and Liu (1993) from El Kef and by Huber (1996) at high-latitude sites, who interpreted them as reworked specimens. Likewise, in a review of K-T boundary sites, Gartner (1996) reported a pattern of calcareous nannofossil survivorship similar to that at Site 1049. At Site 1049, typical Cretaceous fossil assemblages are intermixed with Paleocene assemblages that include abundant calcareous dinoflagellate cysts ( Thoracosphaera ) and blooms of nannofossils in the genera Braarudosphaera, Futyania, Neobiscutum , and Prinsius. DISCUSSION Impact Origin of the Spherule Bed The clast composition within the spherule bed suggests that the grains were derived from metamorphic basement or a continental source area associated with a carbonate platform. Similar suites of exotic grains have been reported from other K-T boundary sections from North Amer420 GEOLOGY, May 1999 S ph er ul e B ed W ith C ha lk C la st s Ir -R ic h, S ilt y O oz e F or am . N an no . O oz e F or am . N an no . O oz e A bt . m ay ar oe ns is P α C P 1a C C 26 b Lit ho log y