(Table 1) Clay mineralogy at DSDP Sites 86-576 and 86-578

Late Cretaceous and Cenozoic sediments deposited on the eastern and western sides of the Shatsky Rise show similar clay assemblages of chlorite, illite, mixed layers, smectite, kaolinite, and palygorskite, and a similar vertical differentiation into three units. The deep-sea pelagic red clay is dominated by terrigenous components, which suggests that diagenesis is minor in this part of the western North Pacific and that eolian supply is very important. The role of the Shatsky Rise as a submarine barrier is of minor importance. The volcanic contribution to the clay component appears very small, as in some other parts of the Pacific Ocean. Stratigraphic correlations between Sites 576 and 578, which can be made from mineralogical data, partly offset the rareness of biostratigraphic markers.

(Table 1) Elemental abundances in DSDP Core 86-577B-1

An Ir anomaly of 61 ng/cm**2 was found in Deep Sea Drilling Project Hole 577B at the same stratigraphic level as the Cretaceous/Tertiary boundary defined by nannoplankton. This close correspondence supports the asteroid-impact theory for the Cretaceous/Tertiary boundary extinctions.

Geochemistry of DSDP Hole 86-577B samples

THE chemical, mineralogical and isotopic characteristics of deposits at the Cretaceous/Tertiary (K/T) boundary are suggestive of a large impact event, the prime candidate (Sharpton et al., 1992, doi:10.1038/359819a0) being the Chicxulub crater in Yucatan, Mexico. Spinel-bearing spherules, which may be associated with such impacts, have been reported (Smit and Romein, 1985, doi:10.1016/0012-821X(85)90019-6) at several K/T boundary sites worldwide, but their origin is still uncertain. We have examined the spinel-bearing material recovered from K/T boundary deposits at site 577 in the Pacific Ocean (Heath et al., 1985, doi:10.2973/dsdp.proc.86.104.1985) and find two distinct populations of particles: spherules with dendritic spinel textures dispersed throughout the grains and irregularly shaped fragments with spinels essentially confined to the rim. The morphology and composition of the particles are characteristic of melted and partially melted meteoritic ablation debris, but their location is difficult to reconcile with an impact on the Yucatan peninsula, some 10,000 km away. We suggest instead that the spinel-bearing particles at site 577 are derived from the impact of a 2-km asteroid in the Pacific Ocean, and that several accretionary events of this type are required to explain the global distribution of spinel-bearing spherules at the K/T boundary.