Ages at DSDP Site 86-577

A synthesis of paleomagnetic and calcareous nannofossil stratigraphies for the sedimentary sequences recovered at Deep Sea Drilling Project (DSDP) Site 577 on the Shatsky Rise is presented. Numerical ages are estimated for a series of nannofossil datum levels from the late Maestrichtian to middle Eocene period ( about 68 to about 52 m.y. ago) and the late Cenozoic (last about 5 m.y.). Absolute age control is obtained on the basis of the revised geomagnetic polarity time scale of. The results are compared with various sets of data reported in the literature, in particular to magnetobiochronologies derived from marine sections accessible on land in Italy and from recent DSDP boreholes in the South Atlantic, and with the summary by Berggren et al. Although a number of minor discrepancies remain to be resolved, the remarkable general agreement of the data validates the basic concept of this approach to the elaboration of a calibrated geologic time scale.

(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.