Calcareous Nannofossils from Cores Recovered during Leg 18, Deep Sea Drilling Project: Biostratigraphy and Observations of Diagenesis

Abstract

Calcareous nannofossils were recovered at all sites drilled during DSDP Leg 18 on a traverse from Hawaii to California to Alaska. Middle-and high-latitude coccolith assemblages are characterized by sharp reductions in species diversity due to ecological restrictions imposed by cool surface water temperatures. At Site 173 (northern California), combination range zones based on silicoflagellate and calcareous nannofossil datum levels are used to subdivide portions of the section where key latitude coccolith index species are absent. New species described from middle high latitude assemblages are Discoaster mendomobensis, Coccolithus pliopelagicus, and Cyclicargolithus bukryi. Early diagenesis resulting in in situ dissolution and reprecipitation of skeletal calcite is responsible for heavy secondary calcite overgrowths on discoasters and some placoliths at Site 172. A similar phenomenon promoted early cementation of a lower Miocene coccolith ooze at Site 178. This resulted in formation of a chalk unit sufficiently well lithified to withstand compactive deformation during the subsequent loading of many hundreds of meters of Neogene clastic sediments.