Western North Atlantic Upper Jurassic Calcareous Nannofossil Biostratigraphy

Abstract

Stratigraphic ranges of key Upper Jurassic and lowermost Cretaceous nannofossil species are revised on the basis of a comprehensive review of previously published biostratigraphic studies. Both the known ranges and the highest and lowest possible occurrences of key taxa, on the basis of an assessment of the paleo-ecological and preservational phenomena affecting Upper Jurassic and lowermost Cretaceous nannofloral composition, were considered in preparation of the proposed biostratigraphic revision. The identification of several Upper Jurassic evolutionary lineages and the synonymy of Upper Jurassic and Lower Cretaceous forms indicate that several previously recognized biostratigraphic datum planes are incorrect or unreliable as age indicators. Stage assignment for Upper Jurassic sediments from DSDP Leg 11, Site 105, are revised in light of the proposed biostratigraphic revision. During DSDP Leg 44 well-preserved Upper Jurassic sediments containing a diverse nannofossil assemblage were recovered at Hole 391C. The oldest samples recovered are lower (?) Kimmeridgian. The Jurassic/Cretaceous boundary is placed within an extensive interval of sediments containing poorly preserved nannofossils, at a point between the highest occurrence of Hexalithus noelae and the lowest occurrence of Lithraphidites camiolensis. Microstaurus quadratus Black is transferred to Cretarhabdus.