Middle Eocene to early Oligocene foraminifers from the Izu-Bonin Forearc, Hole 786A

Abstract

Drilling at Site 786, penetrated an apparently continuous section of middle Eocene/lower Oligocene volcaniclastic breccias and nannofossil oozes. Planktonic foraminiferal faunas underwent a gradual transition from relatively high-diversity middle Eocene through late Eocene tropical or warm-water assemblages to a cooler-water, less diverse assemblage during the early Oligocene. In the cosmopolitan benthic foraminiferal faunas, the major transition occurred during the early late Eocene. The progression from a middle Eocene bathyal "Lenticulina' fauna, rather than an abyssal "Nuttalides truempyi' fauna, to an abyssal "Globocassidulina subglobosa' fauna during the early late Eocene, suggests that a bathymetric deepening occurred at Site 786. Increased water depths may have resulted from tectonic subsidence. -from Author