Middle Eocene to Early Oligocene Foraminifers from the Izu-Bonin Forearc, Hole 786A

Abstract

Drilling at Site 786, located in the center of the Izu-Bonin forearc basin, 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. Middle Eocene benthic assemblages resembling the bathyal "Lenticulina" fauna (characterized by Osangularia mexicana, Cibicidoides eocaenus, and several buliminid species) changed to an upper Eocene abyssal "Globocassidulina subglobosa" fauna (characterized by Cibicidoides praemundulus, Globocassidulina subglobosa, Gyroidinoides girardanus, Oridorsalis umbonatus, and Siphonodosaria aculeata). Even though no large, abrupt faunal changes appear to have been associated with the assumed Eocene/Oligocene boundary, benthic species turnover continued through the late Eocene and into the early Oligocene. This resulted in a slightly lower diversity early Oligocene fauna dominated by three species: Laevidentalina sp., Bulimina jarvisi, and Gyroidinoides girardanus.The progression from a middle Eocene bathyal "Lenticulina" fauna, rather than an abyssal "Nuttallides 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.

Relationship between outer forearc subsidence and plate boundary kinematics along the Northeast Japan convergent margin

“…Tectonic subsidence of the Tonga forearc at Site 841 [ Clift and MacLeod , ] initiates 2–4 Ma prior to the onset of seafloor spreading in the South Fiji basin and prior to seafloor spreading in the Lau basin (Figure ). Tectonic subsidence of the Izu‐Bonin forearc at Site 786 [ Lagabrielle et al ., ; Milner , ] begins 5–7 Ma prior to the onset of seafloor spreading in the Shikoku Basin (Figure ), while tectonic subsidence of the Mariana at Site 460 [ Hussong and Uyeda , ] occurred during backarc spreading associated with the opening of the Parece Vela basin and Mariana trough (Figure ). These periods of rapid forearc subsidence are also associated with accelerations in the absolute trench‐orthogonal velocity of the Pacific plate (MHR), and with a local increase in plate convergence rate (Figure ).…”