Abstract:
Changes in ratios of detrital to biogenic sediments and in mean grain sizes of detrital grains of samples from DSDP sites 280 and 281 reflect Middle Cenozoic development of oceanic circulation on and near the southern edge of the South Tasman Rise. The South Tasman Rise, a continental block with quartz-mica schist basement, subsided in the Late Eocene during the final stages of rifting between Australia and Antarctica, producing a shallow marine connection between the southeast Indian Ocean and the southwest Pacific Ocean. The oldest sediment on the southern part of the South Tasman Rise is a nearshore shallow marine breccia, which is overlain by lower-energy, deeper-water sediments with minor amounts of silt- and and sand-size detritals, reflecting subsidence of the South Tasman Rise below wave-base. Through the late Late Eocene, detritals generally increase in abundance and coarseness, apparently in response to an increase in bottom current velocities across the rise. The current intensified, producing a disconformity spanning most of the Oligocene. In the Late Oligocene, deposition of coarse-grained, detrital-rich sediments resumed on the southern part of the rise, but throughout the Miocene a distinct reduction in detrital sedimentation occurred with a concomitant increase in biogenic sedimentation. The deep basin south of the rise received fine-grained, poorly sorted detrital sediments with little biogenic material through much of the Middle to Late Eocene. In the Latest Eocene, biogenic sedimentation increased, reflecting less restricted, oceanic conditions in the basin. Neogene unconformity probably reflects the development of the deep-ocean connection between the southeast Indian Ocean and the southwest Pacific Ocean. This caused the intense current that previously was flowing over the relatively shallow rise to migrate southward into deep water, possibly in conjunction with the onset of the Antarctic Circumpolar Current.