Micropaleontology, biostratigraphy and paleoenvironmental studies of DSDP Site 270 (late Oligocene-Quaternary) Ross Sea, Antarctica

Includes bibliographical references.Includes illustrations and maps.The Ross Sea continental shelf is a key area in understanding the Late Mesozoic-Cenozoic history of the Ross Sector of Antarctica. In the central part of this area the shelf consists of an Early Paleozoic metasedimentary basement (Unit 6), overlain by a succession of Oligocene subaerial and near-shore marine sediments (Units 3, 4, 5), and capped by a thick late Oligocene to Recent glaciomarine sequence (Units 1, 2). This succession was first described during Leg 23 of the Deep Sea Drilling Project. Benthic foraminiferal evidence indicates that there was gradual subsidence in this area of the Ross Sea reaching depths of 300-500 m by the early Miocene. Barrett (1975) designated ten lithologic subunits in the 365 m thick glaciomarine sediments of Unit 2 at Site 270. Biostratigraphic and population trends observed in the benthic foraminiferal assemblage indicate a close relationship to these lithofacies subunits. Five foraminiferal assemblage zones are proposed. These assemblage zones represent the first detailed mid-Tertiary biostratigraphy to be established in the Ross Sea. As major glaciation was initiated on the continent, gradual hydrological changes occurred in the Ross Sea, resulting in the sequential development of contrasting foraminiferal populations. These assemblages reflect a strong endemic character and correlation beyond Antarctica is difficult. Only the earliest zone, Zone I, appears to have affinities to New Zealand mid-Tertiary shallow water assemblages. The paleoenvironmental interpretation advanced herein provides evidence in support of the argument that the Ross Sea was undergoing significant bathymetric and oceanographic evolution in the late Oligocene-early Miocene. The transgression evident in Zone I could be of tectonic or glacial isostatic origin, since extensive glaciation in Antarctica began about 25 my ago (Hayes, Frakes, and others, 1975). Deteriorating climatic conditions stemming from ice build-up in the Ross Sector of Antarctica is the main influence on hydrological events controlling the microfauna of Zones II, III, and IV. Therefore, in the lowest intervals of Site 270, the faunal regime is bathymetrically controlled. In Zone II changing oceanographic conditions become prominent over increasing paleobat’nymetry. Zones II, III and IV represent paleoenvironmentally controlled assemblages. These interpretations are supported by the chemo-stratigraphy of Site 270 and parallel trends observed in oxygen isotope paleotemperature curves from the sub-antarctic. The late Oligocene-early Miocene paleoenvironmental trend is one of the gradual development of the polar marine environment, culminating in a major glacial incursion, possibly the first, into the Ross Sea.M.S. (Master of Science