ORBITAL FORCING OF FORAMINIFERAL BIOSTRATIGRAPHY IN UPPER OLIGOCENE-LOWER MIOCENE GLACIMARINE SEQUENCE STACKS, VICTORIA LAND BASIN, ANTARCTICA

The upper 1200 m of an Oligocene-Lw Miocene succession is subdivided into 54 unconformity-bound stratigraphic sequences. Each sequence represents deposition in a complex of nearshore glacimarine environments during margin oscillations of the East Antarctic Ice Sheet. Chronostratigraphy for three contiguous sequences spanning the Oligocene-Miocene boundary indicates durations of <100,000 years for each, with orbital forcing of glacier advance-retreat over marine water columns and associated sea floor lithofacies development. Meter-scale sampling (~1000-2000 years sample separation) for benthic foraminifera allows monitoring of foraminiferal responses to perturbations in both water column and sea floor environments. Responses are forced by subtle changes of lithofacies and levels of IRD, by bottom current sediment sorting, by trophic relationships amongst benthic foraminifera, other benthos, phyto- and zooplankton, by fluxes in biotic and abiotic-derived nutrients from surface waters, and by water mass properties including salinity variations. A lithofacies-dictated assemblage composition and species diversity pattern is repeated in each sequence. Low levels of species origination, extinction and assemblage overturn characterize contiguous sequence stacks separated by unconformities of 103 to 105 yrs duration. More pronounced assemblage overturn is apparent in sequences separated by 106yrs duration hiatuses. Biostratigraphic signatures for benthic foraminifera in multi-sequence stacks include intermittent and repetitive partial ranges. First Appearance/Last Appearance Datum (FAD/LAD) procedures in ice-proximal in-shore sequences should be applied with caution. Significant datums often coincide with sedimentary hiatuses, and because of lateral facies changes and variable sequence preservation along strike and dip in glacimarine basin margins, there is the potential for pseudo-isochroneity and -diachroneity associated with FAD, LAD, and other range data.