Miocene Strata in CRP-1, Cape Roberts Project, Antarctica

Based on the alternation of diamicts with other clastic sedimentary facies, the predominantly lithified Miocene section of CRP-1 has been divided into lithostratigraphic Units 5 to 7, with Unit 5 further divided into 8 subunits and Unit 6 into 3. Petrological investigations of extraformational clasts indicate provenance from probable Cambro-Ordovician granites, metamorphic rocks (Koettlitz Group?), Ferrar dolerite and rhyolites of uncertain affinity. The sand fraction is dominated by grains derived from crystalline basement and the Beacon Supergroup, but volcanic glass, abundant above 62 mbsf, is correlated with the McMurdo Volcanic Group. X-ray diffraction analysis of mud samples shows a similar change in mineralogy over the interval 60-65 mbsf from smectite-dominated above to illite and chlorite-dominated below. The Miocene section is dated by diatom biostratigraphy at 17.5-22.4 Ma (early Miocene), which suggests an average sediment accumulation rate of about 21 m/my. The most prevalent fossil groups are diatoms, foraminifers and palynomorphs. The well-preserved marine palynomorph assemblage contains many new acritarchs, and apparently represents the first known palynomorph record from in situ lower-Miocene Antarctic sediments. Calcareous nannofossils (Thoracosphaera) are rare to few, as are macrofossils (serpulid worm tubes, echinoid spines, bryozoans, and scallop shells). Palaeoenvironmental interpretations suggest a glacimarine setting warmer than present that underwent some significant fluctuations in water depth and proximity to the ice margin. A magnetic-polarity stratigraphy has been measured for the lower c. 90 m. Four magnetozones defined below 89 mbsf are correlated within the range of Chrons C5Dr to C6 of the Magnetic Polarity Time Scale. The Miocene section has also been divided into six recurrent lithofacies: diamictite, rhythmically interlaminated finegrained sandstone and siltstone, well-stratified sandstone, poorly-stratified muddy sandstone, coarse-grained siltstone and fine-grained siltstone. No major changes in facies assemblage have been identified, implying similar variations in environmental conditions throughout. Last, the succession has been divided by sequence stratigraphic analysis into 10 units, wherein each sequence passes upward from a basal diamict into progressively finer-grained facies, interpreted as a flooding event. Each sequence is thought to record the cyclical advance and retreat of grounded ice across the site, although there is little or no record of glacial advance. The only positive evidence for sediment accumulation under grounded ice has been found in a directional clast fabric in a diamictite at 62.64 mbsf.