PROVENANCE OF ROSS SEA TILL

The coarse sand fraction of till samples from cores retrieved beneath the Whillans Ice Stream and Ice Stream C in West Antarctica and samples collected from six locations in East Antarctica have distinct petrologic signatures. Thirty-three samples of Last Glacial Maximum (LGM) till from across the Ross Sea exhibit compositional similarities to the East and West Antarctic source terranes.

The coarse sand fraction of East Antarctic till is dominantly composed of angular intermediate to mafic lithic fragments, fine-grained quartz-rich detrital sediments, rounded quartz, rounded metamorphosed carbonate lithic fragments, angular pyroxene, and rounded feldspar grains. Adjacent western Ross Sea till exhibits compositional similarities with East Antarctica tills with the exception that several western Ross Sea samples containing higher percentages of felsic and sedimentary lithic fragments.

The coarse sand fraction of West Antarctic till is dominantly composed of rounded quartz grains, rounded feldspar grains, sedimentary lithic fragments containing primarily quartz, and felsic lithic fragments containing predominantly quartz and phyllosillicates (biotite and chlorite). Adjacent eastern Ross Sea tills exhibit compositional similarities with West Antarctic till.

The central Ross Sea till exhibits compositional similarities to both the East and West Antarctic source terranes. The coarse sand fraction includes: rounded quartz; rounded feldspar; angular felsic - mafic lithic fragments; quartz-rich detrital sediments; rounded limestone, some of which contain ooids; angular pyroxene; and extrusive lithic fragments. Although absent in the source terrane till samples, oolitic limestone has documented occurrences in the Holyoake Range, which flanks the Nimrod Glacier in East Antarctica. Expansion of the East Antarctic ice sheet into the central Ross Sea is also supported by the presence of mafic lithic fragments, which are unique to the East Antarctic tills. These qualitative observations linking Ross Sea till to source terrane samples are also supported by cluster and discriminant analysis. Therefore, we conclude from the sand petrography of Ross Sea till that both the East and West Antarctic ice sheets contributed to the expansion of LGM-age ice into the central Ross Sea.