PRELIMINARY TILL PROVENANCE STUDY IN THE ROBERTSON ISLAND/CAPE MARSH REGION, EAST COAST ANTARCTIC PENINSULA

The Antarctic Peninsula is undergoing greater warming than almost anywhere else on earth. This accelerated regional warming has caused glacier ice to thin rapidly. Gilbert and others (2003) propose that the large Greenpeace Trough was scoured subglacially during the Last Glacial Maximum as an ice stream flowed northeastward across the inner shelf. As the ice sheet thinned, probably during the late Pleistocene or early Holocene, the high Seal Nunataks area west of Robertson Island became a barrier that redirected flow from the Antarctic Peninsula toward the southeast across the shelf and the Greenpeace Trough. The Seal Nunataks separated the thinning ice sheet into the partially floating Larsen A and B Ice Shelves in the Weddell Sea. Till lithologies should be a useful tool to determine if this flow-pattern hypothesis is correct.

Bedrock lithologies vary in the region. Upper Cretaceous, thinly bedded siltstones with concretionary horizons exist at Cape Marsh on Robertson Island. To the west, the Seal Nunataks contain late Cenozoic olivine-rich basalt. Farther west, rocks of the eastern Antarctic Peninsula contain granitoid intrusive rocks and middle Jurassic to lower Cretaceous basalt-andesite-rhyolite assemblages with abundant pyroclastic rocks. Upper Paleozoic to lower Mesozoic marine siltstone and shale are located on the eastern side of the peninsula and also within the Greenpeace Trough. If Gilbert and others (2003) are correct, older till lithologies should be locally derived (basalt and siltstone), and younger till units should reveal more distal lithologies (andesite, rhyolite, and granitoids).

Glacial till samples were collected from the eastern edge of the Robertson Island Ice Cap at Cape Marsh. Sieving was performed to obtain the greater-than-63 micron sand and gravel fractions. In order to determine till provenance, the lead author will perform petrologic analyses of the Robertson Island/Cape Marsh glacial till samples. A binocular microscope will be used to identify and count shale, feldspar, quartz, mafic and felsic igneous, and metamorphic rock grains in the sieved sand and gravel fractions. The authors predict that the youngest till on Cape Marsh should have more variable, distal lithologies than older till. Preliminary results will help determine if further study is warranted.