CHARACTER OF CLASTS IN GLACIOMARINE SEDIMENTS AS AN INDICATOR OF TRANSPORT AND DEPOSITIONAL PROCESSES, WEDDELL AND LAZAREV SEAS, ANTARCTICA

The gravel component in marine sediments of the continental margin of Antarctica is almost entirely from transport by grounded ice, ice shel ves, ice tongues, and icebergs. About 2000 gravel clasts from about 40 sites were examined for roundness, Zingg shape, and Krumbein spherici ty. Surface characteristics, like faceting, striations, and other spec ific glacigenic shapes, were recorded as well as lithology. The sample s were from both shallow and deep waters along > 1500 km of the Antarc tic margin bordering the eastern Weddell Sea and Lazarev Sea. In addit ion, onshore observations were made in ice-marginal areas at Schirmach er Oasis. Few systematic differences in clast shape in modern sediment s could be detected among the various glaciological environments: vari ation in shape within a given environment is greater than that between different environments. Abrasion of clasts at the interface between g lacier and bedrock, before transport into the ocean, is less important than in temperate regions. This reflects the cold thermal regime of A ntarctic glaciers, a view confirmed by the similarity in shape of debr is from basal ice and from sandy basal tills at Schirmacher Oasis. Mor e angular debris was recovered off grounded-ice margins in Coats Land, but given the absence of supraglacial sources this debris seems to be the product of rock fracturing at a relatively dry, frozen ice bed. M ore than half, and sometimes nearly all, the clasts are faceted. Facet ing, roundness, and sphericity are independent of lithology. In contra st, striae on clasts are strongly dependent on lithology: few gneissic clasts have striae, but nearly half the clasts of fine-grained basic igneous or metavolcanic rocks are striated. Each area studied has a di stinct population of rock types of limited variety, suggesting that de position is predominantly from the nearest land-ice source, and that m ixing of sediment with that from far-traveled icebergs is unimportant.