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Character of Clasts in Glaciomarine Sediments as an Indicator of Transport and Depositional Processes, Weddell and Lazarev Seas, Antarctica
Gerhard Kuhn, Martin Melles (1), Werner U. Ehrmann, Michael J. Hambrey (2), Gerhard Schmiedl
The gravel component in marine sediments on the continental margin of Antarctica is almost entirely from transport by grounded ice, ice shelves, ice tongues, and icebergs. About 2000 gravel clasts from about 40 sites were examined for roundness, Zingg shape, and Krumbein sphericity. Surface characteristics, like faceting, striations, and other specific glacigenic shapes, were recorded as well as lithology. The samples were from both shallow and deep waters along > 1500 km of the Antarctic margin bordering the eastern Weddell Sea and Lazarev Sea. In addition, onshore observations were made in ice-marginal areas at Schirmacher Oasis.
Few systematic differences in clasts shape in modern sediments could be detected among the various glaciological environments; variation in shape within a given environment is greater than that between different environments. Abrasion of clasts at the interface between glacier and bedrock, before transport into the ocean, is less important than in temperate regions. This reflects the cold thermal regime of Antarctic glaciers, a view confirmed by the similarity in shape of debris from basal ice and from sandy basal tills at Schirmacher Oasis. More 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. More than half, and sometimes nearl all, the clasts are faceted. Faceting, roundness, and sphericity are independent of lithology. In contrast, 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 distinct population of rock types of limited variety, suggesting that deposition is predominantly from the nearest land-ice source, and that mixing of sediment with that from far-traveled icebergs is unimportant.
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