ROLE OF GLACIAL POLISH IN THE FORMATION OF LOESS DEPOSITS

Large loess deposits in Earth’s deep-time past might reflect a glacial origin for this fine-grained material, wherein powerful glacial weathering processes grind rock into dust. To test this interpretation proximally (at the ice-bedrock interface), and on a microscale, we conducted quartz grain size and micro-textural analyses on samples of glacially polished granite collected from the Lake Tahoe region (glaciated during the Last Glacial Maximum), samples of its host rock (granite), and samples of Palouse glacial loess (Pleistocene). Glacial polish is a thin, smooth coating that forms at the contact between the bedrock and the glacier, which records important information about subglacial processes, but it remains poorly studied. Here we present data on grain shape and composition of glacial polish to assess whether it could play a role in the formation of the particles that form loess. In this case, quartz grain size analysis is conducted by applying image processing on back-scattered electron images (acquired on electron microprobe). We will present preliminary results and compare quartz grain size distributions found in the glacial polish to that of its underlying host rock and of typical glacial loess. If the polish forms the grains that compose loess, then the quartz grain size of the polish and its textural characteristics should approximate that of glacially-derived loess deposits; if the polish does not have similar textural characteristics to the loess, then it might not constitute a major source for the loess sediment.