| Summary: | A petrographic study of the Sirius Group from two locations in the Antarctic Dry Valleys shows distinct differences in both the detrital and diagenetic components of the sediments. The predominantly glaciofluvial depositional environment at Table Mtn. produced a sediment with less matrix and better sorting than did the subglacial depositional environment at Mt. Feather. This set the stage for different diagenetic events to occur because burial histories at both locations are similar. Diagenesis in Sirius Group sediments consists of major modification to primary porosity, dissolution of mostly feldspar framework grains, and precipitation of authigenic chabazite and calcite. Dissolution porosity and authigenic cement dominate at Table Mtn., whereas the development of primary porosity and matrix fabric dominate at Mt. Feather. A working model suggests that at Mt. Feather the pores and matrix fabric developed under wet, periglacial conditions through repeated cycles of freezing and thawing or wetting and drying. This is inferred to have occurred when the diamictite was soft and moist during a period when the climate was warmer than present and when water was more abundant. At Table Mtn. dissolution porosity and precipitation of the authigenic cements, calcite and chabazite, occurred under permanently frozen conditions. This is possible because ionic migration may occur in frozen sediments along interfacial films of brine. These highly saline and alkaline films provide the chemical conditions under which calcite and chabazite can precipitate in dry, subzero conditions.
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