| Summary: | The Aztec Siltstone (late Devonian) crops out for 150 km along the Transantarctic Mountains, between the Mawson and Mulock Glaciers of southern Victoria Land, Antarctica. It is the uppermost formation of the Taylor Group, the lower of the two subdivisions of the Beacon Supergroup of southern Victoria Land. The formation consists largely of fine to medium-grained sandstone, and greyish red (10R 4/2), grey (N5), and greenish grey (5G 6/1) siltstone and claystone. Other lithologies include carbonaceous siltstone and claystone, limestone and intraformational conglomerate. Conchostracans, fish fossils, plant fragments, and gypsum lenses are present also. Cross-stratification, horizontal stratification, channelling, and "fining-upwards" cycles indicate deposition from shallow, high sinuosity (tortuous), meandering streams that migrated laterally across a broad alluvial plain. The sandstone beds are laterally accreted channel deposits, whereas the siltstone and claystone beds represent overbank deposition by vertical accretion in the interchannel areas of the floodbasin. Other floodbasin deposits include lacustrinal sediments from pluvial ponds and ox-bow lakes, and palustrinal sediments from the backswamps. Overbank deposition of bed load material formed levees, and stream avulsion and crevassing during flood stage produced crevasse-splay deposits. The sandstone beds are quartzarenites, with detrital grains consisting largely of plutonic quartz; other grains include chert, feldspar, metamorphic quartz and a trace of heavy minerals. Sandstone textures average fine-grained and well sorted, although sandstone with textural inversion is common. Compositional and textural characteristics indicate that the sandstone is a product of the reworking of older quartzarenite in the source area. Subaerial exposure was a feature of the fine-grained floodbasin sediments; the evidence includes the abundance of mudcracks, and a variety of soil features. The latter include extensive kankar ('calichea') horizons, pseudo- or wavey bedding ...
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