Abstract
Abstract The Aztec Siltstone is a redbed sequence deposited on a broad, low-lying alluvial plain during Late Devonian times in southern Victoria Land, Antarctica. It outcrops between the Mawson and Mulock Glaciers and is the uppermost formation of the Taylor Group, the lower of the two Beacon Supergroup subdivisions in southern Victoria Land. The formation consists largely of fine- to medium-grained, well sorted quartzarenite, greyish red (10R 4/2), grey (N5) and greenish grey (5G 6/1) siltstone and claystone. Minor lithologies include carbonaceous siltstone, limestone, and intraformational conglomerate. Conchostracans, fish fossils, plant fragments, and gypsum lenses are also present. Fining-upwards cycles with the characteristic channelling, intraformational conglomerate, crossstratification, and horizontal stratification are common in the formation and indicate deposition from shallow, high sinuosity (tortuous), meandering streams that migrated laterally across a broad alluvial plain. Laterally accreted channel, and vertically accreted overbank sediments are represented by sandstone and the finer-grained lithologies respectively. Levee and crevasse-splay deposits are also represented. Other floodbasin deposits include lacustrine sediments from pluvial ponds and ox-bow lakes and palustrinal sediments from the backswamps. Prolonged subaerial exposure of the overbank sediments gave rise to soil development in some, characterised by a compositional and textural similarity to modern soils, calcrete (caliche), vein networks, burrowing, and root horizons. Reddening of these deposits also took place during subaerial exposure, influenced by soil processes and a hot and seasonally wet and dry (savanna) climate. The hiatus, represented by soil development and reddening, was probably in the range 4000 to 10 000 years.
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