The Aztec Siltstone: an Upper Devonian Alluvial Plain Red Bed Sequence, Southern Victoria Land, Antarctica

Abstract

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 (â calicheâ ) horizons, pseudo- or wavey bedding structures, a compositional and textural similarity to modern soils, vein networks (considered to be a product of deep cracking in the unconsolidated. substratum), burrowing, and root horizons with in some cases associated plant fragments. The kankar (â calicheâ ) suggests that there was a period of prolonged subaerial exposure and soil development which followed the deposition of fine-grained, fine member lithologies of the "fining upwards" cycles. This period was probably in the range 5,000 to 50,000 years. The Aztec Siltstone is a typical â variegatedâ red-bed sequence, containing interbedded red and drab fine-grained lithologies. The fine-grained drab lithologies consist of quartz grains set in a matrix of green illitic and chloritic clay. The colour in the interbedded red and grey siltstone and claystone results from a haematite pigment, which in the red samples is present in a concentration sufficient to completely mask the green colour of the clay matrix. The reddening is believed to have been a penecontemporaneous process that took place in the floodbasin sediments during their prolonged subaerial exposure under a hot and seasonally wet and dry (savanna) climate. The haematitic pigment was derived from the in situ and progressive dehydration of detrital amorphous and poorly crystalline brown hydrated ferric oxide in those sediments which maintained an oxidizing environment and were above the ground-water table during the dehydration process. Sediments which remained in a water-logged state, below the water table, and in association with organic matter, were invariably reduced and lost their iron oxide in solution. Later post-depositional reduction of some red lithologies produced reduction spheres and channels, reduced burrows and vein networks, and the reduced layers immediately underlying the scoured surface at the base of the channel sandstones. Some chemical redistribution of iron contributed to the variegated and mottled horizons of the formation. The mineral composition of adjacent red and drab lithologies is essentially the same except for the haematite constituent. The red average 5.86 (ó = 1.09) percent total Fe (as Fe2O3), of, which 3.01 (ó = 0.63) percent is as haematite pigment, 1.57 percent Fe2O3 is in a combined form (probably as ferric silicates), and 1 14 (ó = 0.57) percent is as FeO. The green average 3.65 (ó = 1.81) percent total Fe (as Fe2O3) of which approximately 0.27 percent is as haematite pigment, approximately 1.4 percent Fe2O3 is in a combined form, and 1.77 (ó = 1.37) percent is as FeO. In the majority of the green lithologies the free ferric oxide (as haematite or hydrated ferric oxide) was removed in solution during reduction, and at the same time minor amounts of clay matrix were also leached out.