Genesis of variegated redbeds in the fluvial Aztec Siltstone (late devonian), southern victoria land,Antarctica

The Aztec Siltstone (Late Devonian) formed in a meandering fluviatile environment and is a variegated redbed sequence consisting of red or drab overbank mudstones and drab channel sandstones, commonly arranged in fining-upwards cycles. The pigmentation in the red mudstones is caused by euhedral cryptocrystalline haematite which is dispersed throughout the matrix, masking the green colour of the underlying illitic and chloritic clays. Textural, mineralogical and chemical analyses suggest that the pigment formed in situ by dehydration and crystallization of a detrital, amorphous or poorly crystalline, yellow-brown ferric oxide precursor. The hydrated precursor apparently was derived from the weathering of iron silicates in a source area regolith, and transported in suspension with the Aztec alluvium, commonly by attachment onto surfaces of clay platelets. The association of the precursor with the clays is a contributing reason for the absence of red sandstones in the formation.Dehydration and crystallization of the yellow-brown ferric oxide to form the red haematite pigment occurred prior to deep burial of the host sediment. At least part of it occurred before burial, in an oxidising environment in well-drained and well-aerated overbank soils which were exposed for prolonged periods of time (4,000–10,000 years) under a hot and at least seasonally arid climate. Crystallization of the red pigment may have been completed during early diagenesis.Reducing conditions, induced by the presence of organic debris in the sediment and a relatively high water table, affected much of the deposit by removing the “free” hydrated ferric oxide, either partially or completely, from the in-channel sands, the low-lying and poorly drained overbank sediments, and the lacustrine sediments. This reduction was both a syndepositional and an early post-depositional process.Iron analyses indicate that, on average, the red mudstones are enriched in total iron (Fe) and Fe³⁺ by 2%, all of which is attributable to the inherited “free” ferric oxide. There is no other significant difference in whole-rock mineralogy or major-element chemistry between the red and non-red mudstones.