Calcrete (caliche) palaeosols in fluvial redbeds of the Aztec siltstone (upper Devonian), Southern Victoria Land, Antarctica

Abstract

Palaeosols in the Upper Devonian Aztec Siltstone of southern Victoria Land closely resemble pedocal ‘calcareous red earth’ and ‘red earth’ type modern soils and were formed on a low-relief surface under a hot and seasonally wet and dry, semi-arid climate. The Upper Devonian Aztec Siltstone of southern Victoria Land is interpreted as a fluviatile redbed sequence deposited on a broad, low-lying floodplain from high-sinuosity meandering streams which gave rise to characteristic fining-upwards sedimentary cycles. The fine-grained, fine-member portions of the cycles represent vertically accreted overbank sediment laid down on the interchannel floodbasin areas. With the development of relief, these sediments were subaerially exposed for extended periods (4000–10 000 years) during which time they underwent in-situ modification by pedogenic processes. This is evidenced by the common occurrence of carbonate nodules (calcrete), wavy bedding or pseudo-anticlines, colour mottling or gleying, and extensive bioturbation in these unbedded clay-rich sediments. Plant roots and associated plant fossils and spores are also present. The textural character of the fine-grained rocks is closely comparable to that of modern soils including a skel-lattisepic fabric with skew-planes. Mineralogical and detailed chemical analyses, including down-profile element variations, show that there was an in-situ reorganization of ‘soil plasma’ consequent on clay illuviation, translocation and deposition of carbonate, and mobilisation and segregation of iron oxide.