Abstract

Three types of paleosol profiles are described as representative of the range of soil types that developed on the ancient alluvial plains of the southwestern Karoo Basin during the Late Permian. Continuous 3-dimensional exposures of the Lower Beaufort (Adelaide Subgroup) strata allow detailed sedimentological, taphonomic, and paleopedological observation of thick floodplain sequences that contain numerous "stacked" paleosol profiles. Various floodplain strata contain characteristic paleosol profiles that are regulated by the periodicity and competence of overbank flooding, both of which decrease with increasing distance from the contributing channel. Paleosols developed in levee or channel-bank deposits are immature calcic Entisols displaying evidence of early carbonate translocation but without well-developed horizonation. Proximal floodbasin paleosols, at the foot of the meanderbelt slope, are mature calcic Vertisols with more fully developed profiles containing zones of both carbonate and clay illuviation as well as some groundwater calcretes. Slickensided stress argillans in the B-horizons are indicative of multiple cycles of wetting and drying. Distal floodbasin paleosols are dominated by hydromorphic and evaporitic processes as a result of their axial position in the floodbasin. Sheet-like networks of mudcrack casts composed of calcretized aeolian sand and silt suggest that dust was a major source of calcium carbonate to these soils. The paleosols are interpreted as having formed under warm to hot, semi-arid climatic conditions with a strongly seasonal rainfall of 500 to 800 mm per annum and a mean annual moisture deficiency. The geomorphic setting was similar to the Holocene Indo-Gangetic alluvial plain. Vertebrate burrow casts in the proximal floodbasin facies indicate a minimum depth to watertable of 150 cm. The distribution of rooted horizons reflects dense vegetation flanking major drainage channels and sparse cover in the distal floodbasin. A simple stratigraphic sequence of once laterally continuous paleosols is termed a pedofacies. Stacked sequences of pedofacies within approximately 20-50 m thick, and 100-250 m thick stratigraphic successions reflect second- and first-order sedimentary cycles respectively. Both scales of cyclicity are attributed to autocyclically controlled avulsions of differentially aggrading channels in the first instance and of complete drainage networks in the second.

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