Palaeoenvironmental models in the Eastern Karoo Basin

Hobday, D. K.
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Bernard Price Institute for Palaeontological Research
Palaeoenvironmental models are based on a three-dimensional conception of sedimentary rock units and their internal geometry. These models are process-oriented and are interpreted by comparison of their attributes with those of modern sedimentary environments. Six models are proposed as a result of observations in the eastern Karoo Basin, three in the Ecca and three in the Beaufort, although some are common to both. Both regressive delta and beach models are upward-coarsening, but they are readily distinguished on the basis of sandstone composition, texture and sedimentary structures. Beaches probably developed along a non-tidal or micro-tidal coast, but in most areas the relatively rapid sediment influx favoured the formation of deltas which prograded across the shallow shelf. Incised into the delta front sandstones are channels of distributary and alluvial origin. Large fluvial channels were generally meandering, and their deposits record a vertical reduction in flow energy from thalweg through point bar to levee, with the capping coal seams representing an hiatus in detrital sedimentation. Delta front sandstones within the Beaufort Group resemble superficially those of the Ecca, but display differences in vertical sequence which are tentatively ascribed to changes in density of the basin waters. Whereas the northern and eastern basin margins were characterized by persistent, moderate energy fluvio-deltaic sedimentation, with small prograding lobes separated by shallow embayments subject to crevasse splays, the southern part of the basin was the locus of major fluvial deposition as a consequence of orogenic uplift to the south. High energy braided stream conglomerates and sandstones were deposited contemporaneously with finer-grained meanderbelt and floodplain sediments, which accumulated farther basinward in an area of reduced gradient and more constant discharge. The value of these models is that most outcrops in the study area can be explained in terms of their relationship to one or more of the models. Future palaeoenvironmental synthesis should incorporate the great variety of biological information available from the Karoo Basin.
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Karoo basin; Palaeoenvironmental models; Ecca; Beaufort