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Late Paleozoic Fluviatile and Deltaic Deposits in the Northeast Karroo Basin, South Africa
Six sedimentary facies, each distinguished by their lithology and structures, occur within the Ecca Group of the northeastern Karroo Basin. Prominent upward-fining cyclic sequences with abundant coal seams are present in the north. These are replaced to the south by thicker, finer-grained upward-coarsening sequences, in which coals are rare. Markov chain analysis of vertical transitions in selected outcrops provided probabilistic descriptions of preferred facies arrangements, and confirmed two distinct depositional models. The upward-coarsening sequences are attributed to southwestward progradation of shallow water deltas over a broad, gently subsiding shelf. Associated trace fossils include representatives of Seilacher's Cruziana and Skolithos assemblages, with Nereites-type trails in the equivalent deeper water deposits. Sedimentary criteria, supplemented by trace fossil evidence concerning bathymetry or energy level, permit recognition of prodelta, distributary mouth bar, interdistributary bay, levee, and crevasse deposits. The upward-fining cycles were probably deposited by meandering rivers, which dissected and modified the delta plain as the environments moved basinward during regression. Fluvial processes obliterated much of the deltaic record in the north, but farther south these were restricted to local channeling of the distributary mouth bar sands. The vertical models of fluvial and deltaic sequences, which are derived from field observation and supported by nonparametric statistical analysis, will assist in paleoenvironmental reconstruction during subsurface exploration of the Karroo Basin.
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