Architectural styles and sedimentology of the fluvial lower beaufort group, Karoo Basin, South Africa

A. Wilson, S. Flint, T.H.D. Payenberg, Eric Tohver, L. Lanci

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    The Permo-Triassic Beaufort Group comprises in excess of 3000 m of sandstone and mudstone-rich fluvial deposits that vary both laterally and vertically in abundance, architecture, and clustering styles. The lower 1370 m has been studied in detail. Detailed stratigraphic logging and photographic interpretation have been carried out on river cliffs and hillside sections, and sedimentary structures have been mapped by close inspection of clean outcrops. Seven fluvial architectural styles have been identified, encompassing sand body and sand body cluster scales: 1) flat-topped sheets dominated by lateral accretion, 2) concave-up topped sheets dominated by lateral accretion, 3) heterolithic ribbons dominated by lateral accretion, 4) flattopped ribbons with attached splays dominated by downstream accretion, 5) unconfined moderately amalgamated concave-up topped complexes, 6) unconfined strongly amalgamated sheet complexes, and 7) valley-confined strongly amalgamated ribbon complexes. The internal architecture of each indicates mixed lateral and downstream accretion modes on several scales with local variability. Lateral-accretion deposits differ from typically published facies models in that trough cross-bedding is rare, possibly the result of an inability to generate dunes due to the dominant very fine sandstone grain size. The observed architecture and sedimentary structures are the product of: 1) a semiarid climate, producing fluvial and debris flows, and cementation of sediment packages at the surface (which exerts a strong control on the magnitude of channel scour, the nature of channel fill, and compaction history); 2) local variability in subsidence rate causing variation in the vertical spacing of channel belts, channel slope, and plan form, and variation in preservation of topographically higher features (e.g., levees); 3) relative position in a fluvial megafan influencing suspended load to bedload partitioning from proximal to distal locations, and formation of channel belt complexes; and 4) relative sea-level changes causing incised-valley formation and fill only in the most distal parts of the system. Copyright © 2014, SEPM (Society for Sedimentary Geology).
    Original languageEnglish
    Pages (from-to)326-348
    JournalJournal of Sedimentary Research
    Issue number4
    Publication statusPublished - 2014


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