Testing Applicability of Models Of Distributive Fluvial Systems Or Trunk Rivers In Ephemeral Systems: Reconstructing 3-D Fluvial Architecture In the Beaufort Group, South Africa

Abstract

Abstract Multi-kilometer-scale strike and dip exposures of fluvial deposits from the Permo-Triassic Beaufort Group enable detailed analysis of spatial and temporal changes in fluvial style and stacking patterns. A study succession 145 m thick in the Abrahamskraal Formation comprises a hierarchy of channel-related deposits, from stories through channel belts that are stacked into nine channel-belt complexes in four complex sets. Channel belts show evidence for both downstream and lateral accretion and include common upper-phase plane bedding. Floodplain deposits comprise crevasse-splay sandstones and siltstone packages showing upward fining from green-gray siltstone into distinctive purple claystone, interpreted as a drying-upward trend in shallow ephemeral lakes. The lower stratigraphy is dominated by splay complexes, overlain by increasingly incised and amalgamated channelized systems with little preserved floodplain material. Paleocurrents are consistently to the northeast. While aspects of the dataset fit a basin-axial trunk-river-dominated system, they better (but not entirely) reflect a prograding distributive fluvial system (DFS). Lines of evidence include the consistent paleocurrents in early (frontal) splays and younger channel-belt complexes, and the presence of splay complexes only in the lower stratigraphy, interpreted as precursors to the prograding fluvial system. Deposition took place under conditions of flashy discharge influenced by high-frequency climate cycles, also expressed in the mudrock color changes and distribution of paleosols. These cycles overprint the expected gradual drying-upward trend in overbank deposits proposed in DFS models. The abrupt increase in channel belt incision, amalgamation, and lack of floodplain preservation associated with Complex 6 is interpreted to reflect sequence-boundary formation and a basinward facies shift that forced progradation beyond the rate predicted in gradualistic DFS models.