Palaeogeographic implications of braid bar deposition in the Triassic Molteno formation of the eastern Karoo Basin, South Africa

Turner, Brian R.
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Bernard Price Institute for Palaeontological Research
The Triassic Molteno Formation in the main Karoo Basin, South Africa, forms a northerly thinning intracratonic clastic wedge deposited by sandy braided rivers of South Saskatchewan type. Deposition of the sandy facies was dominated by channel floor mega-ripples producing trough cross-bedded cosets; transverse bars, represented by solitary, large-scale planar sets are not significant. Departures from this regional pattern of sandstone deposition occur along the northern distal margin of the Molteno basin around Bethlehem in the Orange Free State. Here thickness trends and clast size delineate a deep channel system interpreted as the main braided exit channel from the basin. Because of its depth and constriction by local height differentials the competency and capacity of the flow were able to reproduce features more typical of proximal rather than distal depositional settings. The sandy facies is dominated by fine gravel with lesser amounts of coarse sand. Gravel occurs as longitudinal bars some of which contain low angle foreset stratification whose orientation is consistent with lateral growth and marginal riffle migration. The scale of the bars and simple deposi- tional form imply that they may have been larger than modern equivalents and the flows deeper. The coarse sand occurs mainly as falling water stage features associated with the gravel bars. Shallow channel-fills, bar edge sand wedges, bar top sheet sands and thicker channel sands have been recognised and compared with similar features in modern and ancient braided stream sediments. When traced to the southeast the deep channel sediments contain few longitudinal gravel bars and more transverse bars; the vertical sequence from longitudinal to transverse bars at this locality points to the increasing distality of the depositional site through time.
Orange Free State , Bethlehem , Molteno basin , mega-ripples , clastic wedge