Upward‐thickening patterns and lateral continuity of Permian sand‐rich turbidite channel fills, Laingsburg Karoo, South Africa

Abstract

Abstract Analysis of extensive exposures of the Permian Laingsburg Formation, Karoo basin, South Africa, have enabled a detailed reconstruction of the base of slope stratigraphy and palaeoenvironments in a deep‐water system characterized by a very narrow grain‐size range (fine sandstone). The deposits include an ≈ 4 km wide and 80 m thick channel complex, fringed by sandy sheet deposits that extend laterally for at least 6 km across depositional strike. Within the channel complex, individual channel fills are marked by shallow basal erosion surfaces draped by thin, parallel‐stratified beds of very fine sandstone and siltstone, interpreted as flow tails to largely bypassing flows. These thin beds are overlain by 0·4 to 5 m thick beds of structureless, fine‐grained sandstone that represent the majority of the channel fills. The basal packages may be partially to completely removed by localized scour in the axial zone of the channel complex but can be mapped laterally into overbank areas where they thicken and are dominated by rippled fine sandstones with intercalated siltstones. Axial confinement resulted from subtle topography on the basin floor, whereby the lower, dense parts of the initially erosive and bypassing flows were partially confined in the lows and the more dilute, slower moving upper parts of the flows deposited sheet‐like successions across slightly elevated overbank areas. The narrow grain‐size distribution prohibited the formation ofcoarse‐grained residual bypass deposits during the initial phases of channel formation. With decreasing magnitude, later flows became more depositional, filling remaining axial depressions with thick‐bedded structureless sandstone. The smaller volumes of late‐stage sediment were more axially focused, producing local scour‐and‐fill features and starvation of the overbank areas. Resulting grain‐size vertical profiles are complex. The basal flow tail packages and overlying massive deposits form a thickening and slightly coarsening‐upward trend in the channel fills. The overbank deposits show a thinning‐ and fining‐upward profile as a result of less bypass plus late‐stage starvation of sand. Application of traditional deep‐water facies models could therefore potentially lead to erroneous interpretations of the channel complex as a prograding lobe and the overbank sheets as channel‐fills.