Confined to unconfined: Anatomy of a base of slope succession, Karoo Basin, South Africa

Two contemporaneous weakly confined deepwater systems form Unit B of the Permian Laingsburg Formation are sufficiently well-exposed to allow investigation of the down dip passage from channelized base of slope to distributive deposits over a 25. km dip section, with strike control over some 20. km. A high resolution stratigraphy was established over a 1200. m strike section in the proximal Skeiding locality and extended regionally at a coarser scale. Analysis indicates that Unit B comprises 3 depositional sequences which, at a regional scale, thin towards the N and E. The lowstand systems tract of sequence 1 comprises weakly confined high-aspect ratio, vertically stacked channels cut into a basal frontal lobe system, and are overlain by a regionally correlated condensed hemipelagic mudstone interval (combined TST/HST) that shows evidence of remobilisation in up-dip areas The LST of sequence 2 includes two superimposed channel complexes of different styles that become deeply entrenched 5. km down dip and pass basinward into tabular, distributive lobe sandstones. Sequence 3 is marked by a regional 100. m thick levee complex related to a lower slope channel system and marks a basinward shift in facies. Channel fills in the lowstand systems tracts of all 3 sequences commonly include a thin drape of mudstone-clast conglomerate over the basal erosion surface, onlapped by thin-bedded sandstones with tractional structures that exhibit a characteristic axis to off-axis transition in facies. These deposits accumulated during periods of sediment bypass and are overlain by amalgamated structureless sandstones which thin from the axis into characteristic wings that extend laterally up to 200. m. Levees are absent in the lower two sequences and flows appear to have been only weakly confined by basal erosional keels. The resultant succession is extremely sandstone rich (90%), a much higher percentage than in underlying basin floor fan and overlying slope channel-levee complexes. The levee deposits of sequence 3 are much less sandy (30%), consistent with a more proximal, slope setting. In the 1400. m thick Karoo deepwater succession the base of slope channel sandstones of Unit B represent the maximum sandstone content and connectivity, which has predictive implications for hydrocarbon reservoir development in weakly confined deepwater channel systems.