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Facies Variability in Deltaic Systems of Katjiesberg, Tanqua Karoo


Shelf-edge deltas are critical to understanding shelf-margin accretion and deep-water sediment delivery, but 3-D facies variability along/around the shelf-edge is poorly understood. Recent studies suggest that the shelf edge can be identified in outcrop partly by an increase in clinoform thickness, representing accommodation at the shelf rollover zone due to increased gradient. Pervasive soft-sediment deformation and slump deposits are common in these systems, representing gravitational instability at the shelf edge. Studies have also cited an upsection increase in fluvial, tidal, and wave facies, deposited as the shelf progrades. Examples of fine-scale outcrop interpretations of mixed-influence shelf edge deltas are rare and differ geometrically with those typically interpreted from seismic datasets, typically having higher gradient shelves and narrower slopes than those observed with subsurface data. In the Tanqua Karoo sub-basin of South Africa, the Permian Kookfontein and Waterford Formations record northeastern progradation and evolution of a deltaic system from shelf-slope to delta plain. The system is well-exposed for kilometers along depositional strike and dip. The Karoo paleoshelf margin is considered to be strongly progradational, with a low-gradient, stable shelf and slope. The system is notably fine-grained, with limited syn-sedimentary growth faulting, but widespread amounts of soft-sediment deformation and localized slump deposits. Much remains to be understood about partitioning of fluvial, tidal, and wave faces along the shelf edge, the key factors determining variability, and their potential as local stratigraphic hydrocarbon traps. Questions also remain about the stratigraphic architecture of a progradational shelf comprised of unstable substrate. For example, when are the shelf edge and slope dominated by erosional versus depositional process regimes? This study adds new data from the northern Tanqua sub-basin. Multiple stratigraphic sections in conjunction with extensive UAV-photogrammetry provide detailed outcrop characterization and kilometers of stratigraphic surface correlations. These outcrops allow for description of the vertical transition from slope-shelf deposits in a mixed influence system and of the variability of facies deposited throughout the transition. We document common soft sediment deformation and mass movement in multiple styles throughout the transition, indicating significant collapse was responsible for sediment deposition on and beyond the shelf and slope. Digital outcrop models show thickness and lateral and vertical distribution of these delta front collapse features. The variety of facies present suggest that multiple processes were concurrent and interfingering during deposition and progradation of the shelf edge, with implications for up-system controls on delta type and geometry.