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Spatial-Temporal Evolution of Sedimentary Systems, Transition Zones and Stratigraphic Sequences in a High-Latitude Basin Margin Succession

Abstract

A challenge in hydrocarbon exploration is to understand and predict the spatial and temporal evolution of basin margins over multiple scales. More than 20 years of research in the Permian Karoo Basin has yielded one of the largest outcrop datasets with 200+ km of logged sections and 1000+ km of stratigraphic correlations, complemented by fully-cored research boreholes, radiometric dates, geochemistry and magnetostratigraphic data. Detailed outcrop and core observations, combined with geometric analysis from correlation panels, have provided criteria to define the base-of-slope, shelf-edge and shoreline position through time, from which minimum shelf widths (20-100 km) and slope lengths (>40 km) have been reconstructed. Facies associations and average net-to-gross values were obtained for each depositional setting, and architectural element dimensions were deduced from the mapped extent of sedimentary bodies. The 2 km-thick succession has been subdivided in 6 composite sequence sets. In sets 1-3, basin floor sand-rich deposition occurred in an area of increased subsidence around Laingsburg, and correlate to a thick mud-rich succession 40 km along margin in the Tanqua area. Sets 4 and 5 comprise sandy basin floor to slope deposits in Tanqua with equivalent erosion-dominated and mud-rich slope systems in Laingsburg. This significant variability in sediment distribution evidences different basin margin physiography over relatively short distances. This configuration was progressively healed by progradation of set 6 basin margin clinoforms, which are correlated throughout the study area. Shelf deposits show lateral variability in margin progradation, process regime and soft sediment deformation, but their associated slope deposits are sand-poor. High sand aggradation and absence of valley systems incising shelf strata result from the combination of intrinsic compensational processes and extrinsic controls with high accommodation and sediment supply rates in a high-latitude, greenhouse setting. This contrasts with the underlying slope and basin-floor composite sequences that show major shifts in sediment supply attributed to basin margin configuration and significant changes in relative sea-level on the coeval shelf. Therefore, the studied shelf strata are a poor analogue to the nature of the shallow-marine systems that fed the Karoo deep-water succession, and this highlights potential risks in applying simple models in exploration of progradational basin margins.