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Lateral Variability in Upper Slope, Shelf Edge and Shallow Marine Stratigraphy Along a 70-km Strike Transect: Karoo Basin, South Africa

Abstract

The character of key transition points in the depositional profile occurs through the interaction of numerous processes resulting in a complex heterogeneity in ancient basin margin successions. This is well studied along depositional dip profiles, but lateral (strike) variability and consequent implications for reservoir distribution is less constrained. In the Karoo Basin, continuous NW-SE-oriented exposure over 80 km- has been characterized by 53 logs with 9910m of cumulative thickness, 2500 paleocurrent measurements, and ground- and helicopter-based photo panels. Dominant sediment transport was to the N-NE with E-W and NE-SW bidirectional components consistent with the strike orientation of the outcrop belt. In the south, upper slope to shelf-edge parasequences (50-75 m-thick) show abundant current ripples and inverse-to-normal grading in dirty siltstones and sandstones. They are interpreted as river-dominated, wave-influenced prodelta and mouth bar deposits incised by distributary channels (up to 25m-thick, 700m-wide). Parasequences are partly truncated by 10s of m-thick upper slope gullies and by 100m-thick, 1.5km-wide shelf-incised canyons. Overlying shelf parasequences are thinner (15-50 m) with symmetrical ripple tops, HCS, SCS or low angle cross bedding, interpreted as wave-influenced deltaic or shoreface deposits. Transition to terrestrial deposits includes channels and crevasse splays within delta plain mudstones. Along strike to the north, upper slope to shelf-edge parasequences are thinner, include wave reworking indicators and no evidence of gullying or incision. Overlying shelf parasequences are thicker, sandier and more amalgamated, interpreted as offshore-shoreface, shoreface, foreshore and strandplain deposits. Although the succession reflects the NE progradation of a basin margin, the mixed influence of coastal processes results in complex along-strike architecture. Southern nearshore environments were river-dominated with bypass and sediment delivery to deeper parts of the basin across a steeper and more erosive margin. Waves and storm current redistribution towards the north basin resulted in higher net-to-gross and sand connectivity on the shelf but no incision, bypass and sand supply to the slope. This work provides a good analogue for capturing along-strike heterogeneity in mixed influence shallow marine reservoirs.