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Lateral Variability in the Stratigraphic Transition From Open Shelf to Non-Marine Deposits Along a 70 km Strike Transect, Tanqua Depocenter, Karoo Basin, South Africa

Abstract

Coastal plain to open shelf sediments are transported and deposited through the complex interaction of continental and marine processes. The physical transition from non-marine to marine process regimes is documented along dip profiles of ancient regressive systems. Less commonly documented, however, is the lateral (strike) variability of transitional successions and basin margin progradation, although this can impact reservoir architecture. In the Karoo Basin, continuous NW-SE oriented outcrop exposes more than 200 km of the basin margin. Eight 500m logged sections with 4–15 km spacing were correlated and tied to detailed photo panoramas along a 70km long transect. Successions of 5–25m thick packages of current ripple laminated, thin-bedded inverse to normally graded and dirty siltstone (St)-sandstone (Sd), are interpreted as prodelta lobes of a river-dominated partly wave-reworked system. Where these are sand-prone with climbing ripples they are interpreted as distal mouth bars. Structureless Sd in channel-forms that incise into fine-grained St is interpreted as distributary channels cutting into bay-fill mudstones in the lower delta plain. Laterally, these successions pass into thickening upwards units with thin-bedded and lenticular Sd with symmetrical ripple forms overlain by clean Sd containing convex-up lamination or to amalgamated Sd. These are interpreted as shoreface-to-offshore transition to shoreface units. More than 2500 paleocurrents show dominant transport to the N-NE, with local E-W and NE-SW bidirectional components confirming that the section is a regional strike to oblique-strike orientation. Seven correlated units summarize the stratigraphic evolution. The overall succession, and regional context, reflects the progradation of the basin margin to the NE. However, the mixed influence of river- and wave-dominated process regimes leads to a more complicated depositional architecture along strike. Although the nearshore environments were river-dominated, the paleocurrents and lateral changes in depositional environments and sand cleanness suggest that there was across shelf sediment transport by waves and storms. The marine to non-marine transition shows mixed influence shoreline systems evolving through time in a non-coal-bearing, moderate to high-latitude paleo-climatic setting, providing a good example of architecture heterogeneity in paralic reservoirs.