Relict and contemporary influences on the postglacial geomorphology and evolution of a current swept shelf: The Eastern Cape Coast, South Africa

Abstract

Few stratigraphic models of continental shelves incorporate the process of geostrophic current-sweeping, consequently its role in the stratigraphic record is often overlooked. We examine the narrow, current-swept Eastern Cape shelf of South Africa using a combination of geophysical techniques, seafloor sampling and video observations and interpret the role of current action on the transgressive stratigraphy of this steep subtropical shelf. During the Last Glacial Maximum, fluvial valleys incised the acoustic basement rocks. During the subsequent transgression, two distinct shorelines were formed and preserved at −105 m and − 60 m. Their development and preservation is linked to (i) high sediment supply from adjacent fluvial sources, (ii) early diagenesis and (iii) alternating sea-level stillstands and periods of rapid sea-level rise during melt water pulses 1A and 1B, respectively. The deeper shoreline formed in a sandy, wide coastal plain setting with limited bedrock influence, whereas the shallower shoreline comprised alternating rock headlands and embayments like the contemporary coast. Differences in antecedent topography and geology are responsible for the temporal variability in shoreline type; coastal squeeze is exacerbated with increasing bedrock control as the shoreline migrates landwardBetween the two shoreline complexes, in the mid-shelf, the transgressive stratigraphy records initial valley infill by progradation of coast-parallel sandy spits. These are capped by a stiff lagoonal mud deposited as ongoing sea-level rise overspilled the valley interfluves, onlapping the adjacent aeolianites. The uppermost stratigraphy comprises mounds of rhodoliths which interfinger with a sandy inner to middle shelf highstand wedge.After sea-level reached its present position ca 7.4 ka yr BP, the shelf became subject to reworking by the high-energy geostrophic Agulhas Current. This has had the following major effects on the shelf stratigraphy: 1. The topographic relief of the cemented palaeo-shorelines has been emphasised by removal of the post-transgressive cover; and 2. The shelf no longer acts as a depocenter; instead, the seabed consists of rhodoliths, gravel streamers, bedrock or gravel hash of the wave ravinement surface. Given the necessary antecedent conditions such as accommodation, sediment supply and favourable diagenetic climate, prominent shorelines can form and be preserved on the shelf. Strong current sweeping emphasises these morphological features on subtropical shelves.