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Abstract: The evolution of several incised valley systems from the KwaZulu-Natal shelf is compared over time. These represent the shelf extension of the collective Durban Harbour drainage system, and the Mgeni, Mhlanga, and Mdloti Rivers. Two main ages of incision are apparent; early Santonian (Cretaceous), and late Pleistocene/Holocene. The early valleys formed by tectonic controls, namely phases of higher frequency base level fall superimposed on lower frequency, higher order transgression. The results are complex networks of stacked compound valleys that have been exploited by some of the younger networks formed by glacio-eustatic fall prior to the last glacial maximum (LGM) of ~18Ka BP. The valley fills have evolved from high energy basal fluvial deposits, low-energy central basin fines, mixed-energy estuarine mouth plug deposits, clay-rich flood deposits through capping sandy shoreface deposits. The earliest fills are dominated by central basin deposits and were underfilled as a result of low gradient and limited sediment supply. The more recent late Pleistocene/Holocene fills have significantly thicker fluvial deposits as a result of increased gradient and stream competence during the later stages of valley and shelf evolution. The youngest valleys show a situation of differential evolution along the valley length due to varying rates of sea level rise in the Holocene. Initial rapid sea level rise caused drowning and overstepping of the outer segment of the incised valley, whereas slower rates of sea level rise in the late Holocene caused shoreface ravinement of the inner-mid segments of the valley. This differential exposure to accommodation has resulted in a sedimentological partitioning between tide-dominated facies in the outer valley segment and river-dominated facies in the inner segment (cf. Cooper, 2001). [Copyright &y& Elsevier]

Abstract: The late Holocene evolution of the Durban Bight shoreface is documented on the basis of core, seismic and micropalaeontological data. From a sedimentological perspective the shoreface comprises three fining-upward packages resting on either granule or pebble layers with well-defined erosional bases. The best developed of these is a lowermost pebble conglomerate that has been tied to a regionally developed erosion surface (g). The surfaces are interpreted as having formed on the lower shoreface during sediment exchange with the upper shoreface via storm-driven erosion. The succession is capped by a monotonous series of small sandy climbing bedforms, terminating in a thin, mudball-rich layer. The mudballs are the product of a large-scale fluvial flood that occurred in 1987. From a palaeontological perspective, the shoreface shows fluctuations in nutrient supply and temperature, identified by the relative abundance of F. profunda in terms of the high small placolith to lower photic species ratios, in addition to the changing bridge angle of Gephyrocapsa sp. Where storm-event layers are recognised, corresponding peaks in sea surface temperature (marked by high-bridge Gephyrocapsa assemblages) and low nutrient contents (low relative abundances of F. profunda) occur. Periods between storm events are characterized by cooler sea surface temperatures and increased nutrient contents. The controls on such fluctuations are the presence of the warm, oligotrophic Agulhas Current that occasionally meanders and impinges on the shelf; the local coastal upwelling triggered when the current migrates offshore; and the relatively lesser influences of the fringing fluvial sources during flooding. We consider that the event layers recorded in the shoreface succession are persistent stratigraphic features formed at times of increased storminess associated with the inshore meandering of the Agulhas Current. Where significant exchange of sediment occurs in the offshore as in the present case, this sediment is lost from the system and the shoreface recovery is stunted. [Copyright &y& Elsevier]