Palaeo-drainage, incised valley fills and transgressive systems tract sedimentation of the northern KwaZulu-Natal continental shelf, South Africa, SW Indian Ocean

Abstract

Two sequence boundaries are recognised in seismic data collected from the narrow northern KwaZulu-Natal continental shelf. These correspond to late Cretaceous and latest Pleistocene–Last Glacial Maximum (LGM) periods of sea-level fall where fluvial incision into the shelf resulted in several incised valleys having formed. In both examples, the ensuing transgressive valley fills document an evolution from fluvially dominant sedimentation to classic backbarrier estuarine conditions. Central estuary basin sedimentation is most prominent within each fill and is separated from the underlying fluvial sediments by bayline ravinement processes. Tidal-ravinement surfaces separate the upper estuarine sequences from barrier and shoreface deposition, most easily observed in the latest Pleistocene/Holocene valley fills where a Holocene unconsolidated sediment wedge overlies the estuarine fill. The wedge itself is characterised by two packages of sediment separated by a regionally occurring wave/shoreface transgressive ravinement surface which crops out as a semi-indurated shell hash in the mid to outer shelf. Late Cretaceous examples are capped by deposits exhibiting oblique parallel to wavy reflectors interpreted as increasingly energetic deposits analogous to flood-tide deltaic sedimentation. Valley incision and fill are linked to two onshore coastal lake systems, Lake Sibaya and Lake St Lucia and appear to be the product of barrier breaching and fluvial re-adjustment to baseline shifting of these systems during forced regression. Lake St Lucia exhibits a composite palaeo-drainage channel where both sequence boundaries merge within the lake margins. Lake Sibaya has two distinct palaeo-channels for each sequence boundary episode, a late Cretaceous channel which fed a small incised valley within which a modern day submarine canyon is located, the other an LGM channel directed in towards the head of Wright Canyon, the subsequent input of sediment causing this canyon to breach furthest into the shelf than any other canyon in the area. Offshore of Lake St Lucia, the most proximal portions of the associated Leven Canyon occupy the mid shelf and are underlain by Lake St Lucia palaeo-drainage channels of LGM age where erosion along the initial LGM palaeo-channel has occurred. Freshwater exchange between the shelf and the fringing lake system has resulted in the development of sinuous features of Leven Canyon which have eroded in a downslope-fluvial like manner, thus occupying palaeo-drainage lows as erosive flows were entrained. Models proposed here indicate that 1) the limited accommodation space offered by a narrow continental shelf may allow for a greater preservation potential of wave-dominated estuarine facies; 2) the seismic expression of these fills may vary, yet still satisfy similar sedimentological conditions and 3) antecedent valley topography increases the preservation potential of these valley fills by providing a preferred space in which to re-incise and then fill during sea-level rise.