The late Cenozoic evolution of coastal water bodies in Northern Kwazulu-Natal, South Africa

Abstract

Abstract Three large coastal water bodies (St Lucia, Sibaya and Kosi) of markedly different contemporary morphology occur on the eastern margin of the northern KwaZulu-Natal coastal plain, South Africa. This paper examines their Late Cenozoic evolution and investigates the controls on their evolution that have produced the observed differences in morphology. All three systems origins were due to Mio-Pliocene low sea-level still stands allowing rivers to scour channels into Cretaceous and Palaeocene sedimentary sequences. During the Last Interglacial (oxygen-isotope stage 5e), sea level was approximately 5 m above present level and the morphology of the three systems was similar. An offshore barrier archipelago partially isolated a back-barrier environment that comprised intertidal sand-flats, localised reefs and tidal channels. The area around Bazaruto Island, Mozambique, provides a modern analogy. River rejuvenation and associated incision up to and during the Last Glacial Maximum (16 000–18 000 BP) caused a minimum of −40 m scour which is still reflected in the bathymetry of Lake Sibaya and Kosi. Accumulation of barrier sands partly against pre-existing Interglacial barrier remnants during the Holocene marine transgression (16 000–4500 BP, Ramsay, P.J., 1995. 9000 Years of sea-level change along the South African coastline, Quaternary Int. 27, 1–5.) was responsible for lagoon isolation, although the extent and nature of separation from the sea varies markedly between the systems. The northern part of Lake St. Lucia was closed from the sea during the late Holocene, probably during a sea-level fall from a Holocene high stand of ca. 3 m, at 3000 BP. The Kosi system had a late Pleistocene to early Holocene outlet situated at Bhanga Nek, Lake Nhlange, but this inlet mouth closed approximately at 3000 BP. The present mouth was established by 1600 BP. Lagoonal conditions within Lake Sibaya terminated ca. 5000 BP when coastal barrier dune formation caused complete impoundment and enabled the lake water level to rise to 20 m above mean sea-level. Lake St. Lucia's large catchment has meant that fluvial sediment input has kept pace with the Holocene transgression, whereas the localised drainage associated with Sibaya and Kosi has resulted in a negligible fluvial sediment input allowing for deep lacustrine conditions to develop. Lagoonal segmentation is present to some extent in each system as a result of mobile coastal sediments and their reworking by short-period wind waves. The differential stages of lagoonal segmentation in the three systems can be explained by (a) the presence of cohesive sediment and bedrock around St Lucia which reduce shoreline mobility, and (b) the great water depths in Sibaya compared to Kosi which require longer periods of infilling before segmentation is complete