The stratigraphic evolution of a large back-barrier lagoon system with a non-migrating barrier

Lake St Lucia, the largest estuarine system in Africa, is enclosed by a 120 m-high compound Quaternary barrier-dune system in northern KwaZulu-Natal, South Africa. It comprises several discrete sedimentary basins within a single shallow back-barrier water body. This paper reports the first very-high-resolution seismic study of the system. Seven seismic units (A-G) are identified and interpreted based on their geometry, acoustic properties and a sediment coring programme. The units are bounded by regionally developed sequence boundaries and lower order unconformity surfaces corresponding to bay and tidal ravinement and hiatus surfaces. The lowermost subaerial unconformity formed during regression related to late-Pliocene hinterland uplift. Initial infilling of this surface in the proximal areas reflects estuarine sedimentation in a mixed wave- and tide-dominated system during the subsequent lowstand and early transgressive systems tracts, overlain by prograding highstand deltaic deposits that developed as sea-levels began to stabilise. Distally, this cycle of sedimentation is initially reflected by the infilling of a similar estuarine system that gradually transitioned to a back-barrier lagoon. Superimposed stillstands drove imbalances in the rate of sediment supply and accommodation space, and this, coupled with the development of an early barrier complex, caused lagoon aggradation and shallowing. A second similar trend is repeated for the recent postglacial sea-level cycle. There appears to be no preservation of the intervening Pleistocene cycles of erosion and deposition. The system is capped by estuarine-lacustrine deposits. The onset of lower energy estuarine-lake conditions began ~ 6235 cal BP in North Lake and started earlier in the proximal False Bay owing to sheltering by the antecedent barrier of a rocky peninsula. This study reveals a markedly different evolutionary history in adjacent basins of the same back-barrier system during two full sea-level cycles. These are attributed to a long established, non-migrating barrier, and to the influence of antecedent conditions on the system response to transgression. Unlike the US Atlantic margin, where migrating barriers truncate the infilling sequences, the main stratigraphic unconformities within each fill sequence correspond to tidal and bay ravinement surfaces.