Number One Reef: An overstepped segmented lagoon complex on the KwaZulu-Natal continental shelf

Abstract

This study of the bathymetry of the mid-shelf of the Durban Bight, KwaZulu-Natal revealed a series of previously undocumented seafloor features. These features were mapped using a high-resolution multibeam bathymetric echosounder and a detailed map of the seafloor topography was produced. We recognised several features that closely resemble features of contemporary segmented lagoon and lake systems: semicircular seafloor depressions, arcuate ridges, cuspate spits and prograding submerged barriers. Based on the striking similarity in morphology to Kosi Bay -a segmented lagoon system from the sandy northern KwaZulu-Natal coastal plain -a similar evolutionary model is proposed. This model is of an incised valley formed following a sea level lowering to the Last Glacial Maximum at about 18 000 BP. Thereafter, continued transgressive infilling occurred to a point where an extensive lagoon and back-barrier system was established. At this point, sea levels remained static, causing the net segmentation of the system and the slow closure of the tidal basins or circular depressions. This type of seafloor topography is rarely preserved and is the result of fortuitous cementation after deposition and the later removal of sediment that would ordinarily bury such features.

Submerged shorelines and landscape features offshore of Mossel Bay, South Africa

“…This complex interplay of processes has led to the preservation of offshore features that are expected to be dominated by aeolianite and back-barrier sediments. Numerous subaerially exposed superimposed retrogressive barrier systems have been described along the wave-dominated south coast (Bateman et al 2004(Bateman et al , 2011Carr et al 2010), offshore on the east coast (Bosman et al 2005;Cawthra et al 2012;Green et al 2012) and, recently, off the south coast in this study. The morphology and relief of the shelf banks/shoals on the shelf of Mossel Bay (Fig.…”

“…Geomorphological, stratigraphic and sedimentological models of the shelf therefore remained relatively poorly advanced. Numerous geophysical studies carried out on the South African continental shelf (Ramsay 1996;Bosman et al 2005;Cawthra et al 2012;Green et al 2012) have documented marine geological evolution of the margin, but our study is the first to stress the significance of these geomorphological elements of the continental shelf as features that formed on an exposed coastal plain within a South African context. Foothills grading into dissected plains of the south coast dominate the landscape south of the Cape Fold Belt.…”

Topographic and hydrodynamic controls on barrier retreat and preservation: An example from Dogger Bank, North Sea

“…Forbes et al, 1991;Jensen and Stecher, 1992;Forbes et al, 1995;Kelley et al, 2010;Mellett et al, 2012;Billy et al, 2018) or early cementation in warmer climates (e.g. Jarrett et al, 2005;Green et al, 2012;Albarracín et al, 2013;Green et al, 2013;Salzmann et al, 2013;Green et al, 2018). Barrier phase B is a rare example of a well-preserved sand barrier that fits neither of these models.…”