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This study examines a Holocene-aged submerged shoreline, Limestone Reef, located in the shallow subtidal zone of South Africa's east coast. It comprises an elongate, coast-oblique, slab-like outcrop of beachrock situated above the contemporary fair-weather wave base. It is currently undergoing mechanical disintegration. Its unique and rare preservation in a high-energy setting affords an opportunity to examine the mechanical processes occurring during wave ravinement associated with rising sea level. The submerged shoreline and the adjacent shoreface were examined using high-resolution seismic reflection, side-scan sonar and shallow-water multibeam echosounding techniques. Limestone Reef rests on top of unconsolidated Holocene deposits. The structure's surface is characterised by reef-perpendicular gullies with rubble derived from the slab fringing its seaward edge. Limestone Reef slopes gently seawards and has a steep landward-facing edge where gullies are most prominently developed. Teardrop-shaped rippled scour depressions, marked by high backscatter, are located seawards of the submerged shoreline. These elongate in a seaward direction and are filled with bioclastic gravels and residual rubble from Limestone Reef. The gullies in the upstanding structure are indicative of wave plucking and abrasion of the shoreline. The material exposed by the rippled scour depressions is identical to that comprising the postglacial ravinement surface identified in the offshore stratigraphy. These deposits are considered to represent the contemporary, actively forming wave ravinement surface. The results suggest that wave ravinement of submerged shorelines is a discontinuous process dominated by the seaward entrainment of material from its landward edge controlled by high-energy drawback during storm surges. The ravinement process appears to operate at the seasonal scale and averages out over the long-term millennial scale for the continuous surface. [ABSTRACT FROM AUTHOR]

Relict rocky shorelines are seemingly rarely preserved in the geological record, often underrepresented in the literature and the reasons for their scarcity, as well as the conditions needed for preservation are not well understood. On the "Wild Coast" shelf along the subtropical east coast of South Africa a series of submerged shorelines are present. High-resolution multibeam bathymetry, side scan sonar, ultra-high-resolution seismic profiling and legacy seismic reflection data show several fluvial channels incised into the shelf, associated with the Last Glacial Maximum (LGM) lowstand. Palaeo-shorelines are preserved at −85, −75, −60 and −40 m. The −85 and −75 m shorelines reflect sediment abundance and depositional features preserved as lagoonal depressions, lithified barrier-spits and coastal barrier dunes with limited bedrock influence. The shallower shorelines at −60 and −40 m occur on a steeper shelf gradient and reflect the development of rocky headlands, palaeo-cliffs and rock shore platforms. Their occurrence implies increasing bedrock control, erosion and sediment bypass during transgression. The preservation of rocky shoreline signatures on the inner shelf is aided by competency of the bedrock lithologies, in addition to their depth relative to sea-level stillstands and rapid rises in sea level associated with meltwater pulses (MWPs) (notably the −60 m shorelines and MWP1B). MWPs may have helped facilitate the overstepping and eventual preservation of these shoreline landforms on the seabed. Preservation of the −40 m shoreline is considered to correlate with accelerated rates in sea- level rise prior to MWP1C. The preservation of the −85 and −75 m aeolianite shorelines are not linked to meltwater pulses, however, their substantial sediment volume precluded total loss through transgressive erosion and this, coupled with early cementation, promoted their partial preservation. Such deposits rarely survive subaerial exposure but could be preserved if covered by marine sediments. Conversely, the existence of well-preserved, Pleistocene-age rocky shorelines on the shelf points to their high potential for inclusion into the longer-term geological record under subsequent high or low sedimentation. The formation of distinctive rocky shorelines, however, may reflect alternating slow and fast rates of sea-level rise. [ABSTRACT FROM AUTHOR]

The literature on incised river valley sedimentology is dominated by studies of sediment-rich systems in which the valley has been filled during and/or shortly after drowning. In contrast, the Holocene evolution of the Kosi Lagoon, South Africa (an incised coastal plain river valley) took place under very low sedimentation rates which have produced a distinctive stratigraphy and contemporary sedimentary environments. The findings are based on a synthesis of the results of studies of seismic stratigraphy, sediment distribution, morphodynamics and geomorphology. Barrier migration was prevented by a high pre-Holocene dune barrier against which Holocene coastal deposits accumulated in an aggradational sequence. Holocene evolution of the back barrier involved: (i) drowning of the incised valley; (ii) wave-induced modification of the back-barrier shoreline leading to segmentation during the highstand; and (iii) marine sedimentation adjacent to the tidal inlet. Segmentation has divided the estuary into a series of geochemically and sedimentologically distinctive basins connected by channels in the estuarine barriers. The seismic stratigraphy of the back barrier essentially lacks a transgressive systems tract, shoreline modification and deposition having been accomplished during the highstand. The lack of historical geomorphological change suggests that the system has achieved morphological equilibrium with ambient energy conditions and low sediment supply. This study presents a classification for estuarine incised valley fills based on the balance between sea-level rise and sedimentation in which Kosi represents a 'give-up' estuary where much of the relict incised channel form is drowned and preserved. It exhibits a fundamentally different set of evolutionary processes and stratigraphic sequences to those of the better known incised valley systems in which sedimentation either keeps pace with sea-level ('keep-up' estuaries) or occurs after initial drowning ('catch-up' estuaries). [ABSTRACT FROM AUTHOR]