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Beachrocks are a common characteristic of tropical and subtropical coastlines. It is known that they have a substantial influence on beach morphodynamics and they are commonly utilised as indicators of palaeo‐sea levels. At the same time, facies variability in beachrocks is understudied and their effect on shoreline evolution is poorly understood. At Mission Rocks on the KwaZulu‐Natal coastline of South Africa a narrow beach with isolated sand patches occupies low points of an otherwise continuous 3 m thick, raised shore platform of sandy and gravelly beachrocks. These beachrocks are in the process of disintegrating into megagravel deposits through chemical and mechanical weathering in a wave‐dominated, high‐energy setting. The breakdown is potentially slowed by a contemporary, fast‐forming beachrock facies, that blankets the surface and fills fractures and potholes within the older platform. The accumulation and cementation of this recent beachrock is the focus of this study. The beachrock is dated by historical evidence to post‐World War II. Data from field observations, petrographic and geochemical methods reveal that the cementing agents of the beachrock were precipitated from marine water in a phreatic setting despite its position above the intertidal zone. Not only does this facies have implications for the interpretation of palaeo‐beachrock as a sea‐level indicator, it also raises further questions regarding modelling of coastal erosion of beaches associated with outcrops of beachrock. [ABSTRACT FROM AUTHOR]

Evidence for sea-level change during and around Marine Isotope Stage (MIS) 5e (ca. 125 ka) in southern Africa derives from a wide variety of geomorphic and sedimentological sea-level indicators, supported in the past 2 decades by absolute chronological control, particularly on littoral deposits, some of which have a quantifiable relationship to former sea level. In addition to these proxies, data provided by both terrestrial (dune sediments and archaeological remains) and marine (lagoonal and nearshore littoral sediments) limiting points provide broad constraints on sea level. Here, we review publications describing such data points. Using the framework of the World Atlas of Last Interglacial Shorelines, we insert in a standardized database (10.5281/zenodo.4459297, Cooper and Green, 2020) all the elements available to assess former palaeo-relative sea level (palaeo-RSL) and the chronological constraints associated with them (including uncertainties). Overall, we reviewed 71 studies, from which we extracted 39 sea-level indicators and 26 limiting points. As far as age attribution is concerned, early analysis of molluscs and whole-rock beachrock samples using U series allowed dating of several sea-level indicators during the 1980s, but the more widespread application of optically stimulated luminescence (OSL) dating since 2004 has yielded many more (and more accurate) sea-level indicators from several sites. This has helped resolve the nature and timing of MIS 5e shorelines and has the potential to further elucidate the apparent presence of two or more sea-level peaks at several South African sites during this interval. The standardized sea-level database presented in this paper is the first of its kind for this region. Future research should be directed to improve the stratigraphic description of last interglacial shorelines and to obtain better dating, high-accuracy elevation measurements with better palaeo-RSL interpretation. [ABSTRACT FROM AUTHOR]

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]

The seismic stratigraphy, architecture and evolution of a large subtropical embayment, Maputo Bay, Southern Mozambique is described. Nine seismic units are documented; and show repeated phases of incised valley formation and transgressive infilling. Trangressive infilling alternates between the classic wave-dominated transgressive succession of fluvial and central estuarine bay deposits and the filling by intertidal flat and tidal bedforms developed during stages of inlet restriction and shallowing associated with barrier growth at stillstand intervals. Imprinted on the succession are several key facets of subtropical stratigraphy, hurricane-generated erosional surfaces and beachrock horizons. The upper stratigraphy comprises the highstand bayhead delta, a prograding feature constructed by several flood events. In comparison with literature from other similar systems such as lagoons, coastal lakes and estuaries, a generic three stage stratigraphic model of large marine embayments is proposed that includes incision during glaciations; transgressive infilling during deglaciations; and shallowing during the stillstand episodes of the recent deglaciation. [ABSTRACT FROM AUTHOR]

Kelly, C.S., Green, A.N., Cooper, J.A.G. and Wiles, E. 2014. Beachrock facies variability and sea level implications: a preliminary study. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 736-742, ISSN 0749-0208. In spite of the worldwide abundance of beachrocks and their acknowledged utility as an indicator of former sea level position, some studies have expressed doubt as to their position of cementation on paleo shorelines. These criticisms are not, however, coupled with nuanced sedimentological studies of beachrocks. Instead, few beachrock studies acknowledge any facies and therefore disregard important signatures of the depositional environment and, consequently, utility as paleo sea level indicators. This study presents detailed sedimentological descriptions and interpretations from two beachrock localities along the subtropical, microtidal, wave-dominated eastern coastline near Durban, South Africa. The outcrops record the migration of a paleo inlet and deposition in sub, inter, and supra tidal environments. Understanding the inferred depositional environment, and observed stratigraphic relationships between various beachrock facies is critical to teasing out the local evolution of shoreline and relative sea level. The outcrops studied here record multiple episodes of Holocene sea level rise and fall. [ABSTRACT FROM AUTHOR]

The duration of shoreline occupation at a given sea-level, coastal response to sea-level change and the controls on preservation of various shoreline elements can be recognized by detailed examination of submerged shorelines on the continental shelf. Using bathymetric and seismic observations, this article documents the evolution and preservation of an incised valley and lithified barrier complex between −65 m and −50 m mean sea-level on a wave-dominated continental shelf. The barrier complex is preserved as a series of aeolianite or beachrock ridges backed by laterally extensive back-barrier sediments. The ridges include prograded cuspate lagoonal shoreline features similar to those found in contemporary lagoons. The incised valley trends shore-parallel behind the barrier complex and records an early phase of valley filling, followed by a phase of extensive lagoonal sedimentation beyond the margins of the incised bedrock valley. Sea-level stability at the outer barrier position ( ca −65 m) enabled accumulation of a substantial coastal barrier that remained intact during a phase of subsequent slow sea-level rise to −58 m when the lagoon formed. These lagoonal sediments are stripped seawards by bay ravinement processes which caused the formation of several prograded marginal cuspate features. An abrupt rise in sea-level to −40 m, correlated with melt-water pulse 1B, enabled the preservation of thick lagoonal sediments at the top of the incised valley fill and preservation on the sea bed of the cemented core of the barriers. This situation is unique to subtropical coastlines where early diagenesis is possible. The overlying sandy sediment from the uncemented upper portion of the barriers is dispersed by ravinement, partly burying the ridges and protecting the underlying sediments. The high degree of barrier or shoreline preservation is attributed to rapid overstepping of the shoreline, early cementation in favourable climatic conditions and the protection of the barrier cores by sand sheet draping. [ABSTRACT FROM AUTHOR]

Abstract: Late Pleistocene–Holocene shoreline sequences and associated shelf stratigraphy are described from a high gradient, high wave-energy shelf. Although shoreline preservation by overstepping is considered unlikely in such an environment as a result of intense ravinement during shoreline translation, the development and preservation of two distinct shorelines at −100m and −60m are evident and attributed to century to millennial scale periods of stillstand. During these periods shoreline equilibrium forms developed, characterised by early diagenesis of beachrock and aeolianite. Between these features the shelf is dominated by erosional surfaces reflecting ravinement during periods of slowly rising sea-level. Where shorelines are not preserved, areas of scarping in the ravinement surface, at similar depths to the adjoining shorelines, represent rocky headlands separating the sandy coastal compartments where the depositional shorelines formed. Shoreline preservation by overstepping was facilitated by rapid relative sea-level rise associated with meltwater pulses 1A and 1B, although the features preserved are likely to represent only the cemented cores of the coastal dunes and linear beachrock outcrops formed. The steep antecedent shelf gradient fostered the erosion of significant portions of the shorelines by ravinement, with loose sediment re-organised into a thick post-ravinement drape. The removal of shorelines during ravinement was countered by the rapid rate of sea-level rise and early sub-tropical diagenesis. It is clear that due to the combined effects of subtropical diagenesis, long periods of stillstand and drowning by meltwater pulses, the governing controls on shoreline preservation can be over-ridden by combinations of local (climatic) and global (eustatic) factors. [Copyright &y& Elsevier]