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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]

The morphodynamics and structure of barriers with persistent tidal inlets have been well studied. In contrast the stratigraphy and functioning of barrier systems with ephemeral inlets is poorly understood. This article examines the barrier‐inlet systems of two intermittently closed open lagoons or temporarily open closed estuaries on the east coast of South Africa. Multiple geophysical surveys using ground‐penetrating radar (GPR) were correlated with exposed sections of the barrier where inlet formation revealed the internal stratigraphy. Stratigraphic observations were placed in the context of the contemporary wave dynamics and mesoscale geomorphic evolution. The integrated databases reveal an absence of migrating channel features. Instead the stratigraphy is dominated by landward dipping sheets of alternating high‐ and low‐amplitude reflectors. These correlate with gravel, shell debris and heavy mineral‐lined beds formed by overwash processes. Where ephemeral inlet structures are preserved in the stratigraphy, their fills comprise aggrading, high‐amplitude reflectors, linked to washover infilling of the inlet mouth. Multiple small channels in the more distal portions of the barrier in georadar stratigraphy are related to channelized washover flow. These barriers often breach during high swell and are subsequently sealed during fairweather wave conditions. Time series analysis of waves and satellite imagery shows a link between storms from the south and breach events. This is consistent with the truncations in subsurface images and inferred barrier lowering by overwash channelling. These barriers experience quasi‐stable oscillations in their landward and seaward shore position, punctuated by periods of barrier rollover associated with the most intense storms. As overwash is responsible in part for both the constructive and destructive phases of the barrier, these barriers have low preservation potential. Persistent rollover driven by overwashing will terminate once accommodation space is eliminated and the barriers are eroded by storm activity. © 2019 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Terrestrial signals in marine sediment archives are often used for paleoclimatic reconstructions. It is therefore important to know the origin of the different terrestrial sedimentary components. The proximity to a river mouth is often the key location to determine the source. Especially in regions with strong ocean currents, such an assumption might, however, lead to considerable misinterpretations. To investigate the source of various terrigenous sediment fractions in southeastern Africa, a region with strong sediment redistribution, we have performed an extensive comparison between terrestrial material (pollen, plant lipids, detrital modes, and heavy minerals as well as bulk inorganic geochemical composition) from potential source regions and the same components in the adjacent coastal and continental shelf sediments. Onshore the proxy‐indicators reflect small‐scale diversity in sampling locations and associated environments (riverbank sediments, flood deposits, suspension loads, and soils). Nevertheless, the overall trends reflect significant environmental gradients along a SW to NE transect. We note a general comparability of the studied parameters between the continental and marine sediments regardless of their specific differences in transport and depositional characteristics. We propose that the influence of the Agulhas Current affects sediment deposition and distribution only seaward of the midshelf and that pockets of sediment remain preserved in the lee of coastal protrusions where they are protected from erosion. This study provides the essential prerequisite to allow the attribution of temporal variations of compositional changes in marine sediment cores to environmental changes in southeastern Africa. [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]

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]