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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]

Antecedent conditions and geology influence bedform dynamics and other sea-bed bottom current interactions on the continental shelf. However, the nature of this influence has not been well constrained, particularly on current-swept shelves. A variety of bedforms (narrow and broad sediment ribbons, comet marks and subaqueous dunes) are revealed by high resolution multibeam bathymetry and side scan sonar surveys of the mid-to outer "Wild Coast" shelf along the east coast of South Africa. The bedforms demonstrate a relationship to the geological framework of the shelf. The various bedforms occur in conjunction with bedrock-incised valleys and topographic depressions filled with transgressive sediment and bounded laterally by prominent rocky outcrops. These sediment sources are siphoned and mobilised by the Agulhas Current to create the contemporary shelf bedforms. Mechanisms of bedform development are associated with the interplay between the loci of sediment supply and associated grain size on the one hand and the current velocity and pathway of the Agulhas Current on the other. Sediment ribbons form parallel to the flow path of the Agulhas Current in narrow or broad forms that can be differentiated primarily by sediment grain size. With increased sediment availability from nearby bedrock-bound depocenters these transition along shelf to 2D dunes. A cross-shelf transition to 3D dunes on the outer shelf is marked by increased current velocity, grain size and decreased sediment availability outside of rock-bound depocenters. Rock outcrops affect current velocity and flow paths sufficiently to favour localised deposition over sediment stripping from the shelf. Comet mark forms develop from interactions between availability of finer sediment and perturbations of current flow paths by rock outcrops. This creates conditions that favour localised sediment scouring over deposition. With variability in sediment size, grain size and current flow, dunes fluctuate between active (sharp crested), inert (round crested) and degraded (flat crested) states. A lack of available sediment to replenish outer shelf dunes sees prolonged stasis, eventual dune splitting and breakdown under progressive current influence. Mid-shelf dunes have a greater capacity to recover from periods of stasis and short-term degradation due to greater availability of sediment. Dune migration rates are slower than previous regional estimates. Reasons for this are inconclusive and may be multifaceted. Possible reasons include inertial constraints, prolonged downcurrent reformation periods and longer dune relaxation associated with coarser grain sizes and a paucity of sediment on the outer shelf coupled with the Agulhas Current core migrating further in and offshore. This may explain the stability of the large barchan dunes and reduced movement of other sharp crested dunes relative to the regional migration rates. • Document sediment dynamics on a geostrophic-current dominated shelf. • Reveal influence of geological control on dune development. • Core of Agulhas Current directed by changes in shelf physiology. [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]

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]

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]

Abstract: The evolution of several incised valley systems from the KwaZulu-Natal shelf is compared over time. These represent the shelf extension of the collective Durban Harbour drainage system, and the Mgeni, Mhlanga, and Mdloti Rivers. Two main ages of incision are apparent; early Santonian (Cretaceous), and late Pleistocene/Holocene. The early valleys formed by tectonic controls, namely phases of higher frequency base level fall superimposed on lower frequency, higher order transgression. The results are complex networks of stacked compound valleys that have been exploited by some of the younger networks formed by glacio-eustatic fall prior to the last glacial maximum (LGM) of ~18Ka BP. The valley fills have evolved from high energy basal fluvial deposits, low-energy central basin fines, mixed-energy estuarine mouth plug deposits, clay-rich flood deposits through capping sandy shoreface deposits. The earliest fills are dominated by central basin deposits and were underfilled as a result of low gradient and limited sediment supply. The more recent late Pleistocene/Holocene fills have significantly thicker fluvial deposits as a result of increased gradient and stream competence during the later stages of valley and shelf evolution. The youngest valleys show a situation of differential evolution along the valley length due to varying rates of sea level rise in the Holocene. Initial rapid sea level rise caused drowning and overstepping of the outer segment of the incised valley, whereas slower rates of sea level rise in the late Holocene caused shoreface ravinement of the inner-mid segments of the valley. This differential exposure to accommodation has resulted in a sedimentological partitioning between tide-dominated facies in the outer valley segment and river-dominated facies in the inner segment (cf. Cooper, 2001). [Copyright &y& Elsevier]

Abstract: Medium scale clinoforms within sandstones of the Vryheid Formation, northern KwaZulu-Natal, South Africa have in the past been interpreted as the products of Gilbert-type or river-dominated deltaic deposition. Detailed sedimentary logging of these rocks reveals a tempestite association, overlain by a series of moderate to very large scale climbing bedforms. These are intersected at various stratigraphic levels by massive sandstone filled channels, some with subordinate rippled bases, which are orthogonally orientated to the largest scale bedding surfaces (clinoforms) of the climbing bedform association. The sedimentary architecture is dominated by the south-easterly, south-westerly and north-westerly dipping clinoforms, all orientated oblique to the east–west trending palaeo-coastline. Local dip direction reversals are commonplace. This is at odds with Gilbert-type or river-dominated deltaic settings where: (1) sediment progrades normal to the shoreline; (2) distributary channels do not intersect the underlying deltaic foresets orthogonally; and (3) wave action is typically limited. We subsequently interpret the succession as a shelf sand ridge that evolved on a subaqueous delta, fed by sediment exchange from the upper to lower shoreface via a series of rippled scour depressions. To foster such thick (∼30m) shelf ridge development, it is likely that these remained attached to the shoreface for a significant period of time. In light of these findings, caution should be adopted in applying a blanket deltaic interpretation of shallow marine deposits comprising similar medium scale clinoforms. [Copyright &y& Elsevier]

Abstract: The late Holocene evolution of the Durban Bight shoreface is documented on the basis of core, seismic and micropalaeontological data. From a sedimentological perspective the shoreface comprises three fining-upward packages resting on either granule or pebble layers with well-defined erosional bases. The best developed of these is a lowermost pebble conglomerate that has been tied to a regionally developed erosion surface (g). The surfaces are interpreted as having formed on the lower shoreface during sediment exchange with the upper shoreface via storm-driven erosion. The succession is capped by a monotonous series of small sandy climbing bedforms, terminating in a thin, mudball-rich layer. The mudballs are the product of a large-scale fluvial flood that occurred in 1987. From a palaeontological perspective, the shoreface shows fluctuations in nutrient supply and temperature, identified by the relative abundance of F. profunda in terms of the high small placolith to lower photic species ratios, in addition to the changing bridge angle of Gephyrocapsa sp. Where storm-event layers are recognised, corresponding peaks in sea surface temperature (marked by high-bridge Gephyrocapsa assemblages) and low nutrient contents (low relative abundances of F. profunda) occur. Periods between storm events are characterized by cooler sea surface temperatures and increased nutrient contents. The controls on such fluctuations are the presence of the warm, oligotrophic Agulhas Current that occasionally meanders and impinges on the shelf; the local coastal upwelling triggered when the current migrates offshore; and the relatively lesser influences of the fringing fluvial sources during flooding. We consider that the event layers recorded in the shoreface succession are persistent stratigraphic features formed at times of increased storminess associated with the inshore meandering of the Agulhas Current. Where significant exchange of sediment occurs in the offshore as in the present case, this sediment is lost from the system and the shoreface recovery is stunted. [Copyright &y& Elsevier]

Acoustic anomalies in seismic records have revealed that gas-charged sediments are very common features in the coastal environments around the world. The ubiquitous gassy sediments challenge the effective acoustic mapping of shallow stratigraphy by seismic means, as well as having an important influence on environmental issues related to the coastal zone occupation and management. This paper documents examples of gassy sediments from coastal lagoons, estuaries, rivers, bays and the inner shelf and nearshore environments of Brazil, Argentina and South Africa. Seismic echograms from selected areas show several gas-related anomalies, which present distinctive morphologies for sediment-trapped gas, leaking or free gas discharge into the water column. In several places the gas-charged sediments occur in areas of palaeo-topographic lows related to fluvial channels and valleys that developed in the coastal zone due to sea level oscillations during the Quaternary period. This forcing by palaeo-topographic features results in the occurrence of shallow gas being controlled in most coastal sites by the previous environmental scenario, the stratigraphic arrangement of the transgressive infilling elements, and the local hydrodynamic conditions. [ABSTRACT FROM AUTHOR]

The intertidal zone and raised shore (supratidal) platform at Mission Rocks, northern KwaZulu-Natal coast, South Africa, consists of fracture-bounded Holocene beachrock units, and megagravel (boulder) deposits. This area is microtidal and wave-dominated, with swells approaching from the southeast. The area is also influenced by storm wave events. The seaward edge of the intertidal platform is undercut, leading to the formation of overhangs, which are modelled here as cantilevered beams. This analysis shows that beam volume/weight does not control failure because relatively short, thick beams (l:h < 2) are less susceptible to failure, and the beam length-to-thickness ratio is important. This accords with tensile strength testing which demonstrates that the bedding planes are weak and that saturated beachrock (i.e., at the platform edge) is ~25% weaker than unsaturated rock. Fracture, block and boulder analyses of seven coastal sections, and their geomorphological features (elevation, slope, terrain ruggedness), reveal three key relationships: (1) The number of fracture intersections and fracture density are higher in the raised shore platforms than in the intertidal, which is relatively unfractured, lower in elevation and less rugged than the raised shore platform. These parameters correlate positively with the number of fracture-bounded blocks. (2) Fracture-bounded blocks on the raised shore platform (n = 145) have a mean shape ratio of 2.25 and are 10–20 m2 in size, while the disarticulated boulders (n = 172) have a mean shape ratio of 1.3–1.5 and are 2–3 m2 in size. It is considered that the reduction in shape and size of the boulders is the result of fracture-bounded blocks being quarried and transported from the raised shore platform, and not from the intertidal platform, during high-energy storm events. (3) Coast-perpendicular fractures in the intertidal platform deepen and widen towards the sea, forming gullies in which wave run-up and retreat was observed. A relatively good negative correlation (r2 = 0.7) was found between the number of these gullies and values of maximum terrain ruggedness which suggests that platform lowering occurs when an appropriate fracture network (relatively dense and suitably orientated) receives unimpeded swells and waves. Overall, we find that the evolution of the beachrock platform in terms of ruggedness and downwearing is influenced, at least in part, by the fracture networks present, most notably in terms of their orientation, density, persistence and intersection geometry. • Intertidal platform ruggedness decreases with increasing gully formation. • Disarticulated boulders most likely derive from quarrying of raised shore platform. • Platform slope correlates weakly with fracture length and density. • Collapse of cantilevered beachrock beams controlled by fracture geometry, not mass. [ABSTRACT FROM AUTHOR]