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Abstract This paper investigates the complex interplay between allocyclic, autocyclic and antecedent controls on the development of the proximal portions of a wave-dominated, gravel-rich submerged delta. The inner shelf of the Orange River delta of southern Namibia is investigated using seismic stratigraphic methods and vibracore data, in conjunction with onshore electromagnetic data. The aim is to define and interpret the major seismic stratigraphic units and to examine the controls on the stratigraphic evolution of the area. The stratigraphy comprises a crystalline bedrock and sedimentary rock basement, the contact of which exhibits a break-in-slope in the palaeo-bathymetry surface. The basement is truncated by a subaerial unconformity which includes two palaeo channels incised into a bedrock palaeo-promontory offshore of the modern Orange River Mouth. The incision of these channels is attributed to episodic periods of river rejuvenation linked to tectono-eustatic events of the late Cretaceous and end Neogene, during which sea level dropped below the shelf break. The ensuing smaller glacio-eustatic sea-level oscillations of the Quaternary led to the formation of bypass valleys which did not incise the existing channels and allowed remnants of original (old), course channel fill to remain in the lower sequences of the valley fill. North and offshore of the bedrock palaeo-promontory, the basement is overlain by a widespread, thick (up to 3 m) gravel lag associated with repeated ravinement events that represent multiple cycles of subaerial and wave ravinement erosion including those of the last post-glacial transgression. This gravel lag represents the proximal portion of the world's largest diamond placer deposit. Gravel spit, barrier and linear beach facies occur between −70 and −55 msl and are interpreted to have developed under high rates of coarse sediment supply during a stillstand preceding MWP-1B. A constructional phase of delta progradation followed, interposed with backstepping which we consider representative of the final shoreline trajectory from MWP-1B through 1D. The overlying thick (up to at least ~70 m), fine-sandy to muddy clinoform unit is the proximal portion of the modern delta of the Orange River which has prograded during the Holocene and episodically accumulates fine sediments during flood events in modern times. Gravel deposits are winnowed by cyclical erosion processes in combination with the oceanographic regime to strongly partition the grain sizes of the shelf stratigraphy. Gravel deposits (e.g. lags and barrier beaches etc.) are preserved by inheritance of the sediment stock and by later sheltering from wave energy due to antecedent topography. Where gravel beaches and deltas formed, their preservation in the stratigraphy is enhanced due to existing low-gradient palaeo-slope and punctuated rises in sea level which overstepped the shelf geomorphology. Highlights • Examine controls on nearshore stratigraphy of Orange River's wave-dominated delta • Stratigraphy fractionated into gravel and mud • Related to sediment supply, multiple erosion cycles and energetic wave regime • Show evidence for meltwater pulses and wave sheltering from antecedent topography [ABSTRACT FROM AUTHOR]

On the SE African shelf, a submerged shoreline at a depth of 60 m is examined and its attributes compared between two shelf sectors with different morphologies, yet similar energy regimes. The aim is to assess the controls of antecedent conditioning on shoreline development and later preservation from transgressive ravinement. Using a combination of multibeam bathymetry and single-channel seismic profiles, the stratigraphy and morphology of the shoreline is investigated. Low-gradient bedrock examples reveal several distinctive seismic facies, including onlapping chaotic reflector packages which are interpreted as calcarenite rubble fields. These palaeo-shorelines possess planform equilibrium morphologies, including parabolic dunes and blowout forms along with relict shore platforms. They are strongly associated with incised valleys of last glacial maximum age which underlie the shoreline locations; these provide wide, back -barrier accommodation space during transgression. In contrast, palaeo-shorelines on the steeper-gradient shelf have a simpler stratigraphic arrangement. They are not as well preserved, are generally covered by thick drapes of sediment, and lack the elaborate planform morphologies of their lower-shelf gradient equivalents. Isolated incised valleys and the steep bedrock gradient limit accommodation space. The comparison indicates that antecedent bedrock slope and available accommodation are amongst the dominant controls on overstepping, and thus potential preservation, of palaeo-shorelines on the shelf. Lower-gradient shelves not only promote rapid shoreline translation but, together with wide, sandy back -barrier accommodation, also foster larger barrier volumes. In suitable climates such as in the Mediterranean and other sub-tropical areas, the ensuing shoreline stability promotes rapid and effective cementation of the barrier. In comparison, steep bedrock profiles with limited back -barrier accommodation have much lower preservation potential. Transgressive ravinement is more focussed on steep slopes, effectively removing more material during the ravinement process. The more dynamic environment may also reduce the effectiveness of diagenesis. The potential of beachrock and aeolianite palaeo-shorelines as submerged sea-level indicators may be optimal in low-gradient settings in Mediterranean to subtropical environments. [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]