The seismic stratigraphy of the inner shelf of southern Namibia: The development of an unusual nearshore shelf stratigraphy

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 palaeobathymetry 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 glacioeustatic 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 MVVP-1B through 1D. The overlying thick (up to at least similar to 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.