Anatomy and stratigraphic evolution of a shelf bypass valley system: Lessons from the Namibian continental shelf

Abstract

The stratigraphic architecture and evolution of a bypass alluvial valley system on the wide and deep Namibian shelf is investigated using a dense, pseudo-3D grid of ultra high-resolution sub-bottom profiler data, high-resolution multibeam and backscatter data, and >4900 cores. The channel morphology is defined by changes in terrain slope, with steeper slopes hosting more sinuous and narrower channels. Nine seismic units were identified in the <10 m-thick fill. Acoustic basement of the area comprises Precambrian gneisses and schists of the Namaqua Metamorphic province, and schists, phyllites and quartzites of the Gariep belt. These crop out on the shelf and form the framework within which the valley is located. A subaerial unconformity developed during an episode of Late Campanian hinterland uplift during which the channel system was initiated.

The basal fills of the channels (Units B and C) comprise weathered and reworked derivatives of Unit A, namely saprolites and balls of mudrock that were reworked by tidal and wave processes. These represent a long-term hiatus (weathering episode and landscape exposure of the Lower Palaeocene), followed by the transgressive flooding and filling of the channels during surfzone migration. After a subsequent hiatus shallow marine sandstones were deposited during the Early Eocene. The remaining valley fill comprises unconsolidated sediment of varying size classes. A gravel body (granules to boulders) dominates this fill, and represents initial fluvial deposition, followed by reworking during multiple phases of transgression and regression up to and including the Holocene. The capping succession of sands and muds represents the modern sediments of the shoreface and shelf; however, these are constrained to the original valley form and reflect strong geological control and lack of accommodation on the valley interfluves.

The fill succession and architecture of this shelf bypass valley contrast markedly with incised valley fills, where multiple regressive transgressive cycles form compound valleys. It developed over a much longer period of geologic time, is dominated by transgressive sediments and stratigraphic surfaces, is exceptionally thin and lacks multiple incisions. The lack of connection to a terrestrial drainage system for at least the past million years has contributed to the lack of incision and scour of accumulated sediment during lowstands. Such a sequence in the geologic record could easily be misinterpreted if the overall valley depositional setting was not recognised. Given the occurrence of many contemporary low gradient and deep shelves along passive margins globally, as well as in the geologic record, such systems are certainly more common than presently thought.