Shelf architecture and recent sediment stratigraphy of the Chameis Bay area, southern Namibia

Abstract

Chameis Bay is located about 115 km north of the Orange River mouth and falls within the Sperrgebiet, an area which hosts the world’s largest gem diamond deposit. Although significant quantities of diamonds have been recovered both on land and from offshore deposits in the Chameis Bay area, the marine geology of this important tract of coastline has not previously been described in the scientific literature. Here, we report the nearshore geomorphology and seismic stratigraphy offshore of Chameis Bay through analyses of bathymetrical and seismic datasets. These data have been complemented with lithogical data obtained from 70 reverse-circulation boreholes which helped to confirm and constrain the sedimentary stratigraphy of the study area. These datasets identified four major lithological units; viz. a Precambrian basement which predominates as the footwall in the nearshore regions of the study area, a Cretaceous clay unit that represents the offshore footwall lithology and two unconsolidated Cenozoic sedimentary units. The distribution of these unconsolidated sediments is strongly controlled by the ambient accommodation space which can be quantified by considering the architecture of the respective footwall units. Architectural features within the study site include two prominent wave-cut platforms, two coast-parallel sea cliffs, and a shelf-break formed at the contact between the Precambrian basement and the Cretaceous clay footwall. Accommodation space exists on the seaward of the two wave-cut platforms, which is cut into the Precambrian basement footwall and which lies below the fair-weather wave base, and at the break in slope at the contact between the two footwall units. The former accommodation space is most notable for gravel entrapment and preservation since gulley-controlled jointing and erosional depressions at lithological contacts represent ‘fixed’ trapsites from which coarse material is less likely to be remobilised. In contrast, the trapsites formed on the soft Cretaceous clay footwall are regarded as ‘mobile’ trapsites since they can be easily reconfigured by continuing erosional processes. As a result, the gravel bodies found above the Cretaceous clay are generally thin and poorly developed. The implications of these two different trapsites are briefly discussed in terms of diamond preservation potential, where anticipated diamond sources to the Chameis Bay near-shelf include the Orange River mouth as well as material that has been reworked from proximal sources. These results represent the first detailed description of the marine geology of the Chameis Subterrane thrust sheet and complement existing understanding of the Sperrgebiet’s marine geology which largely derives from study sites on the Oranjemund Subterrane where linear beaches predominate.

Graphical abstract