Mapping diamondiferous palaeo-shorelines in complicated terrain: seismic and GIS-based methods from the inner shelf of southern Namibia

Abstract

The inner shelf of Namibia’s southwestern coastline comprises the submerged extension of the world’s largest diamond placer deposit. The onshore raised beach deposits, which are constrained by wave-cut cliffs and platforms, bevelled into the schist bedrock during palaeo-sea level stillstands, are largely mined out. As land-based operations near their practical limit, spatially accurate resource estimation of the inner shelf becomes increasingly critical to extend the life of mine of this extensive but low-grade resource. This study analyses a comprehensive seismic-reflection dataset, to produce a 50-m cell size bedrock morphology surface for the inner shelf. The data reinforce previous results and show that inherited structural influence exerts the overarching control on bedrock morphology at the scale of the study area. However, within this framework, the detailed data have now facilitated the zonation of the study area into discrete structural zones from which subtle coast-parallel bedrock gradient changes, that we interpret to represent submerged palaeo-shoreline morphologies (i.e. bedrock gradient changes associated with wave-cut cliffs and platforms), have been extracted. These bedrock gradient variations are contextualised within areas of similar glacial isostatic adjustment (GIA) response, which lends weight to the GIS-based palaeo-shoreline interpretation. Although these may be considered composite features that are the product of 42 million years of shoreline transgression and regression, the more recent occupation of the shoreline at these depth intervals, during the late Pleistocene/Holocene, forms a critical upgrading component to the offshore resource. Our study proposes a\u2009−\u200920 msl shoreline, the most recent occupation of which we link to the peak of marine isotope stage (MIS) 5c, and a\u2009−\u200930 msl shoreline that is linked to the time shortly after MIS 5c and the peak of MIS 5a. The most recent period of shoreline occupation for the deepest shoreline at\u2009−\u200940 msl is attributed to slowly rising sea level between 10.8 and 10.6 ka. This information is critical to the development of the resource estimation philosophy for the offshore extension of mining operations.