Bathymetry of the South African Continental Shelf

de Wet, Willem Myburgh

Bathymetry of the South African Continental Shelf

Master Thesis

2013

Publisher

University of Cape Town

Department

Department of Geological Sciences

Faculty

Faculty of Science

Abstract

South Africa has an extensive coastline offshore of which lies the prominent South African continental shelf, a relatively flat extension of the onshore coastal plain. The continental shelf is host to major mineral and petroleum deposits, home to South Africa’s major sea fisheries and full of navigation hazards. Therefore, knowledge of the seafloor features, or bathymetry, of the continental shelf is essential to understanding both its long-term geological evolution and present-day use for resources and navigation. Unfortunately there has been little advancement in our knowledge of the South African continental shelf since the marine studies of the 1970’s and 1980’s which culminated in the “Bathymetry around Southern Africa” map of Dingle et al. (1987). Although bathymetric mapping equipment and techniques have greatly improved during the last few decades, very little high resolution bathymetric data of the South African continental margin are currently available for scientific use, with the majority of the high resolution multi-beam echo-sounding bathymetric surveys being undertaken by privately owned mineral exploration and mining companies (such as De Beers, Alexkor, Petro SA, Petroleum Agency of South Africa, etc.), the Council for Geoscience and the South African Navy and Hydrographic Office. More recent advances in satellite altimetry have had a major impact on ocean floor bathymetric mapping especially in deep ocean areas where the sea surface generally reflects the underlying bathymetry. The Department of Agriculture, Forestry and Fisheries (DAFF) annually collect single-beam echo-sounding data in order to monitor the abundance of fish species along the South African continental shelf and along with that collect seafloor bathymetry as an additional benefit. The aim of this project is to create a detailed bathymetric map of the continental shelf of South Africa by using digital single-beam echo-sounding data collected by the Fisheries Division of the DAFF over the last two decades. The bathymetric dataset of ±7 million single-beam echo-sounding data points was manually processed, gridded and exported to produce a detailed bathymetric map of the entire South African continental shelf between the Orange River mouth and Kosi Bay complemented by Satellite Altimetry data from the ETOPO 1 – 1 Arc-Minute Global Relief Model (Amante and Eakins 2009) for the deep ocean area adjacent to the continental shelf. The single- beam bathymetric data were collected by the F.R.S. Africana II and F.R.S. Algoa vessels using SIMRAD EKS-38, EK 400, EK 500 and more recently the EK 60 single-beam echo-sounders along with a the SIMRAD ES38B split beam transducer. The West Coast and South Coast margins have the greatest bathymetric detail due to DAFF’s Cape Town base of operations, whilst the East Coast margin is less detailed due to fewer research campaigns in this area. The Bathymetric Map of the South African Continental Margin produced in this thesis reveals several new and more detailed bathymetric features. New bathymetric features include the northern extension of the Olifants Valley submarine canyon, details of the rocky inner shelf related to glacial period sea level lowstands, as well as the coast parallel wave cut terraces and palaeo dune ridges on the middle shelf between Cape Seal and Cape Recife. Other prominent bathymetric features such as Childs Bank, Cape Canyon, Cape Point Valley, the offshore submerged river valleys of the Breede and Gouritz Rivers and the east-west trending, basement anticlinal ridges situated at the southernmost extent of the Agulhas Arch were revealed in greater detail by this study. The underlying geology, physical Oceanography, drainage patterns as well as eustatic sea-level fluctuations can all be linked to the bathymetry of the continental shelf, which is why this thesis examines the influences of each of these factors on the seafloor morphology around the South African coastline.

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