The marine geology of Mossel Bay, South Africa
Doctoral Thesis
2014
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University of Cape Town
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Abstract
This thesis presents work undertaken to better understand the complex evolution of the terrestrial landscape now submerged by high sea levels offshore of Mossel Bay along the South Coast of South Africa. Three marine geophysical surveys and scuba diving were used to examine evidence of past sea-level fluctuations and interpret geological deposits on the seafloor. Additional geological mapping of coastal outcrops was carried out to link land and sea features and rock samples were dated using Optically Stimulated Luminescence (OSL). Geophysical investigations include a regional seismic survey extending from Still Bay in the west to Buffels Bay in the east out to a maximum water depth of 110 m; a high-resolution investigation of the Mossel Bay shelf using multibeam bathymetry, side-scan sonar and sub-bottom profiling; and a shallow seismic pinger survey of Swartvlei, the most prominent coastal lake in the Wilderness Embayment. This study presents 9 discrete seismic sequences, and describes major offshore geomorphic features such as submerged sea cliffs, palaeo-coastal zones and fluvial systems. Oscillation in sea level between ca. 2.7 and 0.9 Ma likely resulted in the formation of the prominent -45 m terrace, which separates a relatively steep inner from a low-gradient mid shelf. Beach and dune deposits span from Marine Isotope Stage 15 (MIS 15) (582 ka) to Recent based on an age model that integrates OSL ages and the established eustatic sea-level record. The most prominent deposits date from the MIS 6 glacial to MIS 5 interglacial periods and include incised lowstand river channels and regressive aeolianites that extended at least 10 km inland from their associated palaeoshorelines. The MIS 5 deposits include transgressive beachrock, an extensive foreshore unit which prograded on the MIS 5e highstand, and regressive beach and dune deposits on the shelf associated with the subsequent fall in sea level. MIS 4 lowstand incised river channels were infilled with sediment truncated during rapid landward shoreface migration at the MIS 4 termination. Lowenergy, back-barrier MIS 4/3 sediments are preserved as a result of overstepping associated with meltwater pulses of the MIS 2 termination. The MIS 1 sediment wedge comprises reworked sediment and is best developed on the inner shelf. Holocene highstand sedimentation continues to prograde. Accommodation space for coastal deposits is controlled by antecedent drainage pathways and the gradient of the adjacent inner continental shelf. The geological deposits on the emergent shelf indicate a greatly expanded glacial coastal plain that potentially received more rain feeding low-gradient meandering rivers and wetland lakes. These extensive wetland environments provided a rich source of diverse food types which along with abundant marine resources on the shoreline made the Southern Coastal Plain an ideal habitat for our ancestors.
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Includes bibliographical references.
Reference:
Cawthra, H. 2014. The marine geology of Mossel Bay, South Africa. University of Cape Town.