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Depositional landforms and environmental change in the headward

vicinity of Dias Beach, Cape Point

SOUTH AFRICAN JOURNAL OF GEOLOGY, 2001, VOLUME 104, PAGE 101-114

Alexander Shaw & Peter Holmes

Department of Environmental and Geographical Science, University of Cape Town

Rondebosch 7701, South Africa

John Rogers

Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa

ABSTRACT

The southern portion of the Cape Peninsula is characterised by a number of geomorphic features that reveal evidence of fluctuating sea levels and past environments. These include pronounced wave-cut platforms and the remnants of fossilised dunes. This paper reports on a unique Late Quaternary sedimentary sequence preserved within a steepsided ravine between Cape Point and Cape Maclear at the southern tip of the Cape Peninsula. The site is situated headward of Dias Beach, where erosion has exploited a Cretaceous dolerite dyke that has intruded the Ordovician Table Mountain Group sandstone. The sediments have been incised during the Holocene, and the resultant gullies present exposed faces whose lithostratigraphy attests to a variety of geomorphic processes, which were active during the Late Pleistocene. The two main deposits reveal contrasting sedimentary structures, representing very different depositional environments. Immediately above the active dune cordon, which flanks the modern beach, a sequence of aeolianite (calcarenite) displays a variety of low-angle planar and high-angle cross bedding. This calcarenite is moderately cemented, but not completely lithified. Overlying this sequence, above an abrupt unconformity, is a massive colluvial deposit comprising unconsolidated, quartzose sand, as well as angular sandstone clasts in a number of lags. This deposit does not display any sedimentary structures. The uppermost section of the aeolianite was dated by thermoluminescence to 185 ka. The aeolian lithofacies is interpreted to be the product of aeolian deposition by onshore winds during the transition period following the 220 ka Penultimate Interglacial, the period when sea-level initiated its retreat towards the 150 ka Penultimate Glacial Lowstand. The colluvial deposit is interpreted as the midfan remains of an alluvial/colluvial fan formed by debris flow and sheetwash. Optically stimulated luminescence (OSL) dates from the base (83 ka) and the top (32 ka) of the main fan exposure suggest emplacement during the Last Glacial. Whereas the aeolianite is regarded as the product of an onshore depositional phase, the emplacement process of the colluvial fan suggests the increased presence of moisture. This, in turn, may imply a wetter environment for the southwestern Cape during the Last Glacial. It is known that the Last Glacial was generally drier over the summer rainfall region of South Africa, but the situation for the present winter-rainfall region remains unclear. The findings from this study support the notion that conditions for the winter-rainfall region of the southwestern Cape may have been wetter prior to the termination of the Last Glacial.