Pleistocene Shoreline Deposition and Sea-level History at Swartklip, South Africa

Abstract

ABSTRACT A 50-m-thick succession of poorly cemented sandstone is exposed along the northern shore of False Bay, South Africa, This Late Pleistocene succession is attributed to shorezone-estuarine deposition in the last interglacial and dune activity during the highstand and early in the succeeding sea level withdrawal. Facies analyses allow us to establish that the last interglacial sea level high (120± ka) locally reached an elevation between the upper and lower boundaries of a washover sequence that is 5.1 to 8.0 m above present mean sea level. Four depositional facies are recognized on the basis of distinct lithologies and assemblages of sedimentary structures. Beachrock is present intermittently in the present intertidal zone as a seaward-dipping unit of flatbedded sandstone capped by calcrete. We interpret this facies as a beach foreshore wedge which originated in warmer climates of the 135± ka sea level high, and which developed its calcrete crust in the lowstand which immediately followed. Subsequent sea level rise at 120± ka and shoreline progradation is reflected in a two meter-thick estuarine fill containing Tomichia ventricosta. A third facies comprises landward-dipping sheetsands which overlie the estuarine fill. On the basis of facies relationships and primary sedimentary structures, this unit is attributed to washover fan deposition. The last and dominant facies comprises calcite-cemented sandstone arranged in sets of very large-scale crossbeds. Pulmonate gastropods occur throughout. Poorly developed calcretes interrupt the succession. Deposition of this facies is believed to have occurred as parabolic dunes in a coastal foredune complex near the start of the Wurm (Wisconsinan) glacial climate when atmospheric circulation was more intense. Modern pedotubule and hardpan calcretes cap the succession.