Late Cenozoic Evolution of the Cape Coast Between Knysna and Cape St. Francis, South Africa

Abstract

The southeastern coast of South Africa's Cape Province underwent complex geomorphic evolution during the late Cenozoic, leaving a variety of erosional forms and a detailed, complementary record of distinctive sediments. The latter include several new lithostratigraphic units and paleosol horizons. An almost ubiquitous planation surface, the 200-m Coastal Platform, is associated with the fanglomeratic and deltaic Keurbooms Formation (late Tertiary?). A major sea level at +120 m truncated a laterite paleosol and was followed by accumulation of land rubble and littoral deposits to 101 m, constituting the Formosa Formation. Then follow sea level stages at +60, +30, and +15–20 m, as well as several generations of weathered eolianite, including the Brakkloof Formation with the deep superposed Brakkloof Soil. Beaches at +5–12 m, with thermophile mollusca and C14 dates of >40,000 yr mark the Swartkops horizon, probably of Eem interglacial age. Next are cryoclastic screes and cave deposits, and ultimately the podsolic Brenton Soil of the Würm Interpleniglacial. Transgressive eolianites and coastal dunes after 16,000 BP were interrupted by pedogenesis ca. 7500 BP and stabilized after 4200 BP when sea level reached +2.5 m. Geomorphic instability in stream valleys after 1000 BP was followed by man-induced activation of the coastal dunes, since the late 18th century. Environmental patterns accompanying this succession at various times included (1) semiarid pediplanation, (2) interior dune formation, and (3) intensive, cold-climate denudation, all under open vegetation in what is now closed, humid forest; by contrast, some of the more aberrant paleosols indicate warmer, perhumid conditions.