The stratigraphy and sedimentology of the Bredasdorp group, southern Cape Province

Abstract

The Cainozoic Bredasdorp sediments along the south coast of the Cape Province, South Africa, have come under investigation through a few intermittent studies undertaken since the early part of the century. In this presentation the literature is reviewed and a stratigraphic subdivision, based on both lithostratigraphic and biostratigraphic principles, is applied to the sediments of the Bredasdorp Group. The Bredasdorp deposits can be classified, according to origin, as shallow marine and aeolian. The marine deposits are subdivided into the Pliocene De Hoopvlei and the Middle to Late Pleistocene Klein Brak Formations. The Late Pliocene to Early Pleistocene Wankoe Formation, the Late Pleistocene Waenhuiskrans Formation and the Holocene Strandveld Formation constitute the coastal aeolian deposits. These marine and marine-related (aeolian) formations, characterised by calcareous elastics have been grouped together in a newly defined Bredasdorp Group. In order to construct a depositional model for the Bredasdorp Group, various facies have been identified on the basis of geometry, lithology, fossil contents, palaeocurrent data, biogenic and sedimentary structures. The facies are related to environments constituting a shoreline setting with offshore, transitional, shoreface, foreshore and backshore zones, followed by back-barrier lagoons, estuaries, backshore dunes and coastal dunefields associated with transgressive/regressive shorelines. The various deposits, as well as distinctive geomorphological features, are correlated with the relative sea-level movements throughout the Cainozoic, which have shaped the southern Cape coastal plain. Sea-level curves for Southern Africa, drawn by several authors are compared. A relative sea-level curve is constructed for the south coast of South Africa. Several Early Cainozoic transgression/ regression cycles are recognised at places along the South African coast. The earliest cycle started in the Palaeocene and was followed by a less pronounced transgression/regression cycle occurring in the Late Eocene to Early Oligocene. Remnants of surfaces related to these cycles are recognised in the Southern Cape Province, but these are interpreted as products of subaerial processes. The next cycle, reaching a transgressive maximum of c. 180m, started in the Miocene and terminated in the Early Pliocene. Again, no evidence of marine deposits is preserved on this marine-planed surface. The Early Pliocene transgression reached a maximum present-day elevation of c. 120m. Marine planation of the coastal platform took place during the transgression, whereas the De Hoopvlei Formation situated below 120m, was deposited during the subsequent Late Pliocene regression. The Wankoe Formation was deposited during the same regression as backshore dunes and coastal dunefields. During the Quaternary transgression/regression cycles, of which at least three are indicated, the transgression reaching a maximum of about 50m, in places eroded part of the Neogene Bredasdorp Group. The Klein Brak Formation (preserved below 20m) was deposited during Middle to Late Pleistocene regressions. The Waenhuiskrans Formation, which is extensively developed on the present-day continental shelf, was deposited during this regression with sea-level receding to about -130m below present sea-level. The aeolian Strandveld Formation, which is still being deposited, originated from the Flandrian transgressive maximum at the start of the Holocene.