Abstract

Paleozoic reconstructions of Gondwana show southern Africa and Antarctica juxtaposed, with the wedge-shaped Falkland Plateau intervening in the southwest, the lines of contact between them forming a triple junction. Linear belts of Paleozoic subsidence developed along these lines of incipient fracture. Regional sedimentation patterns in the Ordovician Cape basin reflect this tectonic control.

In the southwestern Cape, bed-load-dominated streams terminated southward in fan deltas. The fan deltas were flanked by barriers with associated tidal flats and lagoons. Transgression by the mixed tide-and-wave-dominated shelf allowed gradual accumulation of 2000 m of quartzose sand. The succession is dominated by tabular and lenticular sandstone bodies with megasets as much as 20 m thick, showing complex internal organization of smaller structures. This facies is compared with the tidal sand ridges and sand waves of the North Sea. These tidal sand bodies formed parallel to the shoreline. Deposition was strongly influenced by storm processes. Reactivation of these tensional failed arms formed an elongate embayment in the east, the Natal embayment. There, fan delta systems built out into a macrotidal gulf, where powerful tidal currents fashioned channels and bars trending perpendicular to the shoreline in a pattern analogous to the modem Ord River delta. Whereas the coastline in the southwestern Cape was relatively straight and open, the funnel-like configuration of the eastern gulf was probably responsible for amplification of the tidal range.