Transgressive-barrier and shallow-shelf interpretation of the lower Paleozoic Peninsula Formation, South Africa

Abstract

The Peninsula Formation of the lower Paleozoic Cape Basin is well exposed on the Cape Peninsula, where it comprises a 750-m-thick quartz arenite complex transgressively overlying back-barrier tidal flat deposits. Five sandstone facies are distinguished on the basis of distinct assemblages of sedimentary structures. The two lowermost facies are relatively thin and consist of northwestward (landward)-dipping plane-bedded sandstones, interpreted as barrier washover deposits, and small-scale back-barrier tidal channels, with cross-bedding of variable direction. At a higher stratigraphic level are large-scale channel deposits, which are ascribed to the lateral migration of tidal inlets. These sequences are characterized by ebb-oriented cross-bedding at the base, with smaller scale sets of longshore-directed or flood-oriented cross-bedding in the upper parts. A facies interpreted as beach foreshore, comprising seaward-dipping plane beds with multiple discordances and heavy mineral laminae, is occasionally preserved above the inlet sequences. The upper part of the Peninsula succession is dominated by tabular and lenticular sandstone bodies with megasets as much as 10 m thick showing complex internal organization of smaller structures. These intrasets are mainly trough crossbeds that are inclined either in the same direction as the major foresets or at a variable angle to them. This facies is compared with the tidal sand ridges and sand waves of North Sea type. A large proportion of the Peninsula tidal sand bodies migrated southwestward parallel to the shoreline; others are directed offshore. The considerable thickness of the Peninsula transgressive sandstone is attributed to gradual subsidence balanced by substantial sediment supply, introduced mainly by a braided alluvial complex to the northwest. This led to vertical stacking of facies. Barrier inlet migration was rapid compared with the rate of transgression, thereby obliterating many of the features characteristic of barrier island deposits. The several-hundred-metre thickness of tidal sand body deposits indicates that the water depth remained within the critical limits of tidal current dominance. This regime was eventually terminated by marine regression and the accumulation of fossiliferous lagoonal siltstones and glaciogenic diamictites. It is suggested that deposition of the Peninsula Formation was strongly influenced by storm processes. Extensive barrier washover sheet sands are attributed to storm surge. Furthermore, the two-part sequence within the tidal sandbar facies of a basal pebbly unit overlain by plane-bedded, fine-grained sandstone without heavy mineral segregation is identical to modern storm-generated sequences off Long Island, New York. The perpendicular to oblique relationship of shoreface channels to the tidal sandbars suggests incision by storm surge ebb currents, which deposited a pebbly lag on the bar toes.