Abstract
The Cretaceous sandstones of the Bredasdorp Basin were investigated to recognize their composition, provenance, and tectonic setting. Ninety-two samples of sandstones from exploration wells E-AH1, E-AJ1, E-BA1, E-BB1, and E-D3 were investigated using both petrographic and X-ray diffraction (XRD) methods. Petrographic studies based on quantitative investigation of the detrital framework grain shows that the Bredasdorp sandstones chiefly consist of quartz (52.2–68.0%), feldspar (10.0–18.0%), and lithic fragments (5.0–10.2%). These sandstones are mostly fine grained, moderately well-sorted, and subrounded to rounded. The modal composition data shows that the sandstones could be classified as subarkosic arenite and lithic arkose. Such a composition of the sandstones perhaps indicates the interplay of pulses of fast uplift of the source area and rapid subsidence of the Bredasdorp Basin, with subsequent periods of calmness within the transgressive-regressive sequence in a rift tectonic regime. The provenance ternary diagrams revealed that the sandstones are mainly of continental block provenances (stable shields and basement uplifted areas) and complemented by recycled sands from an associated platform. The tectonic provenance studies of Bredasdorp Basin revealed that the sandstones are typically rift sandstones and have undergone long-distance transport from the source area along the rift. In the regional context of the evolution of the Bredasdorp Basin, the results presented in this study inferred that the basin developed on a rift passive setting (trailing edge) of the stable continental margins.
Highlights
The Bredasdorp Basin situated in the southeast of Cape Town and west-southwest of PortElizabeth Figure 1 is a south-easterly trending rift basin that is characterized by half-graben structures.These half-grabens are mostly made up of the Cretaceous and Cenozoic rift to drift strata
The ternary plot of quartz-feldspar-lithic fragments (Qt-F-L) on the background provenance diagram of [6] revealed that the samples fall in the transitional continental and craton interior fields of the continental blocks Figure 12, suggesting that the sandstones were derived from stable shields and platforms or uplifted areas, which marks plate boundaries and patterns of intraplate deformation that divide the continental blocks
Petrographic and modal composition studies show that the main detrital components of the sandstones are quartz, feldspar, and lithic fragments
Summary
The Bredasdorp Basin situated in the southeast of Cape Town and west-southwest of PortElizabeth Figure 1 is a south-easterly trending rift basin that is characterized by half-graben structures.These half-grabens are mostly made up of the Cretaceous and Cenozoic rift to drift strata. The Bredasdorp Basin situated in the southeast of Cape Town and west-southwest of Port. These half-grabens are mostly made up of the Cretaceous and Cenozoic rift to drift strata. The Bredasdorp Basin, which is located off the south coast of South Africa, containing mainly sandstones with subordinate mudrocks, was investigated to interpret the provenance. The use of a framework or detrital mineral composition to unravel the tectonic provenance of sandstones was first reported by [2] and has subsequently undergone significant modification [3,4]. Provenance studies of sandstone have shown that different rock types (i.e., sandstone) contain
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