Abstract
Abstract An inorganic geochemical investigation of mudrocks and sandstone from the southern Bredasdorp Basin, off the south coast of South Africa was carried out to unravel the provenance, paleoweathering, and tectonic setting of the basin. Seventy-seven representative samples from exploration wells E-AH1, E-AJ1, E-BA1, E-BB1, and E-D3 underwent geochemical analysis involving major and trace elements. The major oxide compositions show that the sandstones could be classified as sub-arkose and sub-lithic arenite. The provenance discrimination diagrams based on major oxide geochemistry revealed that the sandstones are mainly of quartzose sedimentary provenance, while the mudrocks are of quartzose sedimentary and intermediate igneous provenances. The discrimination diagrams indicate that the Bredasdorp sediments were mostly derived from a cratonic interior or recycled orogen. The bivariate plots of TiO2 versus Ni, TiO2 against Zr, and La/Th versus Hf as well as the ternary diagrams of V–Ni–Th∗10 suggest that the mudrocks and sandstones were derived from felsic igneous rocks. The tectonic setting discrimination diagrams support passive-active continental margin setting of the provenance. Also, the closely similar compositions of the analysed samples and recent sedimentary rocks of the East African Rift System perhaps suggest a rifted basin tectonic setting for the Bredasdorp Basin. Chemical index of alteration (CIA) indices observed in the sandstones suggest that their source area underwent low to moderate degree of chemical weathering. However, the mudrocks have high CIA indices suggesting that the source area underwent more intense chemical weathering, possibly due to climatic and/or tectonic variations.
Highlights
The geochemistry of clastic sedimentary rocks is a vital tool used in the study of provenance, paleoweathering conditions, and tectonic setting as well as to constrain the geodynamic development and composition of the upper continental crust (UCC) [1,2]
The milled samples were dried at 100°C (Weight A) and heated at 1,000°C (Weight B) for a period of at least 3 h to oxidize S and Fe2+ in order to determine the loss of ignition (LOI)
The mudrock samples are plotted in both the quartzose sedimentary provenance and felsic igneous provenance fields (Figure 11a), whereas the sandstones fall within the quartzose sedimentary provenance field (Figure 11b)
Summary
The geochemistry of clastic sedimentary rocks is a vital tool used in the study of provenance, paleoweathering conditions, and tectonic setting as well as to constrain the geodynamic development and composition of the upper continental crust (UCC) [1,2]. Some designated trace elements such as La, Y, Sc, Cr, Th, Zr, Hf, Nb, and major oxides especially TiO2 are sensitive indicators of the source rocks, provenance, paleoweathering and paleoclimatic conditions, and tectonic setting [3,4]. This is due to their comparatively low mobility and insolubility during sedimentary processes [5,6,7]. The distribution of these major oxides and trace elements in mudrocks and sandstones gives evidences or clues to the geological processes, provenance, and tectonic settings of their respective sources [6,7,8,9,10,11]
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