Late Cenozoic updated foraminiferal stratigraphic and palaeoenvironmental framework for the southwestern shelf of Africa

Abstract

Marine sediments from the western shelf of southern Africa provide key information on the stratigraphy and palaeoenvironment of the region. Palaeoenvironmental research on Miocene shelf foraminifera is fragmentary. An understanding of the stratigraphy and palaeoenvironment can inform on exploration studies and climatic impact on marine sedimentation. Two marine cores (core 128 at 262 m; core 1408 at 303 m water depth) from the western shelf of South Africa and Namibia were studied for their strontium isotope (⁸⁷Sr/⁸⁶Sr ratios) stratigraphy, foraminiferal biostratigraphy and microscopy. Five depositional units based on their sedimentary and mineral compositions occur in core 128, namely: foraminifera-rich sand; black pelletal phosphorite unit I; glauco-phosphatic unit; calcareous glauco-phosphatic unit and black pelletal phosphorite unit II. Core 1408 contained three main units, namely: foraminifera-rich mud; brown-black phosphorite unit and shelly phosphorite unit. The palaeoenvironment of core 128, based on foraminiferal ecological indicators, show a relatively deeper marine, nutrient-rich environment in the early Miocene (20.45 Ma; indicator species Globigerina praebulloides and Globigerina falconensis), while late Miocene foraminifera in the basal foraminifera-rich mud unit (9.80–5.80 Ma; indicator species Globoquadrina dehiscens) of core 1408 revealed a slightly more oxygenated environment with warmer conditions. Phosphorites and shallow water foraminifera dated to the Pleistocene (2.72–1.09 Ma; indicator species Globoconella inflata and Globorotalia truncatulinoides) in both cores indicate eutrophic conditions and high productivity. Fragmentation of tests (19–54 %) are interpreted to be as a result of upwelling and transport under increasing sea-level amplitude. These two records support key environmental characteristics from upwelling conditions and major temperate water influence in the early Miocene to major fluvial influence in the Pliocene, and strong upwelling and eutrophic conditions in the Pleistocene.