A middle Miocene to Quaternary sedimentary and palaeoenvironmental record from the western continental shelf of South Africa

Abstract

The sedimentary record of the western South African continental shelf is condensed compared to the continental slope and contains erosional unconformities, owing to periods of non-deposition, eustatic sea-level fluctuations, episodic uplift and intensified continental aridity. Despite this, the sedimentary record of the continental shelf provides important information on the depositional history and palaeoenvironmental evolution of the region. A core retrieved from the western shelf of South Africa was analysed for its sedimentary composition, lithological variation, foraminiferal content and its relation to the palaeoenvironment of the region. Four depositional facies were identified along the core, namely quartzitic sand, sandy mud, and glauco-phosphatic sand and a glauco-phosphatic gravel. The basal facies consisting of quartzitic sand is interpreted to have been deposited between 15.90 and 14.60 Ma, corresponding to the timing of the Mid-Miocene Climatic Optimum (MMCO). The highly quartzitic nature of the sediments indicate a high terrestrial influence from fluvial sources. The overlying sandy mud facies was deposited between 14.60 and 13.90 Ma based on planktic foraminiferal biostratigraphy. Foraminiferal analyses of these two facies that were deposited in the Langhian stage of the middle Miocene point to subtropical sea surface conditions and mesotrophic benthic environments. Sea level was noticeably higher during the MMCO and part of the cooling period following the MMCO. An erosional surface that spans 10.77 Myr, equal to the late Miocene (13.90 Ma) to early Pliocene (3.13 Ma), marks the boundary between the two Langhian facies and the overlying two Pleistocene facies, consisting of coarser grained glauco-phosphatic gravelly sand units. The Pleistocene environment on the shelf is interpreted to contrast with the Langhian environment, where cooler, shallower conditions and a more eutrophic benthic environment was prevalent, during a time that Benguela upwelling intensified with higher frequency and higher amplitude sea level fluctuations. Palaeobathymetric interpretations indicate that middle Miocene sea-level in the region were up to 77 m higher than present day and 101 m lower in the Pleistocene, in-line with previous global studies. Glauco-phosphatic content that increase up-core also marks the shallowing of the environment under high productivity conditions.