Radiocarbon-dated evidence for Late Pleistocene and Holocene coastal change at Yzerfontein, Western Cape, South Africa

Stephan Woodborne; Duncan Miller; Mary Evans; Hayley C. Cawthra; Stephan Winkler

doi:10.17159/sajs.2023/15505

Authors

Stephan Woodborne iThemba LABS Johannesburg, South Africa https://orcid.org/0000-0001-8573-8626
Duncan Miller Department of Geology, University of the Free State, Bloemfontein, South Africa https://orcid.org/0000-0001-6765-7339
Mary Evans Department of Geography, University of the Witwatersrand, Johannesburg, South Africa https://orcid.org/0000-0003-1164-9932
Hayley C. Cawthra 1.Minerals and Energy Unit, Council for Geoscience, Western Cape Regional Office, Cape Town, South Africa; 2.African Centre for Coastal Palaeoscience, Nelson Mandela University, Gqeberha, South Africa https://orcid.org/0000-0002-6101-5543
Stephan Winkler Accelerator Mass Spectrometry and Isotope Research, Ion Beam Centre, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany https://orcid.org/0000-0001-8348-3260

DOI:

https://doi.org/10.17159/sajs.2023/15505

Keywords:

radiocarbon dating, sea level change, west coast, palaeolagoon, marine shell

Abstract

We report radiocarbon dates obtained from on-shore marine and near-shore terrestrial deposits near Yzerfontein, on the West Coast of South Africa. These deposits include Late Pleistocene shell concretions from the southern end of 16 Mile Beach and a marine shell deposit inland of the coastal Rooipan (Red Pan); mid-Holocene coastal pan deposits exposed by modern storm erosion of the sandy 16 Mile Beach; and four Holocene storm beach deposits on a rocky shore to the south. We interpret the results in terms of local geomorphology constraints on sea-level fluctuations. The eastern margin of Rooipan is a >40 ka elevated beach deposit in a dune cordon that separates it from the adjacent Yzerfonteinpan. Both pans have gypsum deposits up to 2 m thick formed by repeated marine overwash. Saline pan deposits that are exposed intermittently on the beach are mid-Holocene and indicate a former westward extension of Rooipan. This is in contrast to storm beaches dating 8000–2600 cal BP at higher elevations on a rocky platform further south. This suggests that a dune barrier existed seaward of the present shoreline near Rooipan at this time. The coastal changes described here show that deposition and erosion can be affected significantly by the local palaeogeomorphology and cannot be ascribed solely to sea-level change.

Significance:

Mollusc shells from Yzerfontein, Western Cape Province, South Africa, show radiocarbon ages ranging from >40 000 years to a few decades before present. There is evidence for elevated sea levels between 8000 and 2600 years ago, and sea levels similar to the present in the last 2000 years. Neither the elevation of the deposits nor their ages conform to published sea-level change curves for the Western Cape coast. Inundation by rising sea levels in the Holocene was not spatially uniform. Former and present geomorphology have had a significant effect on deposition and preservation of indicators of sea-level change.