Dating and correlation of neogene coastal deposits in the Western Cape (South Africa): Implications for neotectonism

A northward trending, fossil dune system (Prospect Hill Formation) extends for similar to20 kin parallel to the coast north of Saldanha. Fossil eggshell fragments of the giant struthious bird Diamantornis wardi were recovered from three palaeosols in the Prospect Hill Formation. D. wardi has been assigned to the late Middle Miocene (12 to 10 Ma) in Namibia, making this one of the oldest coastal dune systems in South Africa. The aeolianites rest conformably on a +40m boulder beach, equated with the global Middle Miocene high sea level cycle (HSLC) and the latter stages of the +90m HSLC of Namaqualand. The presence of struthious birds and the dune snail Trigonephrus globulus suggests a relatively dry, Mediterranean climate. A younger +25m beach and associated aeolianites are banked up unconformably on the seaward aspect of the Prospect Hill Formation. The zone fossil (bivalve) Donax rogersi relates these deposits to the +30m HSLC in Namaqualand and probably to a Plio-Pleistocene (similar to2Ma) global highstand. Altimetric discrepancies between the global Mio-Pliocene HSLC in the Western Cape (Varswater and De Hoop Vlei Formations) and Namaqualand suggest crustal subsidence of some 50m along the northern west coast. Comparison of global Middle Miocene sea levels (up to +140m) and the Middle Miocene HSLC in Namaqualand (maximum +90m) yields a similar result. Terrestrial deposits at similar to50m below sea level at Noordhoek on the Cape Peninsula are attributed to eustacy rather than tectonism.