Abstract

The Prospect Hill Formation is well exposed in the Tabakbaai Quarry near Saldanha Bay and represents the southern portion of a 20km long fossilised dune cordon oriented north-northeastward. The Prospect Hill Formation consists of basal sandy beach deposits conformably overlain by up to 70m of aeolian sands. Carbonate shell fragments, foraminiferal tests, echinoderm spines and barnacle plates make up 90% of the sand-size fraction, well-rounded quartz grains make up the remaining 10%. Previously proposed ages for the Prospect Hill Formation range from middle Miocene (~10 to ~12 Ma), based on eggshell of an extinct ostrich species, to latest Pliocene/early Pleistocene based on foraminiferal biostratigraphy. Strontium isotope stratigraphy of various biogenic components from this study, however, indicates a latest Miocene/early Pliocene age for the Prospect Hill Formation, making these dunes the oldest coastal dune system in the Western Cape. Our strontium ages are consistent with the global sealevel highstand at the Mio/Pliocene boundary and appear to correlate with the terrestrial fauna at Langebaanweg Quarry. A reconstruction of the palaeoshoreline in the Saldanha Bay area demonstrates that coastal progradation was dominant in the area up until the latest Miocene/early Pliocene, when sea level was 25m above present day, and strong southerly winds formed the carbonate dune cordon. Evidence of a 23 to 24m highstand during the early Pleistocene is recorded by a coquina bed deposited on a wave cut terrace, similar to other marine terraces preserved along the west and east coasts of southern Africa.

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