Mid- to Late-Quaternary evolution of the Wilderness Barrier dunes, South Africa

Barrier dunes represent potentially long-term, but complex, archives of coastal evolution. The examples occupying the Wilderness embayment, on the southern Cape coast of South Africa, form a regionally unique system of three shore-parallel barriers reaching up to 200 m in height and extending up to ~32 km alongshore. This research combines chronological and sediment provenance analyses to reconstruct the emplacement and evolution of the Wilderness barrier dunes through the Mid- to Late-Quaternary. Thirty-six new luminescence ages collected from ten sites across the three Wilderness barriers are presented, and are combined with a compilation of dates from the literature to produce a high-resolution chronology of barrier accumulation. The record spans at least the last two glacial-interglacial cycles, with notable phases between 245-217 ka, 155-143 ka, 128-121 ka, 91-86 ka and post-6 ka. Analysis of trace element geochemistry, heavy minerals, particle size, carbonate content and offshore topographic evidence all combine to indicate the provenance of the barrier sands has remained constant throughout their formation, and must involve marine transport pathways. The hypothesis that barrier accumulation at Wilderness during periods of low sea level was sustained by terrestrial aeolian activity is thus disproven, and evidence for a regional pre-MIS 5 marine transgression is provided. The terrestrially derived fraction of the barrier sands predominantly comprises quartzitic material derived from Table Mountain Group (TMG) rocks, most likely sourced from the Gouritz River ~75 km west of Wilderness. In addition to sediment from the TMG, the barrier sands also contain contributions of material derived from local geology, of material recycled from previous generations of aeolianite, and of authigenic marine sediment. The extensive coversand deposits inland of the Wilderness embayment, dated to >1.6 Ma using isothermal thermoluminescence, are demonstrated not to have made any significant input of sediment to the barriers. The Wilderness barriers record a complex history of erosion, as well as deposition through the Mid- to Late-Quaternary, and the preserved record clearly reflects the influence of local nearshore bathymetry on the rate of sea-level regression. The importance of previous generations of aeolianite in both fixing the position of subsequent depositional episodes, and protecting them from erosion, is also evident. The barriers exhibit similar behaviour to deposits on tectonically stable coastlines elsewhere, and contrast with the more complete and widely spaced barrier records present on uplifting coasts.