Stratigraphy and optical dating of Pleistocene coastal deposits in the Port Campbell australite strewn field, SW Victoria

Abstract

Alluvial-channel and overlying eolian deposits exposed in coastal cliffs in the Port Campbell australite (Australian tektite) strewn field, SW Victoria, were studied stratigraphically and dated by optically stimulated luminescence (OSL) for quartz sand grains to illuminate the occurrence of australites in this area. The channel deposits, termed the Dog Trap Bay Sand, yielded OSL ages from 420 ± 40 to 230 ± 15 ka for four sites, and the two eolian units OSL ages of 203 ± 16 and 93 ± 11 ka. The ages support field evidence that, contrary to the conclusions of a previous study by others, the channel deposits are stratigraphically distinct from and much younger than the Pliocene Hanson Plain Sand, which forms a regionally extensive system of marine strandlines to the north and NE of Port Campbell. The OSL ages show some preference for times of negative temperature anomalies marking the last four glacial periods of the Middle and Late Pleistocene as indicated by ice-core data from Antarctica. Eolian and at least some alluvial deposition evidently occurred when sea-level was low and rivers extended across the exposed continental shelf. Others have suggested that the Dog Trap Bay Sand contains rare australite fragments in situ. However, these occurrences may have resulted from reworking of sandy material, with the australites possibly derived from overlying younger units known to contain numerous specimens. The OSL ages imply that a discrepancy of ca 350 ka or more exists between the apparent stratigraphic occurrence, as distinct from the terrestrial age, of australites in the Port Campbell strewn field and the australite 40Ar/39Ar age of 770 ± 20 ka. Stratigraphic evidence in this area for the terrestrial age of australites is inconclusive.