Wind-reworked fluvial deposits as an archaeological environment: the Agnes Banks Sand of the Quaternary Hawkesbury–Nepean sequence of southeast Australia

Abstract

ABSTRACT The deposits of Sydney’s rivers are thought to have experienced widespread aeolian modification during the Quaternary. The resultant sediments form archaeologically important landscapes upon which are found several of the oldest archaeological sites in the Sydney district. Unfortunately, little is known of the processes and products of aeolian reworking. The context of the archaeological discoveries is therefore enigmatic. This study focusses on the Agnes Banks Sand, a Cenozoic sand body that may represent the only sedimentary unit in the Sydney area that preserves reliable evidence of the action of aeolian processes on fluvial sediments. Our work confirms that the sands are indistinguishable from inland dune deposits worldwide, with the deposits reworked from alluvial sediments within and adjacent to the channel of the nearby Nepean River. The sands form east–west-aligned linear dunes, whilst the sediment body fines to the east, a pattern consistent with entrainment by westerly winds. Three other aeolian depositional bodies have been identified alongside the Hawkesbury–Nepean River. Together, these form an assemblage of source-bordering dunes that lies downwind of the river across the northern part of the Cumberland Basin. Deposition here began in Marine Isotope Stages 3–2 and continued into Marine Isotope Stage 1, a period corresponding with a general episode of dune activation across southeast Australia. Although it is widely believed that the Agnes Banks Sand was formed sometime between the Pliocene and the Middle Pleistocene, we present evidence that its deposition post-dates 65 ka and that the deposits are contemporaneous with the other fluvio-aeolian units in the catchment. The true importance of Agnes Banks, however, is as a type locality from which it has been possible to derive sedimentological and geomorphological signatures that may be applied to appraisals of the fluvio-aeolian record within (and perhaps beyond) the Sydney area.