The geological setting of ventifacts and wind‐sculpted rocks at Mason Bay, Stewart Island, and their implications for late Quaternary paleoclimates

Abstract

Abstract A complex sequence of late Quaternary beach and dune sands overlies a locally emergent basement of granite at Mason Bay, Stewart Island. Granite outcrops are commonly decorated with striations and grooves formed by abrasion of wind‐blown sand. Ventifacts are widespread and appear to be derived from breccia layers interbedded with and capping the beach sand complex. Palynology and radiocarbon dating of a peat horizon overlying the upper ventifact‐bearing breccia indicate that conditions before 10 000 yr B.P. saw the slow drying out of swamps and encroachment of the forests in response to improving climate until, at Mason Bay, they were overwhelmed by dunefield migration in response to the onset of a strong westerly airstream. This, the most recent aeolian event, also caused sandblasting and resulted in striated outcrops of basement granite. These decorated outcrops also record some local easterly windflow, possibly due to eddying or turbulence. The buried ventifacts suggest that two earlier periods of strong windflow also occurred: one following deposition of the upper beach sand before the deposition of the peat, and the other during the ?periglacial interval between the deposition of the upper and lower sands. The ventifact‐forming episodes on Stewart Island probably correspond to periods of sand and loess deposition in southern South Island, the sand being derived from the exposed coastal plain during periods of lowered sea level.