Recent ventifact development on the central Oregon coast, western USA

摘要

The unusual location of ventifacts, on a boulder-built jetty at the mouth of the Siuslaw River, Oregon coast, western USA, allows ventifact age and wind abrasion rates to be estimated with some precision. The jetty was built mainly between 1892-1901 and extended throughout the twentieth century. Consideration of historical shoreline position and the history of jetty construction and repair suggests the ventifacts have formed since about 1930. Morphologically the ventifacts are aligned south-to-north reflecting winter winds and sediment transport from the adjacent beach. Wind-parallel grooves and ridges with sharp, sinuous crests are developed on inclined boulder surfaces on top of the jetty and reflect suspended sand transport in wind vortices. Deeply pitted surfaces on steep boulder surfaces nearest the beach reflect impact by saltating sand grains. Based on present wind regimes (1992-2000) from three regional weather stations, southerly winds above the sand transport threshold occur for 21.9-29.6 per cent of the time. Based on estimated depth of loss from boulder surfaces, wind abrasion rates are calculated to be on the order of 0.24-1.63 mm a~(-1). This is the first well-constrained field estimate of ventifact age and ventifaction rate from a modern coastal environment.