Distribution of grain sizes across a transgressive shoreface

Abstract

Distributions of seventeen grain sizes in the sand fraction of the shoreface sediments south of Long Island, New York, can be categorized into middle and lower shoreface species (very fine sand), upper shoreface species (fine sand), shoreface-depleted species (medium sand), and swash-zone species (coarse sand). The long-term fair-weather wave base of the study area at a depth of about 16 m is determined on the basis of the characteristic response of grain sizes to the average shoreface hydrodynamics. The crosshore deviations from the mean distribution for each grain size were examined by using empirical orthogonal function (EOF) analysis. These results show differential associations of grain-size deviations with various dynamic zones on the shoreface. The hydrodynamic processes of the modern shoreface are speculated to winnow out fine and very fine sands and transport them onshore, leaving behind on the lower shoreface and inner continental shelf a lag deposit of transgressive sand sheet composed mainly of medium sand.