Environmentally significant sedimentologic characteristics of beach sands

Abstract

Comparative study of five South Island beaches and a beach on the central Texas coast suggests a short list of features that are diagnostic of the coastal beach. Beach sediments are clean (no mud matrix) and are very well to moderately well sorted. Foreshore strata occur as even, internally laminated, thin beds that dip seaward at low angles. The angle of dip is directly related to the slope of the foreshore surface (especially the beach face), which in turn is largely dependent upon the mean size of the beach sediment, that is, the coarser the grain size the steeper the foreshore. Backshore strata occur as lens- and slightly wedge- shaped, internally laminated thin beds. Scour surfaces and cross-stratified lenses are characteristic. Dip of strata varies from 0° to 15°, and strata dip directly or obliquely landward. A transition zone (foreshore or above high tide level plus outer backshore) is characterised by mixed foreshore and backshore stratification, and some strata dip landward, others dip seaward. Contrary to the widely held view that grain size distributions of beach sediments are negatively skewed, many beaches furnish positively skewed distributions. We conclude that skewness is not necessarily an environment-sensitive measure, and that skewness values commonly reflect grain size characteristics inherited from the source rocks.