Genesis of Fire Island foredunes, New York

Abstract

There is speculation concerning the origin of the large foredunes on Fire Island National Seashore, New York. A number of sedimentological techniques were employed to address this question. Samples were taken from the dune top, dune base (of unknown origin), and mid-tide beach area at 30 locations along the seashore for comparison purposes. Cumulative weight percentages for each sample were calculated and plotted on probability paper. The majority of unknown sands have one saltation population, like the dune sands, whereas beach sands characteristically have two saltation populations with a break at or around 2 φ. Statistics show clear separation of the two (aeolian and hydraulic) environments. In all cases, the unknown sands group together with the dune sands, leaving beach sands clearly separate. Linear discriminant analysis and the Mahanalobis D 2 statistic proved to be conclusive techniques, clearly placing the unknown sands in the dune environment. Heavy/light mineral ratios were compared, indicating that dune and unknown sands are of the same origin. Roundness counts were made, but no statistically significant difference in roundness values was found between the environments. Grain surface textures were studied by the S.E.M. in an attempt to establish distinctive surface feature differences. A definite qualitative difference is observed, but this is not substantiated quantitatively. Structural analysis of the dunes proved to be of little significance because few cross-beds and only a small number of sand ripples were found. Where measured, small ripple numbers indicate an aeolian origin for the foredunes. Finally, inferences were made concerning the aeolian nature of the foredunes from a study of Ammophila breviligulata rhizomes and buried tire tracks.