Surf Zone Shape Changes in Quartz Grains on Pocket Beaches, Cape Arago, Oregon

Abstract

ABSTRACT Juxtaposition of modern pocket beach sands, Pleistocene Whisky Run terrace sands, and Eocene Coaledo sandstones at Cape Arago, Oregon, provides a natural setting for studying changes in grain shape as produced by surf action. If the Eocene sandstones can be shown to be the predominant sediment source for both the Holocene and the Pleistocene deposits, then measured differences between shape parameters of quartz grains from the older sandstones and those from the more recently deposited sediments may be attributed to the effects of surf action. Mean roundness and mean sphericity were measured on a total of 13,153 quartz grains (1.50 size) collected from three separate pocket beaches, the Whisky Run sands, and the Coaledo sandstones. Principal components analysis, a multivariate computer technique, was applied to shape data and supplementary size and heavy mineral data. Trends revealed by this analysis show that the sands of one pocket beach (Beach One) are sufficiently similar to those of the bedrock to indicate a definite genetic relationship. Statistical comparison tests of average mean roundness values for quartz grains from the Coaledo bedrock and those from Beach One indicate that the beach grains have undergone a statistically significant increase in roundness. However, there is no significant change in mean sphericity. These results suggest that surf action effectively increases grain roundness, but does not produce an accompanying increase in sphericity.