TEXTURAL VARIATIONS OVER A FORESHORE-BACKSHORE ENVIRONMENT, HAWAII

Abstract

Detailed textural analyses in a carbonate foreshore-backshore coastal environment in Hawaii indicated distinct differences in mean, sorting, skewness, and percent of fines between zones and within zones. A well-developed berm crest partitioned the 1,000 m2 grid into foreshore (n = 79) and backshore (n = 46) zones. As a group, foreshore sediments were finer, better sorted, and more negatively skewed than backshore sediments. This pattern was thought to reflect the decoupling of the foreshore source population by swash-backwash and eolian processes. Spatial variations in texture within the foreshore were also observed, and this led to the subdivision of the foreshore into three zones based on observed textural variations–lower foreshore, mid-foreshore, and upper foreshore. The lower foreshore was found to be statistically coarser, more poorly sorted, and more positively skewed than the other foreshore zones. The mid-foreshore zone was the finest, and the upper foreshore was intermediate in grain size. This patterning was thought to reflect the greatest energy dissipation in the lower foreshore, subsequent competency decrease of swash runup, and deposition of coarsest particles remaining in transport in the upper foreshore, accompanied by infiltration losses in the backwash and deposition of the finest materials in the mid-foreshore zone. Traditional bivariate plots of textural parameters for the foreshore and backshore samples indicated that this approach alone would not be useful in distinguishing subenvironments in paleosequence studies. A mean value approach (MVA) was developed and used in combination with published bivariate plots to identify depositional environments using moment statistics. Encouraging results were obtained from Waimanalo Beach sediments and from data published in the literature. The bivariate (textural) suite statistics approach (Tanner, 1991 b) was used to test for correct environmental recognition using the 125 beach samples. Results at first appeared promising, but assessment of literature data illustrated that the original suite diagrams could usefully be expanded. These results support previous statements that textural studies should not be used as the only tool in paleoenvironmental reconstruction. [Key words: beach, nearshore, backshore, sediment, geomorphology, Hawaii.]