Differentiation of Beach, Dune, and Aeolian Flat Environments by Size Analysis, Mustang Island, Texas

Abstract

Mustang Island is a 16-mile-long barrier island on the south Texas Gulf coast. Sediments consist of very well sorted fine sands (mean size 2.82phi , average sigma I = 0.29phi ), and show exceedingly uniform properties lengthwise of the island. Beach, dune, and aeolian flat environments can be readily distinguished by size analysis. The best means of differentiating them is by plotting skewness versus kurtosis inasmuch as the geologic processes at work have their greatest effect on the tails of the size distribution. Beach sands form normal carves, dune sands are positively skewed but still mesokurtic, and aeolian flat sands are positively skewed and leptokurtic. Sands from the beach are more poorly sorted than those from the other two environments. These differences are extremely significant with P values between .0002 and .0000001. To explain these changes, it is proposed that as sand travels from the beach to the dunes, the coarse end of the parent normal population lags behind, rendering the size curves better sorted and also positively skewed. The aeolian flats are believed to be subject to infiltration of silt from the atmosphere; this imparts a tail of fines, registering a marked increase in kurtosis.