Quartz Grain-Shape of Southern California Beaches

Abstract

Fourier grain-shape analysis (FGSA) was performed for the medium sand (0.25 to 0.50 mm) fraction of 136 foreshore samples from 55 southern California beaches located from Point Arguello to the United States-Mexico border. Beaches were sampled during November 1956, October 1991 and April 1992. Statistical analyses of FGSA results indicate that foreshore quartz grains derived from mostly crystalline rocks exposed in the Peninsular Ranges tend to be more elongate and rougher than those derived from mostly marine sedimentary strata exposed in the Transverse Ranges. The observed variation in grain elongatIon most likely reflects differences in microfracture patterns characteristic of Mojave Desert protoliths from which the sedimentary strata in the Transverse Ranges were derived and that characteristic of the Peninsular Ranges batholith. Marine abrasion and the addition of silica cement during diagenesis are probably responsible for the relative smoothness of foreshore grains derived from the Transverse Ranges. The Redondo Submarine Canyon prevents mixing of foreshore quartz and heavy mineral grains derived from the Transverse and Peninsular Ranges and thus constitutes the most effective littoral sediment barrier in the southern California bight.