QUARTZ GRAIN-SHAPE OF SOUTHERN CALIFORNIA BEACHES

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 C alifornia beaches located from Point Arguello to the United States-Mex ico border. Beaches were sampled during November 1956, October 1991 an d April 1992. Statistical analyses of FGSA results indicate that fores hore quartz grains derived from mostly crystalline rocks exposed in th e Peninsular Ranges tend to be more elongate and rougher than those de rived from mostly marine sedimentary strata exposed in the Transverse Ranges. The observed variation in grain elongation most likely reflect s differences in microfracture patterns characteristic of Mojave Deser t protoliths from which the sedimentary strata in the Transverse Range s were derived and that characteristic of the Peninsular Ranges bathol ith. Marine abrasion and the addition of silica cement during diagenes is are probably responsible for the relative smoothness of foreshore g rains 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 mos t effective littoral sediment barrier in the southern California bight .