DIFFERENTIATING BEACH AND FLUVIAL CONGLOMERATES BY SIZE AND SHAPE CHARACTERISTICS OF QUARTZITE CLASTS IN THE CENOZOIC TECUYA AND KERN RIVER FORMATIONS, SOUTHERN SAN JOAQUIN VALLEY, CA

The size and shape of clasts in conglomerates can be used to infer paleohydrologic conditions, distance to source, and depositional settings. Clast characteristics were evaluated in the late Eocene(?)-early Miocene Tecuya Formation (Ttc) in a 10 km cross section between Tecuya and Tunis Creeks, Tejon Embayment and the late Miocene Kern River Formation (Tkr) at Kern Bluffs. Ttc and Tkr are age equivalent to hydrocarbon reservoir sands in the southern San Joaquin Valley, California. Tkr conglomerates were deposited in a braid plain/delta. Ttc conglomerates were deposited in a beach/fluvial environment but the location of the marine/non-marine transition in the study area is unclear. Mixed fluvial and beach conglomerates are common along the margin of the southern San Joaquin Valley and are difficult to distinguish in the field and core.

Based on published data, quartzite clasts in beach conglomerates tend to be more oblate while quartzite clasts in fluvial conglomerates are more prolate. They plot in separate fields on a Zingg diagram. Shape comparisons were made between quartzite clasts because of their resistance to weathering and lack of anisotropies effecting clast shape. In order to distinguish beach and fluvial conglomerates, shape data (long (L), intermediate (I), short (S) axes) were measured from 1500 quartzite clasts collected from Ttc and 220 quartzite clasts collected from Tkr. Individual clasts and sample means of the dimensional parameters I/L and S/I were plotted on Zingg diagrams.

Most of the Tkr data plot toward the prolate corner of the Zingg diagram as expected. Sample data from central and western exposures of Ttc plot toward the oblate corner. Those from eastern exposures of Ttc plot in both prolate and oblate fields. We interpret the data to indicate that the transition from beach to fluvial environments was present in eastern exposures of Ttc and paleoslope was to the west. This interpretation is supported by paleocurrent data, a decrease in clast size to the west, and facies analysis of sediments. Clast size measurements can be an effective way to distinguish fluvial from beach conglomerates when a statistical approach is used with sample sizes of 200.