Hydrodynamic Significance of Mollusks in Pliocene Turbidites Near Ventura, California: ABSTRACT

Abstract

The distribution and orientation of valves of a small species of pelecypod in Pliocene turbidite sandstone near Ventura help clarify the complicated hydrodynamic history of turbidite events. Because of their relatively constant configuration and size, these pelecypod valves are superior to platy or elongated minerals and rock fragments in this respect. Moreover, it can be shown that valve stability orientations differ according to basic mode of sediment accumulation and possibly flow regime. The preliminary conclusions presented below are based on both laboratory experiments and field observations. The stable orientation of convexo-concave shells seen within the massive, graded basal part of a typical turbidite bed is concave up. Laboratory experiments suggest that the shells are rotated into this configuration by consolidation of the viscous turbidite slurry during the final few seconds of motion before this part of the bed freezes. There is evidence that the opposite configuration will exist if internal shear was present in any significant amount, such as near the base of the flow. Shells located in the upper, laminar parts of the turbidite sequence generally are convex upward; this is the well-known stable configuration for current flow. End_of_Article - Last_Page 470------------