The Axial Ratio C/B as an Indicator of shape Selective Transportation

Abstract

ABSTRACT The sedimentological significance of the axial ratio C/B and the geometricity ratio were analyzed. C/B is a parameter used by Zingg (1935) to differentiate spherical and rod clasts from those of discoidal and bladed form, and the geometricity ratio is given by the equation [rods % + spherical %]/[discs % + blades %]. On this basis it is deduced that the ratios can be employed as scalar (and vectorial in scatter diagrams) properties to distinguish between traction and suspension in shape selective transportation. Because C/B and the geometricity ratio are influenced strongly by original shapes of fragments, it is necessary to minimize the inheritance factor by analyzing these two characteristics in different compositional (and granulometrical) populations. In modern and Holocene gravelly subaqueous sediments under tractive selective transportation, the values of C/B ratio and the geometricity ratio increase with the distance of movement. Inversely, when selective transportation is performed by suspension mechanisms, both C/B and the geometricity ratio decrease downcurrent. Selective sorting also controls the standard deviation of C/B, which diminishes invariably with increasing transport distance. The properties analyzed can be used, in recent and ancient gravelly and sandy rocks, as useful tools in environmental reconstructions.