The Orientation of Sand Grains under Conditions of "Unidirectional" Fluid Flow: 1. Theory and Experiment

Abstract

Recent interest in dimensional orientation of sand-sized particles has indicated that detailed studies of the orienting process are necessary in order to understand better and apply this tool for directional investigations in ancient sedimentary rocks. Based on the laws of fluid mechanics, the theoretical considerations of the present and earlier investigators indicate that the most stable position which an ellipsoidal particle acquires in depositional transport is one in which the long axis lies parallel to the direction of fluid motion, with a slight dip into the current. In the present study, experiments were performed with varied sediment parameters and varied fluid-flow conditions. Although large perturbations exist in the process of sedimentation, these experimental measurements reveal that the theory is valid in a statistical sense. The statistically preferred direction of orientation of sand-grain long axes parallels the fluid-flow direction; in addition, there is a statistically preferred imbricate direction dipping into the current. This orientation is dependent both upon the sediment parameters of size and shape and upon the fluid-flow conditions.