Grain Orientation in Determination of Paleocurrents and Sandstone Trends

Abstract

Measurement of grain orientation in relatively large samples by means of Shell's dielectric anisotropy device is both rapid and reliable. It corresponds quite accurately to other paleocurrent indicators, such as parting lineation, rib-and-furrow structures, sole marks, and cross-bedding. Because grains are imbricated upcurrent, current direction can be inferred accurately from analysis of grain orientation in the vertical plane paralleling the horizontal grain fabric. Dielectric anisotropy measurements of partly impregnated Cimarron River sand depict very well recent local current directions of the river. Grain orientation is normal to the sandstone trend of the lowermost unit of the Eagle Sandstone at Billings, Montana. Grain imbrication in that barrier-bar sandstone suggests that oncoming surf was the most important depositing current during local westward accretion. Grain orientation in three lenticular Permian sandstone bodies in Oklahoma parallels or subparallels the sandstone trends. Because the horizontal component of grain orientation in horizontally bedded or cross-bedded sands statistically corresponds to current trend, remote measurement of grain orientation in a borehole could make possible the determination of sandstone trend from a single well. As suggested by Rodriguez and Pirson, adaptation of the dipmeter log offers an available tool for such a technique.