Paleocurrent Analysis of Trough Cross Stratification

Abstract

ABSTRACT Additional documentation of the economy and superiority as a paleocurrent indicator of trough-axis orientation data in contrast to cross-set orientations is provided from studies of Cambrian sandstones around Baraboo, Wisconsin. Separate concentric polar histogram portrayals of both types of data allowed direct comparison of cross-set and trough-axis distributions, and several interpretable patterns emerged. In many cases bimodal cross-set distributions are bisected by axis azimuths; these reflect symmetrical filling of troughs. In other cases, a single dominant cross-set mode is perpendicular to the axes, indicating asymmetrical filling of troughs, which is commonly observable in cross-section views. In both of these cases, an error of 90° in paleocurrent interpretation is possib e if the trough-origin of such sets goes unrecognized. Sets with a single mode parallel to the axes reflect readings of cross-set dips mainly from the heads of the troughs. Most puzzling are bimodal trough-axis distributions, which are common in Wisconsin. Reflecting the great complexity of sand-wave bedforms, they represent the fillings of canoe-shaped depressions whose counterparts can be seen in modern sediments. Even the bimodal trough-axis data tends to yield better orientation results than do associated cross sets.