Cobble imbrication as a sensitive indicator of subtle local changes in river flow direction

Abstract

Imbrication of cobbles in Pleistocene terrace gravels along the Shoshone River near Powell, Wyoming, shows significant changes in orientation from a lower set to an upper set within a gravel layer. The lower gravels show a mean orientation of 126°, with spreads from s = 38° to s = 56°. The mean orientation of the upper gravels is 070°, with spreads from s = 19° to s = 26°. Change from the lower to the upper deposit is abrupt, with no noticeable variability near the contact. In fact, the contact can easily be overlooked without close scrutiny of the cobble fabric. This suggests that cobble imbrication may be a sensitive indicator of subtle changes in the flow direction of a braided stream. Because the earlier fabric was not disturbed by deposition of the younger unit, there is every reason to believe that such fabrics can and will be preserved even in the consolidated rock record and can be used for paleoenvironmental interpretations.