Disharmonic Folds in Besa River Formation, Northeastern British Columbia, Canada

ABSTRACT Four localities in the Foothills belt of northeastern British Columbia were examined to determine the internal structure of the Devonian-Mississippian Besa River shale and its structural relations to adjacent units. Striking changes in fold style are present across the formation and between it and other units. Although shales of the Besa River are commonly deformed by a mechanism of close cleavage folding, underlying Middle Devonian carbonates have open folds that are probably of the flexural-slip type. Internally the formation displays well-developed axial-plane cleavage and a similar fold style. Dependent on the lithologic character of overlying strata, these structures may be replaced progressively upward by concentric folds in the case of shales interbedded with limestone or sandstone, or by an abrupt change to a concentric fold style where overlying beds are massive limestones. In many exposures a pronounced zone of structural separation may be observed or inferred to exist within the Besa River Formation.