Structure of British Columbia Foothills, Canada

ABSTRACT The British Columbia Foothills between Peace and Liard Rivers comprise an elongate fold belt containing sedimentary rocks principally of Mississippian, Triassic, Jurassic, and Late Cretaceous ages; the structures within these rocks are well expressed on air photographs. Most exposures in the belt belong to the Gething-Debolt structural-stratigraphic unit which in a gross sense displays a concentric fold form. The upper part of this unit, consisting of Lower Cretaceous (Gething) to Triassic (Liard) strata, exhibits curvilinear and subordinate box and cuspate fold profiles as well as other features that suggest compensation for excess bed length at the upper surface of the gross unit. The central part of the unit consists of incompetent Triassic (Toad-Grayling) shales and apparently has deformed passively, whereas the Mississippian Debolt Formation at the base tends to exhibit angular fold profiles and faults suggestive of excess bed length. Zigzag en échelon patterns predominate in the fold belt; these patterns become more complex in the north. In this direction fold amplitude and wave length decrease in response to thinning of the structural-stratigraphic unit. Inclination of folds is dominantly easterly in the west, becoming vertical or westerly with proximity to the Interior Plains.