The geometry of folded and thrusted rocks in the Rocky Mountain Foothills near Grande Cache, Alberta

Abstract

Lower Cretaceous clastic rocks make up the Inner Foothills of the Rocky Mountains along the Smoky River near Grande Cache, Alberta. Shortening of these rocks is accomplished by folding and thrusting and averages 33% over the area studied.Folds in the area are of the chevron variety. They are caused by shortening of a multilayered sequence of alternating competent and incompetent strata, where the thickness of the competent layers is fairly constant. The folds are generally cylindrical and maintain their shapes over distances of up to 2 km along trend. However, at their tapering ends they are conical.The majority of the faults are southwest-dipping thrusts, displaying ramps that cut up stratigraphic section and flats that are parallel to bedding. The best exposed fault in the area is the Mason Thrust. Displacements along this fault increase from 150 m in the northwest to 1500 m in the southeastern part of the area. A prominent anticline below the Mason Thrust diminishes gradually towards the southeast. A conical fold with a half apical angle of 7° describes this geometry. Shortening by this fold decreases from about 30% in the northwest to 10% in the southeastern part of the area. This indicates that a decrease in shortening by folding is compensated by an increase in thrust fault displacement. It is an example of fault-to-fold displacement transfer.