Structural and stratigraphic setting of the Downie slide, Columbia River valley, British Columbia

Abstract

The Downie slide is a late to postglacial rockslide situated on the western slope of the Columbia River valley about 70 km north of Revelstoke, British Columbia. It attains a maximum thickness of 270 m and is estimated to involve 1.5 × 109 m3 of rock and debris. The head of the slide is bounded by a nearly vertical escarpment reaching heights of more than 125 m; its lateral boundaries are defined by a prominent east–west trending scarp on the south and a more subdued linear northeast trending ridge on the north. The toe forms the west bank of the Columbia River in this area.The slide occurs within a compositionally anisotropic formation of high-grade pelitic and semipelitic schists and psammites. The main shear zone at the base of the slide is located in pelitic schists. Minerals in the rock of the shear zone have been mechanically crushed and locally reduced to a fine-grained gouge.Three distinct phases of deformation are recognized in the Downie slide region. The location and attitude of the second and third fold phases and their associated fabrics controlled the external geometry of the slide.Along the western slopes of this part of the Columbia River valley the second phase of deformation has been dominant. Within the formation that contains the slide, bedding is extensively deformed by tight to isoclinal second phase minor folds that exhibit a penetrative axial plane foliation. At Downie slide this foliation dips approximately 20° eastward towards the Columbia River, and nearly parallels the slope of the hillside; the basal shear zone of the slide developed parallel to the axial plane foliation.West of the slide, third phase major and minor folds have been superimposed on the second phase geometry, but they die out eastward, and are of only minor significance within the main body of the slide. The eastern limit of major superposition coincides with the head scarp of the slide. The slide mass broke away along the hinge zone of the first major monoclinal flexure fold associated with this front of phase 3 folding.Late fracturing probably influenced the position of the northern and southern lateral boundaries of the slide.