The Denholm landslide, Saskatchewan. Part I: Geology

Abstract

The Denholm landslide, whose surface is composed of scarps, ridges, and elongated depressions, is 160 m high, 2000 m wide, and up to 100 m thick. The shear zone is in silty, montomorillonitic clay of the upper part of the Lea Park Formation and Upper Colorado Group unit. The Upper Cretaceous Judith River Formation and the Quaternary Empress, Sutherland, and Saskatoon groups were affected by the landslide. Although these sediments were fractured and gravity faulted by tension when the landslide moved, they can be readily traced through the landslide, particularly the upper part. The scarps (gravity faults), ridges (horsts), and elongated depressions (grabens) are the surface expression of tension resulting from the stretching of beds during the landslide.The movement of the landslide is thought to have started when the North Saskatchewan spillway eroded to the level of the present shear zone about 11 000 years ago (established by radiocarbon dating) and is believed to have stopped in recent time. During this time, it moved about 390 m across the North Saskatchewan River alluvium at an average rate of 35 mm per year. As the landslide moved across the valley, it encountered deposition of alluvium at an average rate of about 2.4 mm per year which resulted in the curved shear zone on the alluvium. Keywords: retrogressive landslide, shale-alluvium, displacement, rate, age.