The debris of the Frank Slide and theories of rockslide–avalanche mobility

Abstract

The Frank Slide, a 30 × 106 m3 rockslide–avalanche of Palaeozoic limestone, occurred in April 1903 from the east face of Turtle Mountain in the Crowsnest Pass region of southwestern Alberta, Canada.The reconstruction of the Canadian Pacific Railway (CPR) line created a cut up to 16 m high across the deposit, giving a unique cross section nearly the depth of the debris. The debris shows vertical sorting. The base material is crushed limestone, mainly of sand and gravel size, and contains rounded pebbles from till or alluvial deposits on the surface of separation. The upper surface of the debris is an accumulation of large, predominantly angular boulders. Grain-size analyses by sieving and by "area-by-number" counts demonstrate a gradual increase in grain size with height above the base of the cut. Such inverse grading with fines concentrated at the base of the debris indicates that the landslide was not fluidized by gas pore pressures.The base material has run ahead of the coarse debris of the slide, which usually forms a second distinct scarp up to 300 m from the edge of the slide debris. Lateral ridges and distal rims have formed at only three places on the slide's margins, all close to crests of steep slopes the debris has run up. The Frank Slide then is not a slide of Shreve's Blackhawk type. Support for boulders in the debris probably came from dispersive forces and motion-induced vibration.