THE MCCONNELL THRUST; EVIDENCE FOR PLATE TECTONICS IN THE 1880S

The McConnell Thrust, at the base of the Cambrian cliff-forming carbonates of Mount Yamnuska (65km west of Calgary, Alberta), was first mapped by R.G. McConnell of the Geological Survey of Canada in 1886, eighty years before plate tectonics was fully accepted in North America. McConnell produced a perceptive cross section which depicted this nearly horizontal thrust fault when the very nature of thrust faults was controversial. But what other explanation could there be for this ~1m thick deformation zone separating overlying Cambrian carbonates from underlying Cretaceous Belly River Formation sandstones and shales? Not only was this thrust fault controversial back in 1886, but this field work was completed before the Trans Canada Highway existed, at a time when geologists also created geographic and biological field work.

Today the McConnell Thrust, visible from the Trans Canada Highway, is used in many introductory geology courses as an example of a classic thrust fault that illustrates features such as ramps & flats, klippe & fenestres. It also presents an excellent relative dating problem because movement along the McConnell Thrust at Mount Yamnuska is constrained to being younger than the Upper Cretaceous Belly River Formation which is located underneath the fault surface. The McConnell Thrust is the easternmost surface expressed in-sequence thrust fault in a sequence that first started forming in the Jurassic with the docking of the first superterrane (the Intermontane Belt) onto the western margin of the North American craton. The surface trace of this thrust fault marks the boundary between the Foothills to the east, and the Foreland Belt to the west. Finally, this fault provides a very vivid example of the paradigm shift represented by plate tectonics as the geological community shifted their mental models from only a vertical component to incorporating horizontal components.