THE LEWIS THRUST: WHOSE FAULT IS IT, ANYWAY?

The Lewis Thrust was discovered at Steamboat Mountain, north western Montana and named by Bailey Willis in 1912. Subsequently drilled in Canada in the search for hydrocarbons, the physical expression of Lewis thrust varies along its strike, according to the stratigraphy of its hang-wall section. The most northerly expression of the fault is a well-exposed, large-scale tight fold in the south-east face of Mount Kidd, 75 kilometres west of Calgary, Alberta.

In northern Montana the Lewis Thrust is one of several major east-verging thrust faults in within the east flank of a regional-scale syncline described as the Continental Divide Syncline. Across the border at Crowsnest Lake, the Lewis Thrust sheet takes the form of a basal thrust that placed Palaeozoic carbonates, over Upper Cretaceous clastic sediments. The hanging wall of the thrust is thickened by an infinite number of metre-scale (or less) thrust duplex structures, that dramatically thicken the hanging wall section and absorb probably as much foreshortening as the Lewis Thrust itself.

From Crowsnest Lake northward, the Lewis thrust shows a simpler geometric configuration, dying out as a back-limb thrust to the Opal Range thrust sheet of the Eastern Front Ranges. It’s northernmost expression at Mount Kidd and nearby passing southeastward into a series of small-scale cylindrical folds that mark the western slope of the Opal Range.

The Continental Divide Syncline of northwest Montana may be an analog in Canada of a coal-bearing syncline in Jurassic rocks west of the Lewis Thrust in south-eastern British Columbia.