ABSTRACT

The thin Cambrian sequence in the Flathead-Crowsnest Pass area of the Canadian Cordillera is distinctly different in character from the classical Cambrian succession of the Canadian Rockies to the north. It is unconformable on Purcell (Precambrian) rocks and consists of a basal diachronic sandstone from a few feet to 150 feet thick, a sequence of shales up to 290 feet thick, a lower carbonate unit up to 510 feet thick, and an upper carbonate unit that was initially more than 225 feet thick. These units comprise the Flathead, Gordon, Elko and Windsor Mountain (proposed) Formations respectively. This succession is most closely related to the Cambrian of the southern Alberta Plains and the northern Rocky Mountains of Montana, and is most probably entirely of Middle Cambrian age. It has been bevelled toward the northwest by pre-late Middle Devonian erosion which has locally cut deeply into the underlying Purcell rocks. Several channels in the upper part of the Windsor Mountain Formation filled with fossil plant-bearing siltstone provide a record of earlier Devonian fluvial erosion and deposition.

The contrast between the Cambrian succession in and south of Crowsnest Pass and that in adjacent areas can be ascribed to the influence of the tectonically positive area Montania, which, like a gigantic trapdoor structure, appears to have been bounded on its northwestern and western margins by faults or very steep flexures. Montania was apparently a feature of great antiquity. Coarse detritus accumulated along its northwestern and western flanks in both Windermere (Late Precambrian) and Early Cambrian time. The patterns of thickness and facies variations among the remnants of the Cambrian succession in the Flathead-Crowsnest Pass area that have escaped pre-Devonian and later erosion, indicate that this succession was initially much more widespread and thicker, and that all of Montania was inundated during the Middle Cambrian. The renewed uplift which left its distinctive imprint on the pattern of pre-late Middle Devonian erosion, appears to have assumed the same character as that which occurred in the Precambrian and Early Cambrian. The local absence of Cambrian rocks over the northwestern part of Montania is a result of this later erosion rather than of non-deposition.