Abstract

Three unconformity-bounded clastic sequences in the Foothills of the Canadian Rocky Mountains record the development during Cretaceous time of migrating foredeeps along the eastern flank of the Cordilleran Orogen. They include sediments that grade vertically and laterally from marine shales through prodeltaic and nearshore siltstone and sandstone to deltaic and alluvial sandstones and conglomerate. These sediments, deposited along the western margin of the Western Canada Basin, contain major accumulations of thermal and metallurgical coal and are important sources and reservoirs of oil and gas.

Two major episodes of the Columbian Orogeny are recorded. During Late Jurassic and earliest Cretaceous (Berriasian-Valanginian) time, the regressive sediments of the upper Fernie Formation, Kootenay and Minnes Groups record the first phase and have only limited eastward extent, owing partly to their truncation by a regional unconformity. Major coal deposits within this sequence occur in southeastern British Columbia and southwestern Alberta. The second phase of the orogeny is represented by the Blairmore, Bullhead and Fort St. John Groups and Dunvegan Formation (Barremian-Aptian to Cenomanian). This sequence comprises a series of transgressive-regressive cycles in which some of the prograding sandstone tongues are important hydrocarbon reservoirs. Significant coal resources occur in the Foothills of central Alberta and northeastern British Columbia.

During the Late Cretaceous, the interior of North America was extensively flooded, and the marine shale of the Foothills is included in the Alberta and Smoky Groups. Two vast transgressions during Turonian and Santonian time were interrupted by a regressive phase during which time the oil-bearing Cardium sandstone developed. The succeeding coarse clastic sediments, variously assigned to the Belly River, Brazeau, and Wapiti Formations (Campanian to Maastrichtian age), developed during the early phase of the Laramide Orogeny. Several of the marine sandstone tongues have hydrocarbon potential and thermal coals are extensively developed in the adjacent Plains.

The trends of the axes of maximum preserved accumulation suggest a general eastward migration of foredeeps and also provide evidence of a shift of depocentres northward from Peace River. Eleven major transgressive-regressive marine cycles may be coincident to some degree with world-wide flooding but also reflect extensive mountain building associated with regional tectonism. Three distinct episodes of rapid sedimentation are indicative of two pulses of the Columbian Orogeny and of the initial phase of the Laramide Orogeny.