Important, though not too evident, structural changes took place near the end of the Paleozoic Era along the western edge of the North American craton. They roughly correspond to the orogenic phases in geosynclinal areas all around the continent: "Appalachian orogeny," "Antler orogeny," "Cariboo orogeny." The style was block-faulting (with predominantly normal faults) forming platform edges and grabens, surrounded by shallower troughs. The result is a network of rift valleys along the

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western edge (north-south) and across the platform (east-west), which was itself broken into tilted blocks. Those grabens extending across the continental margin and normal to it, are aulacogens, opening into the geosyncline to the west. All these fractures are rejuvenated, Precambrian basement features. The Liard Plateau in northeastern British Columbia, the Peace River basin in northwestern Alberta, and the Big Snowy trough in central Montana are part of this system, which exhibits analogies with the Ancestral Rockies area farther south (e.g., Paradox-Eagle-Laramie basins, and the Anadarko basin).

Sediments are a mixture of easterly derived fine clastics (shales), coarser clastics with possible sources both from the craton and the Cordilleran belt, and some locally formed carbonate rocks. Chemical sediments, reefal buildups, and coarse, locally derived clastics, are unknown in the British Columbia, Alberta, and Montana area. The thickness of this Mississippian-Pennsylvanian series is usually 100 to 200 m (maximum 700 m).

Excellent hydrocarbon source rocks and reservoirs are present, but their extent and quality are erratic. The petroleum habitats are further complicated in many places by the fact that the whole structural system has been reactivated during the Cordilleran orogeny. Some of the north-south oriented features became reverse faults, whereas most of the east-west aulacogens have undergone some strike-slip movement. In both situations, drape-over folding has resulted. Some earlier formed hydrocarbons may have remigrated and dispersed. Analysis of the remodeled structural pattern in conjunction with the upper Paleozoic reservoir geometry is the main challenge of present exploration on this ancient edge of the North American craton.

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