A paleogeology map with datum below the Devonian unconformity based on data from previous investigations, a revised Devonian palinspastic map, and maps of Devonian units in the Main Ranges west of the Banff-Jasper highway between 51°30^prime and 52°00^prime, provide new data for assessing structural control of reef-bank margins and the western edge of the Fairholme carbonate shelf.

Although recent erosion has removed most of the Devonian strata of the Main Ranges, sufficient outcrops are preserved to indicate definite trends. Two distinct NW-SE trends are evident in the underlying Ordovician and Cambrian strata: (1) a prominent broad positive ridge paralleling the eastern Front Ranges and adjoining Foothills (Alberta ridge), and (2) a prominent negative trough or depression (North Saskatchewan trough) paralleling the eastern Main Ranges and marked by the thickest and youngest Ordovician sediments preserved.

The margins of the Southesk complex and the north margin of the Fairholme complex trend NNE or NE

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almost at right angles to the Alberta ridge and the North Saskatchewan trough. These trends parallel those of the Rimbey-Leduc and Sundance-Windfall subsurface trends and are the most prominent features of this basin. The development of a later Devonian trend is documented by the presence of the southern part of the Southesk complex and the northwest part of the Fairholme complex above the thickest (most negative?) parts of the North Saskatchewan trough.

The positive Alberta ridge exerted a secondary control on the distribution of the Miette, Southesk, and Fairholme complexes. The thickest sections of the Fairholme Group (about 2,000 ft) are at the westernmost outcrops and appear to mark the western shelf margin of the Alberta basin.

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