ABSTRACT

The map area is 50 miles west-northwest of Calgary and includes part of the Foothills belt and a salient of the Front Ranges. The salient is subdivided by large thrust faults into a number of segments which are, from east to west: 1. Orient block of the McConnell thrust sheet, a broad closed syncline; 2. Costigan block, a more typical westerly-dipping homoclinal Front Range structure; and 3. Exshaw thrust sheet, also a westerly-dipping homocline. These segments are limited downwards by the McConnell, Costigan and Exshaw faults. The Ghost River oblique fault further divides the Costigan and Orient blocks into north and south segments.

Analysis of structure suggests that several shortening mechanisms were responsible for the present tectonic configuration, the most unusual resulting from a component of compression parallel to the regional strike. The McConnell fault probably originated as an irregular fracture as evidenced by variations in stratigraphic throw along strike, and was subsequently folded by development of the underlying Panther River anticline of the Foothills. The thrust sheet became divided during this folding, into the Orient and Costigan blocks, by the Costigan fault.

In addition to the overall shortening produced by the Laramide orogeny, shortening has been effected within the McConnell thrust sheet in a direction parallel to the regional strike by dextral movement on the Ghost River fault and by transverse folding of the Orient block. The Exshaw thrust sheet to the west was aparently unaffected by the Panther River structure and shows no manifestation, within the map area, of shortening in a strike direction.

Minor branches of the McConnell and Costigan faults were formed in response to local modifications of the regional stress field in proximity to the master faults.

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