Abstract

The geometry of the structural front and its variation along strike in the Alberta Foothills northwest of Grande Cache are described with the aid of three, balanced regional cross-sections constrained by seismic interpretation, well data and surface geology. The surface expression of the structural front varies from folds in the northern part (Two Lakes), to a mixture of folds and east- and west-vergent thrusts in the central part (Copton Ridge), to west-vergent thrusts in the southern part of the study area (Cutpick Hill). In the subsurface, different styles of tectonic wedging and underthrusting have developed between various structural detachments. One tectonic wedge with a folded, west-vergent, upper detachment in the Kaskapau Formation and overlain by passively folded roof strata has developed in the northern part of the area. In central and southern parts, tectonic wedges with classical triangle zone geometries and emergent, west-vergent faults have developed and a pattern of inner and outer and upper and lower tectonic wedges is interpreted. Structural relationships suggest considerable overlap in the relative timing of tectonic wedge formation with outer wedge formation continuing after inner wedge formation and upper wedge formation broadly synchronous with lower wedge formation. The pattern of inner and outer tectonic wedges is geometrically similar to the pattern of wedging observed in the Oldman River area of southern Alberta. Shortening across the structural front varies from 3 to 7 km at the most shortened stratigraphic levels and is considerably less than across triangle zone structures in the central and southern Foothills of Alberta.

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