Field Guide to the Structural Geology of the Nordegg Area, Alberta

This field trip visits the Foothills and Front Ranges. Structures encountered can be characterized as folds, thrust faults, normal faults and tear faults (related to lateral ramps). A basal detachment above the Precambrian basement places the rocks in the thin-skinned style of deformation (McMechan and Thompson, 1989). A good overview of the structure of the Nordegg area is given by Jones and Workum (1978). Some of it will be summarized here. Folds are generally concentric and are underlain by detachment zones (generally thrusts), indicating that they are detachment folds as defined by Jamison (1987). Some fault-propagation and fault-bend folding is present as a result of the propagation of thrusts through folds that were detached during earlier stages of faulting. This view is somewhat different from the one that sees the Rocky Mountains as a series of rigid thrust sheets, without much internal deformation other than fault- end folding. The latter view is based on the Rich (1934) model of overthrusting and computer simulations such as described by Wilkerson et al. (1991). It is also the basis for fault-bend folding computer models used to characterize the Rocky Mountains by Jones (1987). In contrast, the importance of detachment folding in the deformation of the Rocky Mountains has been previously expressed by-Dahlstrom (1970) and more recently by results from centrifuge experiments by Dixon and co-workers (e.g. Dixon and Liu, 1991). The structure of the area can be described for the six major structural elements: Ancona Thrust sheet, Brazeau Thrust sheet, Bighorn Thrust sheet, McConnell Thrust sheet, Sulfur Mountain Thrust sheet and the Bourgeau Thrust sheet (figures 1 and 2). Some people argue that the Bourgeau Thrust sheet forms part of the Main Ranges, because of the presence of Precambrian strata. A cross section is shown in figure 3, the location of which is shown on figures 1 and 2. A cross section and seismic section through the Nordegg town site between the Stolberg Anticline and the Bighorn Thrust is shown in figure 4.