Structure style influenced by lithofacies, Rocky Mountain Main Ranges, Alberta-British Columbia
Authors: Cook D G 
Published: July 2013

In the Main Ranges of the Canadian Rocky Mountains between Vermilion Pass in the southeast and a line joining Bow Lake and Blaeberry River in the northwest, Middle and Upper Cambrian rocks undergo a facies change from a dominantly carbonate sequence in the
eastern Main Ranges to a dominantly argillaceous sequence in the western Main Ranges. The Chancellor Formation, comprising most of the western argillaceous sequence, contains mappable stratigraphic units that can be correlated with those forming the sequence from Mount Whyte Formation to Sullivan
Formation inclusively in the east. The eastern and western Main Ranges are structural and physiographic subdivisions characterized by the differing structural and erosional styles that have developed in the rocks of the two contrasting facies. The rocks of both subdivisions, in the area studied, lie
within the Simpson Pass Thrust plate. The eastern Main Ranges are characterized by large-scale concentric folds, the westernmost of which, the Cathedral Crags Anticline, persists along the full length of the area. The western limb of the anticline constitutes an abrupt steepening of the over all dip
of the Simpson Pass Thrust plate, and the underlying Simpson Pass Thrust is presumed to be downturned below this zone as well. A complex belt of thrust faults and tight folds occurs along the west limb of the Cathedral Crags Anticline and coincides with the transition zone from eastern to western
facies. The thrusts are back-limb thrusts on the Cathedral Crags Anticline. They and the related tight folds apparently were localized by the coincidence of favourably oriented southwest-dipping bedding and the facies change. Before this study, an enigmatic major fault in the southern part of the
area, the Stephen-Dennis Fault, was believed to separate the Chancellor Formation of the western Main Ranges from Middle Cambrian carbonate rocks of the eastern Main Ranges. The trace of this fault was considered to run parallel with, and close to, the zone of tight folds at the facies change. No
evidence supporting a through-going fault, either normal or thrust, was found along this zone; at a number of localities beds extend unbroken from the eastern carbonate facies into the western slate facies. The concept of the Stephen-Dennis Fault marking the boundary between the eastern and western
Main Ranges should be abandoned. Normal faults in the area trend more northerly than the thrusts, and are confined to the eastern Main Ranges. They cut obliquely across the eastern Main Ranges, and appear to die out in the belt of thrusts and tight folds along the zone of facies transition. The
direction of extension indicated by the normal faults is inclined to the direction of compression implied by the folds and thrust faults. The normal faults appear to have formed later than the thrusts, but subhorizontal slickensides on fractures that apparently are related to the normal faults imply
a phase of horizontal compression that postdates formation of the faults. Cleavage characterizes the Chancellor Formation, and has developed in conjunction with folding as a result of a northeast-southwest compression. Competent beds in the Chancellor have folded chiefly by flexure, whereas
incompetent beds have folded chiefly by pervasive flow parallel to the axial planes through flattening of carbonate grains and recrystallization of micaceous minerals, with resultant development of axial plane cleavage. Beds have been shortened horizontally and consequently extended vertically with
resultant thickening of the depositional units.