McConnell Thrust and associated structures at Mount Yamnuska, Alberta, Canada
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AbstractPrevious observations of rocks associated with overthrusts in the Front Range of the Canadian Rockies have indicated only slight deformation in these rocks. However, the plates bounding the McConnell Thrust at Mount Yamnuska, Alberta, Canada, have been found to be severely deformed, with almost complete loss of mechanical homogeniety in the hanging wall. At Mount Yamnuska massive limestones, with dolomite mottling, of the Cambrian Eldon Formation have been thrust horizontally a distance of from eight to twenty-six miles and vertically about five miles, and now rest on Cretaceous Belly River Formation sandstones and shales. To the west of Mount Yamnuska the thrust, which is commonly considered to be folded, may be a stepped thrust. Megascopic structures have been studied on photo mosaics assembled from photographs taken through a telephoto lens. Mesoscopic features have been studied from close up and attitudes measured by Brunton compass, the attitude measurements later being processed by computer programs. A special program was devised to deal with clustered data such as that pertaining to joints sets. West dipping second order thrusts in the Eldon Formation have attitudes as predicted by the Hafner model for failure of a block that is subjected to a horizontal push that is resisted by sheer along its base. A high pore pressure phase of deformation is indicated by curved splay faults (in the Eldon) that contain slivers of Belly River. The curved splay faults are continuous with some of a group of numerous cleancut offset planes that both parallel and cut bedding, and which are associated with, but postdate, perfectly cylindrical concentric drag folds. In the Belly River Formation folds are due to both drag and buckling. Most of the folds have gently plunging axes bearing approximately 325 degrees, but two minor groups have axes bearing an average of 266 degrees end 37 degrees. The paleo maximum principal stress direction, calculated from presumed transport direction, attitudes of fault step, second order thrusts, folds, joint sets, ranges from 38 degrees to 79 degrees. The minor groups of folds give approximate directions of 127 degrees and 176 degrees for the maximum principal stress. A period of relaxation, which is presumably very late, has produced east dipping normal faults that cut both the Eldon and the thrust plane, but not the Belly River Formation, which is folded around the offset blocks. Present day movement on these faults results from erosion of the Belly River Formation in the footwall of the thrust fault.
Bibliography: p. 103-105.