Structural geometry and kinematic history of the southern Kicking Horse Rim Cambrian carbonate platform: out-of-sequence thrust faulting in the southern Canadian Rocky Mountains

Abstract

Abstract The structure of the southern Kicking Horse Rim Cambrian carbonate platform is spectacularly exposed in the Rocky Mountains southwest of Calgary, Alberta, Canada. Episodic uplift along the approximately 100 km long, north-northwest-trending Kicking Horse Rim during the middle and upper Cambrian controlled the position of the Cambrian carbonate platform to basin facies change. Extensive hydrothermal alteration, with local magnesite and/or Pb-Zn mineralization occurred along the west margin of the Rim. Spectacular slump scars, escarpments, megaconglomerates and slide masses locally mark the edge of the southern Kicking Horse Rim platform. Megaconglomerates form a unique and indisputable stratigraphic tie between the basin and platform and show that there is no stratigraphic gap or suture along this boundary. The change from carbonate- to shale-dominated lithologies resulted in a major change in structural style and the boundary between the Eastern and Western Main Ranges structural subprovinces. Serial and regional cross-sections across the area document the southward plunge of Kicking Horse Rim strata and the Eastern Main Ranges structural subprovince and show that this is due to underlying structure, and not stratigraphic thinning of the Cambrian carbonate platform. Prospective horizons for mineralization extend southward in the subsurface beyond their current southern limit of exposure. Kicking Horse Rim and associated Cambrian strata were carried northeastward along the Fatigue Thrust - blind thrust fault system. Because of a prominent hanging wall ramp through the entire Cambrian succession along the east side of the Kicking Horse Rim, the southern Kicking Horse Rim and Cambrian platform formed an asymmetric, east-facing anticline with a long gently-dipping west limb as they were uplifted and carried northeastward. In the northern part of the study area, displacement was along the Fatigue Thrust and the fault climbed through the Paleozoic section. In the south, most displacement on this fault system went into a blind detachment fault near the base of the thick basinal lower Ordovician section and overlying faulted detachment folds. The Simpson Pass Thrust truncated and overrode in its footwall the east-facing anticline and other structures developed in the hanging wall of the Fatigue Thrust. The Simpson Pass Thrust is out-of-sequence relative to the Fatigue Thrust and is one of the few larger displacement, out-of-sequence thrust faults recognized in the southern Canadian Rocky Mountains. Deformation of the Western Main Ranges and thrust faults near the Cambrian facies change occurred prior to the main motion along the Fatigue Thrust - blind thrust system. Motion on this thrust system, and the formation of faulted detachment folds that extend southward into the eastern part of the Porcupine Creek fan structure, occurred prior to motion on the Simpson Pass Thrust. Deformation of all these structures occurred during Jurassic–Early Cretaceous deformation most likely sometime prior to 125 Ma.