Abstract The front ranges of the Rocky Mountains near the Bow River, Alberta, are a fine example of the thrust-fault type of structure which characterizes the eastern Canadian Rockies. The ranges consist largely of repeated sequences of Paleozoic strata dipping 40° W. and separated by subsequent valleys eroded in Mesozoic or Upper Paleozoic beds. Cambrian to Upper Cretaceous strata are exposed. The map area is traversed by the Calgary-Banff highway and the main line of the Canadian Pacific Railway and, although visited by many geologists since the 188o's, it has not heretofore been mapped. The map area, which consists of three fault blocks that repeat most of the Paleozoic section, is bounded on the east by a major folded thrust fault which throws Middle Cambrian rocks over Upper Cretaceous strata. This fault marks the boundary between the Rocky Mountains and the foothills belt. The Cascade coal basin is the westward limit of mapping. Middle Cambrian strata along the mountain front are overlain unconformably by the Ghost River formation of questionable Devonian age. This is overlain conformably by Upper Devonian dolomites and limestones and these are followed by Lower, Middle, and Upper Mississippian shales and limestones, Pennsylvanian quartzitic and dolomitic sandstones, Permian (?) chert and quartzite, Triassic shaly siltstone, Jurassic black shale, and Lower Cretaceous or Jurassic sandstones, shales and coal. No igneous rocks are exposed in this part of the Rocky Mountains. The great thrust faults, displacements on which are measured in miles, in some places develop drag folds, but the fault planes are clean breaks with little or no gouge, and the strata on both sides approach parallelism with the fault planes.

A sub–Middle Jurassic unconformity is exhumed at Swift Reservoir, in the Rocky Mountain fold-and-thrust belt of Montana. The unconformity separates late Mississippian Sun River Dolomite of the Madison Group (ca. 340 Ma) from the transgressive basal sandstone of the Middle Jurassic (Bajocian-Bathonian) Sawtooth Formation (ca. 170 Ma). North-northwest–trending, karst-widened fractures (grikes) filled with cherty and phosphatic sandstone and conglomerate of the basal Sawtooth Formation penetrate the Madison Group for 4 m below the unconformity. The fractures link into sandstone-filled cavities along bedding planes. Clam borings, filled with fine-grained Sawtooth sandstone, pepper the unconformity surface and some of the fracture walls. Sandstone-filled clam borings also perforate rounded clasts of Mississippian limestone that lie on the surface of the unconformity within basal Sawtooth conglomerate. After deposition of the overlying foreland basin clastic wedge, the grikes were stylolitized by layer-parallel shortening and then buckled over fault-propagation anticlinal crests in the Late Cretaceous–Paleocene fold-and-thrust belt. We propose that the grikes record uplift and erosion followed by subsidence as the Rocky Mountain foreland experienced elastic flexure in response to tectonic loading at the plate boundary farther to the west during the Middle Jurassic. The forebulge opened strike-parallel fractures in the Madison Group that were then karstified. The sandstone-filled karst system contributes secondary porosity and permeability to the upper Madison Group, which is a major petroleum reservoir in the region. The recognition of the fractures as pre–Middle Jurassic revises previous models that have related them to Cretaceous or Paleocene fracturing over the crests of fault-propagation folds in the fold-and-thrust belt, substantially changing our understanding of the hydrocarbon system.