MIDDLE JURASSIC FOREBULGE MIGRATION, NORTHERN MONTANA ROCKIES: EVIDENCE FROM KARSTIFIED AND SAND-FILLED EXTENSIONAL FRACTURE SETS

The sub-Middle Jurassic unconformity is unusually well-exposed along Swift Reservoir, located in the Rocky Mountain front ranges of northern Montana. The unconformity separates Late Mississippian Madison Group carbonate from the transgressive basal sandstone of the Middle Jurassic Sawtooth Formation. The unconformity exhibits a system of karst-widened fractures ("grikes") filled with distinctive black cherty sandstone of the Sawtooth Formation. Middle Jurassic clam borings (Opertochasma?), also filled with Sawtooth sandstone, penetrate the unconformity surface and the sides of some of the grikes, firmly establishing the age of the fractures. The grike fractures have a mean azimuth of 346°, intersecting the axis of the fault-propagation anticline exposed at this site by an acute angle of approximately 24°. Indeed, these fractures were previously regarded as kinematically linked to extension over anticlinal fold crests during Laramide deformation. However, the grikes were filled by sand, stylolitized, and folded in concert with layer-parallel shortening rather than opening over the anticlinal crests. We propose that the grikes record passage of the forebulge through this region during the initiation of the northern Rockies foreland basin. Stockmal and Beaumont (1987) used elastic models of foreland basin flexure to interpret the evolution of the Alberta Basin to the north and locate the forebulge unconformity at the base of the Middle Jurassic Fernie Group. Uplift of the forebulge not only caused erosion at the unconformity, but also generated forebulge-parallel extension fractures in the uplifted bedrock. In northern Montana, at least 80-215 meters of Madison Group, and an unknown thickness of overlying late Paleozoic and early Mesozoic strata, were eroded off the forebulge (Mudge, 1972). The grikes may mark the passage of the forebulge crest, followed by subsidence beneath the transgressing Middle Jurassic sea. The palinspastic position of the Swift Reservoir site lies at least 400 km east of restored positions of correlative Middle Jurassic structures of the western Kootenay Arc. The grikes and connecting network of sandstone-filled bedding-plane cavities may enhance the secondary porosity and permeability of the upper Mississippian carbonate, an important reservoir in this region.