The Blacktail-Snowcrest foreland uplift and its influence on structure of the Cordilleran thrust belt—Geological and geophysical evidence for the overlap province in southwestern Montana

Abstract

The Blacktail-Snowcrest uplift in Beaverhead County, Montana, is a northeasttrending structure of the Rocky Mountain foreland. Models of a detailed gravity traverse across the steep southeastern limb of the structure require the presence of a major thrust-fault system, only subsidiary parts of which are exposed at the surface. Gravity models, consistent with mapped surface structure, are based on density logs and thicknesses of Paleozoic and Mesozoic stratigraphic intervals penetrated in nearby drillholes. Interpretation of the models requires a thicker sequence of low-density Cretaceous rocks southeast of the uplift than was encountered in the wells, and also suggests that the thrust system beneath the uplift involves rocks of both the sedimentary sequence and the crystalline basement with at least 3,700 m (12,000 ft) of vertical separation. The models require the assumption of unusually low densities for some of the Precambrian and Paleozoic rocks; these densities are interpreted as being related to pervasive fracturing of these rocks during thrust deformation.Gravity and aeromagnetic trends associated with the Snowcrest-Greenhorn fault system extend westward beneath the Cordilleran thrust belt, indicating that the fault system is present there in the subsurface. A reentrant in the Medicine Lodge thrust sheet, the termination of the Four Eyes Canyon thrust sheet, and a change in strike of the Cabin thrust sheet, all in the frontal thrust belt, overlie the inferred westward extension of the Snowcrest-Greenhorn fault system, and probably are caused by the impingement of east-directed Cordilleran thrust faults on the older Rocky Mountain foreland structure. Cordilleran thrust sheets appear not only to overlap but also to crosscut older foreland faults, uplifts, and basins. Where crosscutting occurs, both upper and lower plates may exhibit rapid changes in stratigraphic level along strike, and these changes may be used as field evidence of overlap in the subsurface.

Maps showing gravity and aeromagnetic anomalies in the Dillon 1° x 2° quadrangle, Idaho and Montana

“…The western flank of this anomaly marks a north-trending, high-angle subsurface fault; the eastern flank nearly coincides with a succession of connected mapped faults (R~ppel and others, 1981). Immediately south of this region, a major listric fault has been postulated on the basis of gravity modeling and drill-hole data (Kulik and others, 1985;Kulik and Perry, 1988). Near here, other structural features have been inferred by Schofield ( 1981) on the basis of gravity modeling of basin fill and igneous rocks.…”

Mineral and energy resource assessment of the Gallatin National Forest (exclusive of the Absaroka-Beartooth study area), in Gallatin, Madison, Meagher, Park, and Sweet Grass Counties, south-central Montana

“…Small north-northwest-trending anticlines are present in the northern part of the Ruby basin area (Mann, 1960). Other such structures appear to be present farther southwest in the basin as indicated by gravity modeling (Kulik and Perry, 1988) and by seismic profiles. Ruby basin forms the northwestern margin of the late Paleozoic Wyoming shelf (Perry, 1986).…”

Extra-basinal fluid infiltration, mass transfer, and volume strain during folding: Insights from the Idaho-Montana thrust belt

“…This depth is consistent with illite crystallinity analysis of the Scott Peak Formation (Mississippian limestone) in the Lost River Range, which indicates peak metamorphic temperatures of 220°C (Kübler indices average 0.20°2) during Sevier deformation, as well as rare textural evidence (deformation lamellae, subgrains) indicative of dislocation creep in calcite which is thought to require temperatures of ϳ300°C (Rutter, 1976). In contrast, emergent thrusting and fan delta development along the southwest Montana thrust front suggests deformation near the synorogenic ground surface (Perry and Sando, 1983;Perry and others, 1988;Kulik and Perry, 1988;Williams and Bartley, 1988;Skip and Link, 1992;Schmitt and others, 1995;Anastasio and others, 2002). Vitrinite reflectance and conodont color alteration index Influenced by Oliver (1986), Garven (1989) and Hutcheon and others (2000).…”