Geometry and Kinematics of Thrust System Within Central Snowcrest Range: A Major Rocky Mountain Foreland Uplift and Exploration Target in Southwest Montana

Barry C. McBride

The Snowcrest Range forms the southeastern limb of the Late Cretaceous, northeast-trending, southeast-verging Blacktail-Snowcrest uplift. Field mapping and regional balanced cross sections indicate the Snowcrest thrust system consists of two master thrust faults (Snowcrest and sub-Snowcrest thrusts), each involving a system of subsidiary thrust imbricates, splays, and shear zones. East of the Snowcrest Range, the Gravelly Range thrusts are attributed to this system as regional splays.

The Snowcrest thrust dips approximately 39°NW and forms the western boundary of large-scale, upright, and overturned folds within Phanerozoic rocks above the sub-Snowcrest (Greenhorn) thrust. The sub-Snowcrest thrust is buried beneath or within the synorogenic deposit of the Beaverhead group and is inferred to merge with the Snowcrest thrust in Precambrian basement rocks at depth.

Small-scale structures indicate a significant component of right-lateral oblique slip occurred on the sub-Snowcrest thrust system along the central and southern portions of the range. Preliminary strain data indicate that the principal shortening was directed east-southeast.

Large-scale fold interference patterns indicate an intersection of northeast and north-northwest-trending folds. The north-northwest-trending folds are probably controlled by subsurface faults. Late Cenozoic, basin-range extension has inverted the Blacktail-Snowcrest uplift. Listric normal faults parallel the range and probably merge with the Snowcrest thrust system at depth. The listric geometry is supported by rollover of Tertiary sediments into the main normal fault.

Subsurface control is provided by one deep well within the study area. A second well currently being drilled will provide control of the sub-Snowcrest thrust and footwall geometries.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.