ABSTRACT

The frontal zone of the Cordilleran thrust belt in western Montana south of the Helena (central Montana) salient, the southwestern Montana recess, has long been enigmatic. Chiefly Pliocene and younger volcanic rocks of the Snake River downwarp in southeastern Idaho separate the southwestern Montana recess from the better-understood, petroleum-producing Wyoming salient. This separation is a function of the young volcanic cover of the downwarp and does not appear to precede the formation of the downwarp. The southwestern Montana recess originally formed the northern margin of the Wyoming salient; the southern edge of the recess is present south of the Snake River downwarp.

In the Lima, Montana, region of the southwestern Montana recess, the Lima and Tendoy thrust systems occur at the Cordilleran thrust front, respectively southeast of and against a major buttress to latest Cretaceous to early Tertiary Sevier-style thrust advance: the Coniacian to middle Campanian (Late Cretaceous) Blacktail-Snowcrest uplift. The Four Eyes Canyon thrust sheet, the first sheet west of the Tendoy, carries the carbonate shelf sequence of the Mississippian Madison Croup. The McKenzie thrust system, northwest of the Four Eyes sheet, carries time-equivalent Mississippian downslope limestones of a more westerly facies belt. The Medicine Lodge thrust sheet, southwest of the McKenzie system, and directly west of much of the Four Eyes plate, represents a still more westerly miogeoclinal Mississippian limestone facies belt. The northern end of the closely related Cabin thrust sheet contains Ordovician(?) quartzites of the miogeoclinal sequence, closely similar to the Lower Ordovician Summerhouse Formation and Middle Ordovician Kinnikinic Quartzite of east-central Idaho, as well as the eastern wedge edge of Proterozoic sedimentary rocks.