Complexly imbricated Upper Devonian and Mississippian rocks in the northeastern Tendoy Mountains, Montana, form the previously unrecognized McKenzie thrust system, which is south of and structurally above the south-plunging Armstead anticline and north of the Tendoy thrust sheet. The northern margin of the McKenzie system, east of Garfield Canyon, displays a minimum of 4 mi (6 km) of eastward displacement. The southeastern margin is south of Kelmbeck Creek, near McKnight Canyon. The eastern edge of the system is buried under Quaternary to Late Cretaceous cover at or east of Red Rock Valley. East of the McKenzie system, the front of the Montana thrust belt extends north-northeast from Dell, Montana, to the eastern Blacktail Range, on the basis of unpublished mapping by J. . Haley and W. C. Pecora, Jr. The convex eastward curvature of the thrust belt in this area, including the McKenzie thrust system, is herein designated the Blacktail salient.

Imbricates of the McKenzie thrust system comprise two duplex fault zones between Bell and McKenzie Canyons. The lower duplex involves a unique suite of platform to basinal Kinderhookian to lower Meramecian (Mississippian) carbonate rocks as well as Upper Devonian rocks. The

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floor thrust of this imbricate stack appears to lie within the Upper Devonian Three Forks Formation; the roof thrust lies within the middle Meramecian Kibbey Sandstone. The upper duplex involves Upper Mississippian rocks above the Kibbey Sandstone. Its roof thrust closely follows bedding near the top of the Mississippian sequence. The geometry of imbricate stacks within the McKenzie plate demands shortening of greater than 100%, resulting in at least 2 mi (3 km) additional eastward displacement of its trailing edge.

Recognition of the Blacktail salient with its complex structural patterns and unusual platform to basinal carbonate sequence provides new exploration targets in the southwestern part of the Montana thrust belt.

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