The Overthrust Belt of west-central Montana consists of a zone of Laramide (Late Cretaceous-early Tertiary) convex-eastward thrusts and folds that include the major Eldorado-Lombard overthrust that has brought Precambrian Belt rocks eastward over Paleozoic and Mesozoic strata. East of and structurally below the Eldorado-Lombard overthrust is the Disturbed Belt with deformed rocks ranging in age from middle Precambrian (Proterozoic or Y) to early Tertiary.

The Disturbed Belt here forms a salient that extends eastward into the foreland and is bounded on the north by thrusts having a left-lateral component of movement and on the south by folds characterized by right-lateral shift. Deformation is most intense in the western part of the Disturbed Belt and dies out eastward where gentle folds merge with structures of the Rocky Mountain foreland. This salient appears to be underlain by an eastward-yielding decollement fault that merges with tear thrusts on the north and dies out in the subsurface beneath the folds marking the eastern and southern margins of the Disturbed Belt. Deformation resulted from piling up of imbricate thrusts and folds where the major Eldorado-Lombard overthrust sheet of the Cordilleran orogen moved upward and onto the foreland margin. Thus, the strata of the Disturbed Belt have been crumpled and deformed independently of the basement rocks, with crustal shortening above the decollement.

Hydrocarbon exploration in this area should be guided by the tectonic features. Thrusts in the western part of the Disturbed Belt have displacements of several miles and may conceal structures beneath the thrusts, but in the eastern part of the area have smaller displacements and probably do not conceal major structures below. Some anticlines are broken by subsidiary thrusts, resulting in offset axial surfaces at depth. Also, many folds have inclined axial surfaces and the fold aces therefore will migrate with depth.

End_of_Article - Last_Page 574------------