Abstract

The eastern part of the disturbed belt of Montana is in the foothills east of the northern Rocky Mountains. The area studied extends about 190 km from Wolf Creek north to Browning, Montana. It is about 30 km wide in the northern and southern parts, narrowing to about 10 km in the central part. The eastern margin is bounded by a thrust fault or folds.

The eastern part of the disturbed belt contains four structural-stratigraphic subdivisions which are, from west to east: (1) thrust-faulted Jurassic and Lower Cretaceous mudstone and sandstone; (2) folded and locally thrust-faulted Upper Cretaceous mudstone; (3) Upper Cretaceous sandstone and mudstone that are imbricately thrust faulted in the northern part, complexly folded near the central part, and folded and thrust faulted in the southern part; and (4) folded and locally thrust-faulted Upper Cretaceous thin sandstone and thick mudstone. The latter is absent in the central parts.

Structural trends change from northwest in the northern part of the area, to due south in the central part, and southeast in the southern part.

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