The southwest Marias Pass area includes parts of the Lewis and Clark, Sawtooth, and Lewis ranges in the northern Rocky Mountains of Montana.

The western two-thirds of the map area is underlain by a Precambrian sequence, the Belt Supergroup, consisting of argillites, quartzites, and silty microcrystalline carbonate rocks more than 14,000 feet thick. The Belt has been subdivided into seven conformable formations in addition to an unnamed sequence of beds at the top.

About 1300 feet of Mississippian carbonate rocks occur in the southeast quarter. These rocks are delimited above and below by thrust faults.

Jurassic and Lower Cretaceous shales, mudstones, and sandstones of unknown thickness crop out in the eastern part of the map area.

The trace of the Lewis thrust fault extends across the area from north to south separating the Belt strata from the Paleozoic and Mesozoic rocks. The Lewis fault surface cuts upward through the thrust sheet as it is traced southeastward through the map area, and the deformation of the autochthonous Mesozoic and Paleozoic rocks increases southeastward. There is no tear fault cutting the Lewis thrust sheet in the Marias Pass area. At the south end of the Clarke Range salient, erosion has resulted in apparent accentuation of an original shift in the trend of intersection between the fault surface and the beds of the thrust sheet.

The Mesozoic and Paleozoic strata are imbricated by southwest-dipping thrust faults which appear to be genetically related to the Lewis thrust. This zone of imbricate structure plunges at a low angle, N. 50°–85° W. and is the northwest end of the Sawtooth Range structural salient.

The folds within the Lewis thrust sheet and the imbricate structures in the Mesozoic rocks east of the north end of the Sawtooth Range salient (beyond the map area) are parallel and plunge at low angles, S. 20°–30° E. These relationships suggest an early stage of regionally concordant deformation before the Sawtooth Range salient was fully developed.

After movement on the Lewis fault ceased (late Eocene or early Oligocene), normal faulting took place. The largest of the normal faults dip southwest and offset the earlier thrust faults. The easternmost normal fault has a measured stratigraphic separation of 14,000 feet.

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