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Lower Cretaceous strata in southwestern Montana were deposited in a nonmarine facies of the Cordilleran foreland in the west and in a marine shelf facies of the foreland in the east. Some of the strata represent time-equivalent sediments deposited in different environments, and different names have been applied previously to the same strata even within the same area. The four measured sections presented here are in sequences of strata that have been telescoped by thrust faults of Late Cretaceous to early Tertiary age, and facies represented in the measured sections originally were deposited farther apart than they are now. Strata described herein are underlain by the Aptian to Albian(?) Kootenai Formation and are overlain by the Cenomanian to Turonian lower part of the Frontier Formation. In the Snowcrest Range and Lima Peaks area, strata represented by the two westernmost measured sections are assigned to the Albian Blackleaf Formation, which contains the Flood Member and the overlying volcaniclastic-rich Vaughn Member. The Taft Hill and Bootlegger Members of the Blackleaf Formation, in their type sections near Great Falls, are not recognized by us in southwestern Montana. The two easternmost measured sections represent strata in the Madison, Gravelly, and Greenhorn Ranges, where Albian rocks are assigned to the Thermopolis Shale, Muddy Sandstone, and Mowry Shale. In southwestern Montana the Thermopolis Shale and overlying Muddy Sandstone are lithically equivalent to the Hood Member of the Blackleaf Formation, whereas the Mowry Shale is equivalent to the Vaughn Member. Strata in the Snowcrest Range have characteristics in common with both the eastern and western facies. The geographic location for the change in nomenclature is arbitrary, but is pbced along the axis of the Ruby River valley, which separates the Snowcrest Range from the Greenhorn and Gravelly Ranges.

Correlation chart of lower Cretaceous rocks, Madison Range to Lima Peaks area, southwestern Montana