ABSTRACT

A series of Tertiary conglomerates, sandstones and shales approaching 7,700 feet in combined thickness is present between the eastern and northeastern perimeter segments of the Beartooth Mountains and the fine-grained sediments which fill the Big Horn Basin. These alluvial fan sediments are exposed to varying degrees for a distance of 40 miles between the East Rosebud River near Roscoe, Montana and the Clark's Fork Canyon near Clark, Wyoming. Lithologic, petrographic and structural relationships indicate that these sediments were derived from local sources within the Beartooth Mountain complex and that they are both synorogenic and postorogenic. They contain inverted erosional sequences, which resulted from progressive erosion of the Beartooth Block, as well as sequences whose lithologic composition was controlled by local source conditions Basal beds are strongly tilted to overturned but dips become nearly horizontal in the upper most beds within 2 miles of the mountain front. Uncomformities, gravity thrusting, sheared clasts, and differences in grain size, lithologic composition and structural attitude reflect differences in tectonism along the Beartooth Front as well as the culmination of the Laramide Orogeny in this part of the central Rocky Mountains.

The massive, very coarse-grained basal units near the mountains were deposited by debris flows or other water-deficient processes, while more basmward facies were deposited by stream and streamflood activity. Limited paleontologic and paleobotanical evidence indicates a Late Paleocene age for these strata, but no definitive statement can be made that deposition of the upper part of these sediments did not continue into Eocene time.