Abstract

The results of this project represent a further step in the demonstration of the chronologic relationship between time and manner of mountain uplift; time of notable volcanic activity; and time of accumulation of basin-fill sediments in the Bighorn Basin and Crazy Mountain syncline regions of southeastern Montana.

The investigation indicates that the stratigraphic units traced and mapped as Hell Creek, Tullock, Lebo, and Tongue River formations from significant fossil vertebrate localities in the vicinity of Polecat Bench and Heart Mountain, Wyoming, correlate with units designated as Hell Creek, Bear, Lebo No. 1 and Lebo No. 2, and Melville, respectively, in the Crazy Mountain syncline.

A series of agglomerates along the Yellowstone River is overlain by shales and sandstones of the Hell Creek. In some areas the agglomerates truncate beds of the Claggett, Judith River, Bearpaw, and possibly the Lennep; in other areas they are interfingered with these formations. Hence they signify a predominance of volcanic activity along the northern front of the Beartooths during this time interval.

Sedimentary petrographic studies show an absence of minerals derived from metamorphic rocks, which would seem to indicate that the crystalline core of the northwestern part of the Beartooth Range had not been stripped of its cover of Paleozoic sedimentary rocks by the close of Tongue River time.

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