Stratigraphy, Petrology, and Provenance of Cenomanian to Turonian Frontier Formation near Lima, Southwest Montana

T. S. Dyman, W. J. Perry, D. J. Nichols, L. E. Davis, J. C. Haley

More than 7,000 ft of the Cenomanian-Turonian Frontier Formation crop out north of the Continental Divide near Lima in southwest Montana. The Frontier is underlain disconformably by the Albian Blackleaf Formation and is overlain unconformably by the mid-Coniacian to Maestrichtian Beaverhead group. Palynologic data indicate that the Frontier is Cenomanian to Turonian in age, whereas the oldest Beaverhead assemblages indicate an age no older than mid-Coniacian.

The Frontier Formation in the study area is subdivided into three informal lithofacies, all of which were deposited in fluvial environments: a mixed clastic lower unit (1,500 ft), a fine-grained medial unit (5,000 ft), and a mixed clastic upper unit (800 ft). In the lower unit, sandstones contain as much as 50% total feldspar and volcanic rock fragments; conglomerates are rich in quartzite clasts derived from the Belt Supergroup and contain as much as 15% silicic volcanic and 2% limestone clasts. Mudstone, siltstone, shale, and coal are interbedded with the sandstones. The fine-grained medial unit contains nonvolcanic mudstones and siltstones and minor sandstones and pebble conglomerates rich in chert and limestone lithic detritus. The mixed clastic upper unit contains abundant porcel anite and bentonite; sandstone interbeds contain plagioclase and limestone lithic grains.

Sources for Frontier detritus include Proterozoic, upper Paleozoic, and Mesozoic sedimentary and volcanic rocks to the south and west. The Frontier was deposited during a depositional cycle prior to development of the Laramide Blacktail-Snowcrest uplift. Exposures of the Frontier near Lima appear to be among the thickest in the region and were probably deposited near the axis of a foredeep in front of the advancing Cordilleran thrust belt.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.