ABSTRACT

A thick sequence of Cenomanian to Turanian strata crops out north of the Continental Divide near Lima in southwestern Montana. This sedimentary sequence is lithostratigraphically correlative with the Frontier Formation in western Wyoming to which it is assigned. The Frontier Formation is underlain disconformably by the Albian Vaughn Member of the Blackleaf Formation and is overlain with angular unconformity by the Coniacian to Maastrichtian Beaverhead Group.

The Frontier Formation near Lima may be more than 7000 ft (2134 m) thick and is subdivided into three informal lithofacies, all of which were deposited in nonmarine to shallow marine environments: lower mixed clastic; middle mudstone-siltstone; and upper mixed clastic. Palynologic data indicate that the Frontier Formation is Cenomanian to Turonian in age whereas the oldest assemblages from the Beaverhead Group (Monida sandstone unit near Monida) indicate an age no older than mid-Coniacian. The most conservative estimate for the duration of the hiatus represented by the Frontier-Beaverhead unconformity is early to mid-Coniacian time.

The Beaverhead-Frontier contact is identified by a conspicuous lithologic break and an angular unconformity. The base of the Beaverhead Group is defined at the base of the lowest quartzarenite (Monida sandstone unit), or limestone conglomerate (Lima Conglomerate and equivalent limestone conglomerates). Chert-rich sandstones similar to those in the upper Frontier occur above the contact but decrease in abundance upward.

Compositional and paleocurrent data from the Frontier Formation indicate that it was derived from Proterozoic, Paleozoic and Mesozoic sedimentary and volcanic terranes to the west, north, and south. The Frontier Formation was deposited during a cycle of foreland basin deposition prior to development of the Laramide Blacktail-Snowcrest uplift and Sevier-age thrusting in the Lima region. Even though the sequence is faulted and may be in part tectonically thickened, exposures of the Frontier near Lima are among the thickest exposures of Cenomanian to Turonian strata in the region and indicate deposition near the axis of a foredeep in front of the advancing Cordilleran thrust belt.