Correlation chart of lower and upper Cretaceous Blackleaf Formation, Lima Peaks area to eastern Pioneer Mountains, southwestern Montana

Abstract

The Lower and Upper Cretaceous Blackleaf Formation in the Lima Peaks area and the eastern Pioneer Mountains of southwestern Montana was deposited at the western margin of the Western Interior seaway as a nonmarine fades of the Cordilleran foreland basin. Different names have been applied previously to these same strata even within the same area. Five measured sections show sequences of Blackleaf lithofacies that were deposited farther west than their present positions; the strata were telescoped by thrust faults of Late Cretaceous to early Tertiary age. The upper Albian and lower Cenomanian Blackleaf Formation is underlain by the Aptian to Albian(?) Kootenai Formation. At Lima Peaks, the Blackleaf is overlain by the Cenomanian to Turonian Frontier Formation; in the eastern Pioneer Mountains, overlying strata differ in lithology from the Frontier and are unnamed and undivided. Strata of the Blackleaf Formation at Lima Peaks are subdivided into the siliciclastic Flood and overlying siliciclastic and volcaniclastic Vaughn Members, both of which also are present in the eastern Pioneer Mountains. The Taft Hill and Bootlegger Members of the Blackleaf Formation, present in their type sections near Great Falls, Mont., are not recognized by us in southwestern Montana. The Flood Member consists of a mixed clastic sequence of quartzand chert-rich sandstone, mudstone, and siltstone, and dark-gray shale; the member ranges in thickness from a minimum of 495 ft (150 m) at the Lima Peaks section to 1,185 ft (361 m) at the Rock Creek section and 1,270 ft (387 m) at the Trapper Creek section. The Vaughn Member contains abundant porcellanitic mudstone, bentonite, lithic sandstone, and subordinate conglomerate; it varies in thickness from 880 ft (268 m) at the Lima Peaks section to 1,950 ft (594 m) at the Rock Creek section. The Vaughn-Frontier contact at Lima Peaks is disconformable and is marked by a sharp lithologic break. The overlying Frontier Formation contains saT-andpepper sandstone, mudstone, shale, and conglomerate. In the eastern Pioneer Mountains, the upper contact of the Vaughn is considered provisional until furthe field studies are completed. The provisional upper confact of the Vaughn occurs at the top of a porcellanit?. that consistently and directly overlies the highest maroon mudstone-siltstone bed of the upper Vaughn, and is associated with micritic limestone and dark-gray shale. Undivided Upper Cretaceous strata above this contact contain quartz-rich sandstone, quartzite conglomerate, and brown siltstone.