ABSTRACT: Correlation of Lower and Upper Cretaceous Blackleaf Formation, Lima Peaks Area to Eastern Pioneer Mountains, Southwestern Montana Foreland Basin

T. S. Dyman, R. G. Tysdal

The Lower and Upper Cretaceous Blackleaf Formation in the Lima Peaks area and eastern Pioneer Mountains of southwestern Montana was deposited near the western margin of the Western Interior seaway as a nonmarine facies of the Cordilleran foreland basin. Five measured sections show sequences of Blackleaf lithofacies that were deposited farther west than their present position; 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 and in the eastern Pioneer Mountains are subdivided into the siliciclastic Flood and the overlying siliciclastic and volcaniclastic Vaughn Member. The Flood Member consists of a clastic sequence of quartz- and chert-rich sandstone, mudstone, and siltstone and dark gray shale; the member ranges in thickness from a minimum of 495 ft at Lima Peaks to 1185 ft at Rock Creek and 1270 ft at Trapper Creek. The volcaniclastic Vaughn Member contains abundant porcellanitic mudstone, bentonite, lithic sandstone, and subordinate conglomerate; it varies in thickness from 880 ft at the Lima Peaks section to 1950 ft 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 salt-and-pepper sandstone, mudstone, shale, and conglomerate. In the eastern Pioneer Mountains, the provisional upper contact of the Vaughn is placed at the top of a porcellanite that consistently and directly overlies the highest maroon mudstone-siltstone bed of the upper Vaughn. Maroon mudstone is interbedded with freshwater micritic limestone and dark gray shale.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990