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DYMAN, T. S., R. G. TYSDAL, and C. A. WALLACE, U.S. Geological Survey, Denver, CO, and S. E. LEWIS, Westinghouse Savannah River Company, Aiken, SC

ABSTRACT: Correlation Chart of Lower and Lower-Upper Cretaceous Blackleaf Formation, Eastern Pioneer Mountains, Southwestern Montana, to Drummond, Central-Western Montana

The late Albian to Cenomanian Blackleaf Formation in the eastern Pioneer Mountains of southwestern Montana and in the Flint Creek Range and Clark Fork Valley of central-western Montana was deposited near the western margin of the Western Interior seaway as both marine and nonmarine facies of the Cordilleran foreland basin. Five measured sections contain allochthonous sequences of Blackleaf strata that were transported eastward during emplacement of Late Cretaceous and early Tertiary thrust sheets.

Strata of the Blackleaf Formation in the eastern Pioneer Mountains are subdivided into the sandstone- and shale-rich Flood Member and the overlying volcaniclastic-rich Vaughn Member. In the northern Flint Greek Range south of Drummond, the Blackleaf is subdivided into the Flood, Taft Hill, and Vaughn Members. The Flood Member consists of a clastic sequence of greenish gray quartz- and lithic-rich sandstone, mudstone, siltstone, and dark-gray shale. The Blackleaf ranges in thickness from 935 ft (285 m) to 1185 ft (361 m).

The Taft Hill Member is composed predominantly of interbedded tan-weathering lithic-rich flat to rippled beds of sandstone, mudstone, and dark-gray shale. It is the lithic equivalent of the upper sandstone unit of the Flood Member, but is finer grained than sandstones of the upper Flood and contains interbedded silty shales from a few feet to tens of feet thick. The Taft Hill Member is 626 ft (191 m) thick at Barnes Creek, approximately 8 mi (13 km) south of Drummond, but is absent at the Drummond measured section. The Taft Hill Member is not recognized by us in the eastern Pioneer Mountains.

The Vaughn Member contains porcellanitic mudstone, siltstone, lithic sandstone, and subordinate conglomerate. It varies in thickness from 1950 ft (594 m) in the eastern Pioneer Mountains to 2488 ft (758 m) at Drummond. In the eastern Pioneer Mountains, the upper contact of the Vaughn is placed at the top of a porcellanite bed interbedded with limestone and dark-gray calcareous shale, which consistently and directly overlies the highest maroon mudstone-siltstone bed of the upper Vaughn.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.