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Stratigraphic and Structural Relations of Lower Triassic Rocks Within the Frontal Fold-and-Thrust Zone of Southwestern Montana

PAULL, RACHEL K., and RICHARD A. PAULL, University of Wisconsin-Milwaukee, Milwaukee, WI

New sections of Lower Triassic rocks were measured within the frontal fold-and-thrust zone of southwestern Montana at Garfield Canyon, Horse Prairie Creek, Kennison Spring, and Birch Creek to clarify stratigraphic and structural relations. Triassic rocks disconformably overlie Upper Permian units and unconformably underlie younger rocks. From oldest to youngest, they include the Dinwoody, Woodside, and Thaynes formations.

The Dinwoody consists of shale, siltstone, and limestone; thickness varies from 152 to 273 m. Red beds of the Woodside thin northward to zero in the northern Tendoy Mountains. The Thaynes is comprised of limestone, siltstone, and sandstone; thickness varies from 244 m in the south, zero in the central area, to 51 m in the north. North of the Woodside termination, recognition of the Thaynes depends upon recovery of Smithian conodonts.

The Garfield Canyon and Horse Prairie sections are 7 km apart, south and north of the Horse Prairie fault, respectively. Northward across the fault, the Dinwoody thins from 228 to 152 m, and the Thaynes thins from 65 to 28 m. The Triassic from the fault northward to Kennison Spring was also extensively eroded after deposition.

Conodonts provide correlation and biofacies information for this study. From Birch Creek northward, conodonts are "basinal," consistent with lithofacies data. This area is within the McCartney Mountain salient, a depositional basin which may have existed on the craton margin prior to thrusting. However, there is no evidence to support basinal conditions in the Blacktail Mountain salient to the south.

Although thermal alteration values for most conodonts are within the range of oil and condensate production, those from Birch Creek north exceed the stability regime for hydrocarbons.


AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)