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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 68 (1984)

Issue: 7. (July)

First Page: 949

Last Page: 949

Title: Structural Geology of Swift Reservoir Culmination, Sawtooth Range, Montana: ABSTRACT

Author(s): D. Scott Singdahlsen


Northwest-trending, thrusted and folded rocks of Paleozoic and Mesozoic age comprise the Sawtooth Range of northwestern Montana. The Sawtooth Range is a part of the northern disturbed belt and is characterized by thin-skinned deformation. Stratigraphy plays an important role in the location and character of the various thrust sheets. Major decollement surfaces include the Upper Cambrian, the top of the Devonian, and several horizons within the Mississippian.

In the Swift Reservoir area, a broad culmination exposes Cambrian through Cretaceous strata in a series of imbricately stacked, west-dipping thrust sheets. The structural configuration of the culmination appears to be a compound duplex zone with structures north and south of Swift Reservoir dipping away from the culmination. Mapping of the culmination reveals vertical stacking of thrust sheets, and a lateral ramp that forms the southern boundary of the duplex. A structural high in the basement may have resulted in ramping of the thrust sheets and formation of the culmination.

Deformation within the thrust sheets is controlled by structural position and by ductility contrasts between the stratigraphic units. Deformation varies widely, ranging from tight overturned folds in the Cambrian units to broad open folds and fractures within the Mississippian. Because of the variation in lithology and ductility, the most intense deformation is observed within the Cambrian units. This deformation is characterized by overturning of folds in the direction of thrust transport, and the development of small-scale kink folds, cleavage, pencil structures, and boudinage.

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