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The AAPG/Datapages Combined Publications Database

Montana Geological Society



Billings Geological Society: Guidebook: Tenth Annual Field Conference: Sawtooth-Disturbed Belt Area
August 13-15, 1959

Pages 98 - 108


G. WILLIAM HURLEY, Pan American Petroleum Corporation, Casper, Wyoming.


The Disturbed Belt of Montana lies on the hinge-line between the Central Stable Platform on the east and the Rocky Mountain geosyncline on the west. It lies west of the Sweetgrass Arch and includes the Rocky Mountain front ranges of western Montana. It is characterized by a zone of overthrust faulting and folding extending from the Missouri River northward 125 miles to the International Border.

Structurally the Disturbed Belt is divisible into three layers, a regional layer, a high Previous HitangleNext Hit thrust layer, and a low Previous HitangleNext Hit thrust layer. The regional layer is the oldest and comprises the west flank of the Sweetgrass Arch over which the younger high-Previous HitangleNext Hit thrust layer has ridden. The high-Previous HitangleNext Hit thrust layer is complexly imbricated and constitutes the typical Disturbed Belt structures. The low-Previous HitangleNext Hit thrust layer, commonly known as the Lewis overthrust, is the youngest layer and has overridden the high-Previous HitangleNext Hit thrust layer. Subsequent high Previous HitangleTop block faulting has added further complexity to the structures.

Three types of traps have been recognized in Montana's Disturbed Belt. Type number one is the fault traps on the wedge-edge of the Paleozoic thrust sheets; type number two is the drag folds formed as the result of thrusting; and type number three is the folds occurring west of the zone of drag folding. Northern Natural Gas Producing Company's recent Mississippian discovery in sec. 13, T26N, R9W. in Teton County is the first evidence that commercial hydrocarbon accumulations exist in Montana's Disturbed Belt. This discovery is of the wedge-edge type.

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