ABSTRACT

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 angle thrust layer, and a low angle thrust layer. The regional layer is the oldest and comprises the west flank of the Sweetgrass Arch over which the younger high-angle thrust layer has ridden. The high-angle thrust layer is complexly imbricated and constitutes the typical Disturbed Belt structures. The low-angle thrust layer, commonly known as the Lewis overthrust, is the youngest layer and has overridden the high-angle thrust layer. Subsequent high angle 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.