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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 43 (1959)

Issue: 5. (May)

First Page: 1099

Last Page: 1099

Title: Overthrust Faulting and Paleozoic Gas Prospects in Montana's Disturbed Belt: ABSTRACT

Author(s): G. W. Hurley

Article Type: Meeting 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. The Disturbed belt is characterized by a zone of overthrust faulting and folding extending from the Missouri River northward into Canada. Some of the largest gas and distillate reserves in North America have been found in one or more thrust sheets of Mississippian rocks in Canada at Pincher Creek and Waterton Lake and a recent gas discovery was completed in the Devonian formation at Castle River. Northern Natural Gas Company's recent Mississippian gas discovery in Sec. 13, T. 26 N., R. 8 W., may be the first evidence that such accumulations are also present in Montana's Disturbed belt.

Unconformities between the Cambrian and Devonian and between the Mississippian and Jurassic are evidence that the area was tectonically active during Paleozoic time and isopachs of the Jurassic and Cretaceous formations indicate that this activity continued intermittently throughout Mesozoic time. The characteristic overthrust faulting from the west is the result of the Laramide orogeny of early Tertiary time.

The structure of Montana's Disturbed belt is divisible into three layers each younger in age: (1) a regional layer of relatively undeformed rocks comprising the west flank of the Sweetgrass arch over which the high-angle thrust layer has ridden; (2) a high-angle thrust layer of complex faulting and drag folds typical of the Disturbed belt structures; and (3) a low-angle thrust layer, commonly known as the Lewis overthrust, which overrode the high-angle thrust layer. Subsequent high-angle block faulting has added further complexity to the structures.

Three types of traps similar to those of the Canadian Disturbed belt are present: (1) fault traps on the wedge-edge of the Paleozoic thrust sheets; (2) drag folds formed as the result of thrusting; and (3) folds occurring west of the zone of drag folding as typified at Savannah Creek in Alberta. The Northern Natural Gas Company's Mississippian discovery at Blackleaf Creek is of the wedge-edge type.

Structural interpretation of this area is difficult and drilling costs are high. Therefore, much money will have to be spent before the economic possibilities of the hydrocarbons in Montana's Disturbed belt have been adequately evaluated.

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Copyright 1997 American Association of Petroleum Geologists