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Geology and Economic Potential of Montana Aulacogen

Warren Shepard

The Montana aulacogen is one of a dozen or so North American intracratonic megarifts formed in the Proterozoic as part of the breakup of the North American-Siberian-European Precambrian megacontinent. This aulacogen had its inception as the failed arm of a middle Proterozoic triple junction located in western Montana caused by upwelling of a mantle plume; the other two arms became part of the Cordilleran geosyncline. The aulacogen trends east-west across central Montana and connects the Cordillera with the Williston basin. It is 60 mi wide by about 400 mi long (100 by 675 km). Initially, the aulacogen took the form of a pull-apart (keystone) graben, with bounding normal faults and filled with thick Precambrian and Cambrian sediments. During early and middle Paleozoic, it as reactivated several times alternately as horsts and graben, resulting in a complex picture of rift deposition and horst-block truncation. During Mesozoic, recurrent oblique and transcurrent block movement of the aulacogen, in response to compression caused by collision and subduction along western North America resulted in complex structural and stratigraphic maturity of the central Montana area. Mineral and hydrocarbon emplacement occurred during the Mesozoic, resulting in the creation of an economic "sweet spot." Copper, gold, silver, lead, zinc, and hydrocarbons found to date equal $19 billion. The Montana aulacogen has not been heavily explored. Plate tectonic concepts will aid in putting together the complex stratigraphic and structural picture of this giant rift and hopefully lead to discovery of additional economic deposits.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.