MONTANA’S TRIANGLE ZONE

The triangle zone is a complex antiformal structure and hydrocarbon trap that has been traced more than 800 km, from northeast British Columbia to southwest Alberta. It comprises a wedge of thrust plates inserted into the foreland basin at the leading edge of fold-thrust belt deformation. The wedge lifted overlying strata to produce a zone of easterly dips along the west limb of the Alberta syncline. Here we propose that the triangle zone continues into northwestern Montana as far south as Wolf Creek, along the west limbs of the Alberta, Augusta, and Adel synclines. We mapped the detailed structure of the Montana triangle zone by tracing marker beds of brown volcaniclastic sandstone and conglomerate of the Two Medicine Formation, Adel Mountains volcanics, and Horsethief Formation in the foothills of the Montana Rockies. The zone is 10-15 km wide and comprises a complex antiformal array of thrusts and folds that verge both to the east and to the west. Structural plunge varies along the zone to define culminations that separate the Alberta, Augusta, and Adel synclines. The triangle zone makes a natural structural trap for hydrocarbons that were driven from the Rocky Mountain fold-thrust belt by fluid overpressures. It has become highly productive in Canada. However, few exploration wells have been drilled in the Montana portion of the zone. We suggest that the culminations along the Montana triangle zone could make attractive prospects for future exploration efforts.