Origin of the Disturbed Belt in Northwestern Montana

Abstract

The northern part of the disturbed belt in Montana is a northwesterly trending zone of closely spaced westerly dipping thrust faults, many folds, and some longitudinal normal faults and transverse faults. The theory of vertical uplift that results in gravitational gliding is a reasonable explanation of the origin of the disturbed belt of northwestern Montana. The outcropping sedimentary rocks range in age from Precambrian (Belt Supergroup) to Tertiary. All Precambrian, Paleozoic, and Mesozoic stratigraphic rock units thin markedly to the east. Westernmost Montana was a slowly subsiding geosynclme during Precambrian (Belt) sedimentation and a miogeosyncline during much of Paleozoic sedimentation. The miogeosynclinal area was uplifted into a highland during the Jurassic and Cretaceous, and sediment from the highland was deposited in a basin to the east. Periodic uplift and erosion continued through Cretaceous and very early Tertiary. I believe that a decollement was established, in the easterly tilted sediments, and the mass moved eastward under the influence of gravity across the small Mesozoic basin. The decollement migrated upsection to the east. East of the slide mass the rocks were folded, marking the east edge of the northern part of the disturbed belt in Montana. This edge was probably controlled by the erosional edge of the Precambrian (Belt) rocks and the west side of the craton. Additional uplift continued to produce sliding that piled one fault block upon another. The minimum amount of shortening of this upper part of the crust by thrust faulting and folding computed along one line of section is more than 29 miles. The amount of uplift to the west very likely exceeded 45,000 ft during the period from very Late Cretaceous to late Eocene. The main decollement was under an overburden of as much as 25,000 ft of strata—a thickness that would probably permit abnormal fluid pressures to develop in mudstone. The slope of the strata and glide surface by the end of uplift may have been as much as 8.5°. Large Basin-and-Range-type normal faults developed, after thrusting, between the area of maximum uplift and the thrust fault belt. The westernmost of these faults formed the graben and horsts in the Rocky Mountain trench. The total amount of displacement of the normal faults along one line of section is about 43,000 ft. The total thickness of strata eroded from the area of maximum uplift is about 45,000 ft. The theory of vertical uplift and gravitational sliding may also be applicable to the disturbed belt in Alberta and British Columbia. The disturbed belt, Rocky Mountain trench, and areas of uplift are continuous from northwestern Montana to northern British Columbia. Much of the geologic history of western Alberta and eastern British Columbia is like that of northwestern Montana.