ABSTRACT

A composite stratigraphic section ranging in age from late Precambrian to Recent is present in the western Montana overthrust belt. Paleozoic rocks are 5,000 to 15,000 feet thick and are dominated by shallow marine shelf limestone and dolomite facies, much of which is of carbonate bank or reefal origin in the lower and middle Paleozoic, and by shelf marine sandstone and carbonate in the upper Paleozoic. Souce areas for Paleozoic and early Mesozoic clastic facies were in south-central Canada and north-central United States. Mesozoic rocks, between 1,000 and 20,000 feet thick, are dominated by shallow water marine clastic facies in the pre-Cretaceous and by continental and nearshore marine facies in the Cretaceous section, which becomes progressively coarser, volcanic-rich, and more continental in the younger beds. The source area for Mesozoic clastics was in east-central and northern Idaho and westernmost Montana which, beginning in late Jurassic time, became the site of increasingly intense tectonic growth that culminated in the development of the thrust and fold belt, and associated igneous activity, in Middle to Late Cretaceous and early Tertiary time.

The main structural framework of western Montana was developed during late Precambrian Belt Supergroup deposition, and many of the prominent paleostructural elements persisted through most of the remainder of geologic time. The more important of these which influenced the nature and distribution of sedimentary facies during Paleozoic and Mesozoic time are the Lemhi arch, Alberta shelf, Beartooth shelf, Belt Island complex, Boulder high, Coeur D'Alene-central Montana trough, Big Snowy trough, and Ruby trough. In east-central Idaho, the Muldoon trough (north segment of the Sublett basin), which in Paleozoic time lay between the Antler orogenic belt and the Montana shelf province, was a regional area of active subsidence that received a great thickness of Paleozoic shallow to deep water marine sediments.

The lower and middle Paleozoic rock facies in western Montana includes a large thickness of porous dolomite, and the upper Paleozoic contains a large volume of clean shelf sandstone, some of which has good porosity. Potential petroleum source rocks are present in the Devonian, Mississippian, and Permian beds, some of which, despite deep burial of most of the Paleozoic section, are not highly altered thermally. The Mesozoic rock facies is primarily continental in origin, but a broad belt of intertonguing nearshore marine and continental facies is present. The presence of dark marine and nearshore lagoonal or paludal carbonaceous to coaly beds and porous deltaic to marine sandstone bodies in this facies offers reasonable potential for significant biogenic and thermal gas accumulations under adequate trapping conditions. Tertiary continental and lacustrine beds, as much as 5,000 feet or more thick, were deposited in localized downwarped basins. These beds contain substantial thicknesses of discontinuous alluvial sandstone and some carbonaceous to coaly beds and are of interest for their biogenic gas potential.