Middle Devonian to Late Mississippian Geologic History of the Overthrust Belt Region, Western United States

The region of the Overthrust belt from Montana to Nevada was the depositional site for continental-shelf and peritidal upper Middle Devonian and Upper Devonian carbonate rocks and platform, foreslope, and forelandtrough Lower Mississippian to lower Upper Mississippian carbonate rocks. Devonian rocks include many beds of porous dolomite that could serve as petroleum reservoirs. Mississippian rocks contain source, conduit, and reservoir beds. The present position of the late Early Mississippian shelfedge corresponds closely to that of the leading edge of the late Mesozoic Sevier thrust system, and the abrupt changes in thickness, bedding, and competence of Mississippian rocks at this shelfedge may have been a factor in determining the position of later thrusting. The leading edge of the Sevier system also is subparallel to and about 125-400 km east of the present position of the Mississippian Antler Orogenic Highlands. The Middle Devonian to Late Mississippian history of the Overthrust belt region comprises two major transgressive-regressive marine cycles, separated by an episode of continental stability. The history is interpreted using a series of thickness, lithofacies, and paleogeographic maps. The intervals or horizons for which these maps were constructed are dated mainly by a conodont zonation that includes 27 high-resolution Late Devonian conodont zones and 70 moderate-resolution Mississippian zones. Brachiopods provide a supplemental Devonian zonation and corals and calcareous Foraminifera provide supplemental Mississippian zonations. The depositional history is timed in m.y. before (-) orafter(+) the Devonian-Carboniferous boundary (DCB).