Regional Stratigraphic Analysis of Devonian System in Wyoming, Montana, Southern Saskatchewan, and Alberta

Abstract

Devonian strata are traced throughout the Northern Rocky Mountain and Great Plains areas which include Wyoming, Idaho, Montana, southern Saskatchewan, and Alberta. The diverse Devonian nomenclatures between states and provinces are unified, and the areal limits of applicability of established formational names as cartographic units are demonstrated. Isopach and lithofacies studies are employed as a basis for interpreting the sedimentation processes and patterns responsible for the Devonian rocks now observed. In order to work out the dynamic picture of Devonian sedimentation, it is necessary to subdivide the total section into four operational rock units which are recognizable throughout the area on the basis of suitable lithologic criteria; the individual isopach and lithofacies maps of these units are interpreted in terms of the tectonic and environmental conditions responsible for the patterns exhibited. Finally, an integration of these four arbitrary stages of Devonian sedimentation presents the regional Devonian geologic history. The rock units include a basal Devonian unit, a lower limestone unit, a dolomite-evaporite unit, and a post-evaporite unit. Clastics (shale and sand), carbonates (limestone and dolomite), and evaporites (anhydrite and halite) constitute the three significant petrographic end members used in the statistical lithologic triangle. The two indices defining the lithologic variations are the clastic ratio and evaporite ratio. The important tectonic elements which were active during Devonian sedimentation include: Elk Point basin (Middle Devonian age) extending from east-central Alberta into southern Saskatchewan; Central Alberta basin; Southern Alberta arch; Central Montana positive axis; Southern Montana trough; Wyoming shelf; and Cambridge arch of Eardley in southeastern Montana and Wyoming (northwestern extension of Siouxia landmass). Evaporitic sediments are prominently developed in the basal and dolomite-evaporite units; and normal marine deposits, including reef-carbonates, are characteristic of the lower limestone unit.