Devonian strata are traced throughout the Northern Rocky Mountain and Great Plains areas ncluding parts of 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. The basal unit is characterized by evaporites, including halite, locally sylvite, and anhydrite, in association with dolomitized normal marine to reefoid carbonates. Constituting the lower limestone unit are variably dolomitized limestones containing thin biostromal horizons in the south, and large biohermal reef structures in south-central and central Alberta. These bioherms are developed within an increasingly important shaly facies northward. The dolomite-evaporite unit is composed of secondary dolomites (varying areally to partially dolomitized and non-dolomitized limestone) and evaporites (anhydrite and evaporitic dolomites, and locally halite). A widespread uniform dolomitized biostromal horizon is developed within the dolomite-evaporite unit over a large part of the area, especially at the north. Normal marine carbonates in central and southwestern Alberta, and varicolored shaly, silty and sandy beds in the other areas are components of the 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

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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 and adjoining northern North Dakota; 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).