The northern Rocky Mountain region (Figs. 1, 2), contains a relatively complete Phanerozoic stratigraphic section ranging in age from Cambrian to Holocene (Figs. 3 to 5). Thicknesses of Paleozoic rocks are as much as 9,000 m in southeast Idaho, 3,000 m in southwest Montana, 2,500 m in the central part of the Williston Basin, and generally less than 1,500 m in most of Wyoming, South Dakota, and the remainder of Montana. Thicknesses of Mesozoic rocks are as much as 11,000 m in southeast Idaho and western Wyoming, 6,000 m in southwest Montana, and generally less than 3,000 m in northern and eastern Wyoming, South Dakota, and central and eastern Montana. Thicknesses of Tertiary rocks are as much as 300 m in the western part of the Williston Basin, more than 900 m in the deeper parts of the Powder River Basin, and as much as 3,000 m or more in the Tertiary basins of western Montana and southwestern Wyoming.
Cambrian strata unconformably overlie Proterozoic sedimentary rocks in western Montana, east-central and southeastern Idaho, and overlie older Precambrian metamorphic and igneous rocks in the remainder of the region. Thicknesses are more than 1,500 m in south-central Idaho and more than 900 m in western Montana, thinning relatively uniformly eastward and pinching out along the flank of the Transcontinental Arch (Figs. 3 to 6). Cambrian rocks comprise a sequence of marine sandstone, shale, and limestone, which represent the shelf facies of a broad eastward transgression of the Cambrian Cordilleran sea across the
Figures & Tables
Sedimentary Cover—North American Craton
The “sedimentary cover” refers to the stratified rocks of youngest Proterozoic and Phanerozoic age that rest upon the largely crystalline basement rocks of the continental interior. The early chapters of the volume present data and interpretations of the geophysics of the craton and summarize, with sequential maps, the tectonic evolution of the craton. The main body of the text and accompanying plates and figures present the stratigraphy, structural history, and economic geology of specific sedimentary basins (e.g., Appalachian basin) and regions (e.g., Rocky Mountains). The volume concludes with a summary chapter in which the currently popular theories of cratonal tectonics are discussed and the unresolved questions are identified.