The Western Mid-Continent Region
The Williston Basin is an elliptical shaped depression in the western distal Canadian Shield occupying most of North Dakota, northwestern South Dakota, eastern Montana, and parts of southern Manitoba and Saskatchewan, Canada (Fig. 1). This discussion is restricted to the United States portion of the Williston Basin. Glacial or Cenozoic continental deposits mask much of the basin geology. Paleozoic and Mesozoic rocks are exposed infrequently along the basin margin. Therefore, virtually all of the knowledge of this basin is from subsurface geologic data, including cores, samples, well logs, and seismic surveys.
Glaciation during Pleistocene time not only covered most of the basin rocks and structures, but also created a very subdued topography over most of the region. Glacially derived soils form the basis for extensive agricultural land use despite the harsh climate. Fertile glacial Lake Agassiz sediments, in the extreme eastern part of North Dakota, support one of the most productive small grain, potato, and sugar beet farm economies in the country. Less-productive ice-margin deposits support both farming and ranching. Temperatures range from above 100°F to below −40°F, but the northerly latitudes provide long hours of daylight for growing. Areas of exposed Cenozoic rocks in the western part of the state are predominantly ranchlands, although grain farming is also common in river bottoms and where topography is relatively flat. Sharp topographic breaks occur where major drainages cut Cenozoic rocks, exhibited by the Missouri, Little Missouri, Yellowstone, Powder, and Knife Rivers. Badland topography dominates these areas because of the very fine-grained
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.