The Western Interior Basin

Abstract The Western Interior Basin extends north-south over about 35 degrees of latitude, from Texas to the Northwest Territories, a distance of > 3000 km. The basin developed as a result of crustal loading during the westward migration of the North American Plate and the subduction of Panthalassa . Initiation of the Western Interior Basin as a distinctive geodynamic and stratigraphic province is traditionally associated with the deposition of the Upper Jurassic (Oxfordian-Tithonian) Morrison Formation in the United States, and the Fernie and Kootenay formations in Canada. Crustal loading occurred as a series of pulses, as successive terranes arrived at and were obducted onto the western Laurentian margin. These events are represented in the basin as a series of clastic wedges. Westerly sediment sources associated with contractional tectonism appeared for the first time in the Late Jurassic. The first major clastic wedge constitutes the Morrison and Kootenay formations. Much of the Berriasian to Barremian (Neocomian) interval is represented by a regional unconformity throughout the Western Interior Basin. This unconformity corresponds to a “magmatic lull” in the Cordillera. Cretaceous strata, of late Berriasian or Aptian age that lie on the unconformity, typically consist of a sheet of coarse, fluvial gravels throughout much of the Western Interior Basin. Examination of the age of conglomerates deposited at this time and reconstruction of the subsidence histories suggest that a new phase of flexural loading and subsidence commenced shortly after deposition, initiating a new “constructive” phase of development of the Cordilleran orogen. Marine waters did not extend southward into the Western Interior Basin until the Aptian. During the Aptian-earliest Albian, most of the Western Interior Basin was occupied by fluvial and estuarine systems assigned to such units as the Mannville Group in Alberta-Saskatchewan, and the Kootenai Formation of Montana. The giant bitumen reservoir of the Athabasca Oil Sands of northern Alberta, assigned to the Mannville Group, is hosted in the deposits of a large, northward draining, continental-scale, tidally influenced river system. The Upper Cretaceous strata of the Western Interior Basin are characterized by the deposits of several major marine transgressions. Gaps in the stratigraphic record are numerous; some represent millions of years, although most are Provenance studies of the foreland-basin strata, including, in recent years, substantial research into detrital zircon populations, have indicated that most of the detritus was derived from the Sevier orogen to the west, but that at different times, erosion tapped into oceanic-arc and syndepositional continental magmatic rocks of Quesnellia far to the west of the orogenic front, arc-related rocks to the south (Mogollon Highlands), and the Appalachian orogen to the east. Termination of the Western Interior Seaway and foreland basin began during the Late Campanian or Maastrichtian throughout much of the United States, as shallow subduction of the Farallon Plate on the western margin of the continent led to the Laramide Orogeny, the breakup of the basin, and partitioning of the foredeep into discrete fault-bounded basins and uplifted blocks.