Chapter 9 The Western Interior Basin

Abstract

Abstract The Western Interior Basin extends north–south over about 35° of latitude, from Texas to the Northwest Territories, a distance of more than 3,000 km. The basin developed as a result of crustal loading during the western 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, including the Mesocordilleran Geanticline of Nevada. The first major clastic wedge, constituting the Morrison and Kootenay formations, continued into the Berriasian, but much of the Berriasian to Barremian (Neocomian) interval is represented by a regional unconformity throughout the Western Interior Basin. This period corresponds to a “magmatic lull” in the Cordillera. The base of the Cretaceous section, of Late Berriasian or Aptian age, typically consists of a sheet of coarse, fluvial gravels, throughout much of the Western Interior Basin. Provenance studies of the foreland-basin strata indicated that following the regional Mid-Cretaceous episode of tectonic quiescence, erosion tapped into oceanic-arc and related rocks, and syndepositional continental magmatic rocks of Quesnellia, far to the west of the orogenic front. Examination of the ages 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. At least two Cretaceous cycles of transgression occurred in northern Canada, but marine waters did not extend southward into the Western Interior Basin until the Aptian. During the Aptian–earliest Albian interval, 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 Upper Cretaceous stratigraphy of the Western Interior Basin is characterized by the deposits of several major marine transgressions. Gaps in the stratigraphic record are numerous; some represent millions of years, although most are less than one million years in duration. Eustatic sea-level changes were probably partly responsible for this stratigraphic architecture, but regional and local tectonic processes were also important. During the Turonian the sea reached an all-time high, calculated to be at least 300 m higher than at present. Shallow seas may have extended over much of the Canadian Shield, with a connection through Hudson Bay to the north Atlantic Ocean. High-frequency sequence cyclicity in some parts of the basin suggest a control by orbitally forced eustasy, which caused sea-level changes in the range of 10 m over time scales in the range of 10–125 ka. 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 and to the break-up of the basin.