Chronostratigraphic cross section of Cretaceous formations in western Montana, western Wyoming, eastern Utah, northeastern Arizona, and northwestern New Mexico, U.S.A.

The chronostratigraphic cross section presented herein is a contribution to the Western Interior Cretaceous (WIK) project of the Global Sedimentary Geology Program. It portrays the Cretaceous formations at 13 localities in a south-trending transect from northwestern Montana through western Wyoming, eastern Utah, and northeastern Arizona, to northwestern New Mexico. The localities are in the Rocky Mountains and on the Colorado Plateau and contain strata that were deposited along the western margin of the Cretaceous Interior Seaway of North America. These strata are marine and nonmarine, mainly siliceous and calcareous, and of Aptian through Maastrichtian ages. They range in thickness from nearly 19,000 feet (ft) in southwestern Montana to about 1,500 ft thick in northeastern Arizona and northwestern New Mexico.

Cretaceous formations in western Montana, western Wyoming, and eastern Utah disconformably overlie Jurassic rocks and locally include beds of either Aptian and Albian ages or Albian age. The Aptian beds consist of various lithologies of nonmarine origin. Albian beds consist of various nonmarine and marine lithologies. The Lower Cretaceous strata range in thickness from 4,300 ft in northern Utah to 180 ft in southern Utah and are absent at localities in Arizona and New Mexico.

Upper Cretaceous strata at most localities in the transect include beds of Cenomanian through Maastrichtian ages, although at five of the localities in Montana, Wyoming, Utah, Arizona, and New Mexico, younger strata are missing. Beds of Cenomanian, Turonian, Coniacian, Santonian, and Campanian ages in the region are siliceous, calcareous, or bentonitic and were deposited in marine and nonmarine environments. In the following Maastrichtian, siliceous lithologies accumulated in nonmarine environments. Thicknesses of the Upper Cretaceous strata range from more than 17,000 ft in southwestern Montana to nearly 6,000 ft in northwestern New Mexico.

In this transect for time-stratigraphic units of the Cretaceous, lateral changes in lithologies, regional differences in thicknesses, and the abundance of associated disconformities possibly reflect local and regional tectonic events. Examples of evidence of those events follow: (1) Disconformities and the absence of strata of lowest Cretaceous age in western Montana, western Wyoming, and northern Utah indicate significant tectonism and erosion probably during the Late Jurassic and earliest Cretaceous; ( 2) stages of Upper Cretaceous deposition in the transect display major lateral changes in thickness, which probably reflect regional and local tectonism.