Stratigraphic Transects for Cretaceous Rocks, Rocky Mountains and Great Plains Regions

Abstract Four regional east-trending lithostratigraphic and three chronostratigraphic cross sections were prepared for the Western Interior Cretaceous (WIK) project. The transects include the following regions from north to south: (1) northern Rocky Mountains and Great Plains, from southwestern Montana to southwestern Minnesota; (2) middle Rocky Mountains, from northern Utah to south-central South Dakota; (3) southern Rocky Mountains and Great Plains, from the Gunnison Plateau, central Utah, to eastern Denver basin, northeastern Colorado; and (4) southern Rocky Mountains, from east-central Arizona to Oklahoma panhandle and including San Juan basin. Transects depict regional facies relations, sequence boundaries, and biostratigraphic and radiometric correlations for the entire Cretaceous sequence. More than 20,000 feet (6,000 m) of mainly nonmarine strata typify the rapidly subsiding westernmost part of the basin, whereas less than 1,000 feet (400 m) of predominantly marine strata were deposited on the eastern shelf. This asymmetry resulted from different rates of subsidence due to tectonic and sediment loading. Most Cretaceous facies are basin wide; however, Maastrichtian sedimentation was dominated by local sediment sources derived from rising mountain ranges uplifted by early Laramide deformation. Cretaceous rocks consist primarily of sandstone, siltstone, claystone, and shale. However, conglomerate is abundant along the western margin of the basin, and limestone is common on the eastern shelf. Sediment was deposited in both marine and nonmarine environments while the shoreline fluctuated during tectonic and eustatic cycles. WIK is a project of the Global Sedimentary Geology Program (GSGP). The International Union of Geological Sciences established GSGP as a new commission to extend understanding of the history of the earth, surficial processes, evolution of life, and biotic influences on earth processes through global-scaleresearch on sediments, sedimentary rocks, and their contained organisms. The goal of WIK is to have a publicly available data base from which to reconstruct and interpret the depositional history of Cretaceous strata in the Western Interior, U.S.A.