Upper Cretaceous Stratigraphy, Rocky Mountain Area

ABSTRACT Upper Cretaceous rocks of the Rocky Mountains are characterized by intertonguing of marine and non-marine sediments. Along the western margin of the Rocky Mountain area deposits are nearly all nonmarine. Tongues of these sediments extend eastward across the Rocky Mountains and pass by reason of facies change into marine shale sequences. In this intertonguing relationship four principal facies can be recognized in the Upper Cretaceous which are directly related to the environments of deposition. In the marine environments, white or gray limestone or marlstone was deposited in the offshore deeper neritic environment, and gray or black shale with thin beds of limestone, siltstone, or sandstone in the offshore shallow neritic environment. White, gray, or tan thin- to massive-bedded sandstone was deposited in the transitional and shallow neritic environment. In the non-marine swamp, lagoon, coastal plain, inland, and piedmont environments gray clay and shale and tan lenticular sandstones with beds of coal, carbonaceous shale, or conglomerate were deposited. Formation names in the Cretaceous of the Rocky Mountains usually correspond with fades of local areas. Many formation names are used improperly among intermontane basins of the Rocky Mountains. Six diagrammatic restored sections of Upper Cretaceous rocks show that the dominant sedimentary pattern during Late Cretaceous was deposition during regression of the strandline from west to east, across the present Rocky Mountains area. This regressive pattern periodically was broken by sharp transgressions of the strandline, resulting in intertonguing of marine and non-marine beds. Four major transgressions and four major regressions of the strandline can be delineated. The positions of the strandline of the late Cretaceous sea, at maximum advance and retreat, are traced regionally on three maps. The relationship of formation names to Upper Cretaceous lithogenetic units is shown on restored sections and maps. Lithologic sequences were deposited during both regressive and transgressive events, which have been miscorrelated in the past. Regional correlations show that the non-marine coal-bearing shale and sandstone tongue called “Mesaverde” in northwest Colorado and in Wyoming is younger than the Mesaverde formation at its type locality in the San Juan Basin of southwest Colorado. The “Mesaverde” of northwest Colorado and Wyoming is correlative with the Judith River formation of Montana. The Lewis shale of its type locality in the San Juan Basin is correlative with the upper part of the Mancos shale of northwest Colorado and the upper part of the Steele shale of Wyoming. Thus, the “Lewis” shale of northwest Colorado and Wyoming is younger than Lewis shale at the type locality, and is correlative with Bearpaw shale of Montana.