Dakota Group of Colorado Plateau

Abstract

Basal Cretaceous deposits of the Colorado Plateau can be subdivided into two formations on the basis of carbonaceous content. The lower non-carbonaceous unit, the Cedar Mountain formation, consists of mudstones and persistent conglomeratic sandstones which were deposited in an inland floodplain environment. The upper carbonaceous unit, the Naturita formation, consists of carbonaceous mudstone, coal, persistent conglomeratic sandstones, and beach sandstones deposited on or adjacent to the shore of the Mancos sea. Naturita deposits can be traced landward into Cedar Mountain deposits, indicating that they are facies of a larger unit, the Dakota group. Advancing Cretaceous seas reached the eastern edge of the Plateau in late Albian time but did not inundate the entire Plateau until late Greenhorn or early Carlile time. The westward advance of the sea was a halting one. Sharp pulses of basinal subsidence, accompanied by uplift in the source area west of the Plateau, resulted in rapid westward transgressions of the sea. Deposition, essentially confined to periods of quiet following the transgressions, caused some regression of the sea but transgressions exceeded the regressions and resulted in a slow westward advance of the sea. Many pulses of subsidence occurred but five major ones, which were accompanied by uplift in the source area, are reflected in the five widespread orogenic sandstones present in these deposits.