The Dakota Sandstone (Cretaceous) the San Juan Basin area of northwestern New Mexico and southwestern Colorado (fig. 1) includes rocks which were deposited under diverse marine and non-marine conditions. The purpose of this paper is to show that primary sedimentary structures, biotubation, fossils, and lithologies in the Dakota provides basis for recognizing a spectrum of marine and nonmarine environments within this unit and for developing a model to explain the changes in environments in space and during time.