Chapter 6.8, Page 2 of 4

Hydrogeologic Units

The Dakota/Cheyenne Group is composed of the geologic units between the underlying Jurassic Morrison Formation and the overlying Upper Cretaceous Graneros Shale. Due to its deposition in a variety of marginal marine environments, the stratigraphy of this group ranges from well-sorted sandstones to fine-grained shales. While the name Dakota Sandstone and/or Dakota Group is used throughout Colorado, the individual units within the group and the overlying shales are named differently in various parts of the state. The Dakota Sandstone and underlying Cheyenne Sandstone of the Pugatoire Formation represent the dominant aquifer units within the Dakota/Cheyenne Group. A stratigraphic table describing the deeper hydrogeologic units in eastern Colorado is presented as Table 6.8-1.

Table 6.8-1 Deeper Hydrogeologic Units in Eastern Colorado

Modified from Romero, 1994

The Dakota Group ranges in thickness from less than 100 feet in southwestern parts of the state to over 500 feet in northeastern Colorado. The depth to the top of the aquifer ranges from zero where it crops out along the Front Range on the western edge of the Denver Basin, to greater than 9,000 feet below ground surface near the center of the Denver Basin. The depth is less than 2,000 feet in the southern part of Colorado between Pueblo and Trinidad and along the Purgatoire River. It crops out along the margins of most of the major sedimentary basins of the state, including the San Juan and Raton Basins.

Outcrop of the Dakota Sandstone along the hogback west of Denver.
Photo by R. Topper, CGS.

Chapter 6.8, Page 2 of 4