Ground-Water Atlas of Colorado Special Publication 53

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' Colorado Geological Survey HYDROGEOL 0G /C UN /TS The Dakota - Cheyenne Group is composed of the geologic units between the underlying Jurassic Morrison Formation and the over- lying 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. Outcrop colors range from light gray to yellowish -brown to brown- ish -red, depending upon iron content (Romero, 1994). The Dakota Sandstone and underlying Cheyenne Sandstone of the Pugatoire Formation represent the dominant aquifer units within the Dakota - Cheyenne aquifer. A stratigraphic table describing the deeper hydrogeologic units in eastern Colorado is presented as Table 6.8 -1. The Cheyenne Sandstone consists of massive to cross - bedded, white to yellow -gray, fine- to coarse - grained, quartzose sandstone with interbedded shale, claystone, and occasional beds of conglom- Table 6.8 -1 Deeper hydrogeologic units in eastern Colorado. DRAFT erate (Robson and Banta, 1987). The Cheyenne Sandstone Member of the Purgatoire Formation is present in southeast Colorado and represents a westward - thinning fluvial channel deposit. The Dakota Sandstone is a fine- to medium - grained, white to dark brown, and thin - bedded to massive, quartzose sandstone that com- monly forms ledges, cliffs, or hogbacks (Robson and Banta, 1987). The sandstone contains carbonaceous materials and is locally interbedded with claystone, siltstone, or marine shale. The deposi- tional environment ranges from river channel deposits, barrier - island beach sands, and low energy silts, muds, and organic debris. The "D" and "J" sandstone members of the Dakota Group are important oil and gas reservoirs in the Denver Basin. The Dakota Group ranges in thickness from less than 100 feet in southwestern parts of the state to over 500 feet in northeastern Colorado (Pearl, 1982). It is missing over most of the mountainous areas of the state where it has been removed by erosion. Modified from Romero, 1994 a w* * 4 0 111 4 0* 4 9* 011* 0 W O( ' k i GROUND -WATER ATLAS OF COLORADO Outcrop of the Dakota Sandstone along the hogback west of Denver, Colorado. Ripple marks in the Dakota Sandstone along Dinosaur Ridge, west of Denver, attest to its marine deposition. U a T a 0 0 a_ o_ U v c 02 0 0 t o_ SP 53 Strati- Unit Saturated Era System Series graphic Thickness Physical Hydrogeologic Thickness Hydrologic Unit (feet) Characteristics Unit (feet) Characteristics Pierre Shale 0- 4,000+ Black to dark -gray shale Ganfinri layer. Not known to yield water to wells Niobrara Upper unit is yellowish " a Yields water to stock Formation 200+ chalk, lower unit is chalk y FoG= Haye r 50 -60 wells and springs north limestone and marl rnestbne _ of Arkansas River Upper unit is sandy Upper Cretaceous Carlile Shale 200+ shale; middle unit is black, fissile shale; lower Code lSandst 'ne =. 20+ Yields water to a few stock wells unit is chalky shale Greenhorn Upper unit is chalky '- r Cretaceous Formation 65 shale and thin limestone; lower unit is hard, crys- nm Yields no water to wells o talline limestone N E Graneros Shale 85 -100 Gray to black shale :: Yields no water to wells t y Yields sufficient for Dakota Fine-grained, thin - bedded ao _ 150+ domestic and stock use; Sandstone 150 -235 to massive sandstone m °dttone in some areas yields enough for municipal and Lower + industrial use Cretaceous U a Cheyenne Yields for Upper unit is gray to Purgatoire Formation 60 -350 black clayey shale; lower unit is massive, � Y �« andstone 30 -200 sufficient industrial, municipal, and fine- grained sandstone o Membe irrigation use Jurassic Morrison 20 -240 Varicolored marl Cffil>aayer Minimal yield to wells Formation RIM from sandstone lenses Modified from Romero, 1994 a w* * 4 0 111 4 0* 4 9* 011* 0 W O( ' k i GROUND -WATER ATLAS OF COLORADO Outcrop of the Dakota Sandstone along the hogback west of Denver, Colorado. Ripple marks in the Dakota Sandstone along Dinosaur Ridge, west of Denver, attest to its marine deposition. U a T a 0 0 a_ o_ U v c 02 0 0 t o_ SP 53