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Era. them '" U H o N o '" ~ System Series Upper Creta- ceous '" ::> o .., u < ... ~ U Lower Creta- ceous Table 1.--Summary of the geologic formations and groups in the Upper Colorado River Basin and their physical, hydrologic, and chemical characterjstics-~Continued C}Qn:r ~Vy, FO'Cll.ation and group Lance formation Lewis Shale Mancos Shale Pierre Shale Niobrara Formation Benton Sh.ale Baxter Sh,.le 1--- Cody Shale f.-, Steele Shal~ Frontier Sandstone 1-- Howry Shale 1-- Tbermopolis Shale Dakota Sandstone Burro Canyon FOI'1Dation or Cedar Mountain Formation -, Cloverly Formation Hydro- geo- logic unHl Physical characteristics The Mancos Shale consists pri- marily of an approximated S,OOO-foot thick homogenous dark-grey marine shale. It also contains several dis- continuous sandstone layers and limestone beds. 7 The Dakola ~andstone is a per- sistent sandstone unit that extends over much of the basin. Contains lenses of siltstone and shale and has a maximum thickness of 300 feet. 8 Hydrologic characteristics The porosity of shales is very great. and the Mancos Shale con- tains large quantities of ground water. These shales also have a very small permeability which effectively prevents the sub- surface movement of water. Well yields are usually less than 5 gallons per minute. Small amounts of recharge occur in the outcrop areas for direct infiltration of precipitation and snowmelt. Because of tbe low permeability of the Mancos Shale, ground-water discharge to streams is small. Well yields are less than 50 gallons per minute, but it is an important source of water for domestic and stock use. Chemical characteristics Generally the ground water is I highly mineralized and has a dissolved-solids concentration of normally greater than] ,000 to 2,500 mgjL. Locally used for stock watering. The Upper Cretaceous marine shales prob- ably have the greatest effect on salinity or streamflow in the basin than any o~her rocks. The Hancos Shale is highly erodable, and surface runoff from the Hancos Shale usuaLly contains moderately saline levels of dissolved solids. In some areas stream~channel erosion along outcrops of the Hancos Shale can cause signi- ficant increases in the salin- ity of the stream_ Where soils derived from the Hancos Shale are being extensively irrigated, such as the Grand Valley near Grand Junction, the irrigation- return flow to the stream can significantly add to the salt level. The Dakota Sandstone contains large amounts of oil and gas. Oil-field waters from the Dakota have a dissolved-solids concentration of from 5,000 to 30.000 rog/L. In the outcrop ar~as; water from the Dakota generally has a dissolved- solids concentration of from 500 to'),OOO mgjL. Water {rom the deeper parts of the Dakota generally has a dissolved- solids concentration of more than 3~OOO mg/L.