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Era- them System --, ~ ~ 0 H ~ :> ~ '" ~ "' .. I .;,~ H~ "'.. "'.. ~~ "'" , '" ~ '" ~~ ,,~ '.. r:le: Po'" Series ~ ~ ili Ie Table 1. --Summary of the geologic formations and groups in the Upper Colorado River S_sin and their physical. hydrologic, and chemical characteristics--Continued 8991 Formation and group ydro- geo- logic unit1 Pa.rk City fOt1l:lat.ioD Cutler F~~!...i~__ , " 3 e g ."" 0,.-4 ",...~ . 0 " ," n . ~ 0 ..c " '" :n ~ " 0 I 10 e ... 0 ... n 0 n 0 " 0 . . '" " "~ ... n I " 0 ... I n " 0 . '" o ~ e . . e '" 0 ... Minturn Formation Eagle Va Hey Evaporit Leadville and Ma.dison Limestones Sedimentary rocks Physical characteristics Hoat of the Permian aod Penn- sylvanian Formations consist chiefly of sandstone. con- glomerate, shale, siltstone, and mudstone. An exception is the Paradox Member of t~e Hermosa Formation of Penosyl- vanian age, which consists of salt, gypsum, anhydrite, black shale, sandstone, and lime- stone. The format.ion under- lies large areas of western Colorado and eastern Utab and is known to be as mucb as 11,000 feet tbick. Tbe few surface expo5ures of the Para- dox occur mostly in raradox Valley in tbe Dolores River basin and in a few other scat. tered areas in southwest Colo- rado aDd southeast Utah. Tbe Hississippian rocks CODSi5t of I' the Leadville and Hadison Limestones. !be Le~dville crops out along the Colorado II River near Glen'Wood Springs, I in tb.e uppe r r~llch~s of the \ Eagle, Roaring Fork, and Gun- I nison Riyer basins, and in Dinosaur National Monument. It consists largely of dense, impermeable limestone and dolomite. Tbe pre-Hississi~- pian rocks.consist of dolo- mite, limestone, quartzite, sandstone, conglomerate, shale, and chert. The depos- its are 2,500 feet thick in places. Hydrologic characteristics Little is known about most of tbese formatioD5. Some are fairly pro- ductive aquifers, such as tbe Weber Sandstone, Horgan Forma- tion, and the Leadville and Hadi- son Limestones. Water in the Leadville Limestone occurs pri- marily in fractures, caverns. aDd solution cavities. In several areas in the basin these caverns and solution cavities produce very large yields. The largest reported yields are from wells that tap the Leadville Limestone near McCoy, Colo. One well bad a report~d yield of 3,400 gallops per minute of water and a dis- solved-solids Concentration of about 3,000 mg/L. Chemical characteristics Several formations have signi- ficant effec~8 on salinity of slr~a~flow in t.he basin. The Leadville Limestone is a pos- sible source of the Glenwood aDd Dotsero mineral hot springs. These springs hav~ a total discharge of about 17 cubic feet per second and a dis- solved-solids concentration of about 30,000 mg/L. Ground water from the Paradox Member of the Hermos8 Formation has a dissolved-solids CQDcentra- tration of more than 50,000 mgfL. Ground water from tbe Paradox is discharged chiefly along fault zones to shallow alluvial aquifers where it eventually is discharged into streams. The Eagle Valley Evaporite is suspected to be a major source of the salt in the Eagle River.