Field and laboratory studies of ground-water conditions in the carnotite-bearing Morrison formation in southwestern Colorado and southeastern Utah were undertaken to determine possible relations between ground waters and the carnotite deposits.
The ore-bearing sandstone consists of lenticular sandstone strata, interbedded within discontinuous layers of mudstone; these strata were deposited in a stream environment. The porosity and permeability of the ore-bearing sandstone in one area are relatively low, porosity averaging about 15 percent and permeability ranging between 30 and 3300 millidarcys. Permeability studies in this same area show that sandstone classed as favorable for ore is slightly more permeable than and has nearly twice the transmissibility of sandstone classed as semifavorable; outcrop studies also suggest a lower transmissibility for sandstone that is unfavorable.
Samples of water from the ore-bearing and associated strata show considerable variation in chemical charter. These water samples are generally low in uranium, vanadium, copper, and lead, most samples containing less than 1 part per million (ppm) of each metal.
General geologic relations suggest that ground-water movement probably was active through the ore-bearing strata during their deposition and shortly afterward. Movement during this time probably occurred largely through the more permeable sand strata and in the general direction of initial dip and stream flow. During Cretaceous time, when several thousand feet of marine sediments accumulated over the Morrison, the water contained in the ore-bearing strata probably was immobile or nearly so and was protected from escape or contamination. Following Tertiary deformation and erosion, active ground-water circulation no doubt was restored, but because of the low permeability and the lenticular character and therefore low transmissibility of the ore-bearing sandstone, ground-water movement probably was slow. Faulting probably also influenced the direction and rate of ground-water movement during Tertiary time.
Movement and localization of ground water would permit the concentration of metal-bearing solutions during Salt Wash and early Brushy Basin time in the beds that now contain ore deposits. It is more difficult to explain the formation of these deposits, which have a wide geographic distribution at a restricted stratigraphic position, from solutions circulating through the rocks at a later date.
Whatever mode of origin is used to explain the deposits, geologists almost without exception, agree that the metals were transported by solutions that have migrated through the sediments for considerable distances. For this reason, a study of the horizontal and vertical transmissibility characteristics of all exposed sedimentary formations on the Colorado Plateau is planned.
|Publication Subtype||USGS Numbered Series|
|Title||Present and past ground-water conditions in the Morrison Formation in southwestern Colorado and southeastern Utah|
|Series title||Trace Elements Investigations|
|Publisher||U.S. Geological Survey|
|Publisher location||Reston, VA|
|Description||Report: 42 p.; 6 Plates: 31 x 21 inches or smaller|
|County||Mesa County, Montrose County, San Miguel County|
|Other Geospatial||Calamity Mesa, Atkinson Mesa, Slick Rock area|
|Online Only (Y/N)||N|
|Additional Online Files (Y/N)||N|
|Google Analytic Metrics||Metrics page|