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<br />The lower shale member contains massive and
<br />laminated silty-mudstone and sandy-siltstone
<br />parasequences that contain disseminated pyrite,
<br />lenticular kerogen and coal, and pyrite minerals
<br />associated with ash beds. Little information is
<br />available on the Ferron Sandstone Member. The
<br />Mancos B Member (cal]ed the Prairie Canyon
<br />Member by Cole and Young, 1991) is characterized
<br />by lenticular, cyclic, upward parasequences that range
<br />in thickness from 6 to 90 ft and consists of five main
<br />lithofacies: silty claystone, sandstone-claystone,
<br />sandy siltstone, bioturbated muddy sandstone, and
<br />sandy dolomite (as beds and concretions). Volcanic-
<br />ash layers occur as interbeds throughout the Mancos
<br />Shale and range in thickness from less than I in. to
<br />2 ft. The ash layers in the study area could have
<br />originated from the Sevier Orogeny in central Utah.
<br />Volcanic ash could be the source of selenium and other
<br />trace constituents in the Mancos Shale.
<br />The Mesaverde Formation of Upper Cretaceous
<br />age overlies the Mancos Shale (fig. 5) and forms the
<br />caprock for the Book Cliffs that border the northern
<br />edge of the Grand Valley. The formation consists of
<br />massive sandstone, gray shale, and coal, and ranges in
<br />thickness from 60 to 100 ft near the Book Cliffs. but is
<br />about 2,300 ft thick in areas nonh of the Grand Valley.
<br />The Mesaverde Formation has been erodcd away and
<br />is not present in the Uncompahgre Project area.
<br />Terrace deposits on the western side of the
<br />Uncompahgre Project area (fig. 4) consist of glacial
<br />outwash derived from the San Juan Mountains.
<br />These deposits range in thickness from 5 to 60 ft
<br />and are poorly soned materials consisting of fine
<br />rock /lour, sand, gravel, cobbles, and boulders.
<br />The parent material of terrace deposits gencrally
<br />consists of granite, rhyolite, and welded tuff mixed
<br />with weathered and transponed detritus from the
<br />Dakota Sandstone and the Mancos Shale.
<br />Valley-fill deposits in an ancient valley of the
<br />Colorado River are in the central and eastern pans
<br />of the Grand Valley (fig. 58). The deposits were
<br />namcd the "cobble aquifer" (Bureau of Reclamation,
<br />I 986a), although the ground-water quality is poor
<br />and unacceptable for domestic, agricultural, and
<br />industrial uses. The cobble aquifer is late Holocene
<br />to Pleistocene in age, ranges in thickness from 10 to
<br />65 ft, and consists of well-soned pebbles, cobbles, and
<br />boulders in a sandy matrix. Alluvium and eolian sand
<br />may locally cover thc gravels (pan of the Quaternary
<br />clay, silt, and gravels shown in fig. 5A). Mudflows of
<br />
<br />Mancos Shale mixed with detritus from the Mesaverde
<br />Formation overlies the cobble aquifer in some
<br />locations (part of the Quatemary clay, silt, and gravels
<br />shown in figure 5A).
<br />Alluvium overlying the Mancos Shale consists
<br />of clay, silt, and gravel deposits of Holocene age.
<br />Alluvial deposits commonly are a combination of
<br />fluvial sediments and mudflows of Mancos Shale
<br />residuum. In the Grand Valley, the alluvium consists
<br />of Mancos Shale residuum mixed with detritus
<br />weathered from the Mesaverde Formation that forms
<br />the caprock for the Book Cliffs (fig. 5). ]n the
<br />Uncompahgre Project area, alluvium overlying the
<br />Mancos Shale in the eastern and southeastern parts of
<br />the area consists of Mancos Shale residuum mixed
<br />with detritus from the Dakota Sandstone, weathered
<br />ash-flow tuff, and andesite from the mountains east of
<br />the Uncompahgre Project area.
<br />Mineral resources are rich in thc areas
<br />surrounding the Uncompahgre Project area and the
<br />Grand Valley; howevcr, little mineral resource
<br />development has occurred in the irrigated areas,
<br />except for limited natural gas exploration. Uranium
<br />and vanadium have been mined from the Morrison
<br />Formation west of the Uncompahgre Plateau; oil shale
<br />has been developed in the Green River Formation
<br />nonh and northeast of the Grand Valley; coal has been
<br />mined in the Mesaverde Formation and Dakota
<br />Sandstone; isolated deposits of copper, gold,
<br />molybdenum, beryllium, and titanium have been
<br />mined from granitic rocks near Unaweep Canyon
<br />south of the Grand Valley; bentonitic clays have been
<br />mined from the Morrison Fonnation; sulfur has been
<br />extracted from the Mancos Shale and alunite from the
<br />Morrison Fonnation in the upper Gunnison River
<br />Basin; and gypsum, halite, and potash have been
<br />mined in the Paradox and Sinbad Valleys along the
<br />southwestern edge of the Uncompahgre Plateau.
<br />Selenium occurs with many of these mineral deposits;
<br />however, the mineral resources development has not
<br />affected the irrigated arcas on a large scale.
<br />
<br />Irrigation Projects
<br />
<br />Uncompahgre Project
<br />
<br />Irrigation began in the 1880's in the
<br />Uncompahgre Valley, and by 1890 about
<br />30,000 acres of land were irrigated by private
<br />systems that used water from five small diversion
<br />
<br />12 Detailed StUdy of Selenium and Other Conslltuents In Water, Bottom Sediment, Soli, Allalfa, and Biota A.aoclated with
<br />Irrigation Drainage In the Uncompahgre Project Area and In the Grand Valley, Weat-Central Colorado, 1991-93
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