Laserfiche WebLink
<br />0- <br />0: <br />"" <br /><'-- <br />o <br /> <br />sandy shale and mudstone. The Entrada Sandstone and the Summerville Fonnation crop out <br />only near the southwestern edge of the Sagers Wash drainage basin. <br /> <br />The Jurassic Morrison Fonnation, composed of three members, crops out along the southwest <br />edge of the drainage basin. The Tidwell Member (40-100 ft. thick) is the lowest, and is made <br />up of reddish silty shale interbedded with gray limestone and fme-grained yellow sandstone. <br />The middle (Salt Wash) member contains pale yellow-gray cross-bedded, partly conglomeratic <br />sandstone, interbedded with red to grey siltstone, mudstone, and some thin limestone. The Salt <br />Wash Member ranges from 130 to 300 feet in thickness. The upper (Brushy Basin) member is <br />primarily variegated mudstone with thin layers of nodular limestone, conglomerate, and <br />conglomeratic sandstone. Varicolored chert is common in the Brushy Basin Member which <br />varies from 300 to 450 feet in thickness. <br /> <br />The Cretaceous Cedar Mountain Fonnation overlies the Morrison in the southwest part of the <br />drainage basin. The Cedar Mountain Fonnation contains from 100 to 250 feet of variegated <br />silty mudstones with interbedded conglomerates, quartzites, and gritstones. <br /> <br />Overlying the Cedar Mountain Fonnation, and fonning conspicuous east facing dip slopes in the <br />southwest part of the drainage basin is the Cretaceous Dakota Sandstone. The Dakota <br />Sandstone, up to 110 feet thick, is composed of light-yellow to brown (to black desert-varnished) <br />conglomerate, conglomeratic sandstone, and sandstone with some carbonaceous shale, gray <br />sandy shale, and marl. <br /> <br />The Cretaceous Mancos Shale, which varies from about 3500 to 4000 feet thick, overlies the <br />Dakota Sandstone, and crops out from the outlet of the drainage basin up to the base of the Book <br />Cliffs near it's northern end, The Mancos Shale is predominately made up of light to dark gray <br />nonresistant marine shale with some yellow-gray to brown sandy beds. The Mancos Shale hosts <br />veinlets of calcite and gypsum, and exhibits streaks of evaporite salts (usually sodium sulfates) <br />on some weathered surfaces. The Mancos also contains native mercury and arsenopyrite in <br />estimated concentrations of 5ppm to 5Oppm, with the highest concentrations found near the top <br />of the fonnation (Marlatt, 1991), <br /> <br />About 350 to 500 feet above the base of the Mancos, the Ferron Sandstone Member (60 to 100 <br />feet thick) fonns cuestas with more resistant brown to yellow gray thin bedded sandstones, <br />shales, and sandy shale. Some sandy beds are also found in the Mancos below the Ferron. The <br />upper part of the Mancos Shale is sandy, and grades into overlying sandstones at the base of the <br />Book Cliffs. Some resistant, yellowish-brown weathering thin limestone lenses occur in this part <br />of the Mancos. The contact between the Mancos Shale and the overlying Mesaverde group rises <br />stratigraphically eastward as successive eastward pointing sandstone tongues grade into shale <br />(Fisher, Erdmann, and Reeside, 1960). <br /> <br />The Cretaceous Mesaverde group which makes up the major part of the Book Cliffs contains <br />several fonnations comprised of alternating layers of buff to gray, fme to course-grained <br />sandstone and gray shale. In the Sagers Wash area, the lowest recognizable part of the <br /> <br />8 <br />