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Whirlwind Mine Groundwater Characterization Report <br /> 3.1 Lithology <br /> Regionally, the bedrock geology of the area is comprised of gently dipping Mesozoic age <br /> sedimentary rocks common to the Colorado Plateau unconformably overlying basement rock <br /> comprised of Precambrian granitic gneiss which outcrop in Unaweep Canyon several miles to <br /> the northeast. Sedimentary formations range from Permian age Cutler Formation which <br /> outcrops in the Dolores River Canyon near Gateway to Cretaceous-age Dakota Sandstone just <br /> west of the Whirlwind Mine on Beaver Mesa. General descriptions of these formations are as <br /> follows. <br /> • Dakota Sandstone (Kd); Cretaceous; Up to 200 ft thick regionally, but well logs in <br /> Beaver Mesa area indicate the thickness of the Dakota Sandstone is only a few tens of <br /> ft. A basal yellow-brown, orange-brown medium grained sandstone, plant debris, <br /> possibly basal discontinuous conglomerate; a middle unit of light gray carbonaceous <br /> shale, siltstone and mudstone, some coal; and an upper unit of light-brown medium to <br /> fine-grained sandstone. <br /> • Burro Canyon Formation (Kbc); Cretaceous; Up to 260 ft thick regionally, 100-210 ft <br /> thick locally. Basal light brown and white to gray, medium to coarse-grained sandstone <br /> with pebble conglomerate; maybe overlain or interbedded with light-green mudstone, <br /> siltstone, and shale, with beds of limestone, limy sandstone, limestone conglomerate, <br /> and chert that separates the formation from the Dakota Sandstone. The Burro Canyon <br /> and Dakota Sandstone forms large ledges or cliffs and the caprock of Beaver Mesa. <br /> • Morrison Formation (Jmb); Jurassic; Regionally up to 700 ft thick. The formation <br /> consists of the upper Brushy Basin Member(350 to 450 ft thick) and the lower Salt <br /> Wash Member(300 ft thick or more). Other lower members of the Morrison Formation <br /> do not exist in this area. <br /> • Brushy Basin Member. In the study area, the Brushy Basin Member is 250 to <br /> 400 ft thick and consists of variegated siltstone, mudstone, bentonitic clay with <br /> minor sandstone and conglomerate. Some lenticular sandstone beds occur in <br /> the lower portion. The Brushy Basin Member is a prominent slope former and <br /> Western Water S Land, Inc. 7 <br />