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• 4.1 Litholo~y <br />Figure G3 shows a geologic cross section through the Whirlwind and Packrat area from the southwest <br />(top of mesa) to the northeast (Lumsden Canyon). Each of the geologic units, starting at the top of the <br />mesa is described below (Umetco, 1995b). A more detailed stratigraphic column for the area is <br />provided on Figure G2. <br />Dakota Sandstone (Kd) <br />Erosional remnants of the Dakota Sandstone Formation cover about ten percent of the top of Beaver <br />Mesa above the Whirlwind Mine area. The Dakota Sandstone Formation is only a few tens of feet <br />thick in this area and consists of yellowish-brown and gray friable to quartzitic sandstone and <br />conglomeratic sandstone with interbedded gray to black carbonaceous shale. <br />Burro Canyon (Kbc) <br />The Burro Canyon Formation is the primary surface outcrop on top of Beaver Mesa and is <br />approximately 100-210 feet thick on the mesa. The Burro Canyon Formation consists of white, gray <br />• and light-brown fluvial sandstone and conglomerate interbedded with green and purple lacustrine <br />siltstone, shale and mudstone and thin impure limestone beds. It forms cliffs, often about 100 feet high. <br />The sands and conglomerate occur in lenticular, very thick beds that display crossbedding. The <br />conglomerates contain generally well-rounded chert pebbles. <br />Brushy Basin (Jmb) <br />Much of the Brushy Basin Member of the Morrison Formation (90%) is mudstone, claystone, and <br />siltstone composed of bentonitic clays derived from detrital glassy volcanic debris from volcanic <br />activity to the southwest. This material settled on a large floodplain and deposited fine-grained clastic <br />material interbedded with a few channel sandstones and conglomerates. Occasional beds of lenticular <br />coarse-grained sandstone are fairly common in the lower part of the formation. The Brushy Basin also <br />contains a few thin fresh-water limestone beds, some of which have been silicified. Devitrification of <br />the volcanic ash may have been a major source of the uranium that leached downward into the Top <br />Rim sandstones of the Salt Wash (see below). The total thickness of the Brushy Basin Member, <br />estimated from several drill holes, is 370 to 410 feet. <br />C~ <br />Whirlwind Mine 07 (rev. April 08) G-4 <br />