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<br />This assessment is divided into the following three major subsections: <br />Description of the E.tisting cnvironment; Probable Hydrologic Consequences of <br />of Mining; and Cumulative hydrologic Impacts or Mining. Surf ac? and ground <br />rater are discussed separately in eacn subsection. <br />Description of the Hydrologic ~nvironmer.t <br />~,. Regional Geology <br />The upper Pampa River Basin is comprised cf sedimentary rocK strata which have <br />been itruCtUrdlly deformed. The area Cc`/er?d by this C9IS lies x;thln the <br />Sand 'dash 3asin, a large structural basin covering nortnwest?rn Coicrado and <br />south cantr.al Wyoming (Figure No. 4>. ?!any smaller structural f?atur?s <br />further define this basin. <br />Seven of the mines considered in this CHIS are located in the Twentymile Park <br />Basin. This structural basin is a subbasin cf the larger Sand Wash Basin. <br />The Twentymile Park Basin consists of a deep central basin flanked by two <br />shallower basins to the north and southwest. The southwestern basin is <br />structurally connected to the Sand Wash Basin by the doubly plunging <br />Hayden Syncline. The Hayden Syncline plunges to the north•.vest into the Sand <br />Wash Basin dnd provides a limit?d outlet fcr the Twentymile Pare 3asin. The <br />tight folding of the sed'mentary rock strata within t:.? Twentymile Park Basin <br />ha; resJited in faulting within the bairn. <br />The three remaining mines lie in structurally separated ar=_as. numerous small <br />anticlines and synclines such as the Sace Creek Anticline and Hayden Syncline <br />isolate these areas. <br />.he geologic units range in age from Late Cretaceous to Quaternary <br />(~igUr~ NO 1). The Oldest ?`(p05?d 52d1menL3ry rCCki 3r? thdt Of the <br />'1ancos Shale, which was dedo;it?d in a de?a marine tc s,nalc'x aarine <br />?nvircnment. After deposition of the Marcos Snale, sediments of the Iles and <br />William; Fork Formations o* the Mesav?rde Groud were decosit?d mcstly in <br />terrestrial environments. Following deposition of the Mesav?rde Group, <br />sediments of the Lewis Shale were deposited in a predominantly marine <br />environment CBIM, 1976). The following description of the geologic units is <br />modified frcm Warner and Dale (1981) and Brogden and Giles (1977). <br />The MancoS Sha'.2 is a thick (acproximat?.y 5,000 ft.) homegenecus light-gray <br />tJ ddrk-gray f05511if?rows marine shale .vi*_h interdedd?; sandstone dnd <br />~imeSt6nB b2~5 dnC i5 Ch2 Oldest Jnlt ?aCGSeJ In Ch2 3`?a. The 53nd5,One; dfe <br />~anora!ly then bedded. " ne grained, tic, anC fossilif?ryJS, and fJrm <br />ras scant ledges in t~? bass' and upp?r carts of the formation. Tne o•rrail <br />ire3 Cc:UC1ed by the '4.inCOi ]hale 15 CndraC'_e rized b,/ r;,iil.'1C hJIP.lGC CKy <br />tod~ grapny. <br />_za_ <br />