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<br />002239 <br /> <br />Durango Pumoing Plant - Stage I <br /> <br />B. The Upper Cretaceous sedimentary rocks that underlie the site area form part of the <br />northern margin of the San Juan Basin, The San Juan Basin is an asymmetric structural <br />basin approximately 100 miles in diameter containing Cambrian to Holocene sedimentary <br />strata that are as much as 15,000 feet thick (Fassett and Hinds, 1971). The San Juan <br />Basin contains coal deposits and gas fields. Northwest of the site area, Permian to <br />Cretaceous predominantly clastic strata containing some limestone and coal are overlain <br />by a discontinuous mantle of Quaternary alluvium and comprise the Four Comers <br />Platform (Haynes et aI., 1972; Tweto, 1979). Late Cretaceous to Early Tertiary igneous <br />rocks intruded these strata, forming the La Plata Dome during the Laramide orogeny <br />(Henkes, 1977; Tweto, 1979). North of the site area, Precambrian igneous, <br />metasedimentary and metavolcanic rocks, Cambrian, Devonian, and Mississippian <br />carbonate and clastic sedimentary rocks, and Pennsylvanian through Cretaceous clastic <br />sedimentary rocks are exposed in the Needle Mountains of the San Juan Dome (Steven et <br />aI., 1974; Tweto, 1979). To the northeast of the site area, Middle Tertiary andesitic to <br />basaltic volcanic and volcaniclastic rocks and felsic to intermediate intrusive rocks of the <br />San Juan Mountains comprise the San Juan volcanic field (Steven et ai" 1974; Tweto, <br />1979). <br /> <br />1.04 SITE GEOLOGY <br /> <br />A. Stratigraphy <br /> <br />1. Surficial Deposits. - Unconsolidated surficial deposits at the proposed pumping <br />plant site consist of man-made fill, Quaternary age slope wash, alluvium, glacial <br />outwash terrace deposits, and Quaternary/recent river alluvium. Several of these <br />deposits have been mixed together at certain site locations as a result ofUMTRA- <br />site cleanup. Soil classifications of the surficial deposits shown on the geologic <br />drill hole logs are based on the Unified Soil Classification System (USCS), and <br />are interpretive visual classifications representing the best estimates of the field <br />geologist. Recovery ofthe surficial deposits was generally poor due to fines and <br />fme sand often washing out during drilling. Maximum sizes of cobbles and <br />boulders, as reported on the drill logs, represent only what was sampled and may <br />not be indicative of the entire deposit. Soil classifications found in the test pit <br />logs are considered the most representative of the various deposits. <br /> <br />a, Man-made fill (Fill). - Man-made fill locally derived from both alluvium <br />(Qal) and terrace deposits (Qt) is present in isolated locations throughout <br />the proposed site in varying thicknesses. Approximately six inches to one <br />foot oftopsoil has been placed over the area by the UMTRA-site <br />contractor and consists generally of sandy clay. These materials are <br />shown as Fill on drawing G_ (69-529-5). <br /> <br />b. Slope Wash (Qsw). - Slope Wash is mapped near the base ofSmeIt~r <br />Mountain and was encountered in test pit TPDPP-J05. The slope wash <br />was deposited primarily by mass wasting assisted by running water not <br />confined to stream channels. It is composed primarily of silty and clayey <br />sand with variable amounts of angular to subangular gravel, cobbles, and <br />boulders measuring up to about four feet in nominal size. The slope wash <br /> <br />Geology <br />00320 - 7 <br />