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time. These sediments consisted of a thick assemblage of fluvial sandstones and <br />variegated shales of the Wasatch Formation. Maximum thickness of the Fort Union <br />Formation in the northern part of the basin is about 1,700 feet (Johnson, 1979a, <br />1979b), while maximum thickness for the Wasatch Formation is about 5,500 feet <br />(Donnell, 1961). <br />In early Eocene time, a system of interconnected (?) fresh-water lakes formed in the <br />central and northwestern part of Piceance Creek Basin and, with time, expanded <br />and coalesced to form a single large body of water, Lake Uinta (Bradley, 1931). <br />Initial deposition consisted of dark shale, ostracodal limestone, mollusk-bearing <br />sandstone, marlstone and low-grade, clayey oil shale. Adjacent to the lake, detrital <br />sediments accumulated in some areas while erosion or nondeposition occurred in <br />others. Lake Uinta expanded rapidly to fill a large part of the basin in late-early to <br />early-middle Eocene time. Deposition at this time changed from fresh-water <br />lacustrine to saline-water lacustrine. <br />The dominant sediment produced was a very fine-grained, organic-rich silty <br />dolomitic limestone characterized by thin laminations and a suite of authigenic <br />silicate and carbonate minerals (i.e., oil shale). During periods of high salinity, large <br />volumes of saline minerals (nahcolite, dawsonite and halite) were deposited with oil <br />shale. Infall of volcanic ash into the lake was also common. <br />During maximum stand, Lake Uinta completely filled the Piceance Creek Basin and <br />the Uinta Basin to the west, submerging the Douglas Creek Arch. At that time some <br />of the richest oil shales in the Green River Formation (i.e., Mahogany Zone of the <br />Parachute Creek Member) were deposited. Following deposition of the Mahogany <br />interval in middle to late Eocene time, Lake Uinta decreased in size, became more <br />saline and began to receive an influx of fluvial and deltaic sediment from the north <br />(Surdam and Stanley, 1979,1980; Johnson, 1981; Donnell, 1982). Infilling of the <br />Piceance Creek Basin by this prograding wedge of sediment resulted in the <br />interstratified sandstones, siltsones and marlstones now recognized as the Uinta <br />Formation, the youngest stratigraphic unit preserved in Piceance Creek Basin. <br />Daub & Associates, Inc. Page 4-14 NSI Mine Plan 2010 Rev. <br />Printed: 7/5/2010 Section 4 Geology