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Continental sandstones, shales, and conglomerates of the Chinle and Morrison <br />Formations, Navajo and Entrada Sandstones, Wingate, Kayenta, Entrada, <br />Summerville, and represent Triassic and Jurassic time periods. These units have a <br />much broader areal extent than underlying Paleozoic units. <br />Cretaceous rocks in the Piceance Creek Basin are represented by the Dakota <br />Sandstone, Mancos Shale, and Mesaverde Group. Collectively, this sequence of <br />sandstone, dark shale and coal attains a maximum thickness of about 9,000 feet in <br />the northeastern part of the basin. The majority of the Cretaceous strata represent <br />marine deposition. However, the Dakota Formation and the upper one-third of the <br />Mesaverde Group are considered nonmarine or marginal marine. <br />The rocks of the Mesaverde Group are exceptionally well exposed along the Grand <br />Hogback (Figure 4-1). In this area, two formations are recognized, the Iles (at the <br />base) and Williams Fork. These two units have a total thickness of about 4,800 feet <br />near New Castle, Colorado, whereas, near Meeker, Colorado, the total thickness is <br />approximately 6,300 feet (Collins, 1976). <br />At the end of Cretaceous time, tectonism associated with the Laramide Orogeny <br />disrupted the old coastal-plain setting that had characterized Cretaceous deposition <br />in Western Colorado, and produced localized large-scale uplifts with adjacent <br />basins, such as Piceance Creek Basin. Elevation of the land above sea level <br />produced erosion and created a broad unconformity which today is marked by a <br />highly weathered, pebbly, kaolinite-rich zone at the top of the Mesaverde Group. <br />This weathered zone was once thought to represent a separate stratigraphic unit of <br />Paleocene age, the Ohio Creek Conglomerate (Donnell, 1961, 1969). However, it is <br />now considered to be a member of the Williams Fork Formation (Johnson and May, <br />1980; Johnson and Keighin, 1981). <br />Tertiary deposition in the newly created Piceance Creek Basin began in early or <br />middle Paleocene time. Deposition began first in the northern part of the basin and <br />consisted of fluvial, paludal and fresh-water lacustrine sediments of the Fort Union <br />Formation. Along the basin margin, deposition did not begin until late Paleocene <br />Daub & Associates, Inc. Page 4-13 NSI Mine Plan 2010 Rev. <br />Printed: 7/5/2010 Section 4 Geology