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produced by climatic changes. The net result was the development of a complex <br />interstratified series of lacustrine marlstone and lean oil shale and fluvial and deltaic <br />sandstone and siltstone. Most workers who have studied this sequence <br />stratigraphically have assigned the lacustrine units (tongues) to the Green River <br />Formation and fluvial-deltaic tongues to the Uinta Formation. Twelve such tongues <br />of the Green River Formation have been identified throughout the basin (Duncan <br />and others, 1974; Hail, 1977; Johnson, 1975, 1981; O'Sullivan, 1974, 1975; Pitman <br />and Donnell, 1973; Donnell, 1982). Intertonguing of the Uinta and Green River <br />Formations is most common in the northern part of the Piceance Creek Basin, while <br />in the southern part, near the Colorado River, intertonguing is rare (Donnell, 1982). <br />As a result of intertonguing, the contact between the Uinta and Green River <br />Formations may be difficult to define. Most workers place the contact at the top of <br />the continuous oil shale sequence (R-8 zone) which overlies the A-Groove. This <br />contact can be identified from Fischer Assay (FA) data and from geophysical logs <br />(gamma and density). In the north-central part of the basin, near the Lease, the <br />contact is about 150 to 200 feet above the Mahogany Zone at a depth of about <br />1,140 feet. <br />The sandstones and siltstones that characterize the Uinta Formation are commonly <br />gray to brown and often contain pyrite. Authigenic analcime (NaAISi2O6•H20) is also <br />common and probably resulted from breakdown of unstable volcanic glass in the <br />original sediment. Sandstone is usually fine to medium-grained and moderately to <br />poorly sorted. Primary porosity is generally low because of intergranular cement <br />(calcite, analcime and dolomite). Fracturing is common in the Uinta Formation and <br />tongues of Green River Formation. <br />4.2.4. Structure <br />The Piceance Creek Basin originated during the Laramide Orogeny, a period of <br />tectonism that began in Colorado near the close of Cretaceous time and concluded <br />in the Eocene (Tweto, 1975, 1980). Orogenic activity in the Piceance Basin area <br />Daub & Associates, Inc. Page 4-25 NSI Mine Plan 2010 Rev. <br />Printed: 7/5/2010 Section 4 Geology