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<br />0939 <br /> <br />3.5.1.2.4 Parachute Creek Member <br /> <br />The Parachute Creek Member of the Green River Formation, named by <br />Bradley (1931), contains the thickest and richest deposits of oil shale in <br />r' <br />the Piceance Creek Basin, and in the world. Because of the magnitude of <br />the resource and the potential for economic recovery, this unit has been <br />studied by numerous workers since Bradley (1931) and subdivided into <br />various informal sub-units. Figure 3-5 summarizes the various strati- <br />graphic nomenclatures found in the literature. Overall, this member is <br />predominantly a thin bedded, ultra-fine grained, light 9rey to black, <br />dolomitic marlstone which contains varying amounts of organic kerogen. The <br />thin bedded nature of this unit is remarkably persistent, such that <br />individual bed sequences can be traced across the entire basin. Numerous <br />tuff beds, ranging in thickness from less than 1 inch to approximately 5 <br />feet, are found throughout the upper part of this unit. These tuff beds, <br />which are generally analcitized, include several important stratigraphic <br />markers, such as the Wavey bed and the Mahogany marker. <br /> <br />Concentrated primarily in the lower portion of this unit below the <br />Mahogany zone, in the structurally deepest part of the basi n ,are beds <br />containing thick sequences of the sodium-bicarbonate mineral, nahcolite. <br />Though the nahcolite decreases and disappears moving laterally southward <br />from basin depositional center, thin zones of nahcolite vugs, both filled <br />and empty, are found along bedding planes, in and above the Mahogany zone. <br />Evidence for the one time presence of this mineral throughout the basin <br />includes empty vugs, vugs lined and filled with replacement minerals such <br />as calcite, and collapsed breccia. Vugs and thin solution zones of this <br />type are found over the entire area of NOSR 1. Stratigraphically, they are <br />scattered in the upper part of the unit, but seem to be concentrated in the <br />uppermost 650 feet. <br /> <br />On NOSR 1, important marker beds in the Parachute Creek Member <br />include, from stratigraphically highest to lowest, the Big Three, the <br />Stillwater, the 4-Senators, the Wavey bed, the A-groove, the Mahogany <br />marker, and the Mahogany bed. Table 3-1 summarizes, the relative <br />stratigraphic positions of these important sub-units on NOSR 1. <br /> <br />3-11 <br />