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Study of the distribution of saline minerals in the Piceance Creek Basin (Roehler, <br />1974; Beard and others, 1974; Trudell and others, 1974; Dyni, 1974a, 1981) has <br />shown a zonal arrangement (Figure 4-7) which is probably related to the relative <br />solubilities of each mineral. Dawsonite has the largest areal distribution, covering <br />most of the northern basin. Nahcolite occurs more to the interior while halite is <br />restricted to only the north-central part. It is in the halite occurrence area that saline <br />oil shale reaches its maximum preserved thickness. <br />The Parachute Creek Member, because of its enormous resources of oil shale and <br />saline minerals, has received considerably more stratigraphic study than other <br />members of the Green River Formation. Most commonly, the Parachute Creek is <br />subdivided into "oil shale zones" (Figure 4-2) which are defined by the relative <br />richness or leaness of the oil shale (Donnell and Blair, 1970; Cashion and Donnell, <br />1972). <br />The maximum number of oil shale zones occurs in the northern basin where seven <br />rich zones (R-8, R-7, R-6, R-5, R-4, R-3 and R-2) are interstratified with six lean <br />zones (L-7, L-6, L-5, L-4, L-3 and L-2). The R-7 oil shale zone is commonly referred <br />to as the Mahogany Zone ([edge on outcrop), while the L-7 and L-6 zones are <br />usually called the A and B grooves, respectively. The Garden Gulch Member is also <br />subdivided into lean and rich oil shale zones: L-1, R-1, L-0 and R-0 (Dyni, 1974a). <br />The contact between the Parachute Creek and Garden Gulch Members (i.e., R-2 <br />and L-1 oil shale zones) represents a lithologic transition from clay-rich (illitic) oil <br />shale, which characterizes the Garden Gulch Member, to the dolomitic oil shale that <br />characterizes the Parachute Creek Member. The lithologic transition is abrupt (Dyni, <br />1974a, 1981) and easily identifiable on geophysical logs (electric, gamma and <br />density). Historically, workers have referred to this log kick as the "Blue Marker'. A <br />similar log response is present near the base of the Garden Gulch Member at the <br />top of the R-0 oil shale zone, and is referred to as the "Orange Marker'. <br />Daub & Associates, Inc. Page 4-20 NSI Mine Plan 2010 Rev. <br />Printed: 7/5/2010 Section 4 Geology