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time with parts of the Douglas Creek and Garden Gulch Members in the southwest <br />part of the basin. A similar facies transition also occurs along the southeastern and <br />eastern margins of the basin. At these localities, the lower Green River Formation <br />consists of a thick interstratified series of sandstone, fresh-water limestone and gray <br />shale. Donnell (1961) called this unit the Anvil Points Member and correlates it with <br />the combined Douglas Creek and Garden Gulch Members and part of the lower <br />Parachute Creek Member (Figure 4-5). <br />The Parachute Creek Member is the thickest and most economically important unit <br />in the Green River Formation. It has a large areal extent and contains all of the <br />commercial resources of oil shale, nahcolite, dawsonite and halite. The Parachute <br />Creek Member ranges in thickness from less than 200 feet south of the Colorado <br />River, to more than 2,000 feet in the north-central part of the basin. Oil shale is the <br />dominant lithology. On the Douglas Creek Arch, however, marginal-lacustrine rocks <br />(sandstone, limestone and marlstone) are also present (Figure 4-5). <br />Saline minerals in the Parachute Creek Member are found primarily in the north- <br />central Piceance Creek Basin. During the Eocene, these minerals, primarily <br />nahcolite, dawsonite and halite, were restricted to the center of the depositional <br />basin. Codeposition of oil shale and saline minerals lasted for many thousands, if <br />not millions of years, and produced a section about 1,800 feet thick (Figure 4-6). As <br />will be discussed in more detail in later sections, part of the saline mineral mass has <br />been removed by descending groundwaters. Today, maximum thickness of saline <br />oil shale is about 1,100 feet and occurs under the Lease. <br />Daub & Associates, Inc. Page 4-18 NSI Mine Plan 2010 Rev. <br />Printed: 7/5/2010 Section 4 Geology