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• Between Unit 4 and Unit 5 is a 40- to 60-foot-thick sequence of the Thirteenmile Creek Tongue (Tgtu) of the Eocene Green River Formation. The upper part of the Thirteenmile Creek Tongue is composed of a light gray to white weathered marlstone. South of the Piceance Site, the Thirteenmile Creek Tongue contains three to four thin, rich oil shale beds. North of the Piceance Site, the marlstone contains a few thin, porous ostracod charophyte-bearing limestone beds. In addition to the Uinta and Green River Formations, Quaternary Holocene alluvium (Qal) is found in the dry washes and the Piceance Creek valley floor. The alluvium consists of silt, sand, and gravel along the floodplains and alluvial fans in the larger valleys. The floodplain deposits are mostly gray, brown, and buff silts and sands. The alluvial fan deposits consist of angular sandstone and marlstone boulders and pebbles mixed with silts and sands derived from nearby hilly terrain. The alluvium is less than 50 feet thick (Duncan 1976). The stratigraphy underlying the Piceance Site is the same as that detailed in Section 7.2.1 for regional geology. Asite-specific stratigraphic section for the Piceance Site is presented in Figure 7-7. It should be noted that the highest concentration of halite in the Saline Zone of the Parachute Creek Member of the Green River Formation is found toward the center of the Piceance Creek Basin. The Piceance Site is located in the dawsonite-nahcolite zone and on the edge of the area containing halite (see Figure 7-1). To better understand the geologic structure underlying the Piceance • Site, two cross sections were prepared using data from existing exploration holes in the area to assess the subsurface geologic structure (Figure 7-8). Exploration holes were selected based on a north-south trend or an east-west trend, as well as on the amount of detail contained in each log. The west-to-east cross section (cross section A - A') presented in Figure 7-9, and the south-to-north cross section (cross section B - B') is presented in Figure 7-10. Both cross sections show the alternating anticlinal and synclinal features buried within the Piceance Creek Basin. Examining both cross sections together shows that the southern portion of the Piceance Site is rising out of a trough. Monitoring well 20-1 appears to be either resting on the northeastern flank of the ascending trough or near the crest of a small anticlinal feature. Pipeline Corridor The majority of the pipeline corridor will lie on the Uinta Formation, which is described in 7.2.1.1, above. At the two locations where the pipeline crosses Piceance Creek, the primary geologic structure is modern alluvium, which includes the Piney Creek alluvium and younger deposits (Tweto 1979). At Davis Point, the pipeline crosses the Parachute Creek Member of the Green River Formation and, at the base of Davis Point, lower parts of the Green River Formation (Tweto 1979). C~ American Soda, L.L.P. 7_(~ Commercial Mine Plan August 18, 1998