Chapter 2 Study area Keith Clarey Melissa Thompson 2-1 he Greater Green River Basin (GGRB) forest and alpine tundra on the higher mountains. includes the Green River Basin, the Great At lower elevations in the basin, the vegetation in- Divide Basin, and the Little Snake River cludes abundant sagebrush, saltbush, greasewood, TBasin (Figure 1-1). The project boundary is deter- and desert shrub. Forested areas contain lodgepole mined by the distal river drainage basin divides. pine, spruce, fir, and aspen. The drainage basin boundaries overlap several geo- logic features within the GGRB. The continental GEOLOGIC SETTING: STRUCTURE divide marks the northern and eastern boundaries The GGRB is bounded by the Overthrust Belt to of the GGRB; it bifurcates and reconverges around the west, the Hoback Basin to the northwest, the the Great Divide Basin (Figure 1-1). Figure 2-1 Wind River Range and Granite Mountains to the shows the townships and ranges in the GGRB. north, the Rawlins Uplift to the east, the Sierra Madre to the east-southeast, the southern Sand The Wyoming portion of the GGRB, as delineated Wash Basin in Colorado to the south-southeast, using GIS databases, has an area of 20,792 square and the Uinta Mountains in Utah to the south. miles (13,306,700 acres). The Wyoming GGRB Within the GGRB, the Rock Springs Uplift and covers 21.3 percent of the area of the state. The Bridger Basin are located in the Green River Basin adjacent 3,821 square miles (2,445,900 acres) of proper; the Wamsutter Arch separates the Great the Colorado and Utah GGRB added to the Wyo- Divide Basin from the Washakie Basin; and Chero- ming GGRB area gives a total basin area of 24,613 kee Ridge separates the Washakie Basin from the square miles (15,752,500 acres) (Figures 1-1 and Sand Wash Basin (Figure 2-2). 2-2). The basin topography follows undulations on the Surface elevations in the Wyoming GGRB range Precambrian basement surface, formed by folding from 13,804 feet above mean sea level on Gan- and faulting of the earth’s crust under compres- nett Peak – the highest point in Wyoming, in the sional stress that began and was most intense dur- central Wind River Range – to 6,040 feet above ing the Sevier and Laramide orogenies. As shown mean sea level, measured as the high-water level in in Figure 2-4, some Precambrian basement areas Flaming Gorge Reservoir on the Green River at the in the GGRB downwarped (subsided) and formed Wyoming-Utah border. The lowest elevation within structural basins, while adjacent areas upwarped to the internal drainage of the Great Divide Basin is form mountains, uplifts, and structural arches and 6,647 feet above mean sea level. ridges. Sediments eroded from the upwarped areas were deposited over the pre-Laramide sediments In 2007, the population of the Wyoming GGRB within the new structural basins. was estimated at 54,760 (WWC Engineering, 2007), giving an average population density of 2.6 The maximum measured structural offset of per square mile. Precambrian basement rocks in the GGRB is on a Laramide fold-thrust fault in the northern Green Average annual precipitation in the GGRB ranges River Basin near Pinedale, where the rocks are from less than 7 inches in central basin areas to structurally offset by approximately 44,000 feet. nearly 60 inches in the high mountain areas above The Precambrian surface, more than 30,000 feet 10,000 feet in elevation (Figure 2-3). Most of below sea level in the deepest part of the structural the lowest parts of the central basin areas receive basin, is faulted against Precambrian rocks uplifted between 6 and 15 inches of precipitation annually. to nearly 14,000 feet at the top of the Wind River The highland and mountain areas receive 21 to 59 Range (Figure 2-4). inches. Figure 2-5 is a geologic cross section across Vegetation in the basin areas is mixed-grassland southwestern Wyoming, from the Overthrust and sagebrush steppe, sagebrush, scrub/brush Belt eastward across the Moxa Arch, Green River lands, and high-altitude desert, ranging to alpine Basin, Rock Springs Uplift, and Washakie Basin. 2-2 The cross section shows the Precambrian basement (Collentine et al., 1981). Figure 2-4 shows the rocks overlain by varying thicknesses of Paleozoic Great Divide Basin with a structural depth more to Cenozoic formations; some large-scale geologic than 25,000 feet below ground surface on the structures; and major faults. The structural basins Precambrian basement contours (more than 20,000 are depressions in the crust in which thicker sedi- feet below mean sea level). The deepest parts of the mentary rock units accumulated. The uplifts and Great Divide Basin are located in the northwestern structural arches mark where the crust upwarped, and southeastern basin; their structural trend is ap- and associated folding and faulting. proximately west-northwest (Figure 2-4). Overthrust Belt. The Overthrust Belt is a large, Washakie Basin. The Washakie Basin has an area elongate fold-thrust complex located in south- of approximately 2,600 square miles. Elevations western Wyoming and adjacent areas of Idaho and range from approximately 6,100 feet above mean Utah. It is a region of complex structural defor- sea level to approximately 8,700 feet at Pine Butte mation of Paleozoic and Mesozoic units: folding, in the southwestern basin (Collentine et al., 1981). reverse faulting, thrust faulting, and overthrust Figure 2-4 shows a structural depth of more than faulting; and a later phase (beginning in the late 20,000 feet below land surface on the Precambrian Tertiary) of overprinted normal faulting that has basement contours (more than 15,000 feet below continued to the present day. The thrust sheets mean sea level in the central basin). This deepest were generally transported eastward during the part of the Washakie Basin is oriented north–south Sevier Orogeny (Late Jurassic to Eocene). The in structural trend (Figure 2-4), parallel to the north–south trending, parallel thrust faults of the major structural trend of the Rock Springs Uplift Overthrust Belt are generally younger to the east. (Figure 2-4). The Moxa Arch is a parallel fold structure east of the Overthrust Belt in the western Green River Rock Springs Uplift. The Rock Springs Uplift has Basin (Figures 2-2 and 2-5). an area of approximately 1,750 square miles (50 miles by 35 miles). Elevations range from approxi- Green River Basin. The Wyoming Green River mately 6,200 feet above mean sea level on Bitter Basin has an area of approximately 14,700 square Creek to 8,680 feet above mean sea level at Aspen miles. West of the Rock Springs Uplift, the local Mountain on the southern half of the uplift and to depression of the Green River Basin between the more than 7,500 feet in the Leucite Hills (Collen- uplift and the Moxa Arch shows a depth of more tine et al., 1981). The maximum structural uplift than 25,000 feet below land surface on the Precam- of the Rock Springs Uplift is estimated at 17,000 brian basement contours (more than 20,000 feet feet (Love, 1961a). The Precambrian basement below mean sea level) (Figures 2-2, 2-4, and 2-5). rocks lie about 8,000 feet below land surface (less than 2,000 feet below mean sea level) in the central Figure 2-4 shows a structural depression in the Baxter Basin area of the Rock Springs Uplift. The Pinedale area deeper than 35,000 feet below land Rock Springs Uplift is a doubly-plunging, anticli- surface on the Precambrian basement contours nal fold (dome) structure with a north–south major (more than 30,000 feet below mean sea level). The structural trend. structural trend on this deepest part of the Green River Basin is northwest–southeast, semi-parallel The deepest and oldest geologic formation exposed to the northwest–southeast structural trend of the at land surface in the core of the Rock Springs Wind River Range (Figure 2-4). Uplift is the Upper Cretaceous Baxter Shale. The surface exposure includes the Baxter Basin area Great Divide Basin. The Great Divide Basin (Plate 2). A north–south trending reverse fault, has an area of approximately 3,500 square miles. east side upthrown, lies along the west flank of the Elevations range from 6,647 feet above mean sea Rock Springs Uplift (Figure 2-5); it is associated level near basin center (Sec. 32, T.24N., R.94W.) with the compressional stress and anticlinal folding to more than 8,000 feet in the adjacent highlands that formed the uplift during the Laramide Orog- 2-3 eny. Most of the structural doming of the Rock feet and a maximum combined thickness estimated Springs Uplift occurred after Eocene time (Surdam at more than 12,000 feet. and Jiao, 2007). CENOZOIC UNITS Sierra Madre. The Sierra Madre ranges in eleva- Quaternary geologic units tion from 6,330 feet above mean sea level on the As shown as outcrop on Plate 2 and Plate 3, Qua- Little Snake River to 11,007 feet on Bridger Peak ternary deposits include unconsolidated alluvial (Collentine et al., 1981). Precambrian basement and colluvial sediments, landslide deposits, dune rock units are exposed in the uplifted core of the sand (eolian), lacustrine sediments, glacial depos- Sierra Madre, and Miocene formations unconform- its, gravel pediment and fan deposits, undivided ably overlie and blanket the flanks of this mountain surficial deposits, and terrace gravels, as well as con- uplift (Plate 2). solidated uppermost Pliocene to Quaternary alkalic extrusive and intrusive igneous rocks (Welder and GEOLOGIC SETTING: ROCK UNITS McGreevy, 1966, sheet 3; Welder, 1968, sheet 2; This section describes the geologic characteristics Lowry et al., 1973, sheet 3; Love and Christiansen, of the geologic units in the GGRB, or rock-strati- 1985).