3.3.1.2 Project Vicinity Geology

3.3.1.2 Project Vicinity Geology

The overall mapped geology of the proposed Project is provided on Figure 3.3-3. Bedrock units are based on WSGS (2020) lithologic descriptions and nomenclature.

The Project is located in the Seminoe Mountains in central Wyoming along the southeastern margin of the Wyoming Basin. The Seminoe Mountains consist of an uplifted Laramide thrust wedge cored by Precambrian metamorphic and plutonic rocks. The Precambrian core consists of Archean metasedimentary and metavolcanic rocks greater than 2.7 Ga6 exposed in a broad, vertically plunging fold. The rocks are of lower amphibolite grade and were intruded and folded by syntectonic granodiorite (more than 2.6 Ga) (Hausel 1994). The Project is located on the southern flank of the Sweetwater Arch within the Seminoe Mountains, an east-west trending anticline that extends eastward for approximately 75 miles from the southern end of the Wind River Range. The core of the uplift, the Granite Mountains, has eroded to Precambrian basement rocks and subsequently has dropped 2,000 feet in elevation by normal faulting due to crustal extension. The southern boundary of the Sweetwater Arch is marked by the Ferris, Seminoe, and Shirley Mountains (Carpenter and Cooper 1951, Bartos et al. 2006). The Sweetwater Arch, along with the Laramie Range, Medicine Bow, and Sierra Madre mountains, is composed of Precambrian, Paleozoic, Mesozoic, and early Tertiary rocks. The Seminoe Mountains were uplifted during the Laramide tectonic event—uplift is bounded on the southwest by a low-angle reverse fault on the north by the Kortes reverse fault, which is a Precambrian shear zone that was reactivated during the Laramide deformation and again in the late Cenozoic (Blackstone and Hausel 1992). These rock layers were compressed into anticlines and uplifted along low-angle thrust faults and highangle reverse faults. The South Granite Mountains fault system bounds the northern extent of the Seminoe Mountains—the fault is an undifferentiated Quaternary fault less than 1.6 million years old. It is a normal fault with a slip rate of 0.2 millimeters per year (WSGS 2020). The total fault length is approximately 85 miles.

The area of the proposed upper reservoir7 is situated in Precambrian (late Archean) rocks of the Seminoe Mountain core located on a flat-topped granite mountain bounded on the west by a steep drop-off to Kortes Reservoir, to the north by a moderately steep drop-off paralleling two gulches, to the east by a steep drop, and to the south by a steep drop-off to Seminoe Reservoir along dip-slope sedimentary bedding (Bennett and Aalto 1982). Approximately 550 million years ago, following uplift of the Precambrian rocks in and around the Project vicinity, the Cambrian Flathead Sandstone was deposited in a shallow sea. In the northernmost Project vicinity, this sedimentary unit occurs as a blue-gray and yellow, mottled, hard, dense limestone interbedded with soft green micaceous shale, with dull-red quartzite sandstone at the base (WSGS 2020). Other rocks in the immediate area of the proposed upper reservoir include the Mississippian through Pennsylvanian age Madison, Amsden, and Tensleep formations, with ages from 360 to 310 million years. The

6 One-billion years (i.e., giga annum)

7 Exhibit A of this license application describes the location of proposed Project facilities in detail.