3.3.1.3 Surficial Geology

formations are distinct, forming steep ridges of light-colored rocks throughout the areas— these rock units formed in shallow oceans and vast sand dune fields (WSGS undated). The Tensleep Formation in this area is characterized by white to gray sandstone with thin limestone and dolomite beds. The Amsden Formation contains red and green shales and dolomite, with a red base. When uplift occurred during the Laramide orogeny (70 to 35 million years), the Cambrian to Cretaceous sedimentary formations in the Project vicinity, which were horizontal at the time of deposition, were uplifted due to faulting associated with this orogeny (Figure 3.3-3 and Figure 3.3-4).

Cambrian through Pennsylvanian sedimentary rocks including sandstones, siltstones, and limestones are found exclusively from Seminoe Reservoir to approximately halfway to the outlet works location in the vicinity of Dry Lake Creek. These include undifferentiated Cambrian rocks as well as the Madison, Amsden, and Tensleep formations (a total sedimentary sequence thickness of approximately 1,300 feet, of which the Madison and Tensleep formations comprise the majority). From this area to several miles beyond the proposed upper reservoir area are crystalline basement rocks of Precambrian age, namely granite with occasional gabbroic dikes (actual composition may range to near diorite in places) (Bennett and Aalto 1982).

The proposed transmission line corridor begins in late Archean-age granite (Wg) and extends across deposits of Quaternary gravels (Qt), alluvium and colluvium (Qa), the Mesaverde Formation (Kmv) (light-colored massive to thinly-bedded sandstone), the Medicine Bow Formation (Kmb) (brown/gray sandstone and shale with thin coal and carbonaceous shale beds), the Hanna Formation (Th) (brown/gray sandstone, shale, conglomerate, coal), the Wind River Formation (Twdr) (variegated red and white claystone and siltstone), Cloverly Formation (Kcv) (rusty sandstone at top, underlain by brightly variegated bentonitic claystone; chert-pebble conglomerate locally at base), the Niobrara Formation (Ks) (light-colored limestone and gray to yellow speckled limy shale [age about 83 Ma]), and Steele Shale (Ks) (gray soft marine shale containing numerous bentonite beds and thin lenticular sandstone) (WSGS 2020) (Figure 3.3-4). The youngest geologic features in the area are the sand dunes located at the southern end of Seminoe State Park – the common wind direction is west to east. Many smaller dunes in the Project vicinity are stabilized by vegetation, but they show an elongate east-northeast shape that indicates the common wind direction (WSGS 2020).

3.3.1.3 Surficial Geology

Quaternary unconsolidated deposits that contain hydrogeologic units in Carbon County include alluvium and colluvium sediment, landslide deposits, dune sand (eolian), lacustrine sediments, glacial deposits, and terrace gravels. Quaternary unconsolidated deposits in Carbon County generally are less than 70 feet thick and occur only in about 16 percent of the land area of the county. Alluvium and colluvium can be found in major drainages of Carbon County, such as the North Platte River, and this sediment also is found in minor drainages. These surficial deposits consist of silt, sand, and gravel, with coarser materials and cobbles occurring near the mountains (Bartos et al. 2006).