2014-01-29_HYDROLOGY - P2009025 (8)

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Baseline Hydrologic Monitoring Plan — Hansen Project 17 2.3 Hydrogeology 2.3.1 Geologic Setting The geology at the Hansen Project consists of approximately 1,000 to 2,000 feet of volcanic and sedimentary deposits of Eocene- and Oligocene -age overlying Precambrian basement rocks. A regional geologic map is provided in Plate 2. A geologic map of the study are is provided in Figure 7. The stratigraphic section is shown in Figure 8. From oldest to youngest, the major geologic units are as follows: • Precambrian Basement. The Precambrian rocks consist of pink to red and white granite gneiss, black and white to tan biotite granodioritic gneiss, and black to green biotite schist. Weathering characteristics of the granitic and metamorphic material vary from very fresh to highly weathered. • Echo Park Formation. The Echo Park Formation is Eocene in age and varies in thickness from 0 to 1,300 feet thick. The formation consists of interbedded sandstone, siltstone, claystone, and conglomerates of fluvial origin. Three distinct facies have been identified: a basal fanglomerate, a sheetwash facies, and a fluvial facies. The basal unit, or fanglomerate facies, consists of cobbles to large boulders of Precambrian granitic gneiss in a matrix of sand, clay, and pebbles. This unit is typically a red -gray to gray pebble to boulder conglomerate with thin, interbedded, silty sandstone lenses and occasional mudstones. These deposits generally occur in the lower part of the Echo Park Formation adjacent to steeply sloping Precambrian rocks (WWE, 1979). The middle unit, or sheet wash facies, consists of red to reddish -brown conglomeratic mudstones, although some green mudstone has been observed (WWE, 1979). This facies may form as much as 70% of the total Echo Park Formation deposited in the Tallahassee- Cottonwood Creeks area. These sediments were derived from mudflows carrying clay and pebble- to boulder -sized clasts from a weathered regolith on the Precambrian surface (Chapin, 1965). The upper unit, or fluvial facies, consists of alternating layers of sandy pebble to cobble conglomerate, pebble sandstone, and relatively clean sandstone. These sandstones and conglomerates commonly contain abundant carbonaceous material in localized sand bodies and thin silty lenses. • Wall Mountain Tuff. The Wall Mountain Tuff is the lowest formation in the Oligocene section and unconformably overlies the Echo Park Formation. It is 0 to 150 feet thick and consists of an ash - flow, welded tuff. It is lavender to reddish -brown in color and is moderately to densely welded. Extensive erosion prior to deposition of the Tallahassee Creek Conglomerate removed much of the Wall Mountain Tuff (WWE, 1979). • Tallahassee Creek Conglomerate. The Tallahassee Creek Conglomerate is composed of three distinct members at the Hansen Project area: (1) the lower Tallahassee Creek Conglomerate Member, (2) the middle Hansen Andesite /Latite Member, and (3) the upper Tallahassee Creek Ash -Fall Member. The lower Tallahassee Creek Conglomerate member is 0 to 350 feet thick and consists of a gray to green volcanic conglomerate in a clayey to sandy matrix. In some places, it has completely scoured away the Wall Mountain Tuff, incorporating the Wall Mountain boulders into the Tallahassee Creek conglomerate. Interbedded tuffaceous to arkosic sandstone and claystones are sometimes present in this member (WWE, 1979). 4153A.140129 Whetstone Associates