Geologic investigations of proposed Sheep Creek, Carlson Creek, and Turner Lake power sites, Alaska

Abstract

Geologic conditions at Sheep Creek, Carlson Creek, and Turner Lake are discussed in relation to possible plans for hydroeletric power development. The proposed sites are on the rugged mainland of Southeastern Alaska along Gastineau Channel and Taku Inlet near Juneau. Bedrock in the area consists of a coastal strip of northwestward-trending foliated metamorphic rocks with steep northeasterly dips. This belt of rocks is adjacent to the Coast Range batholith on the northeast, with a 2 to 3 mile wide zone of injection gneiss between the main batholith and the metamorphic rocks. Unconsolidated glacial and post-glacial deposits of Quaternary age mantle the bedrock over large parts of the area. The valleys of Sheep and Carlson Creeks have been modified by glaciers of Pleistocene age and Turner Lake occupies a rock basin formed by glacial scour. There is an excellent site in greenstone bedrock at Sheep Creek for either a concrete or a rock fill dam. A conduit from the dam to a powerhouse along Gastineau Channel would be on bedrock for most of the distance. Slate bedrock suitable for a powerhouse site is exposed near the mouth of Sheep Creek. To the northwest along Gastineau Channel, bedrock is concealed by a mantle of glacial deposits of unknown thickness. The reservoir is in essentially impermeable bedrock; however, a main haulage adit of the Alaska-Juneau gold mine would probably have to be sealed off to prevent flooding of the mine workings or possible loss of water from the reservoir. The dam, diversion tunnel, and powerhouse at Carlson Creek are all in bedrock consisting of fresh injection gneiss. This rock is well suited as the foundation of a concrete or rock fill dam, but foundation treatment would be required to seal off closely spaced open joints trending perpendicular to the proposed dam axis. The diversion tunnel would stand unsupported except possibly where it would intersect two zones of closely spaced joints. The reservoir would be in essentially impermeable bedrock. Both the main dam and auxiliary structure at Turner Lake would be on an excellent foundation of granitic rock (granodiorite). Loose landslide debris would have to be removed at the dam site to expose fresh, sound bedrock. There is a powerhouse site in bedrock along Turner Creek at a stream elevation of 16 feet. Foundation conditions for a powerhouse at tidewater, near the mouth of Turner Creek were not studied. The conduit would be on sound granitic rock throughout its length, and the reservoir is entirely in relatively tight granitic bedrock.