CHAPTER II. GEOLOGY 19. Regional Geology. (a) Geography and Location. --The Trinity River division is located in mountainous terrain in the northwestern part of California. The main features of this project are situated in the southeastern part of the Klamath Mountain physiographic province. Subsidiary project features are found in the north end of the Great Central Valley. (b) Regional Geology. --The Klamath Mountains record successive periods of sedimentation and volcanic activity including several stages of orogeny, marine sedimentation, induration, uplift, and base leveling by erosion. This history is evidenced by the presence of pre-Paleozoic and Paleozoic rocks of marine sedimentary origin, highly elevated remnants of peneplains, alpine-type physiography, and deeply incised drainage in subdued landscaped. The late Jurassic granitic and ultrabasic intrusions, prominently observed in the eastern Paleozoic belt, compressed, folded, faulted and uplifted the area to form the Klamath Mountains and imposed upon this area a regional structural trend of N. 40° W. to N. 50° W. Range of rock types and age relationships of these formations are as follows: The Devonian Copley meta-andesite represents a period of widespread volcanic flows consisting of meta-andesites, pyroclastics and metatuffs. The Copley formation is thought to have an areal spread of about 1,000 square miles and a thickness of 3,700 feet. Following deposition of these volcanic flows and concurrent with the final stages, siliceous volcanic flows of the Balaklala metarhyolite are recorded in the eastern part of the area. This formation consists of three phases of volcanic activity--porphyritic, nonporphyritic, and pyroclastic rocks which possess a characteristic plated construction. This formation is estimated to be about 3,500 feet thick and outcrops in an area 5 miles wide and 10 miles long in the Whiskeytown area. Sedimentation was resumed in late Middle Devonian and is marked by generally faulted conformable contact between the Copley and deposition of the Mississippian aged Bragdon sediments. Orogeny occurring in Jurassic time was accompanied by intrusion of granitic batholiths and stocks. The Mule Mountain stock was intruded into the Balaklala formation during early Jurassic time and consists of coarse-textured graintoid rock called trohnjemite and an altered phase called albite granite. In last Jurassic a major intrusive body of batholitic size (the Shasta Bally Batholith) invaded all previous deposited formations and developed satellite metamorphic rocks along the intrusive contacts. This batholith covers an area roughly 10 miles wide by 30 miles long and is composed of biotite-hornblendequartz diorite. The granitoid texture is prominently coarse-grained. Along its northeastern border the Shasta Bally Batholith is in intrusive contact with gneissic metasediments of uncertain origin and age (but probably pre-Devonian). The gneisses occur as a narrow 500- to 2, 500-foot-wide band between the irregular intrusive contact on the southwest (sometimes offset by faulting) and the Hoadley fault to the northeast. The Hoadley fault is a steeply dipping normal fault along which a folded and faulted sequence of metamorphic rocks has been dropped down against the gneisses. Intrusive dike activity occurred both preorogenic and post-orogenic time as all formations have been intruded by dikes. Erosion and subsidence during Cretaceous time was followed by uplift. During Tertiary time, probably in the Eocene Epoch, the Weaverville gravels were deposited in ancient channeling of Trinity River basin. Semiconsolidated coarse gravels characterized by prominent red-colored lateritic soil and deep weathering cover an area probably 8 miles wide by 20 miles long in this basin. Deeply weathered rock forms the broad ridges which are characteristic in the area. Following a most recent period of rejuvenation and stream incision, the present drainage system was filled with auriferous gravels, which have been dredged or hydraulically mined. A dendritic drainage pattern has developed both parallel and against the lithic and structural makeup of the region. A. Trinity Damsite Area 20. Damsite, Reservoir, and Project Geology. (a) Physical Geology. --Trinity Dam is located on the Trinity River in the Trinity River basin which provides the main drainage system for the eastern part of the Klamath Mountains. Approximately 3,000 square miles of rugged elevated terrain are included in the Trinity River watershed. Peaks of bare rock rise to elevations in excess of 8000 feet in the northwestern areas. The present Trinity River channel is a result of renewed vigorous incision into progressive uplifting of this area and is illustrated by the steep sloped canyons that are prominent in the river traverse below Trinity Center. At the damsite these slopes rise from the river elevation of 1930 feet to elevations of 3500 feet. Flood plains are formed in areas where the main tributaries junction with Trinity River. Above the dam the main tributaries are Stuart Fork, East Fork, and Swift Creek. (b) General Geology of Trinity Damsite and Reservoir Area. --Rock types found in the Trinity Dam and Reservoir area are the Abrams and Salmon Schists; the Copley formation--a large series of basaltic and andesitic lavas, lapilli tuffs, coarse pyroclastics, and breccias; the Bragdon formation--a series of shales, siltstones, sandstones, and minor conglomerates; the Shasta Bally quartz-diorite; serpentine; basic and acidic dikes; and red, silty gravels of the Weaverville formation. All rock types have been heavily faulted many times and are generally found in a well-structured condition, particularly the Bragdon formation. This formation is commonly involved in landslides where erosion has paralleled the dipping leg of an anticlinal fold, (i.e., left slope of Trinity River). The Bragdon formation has the greatest distribution in the reservoir area of any of the above rocks and is found capping the Copley formation, generally in fault contact. Major downfaulting of the Bragdon into the Copley is observed in several areas along the Trinity River and its tributaries, probably the result of sharp flexures of compression transverse to the axis of folds. The Copley formation is found in wide distribution in the reservoir and forms the hard base of Trinity Dam. Mild metamorphism has altered and hardened much of the rock, but the causative forces of this metamorphism have rendered the rock highly structured. Slides along dipping weathered structures are common. The Weaverville formation is distinctive in that it forms a major red-colored, silty, semiconsolidated gravel body covering large parts of the western reservoir slopes and is arranged in a northerly elongate basin trend which is traceable for over 20 miles. The Weaverville formation was encountered in road relocation construction and required redesign of certain sections to avoid road failures. The serpentine rocks, developed through metamorphism of the Jurassic ultra basic intrusions, were also encountered in road building along the western edge of the reservoir and proved to be highly incompetent when deeply weathered. The inherent slick characteristics of high talc-bearing portions of this rock contain a very negligible shear strength which resulted in numerous large slope failures. (c) Trinity Dam Preconstruction Investigations. --Two locations on the Trinity River were originally considered as sites for the Trinity River Dam. First consideration was given to the Fairview axis, located at the old Fairview gold mine about 1 mile below the Stuart Fork and about 1 mile above Papoose Creek. The Papoose axis, an alternate site, located about 1 mile below the Fairview site, was investigated with a program of 101 diamond drill holes during 1951-1956. This site had the advantage of including waters from Papoose Creek within the reservoir site and thereby adding a fair-sized drainage volume and was eventually chosen as the more desirable site. Extensive diamond drilling, surface mapping, and geophysical investigations followed. Figures 5 and 6 show some locations and logs of exploration for Trinity Dam. (d) Geologic Mapping of Trinity Dam.--The large earth structure and the highly fractured foundation rock emphasized the need for detailed mapping of the area of zone 1. Mapping of the cleaned surface was by planetable on 10 to 1 and 20 to 1 scales; geologic logs were made of both grout trenches. In addition, generous notes of general rock conditions by areas were kept. The diversion tunnel, spillway tunnel, auxiliary tunnel, gate shaft, and spillway glory-hole shaft were mapped also. (e) Damsite Geology. --The Trinity Dam foundation is located in the Copley formation. Immediately above the axis of the dam at approximately elevation 2500 on the west slope, the Bragdon sediments and interbedded tuffs of the Copley formation are in conformable contact and dip about 25° southwest. The Copley meta-andesite rock within the flood plain is hard, fresh, massive to blocky, tough and generally tight. The massive blocks of rocks are rounded and pitted, and contain numerous potholes, some as much as 10 feet deep. Weathering of the bedrock foundation is structurally oriented and follows an irregular pattern, penetrating deeply in areas of highly faulted and broken rock. In some cases weathering had advanced sufficiently to obscure structural trends and such structures are quite compact and tight after |