|The Project area is in the lower Sierra Nevada foothills, which are chiefly underlain by metamorphic and igneous rocks. Most of the metamorphic rocks are meta-volcanics and slates; meta-sandstone, serpentine, and marble are less abundant. The igneous rocks are chiefly granitic and occur in small quantity in the Project area. Repeated episodes of deformation and metamorphism have blurred or destroyed original bedding and compositional variation in the metamorphic rocks. Detailed stratigraphic description and correlation are limited, and the study of structural relations has been possible only in broad outline. The geologic materials of the region are as follows:|
Recent: Soils-and recent stream sediments
Tertiary: Table Mountain latite flow
Old gold-bearing stream gravels
Remnants of unmetamorphosed sedimentary deposits
Mesozoic: Granitic intrusives
Serpentine and related rocks
Mariposa formation: Meta-volcanics, meta-sandstone, slate
Penon blanco (Logtown Ridge) Formation:
Meta-volcanics, Meta-sandstone, slate
Upper Paleozoic: Calaveras formation: Meta-volcanics, Metasandstone, slate, marble
The regional grain of geological features trends about N 45 degrees W, and is produced by major fault zones and by lithologic variations related to major fold structures. Rocks are tightly compressed, tilted, and folded. The project area lies between two large fault systems, the Melones system on the east, and the Bear Mountain system to the west.
The canyon generally is narrow and steep-walled and trends southwestward across the northwest trending regional structure. The southeast canyon wall forms the northern end of 1865 foot tall Peoria Mountain and the left abutment of New Melones Dam. The northwest canyon wall is the southern end of Bostick Mountain, elevation 1814.0 feet, and the right abutment of the dam. Both sides of the canyon slope toward the river at an average of 38 degrees from horizontal.
The dam is founded on five distinct rock groups. All are exposed on the surface. The rock groups comprising the dam foundation are: Meta-volcanics (Mv), Meta-basalt agglomerate (Mba), Meta-sediments (Mvs), Older metamorphics (Omc), and Mariposa slate (Jm).
The rocks within the dam foundation area exhibit two stages of metamorphism, associated folding and faulting as well as features of a melange. The rocks in the Older Metamorphics classification have a moderately to well developed foliation striking East to N 60 degrees E and dipping 70 degrees to 90 degrees North and South. In the meta-sediments (Mvs), the younger slate rock is slightly to moderately foliated and strikes North to N 70 degrees W dipping 65 degrees to 90 degrees NW and SE. The meta-volcanic (Mv) rocks are considered to have been affected during the same episode of metamorphism as the meta-sediments although no foliation has been detected. Considerable evidence of intraformational movement has been noted. The most prevalent grouping strikes N 30 degrees to 70 degrees W and dips 70 degrees to 90 degrees NE and SW, coinciding with the regional structural grain.
The two major faults within the dam foundation are the IF-83 fault and the Powerhouse fault. The Powerhouse fault passes through the Powerhouse foundation, across the canyon floor downstream of the toe of the dam and curves toward the east crossing the left abutment of the dam at about elevation 940.0 feet. The IF-83 fault strikes N 75 degrees W and dips 65 degrees S. It passes through the foundation of the sloping intake structure, under the extreme upstream toe of the dam and continues up the left abutment where it intersects the Powerhouse fault. Two smaller faults occur within the foundation, one located high on the right abutment and the other on the lower left abutment.
Dominant joint sets cross the meta-volcanics with strikes of North to N 48 degrees E, dipping 20 degrees to 43 degrees NW and N 6 degrees to 68 degrees W, dipping 63 degrees to 90 degrees NE and SW. Those joints on the left abutment and dipping toward the canyon form a series of steep faced benches with sloping surfaces. On the right abutment the dominant joints dip into the canyon wall and do not result in any prominent surface expression.