Los Banos Creek drains about 160 square miles of the Diablo Range, one of the coast ranges of California. The damsite is located at a constriction in the Los Banos Creek Canyon where the creek leaves the Coast Range and emerges into the San Joaquin Valley.
The oldest rocks in the region are the Jurassic Franciscan metasediments. These rocks, which underlie a large portion of the drainage basin for the reservoir, are highly contorted but have a general structural trend northwest-southeast parallel to the Diablo Range. The Franciscan rocks are overlain in fault contact along the Ortigalita fault by upper Cretaceous marine sandstones and shales with the Panoche formation dominant. These sediments also strike NW-SE with an average dip northeastward of about 30 degrees. Tertiary sediments overlying the upper Cretaceous are buried beneath Quaternary terrace and alluvial deposits. Remnants of Miocene lava flows occur near the contact of the Franciscan and Panoche formations.
The presence of five or more terrace levels in lower Los Banos Creek; the configuration, elevation, and location of terraces in upper Los Banos Creek; the size and shape of the drainage basins; and the type of materials within the various terraces suggest that the tectonic and related drainage history of the area is quite complex.
Los Banos damsite is located at a constriction in the canyon of Los Banos Creek resulting from differential erosion of a relatively thin sequence (125 feet) of sandstones and conglomerates contained within a much thicker sequence of sandy and silty shales. These beds have been identified as the Panoche sandstones and they strike fairly uniformly about N. 35 degrees W. and dip generally 30 to 45 degrees downstream (northeastward).
The more resistive sandstone and conglomerates form asymmetrical noses steeper on their upstream faces. The sandstone and conglomerate are thick bedded (1-4 feet), lenticular, and discontinuous. The sandstones are clean to shaly and vary from hard, limey, well cemented to poorly cemented, moderately friable. Concentrates up to six feet in diameter are common in some of the OY sandstones and case hardening is present in some zones.
The conglomerates are hard and well cemented. They contain hard volcanic, chert and graywacke cobbles 3-inch or 4-inch in diameter in a matrix of fine sandstone. Most of the cobbles are well rounded and commonly contain incipient fractures. Some zones of the conglomerate are highly fossiliferous.
Minor discontinuous, silty and sandy clay shales occur within the sequence. In the outcrop they are highly weathered and air slaked. In drill holes below the water table they are compact but poorly lithified.
Shales both above and below the sandstone are compact but poorly lithified and air slake moderately. The shales are sandy and silty but more clayey downstream.
The channel section, about elevation 230 and about 300 feet wide, contains outcrops of sandstone and fossil conglomerate which may be traced almost continuously from one abutment to the other. Between outcrops a shallow overburden of sand and gravel overlies bedrock.