Late Cenozoic stratigraphy and structure of the west margin of the central San Joaquin Valley, California

Upper Pliocene and Quaternary deposits were mapped in an area of 1,800 km 2in the west-central San Joaquin Valley and adjacent Diablo Range. The upper Pliocene and Pleistocene Tulare Formation, which consists of alluvial sand, gravel, silt, and clay and locally dips 20°, is overlain by a sequence of six units, each 0 to 20 m thick, each the result of an episode of deposition of alluvium from the Diablo Range. Erosional unconformities and soils between the units record the intervening periods. The units consist of unconsolidated gravel, sand, silt, and clay, with textures and sedimentary structures indicating deposition primarily by flowing water and secondardily by mudflows. The lower three units are grouped into the informally designated alluvium of Los Banos, of middle and late Pleistocene age, the two middle units are grouped into the upper Pleistocene alluvium of San Luis Ranch, and the uppermost unit is the Holocene alluvium of Patterson. Holocene arkosic alluvium, derived from the Sierra Nevada and deposited in flood basins along the San Joaquin River, is informally named the alluvium of Dos Palos. The two older units of the alluvium of Los Banos are coeval with broad pediment remnants preserved across the foothills, which indicates that the present elevation of the foothills is due to late Quaternary deformation. These surfaces are deformed into a series of broad, gentle northeast-trending folds that have been displaced more than 100 m along three northwest-trending fault systems. The Ortigalita fault displaces Holocene alluvium and has predominantly strike-slip displacement. The O’Neill fault system is a group of small reverse faults whose fault planes coincide with bedding in the northeast-dipping Great Valley sequence; these faults are interpreted to be sympathetic displacements associated with continued uplift and northeastward tilting of the foothills. The San Joaquin fault, at the foothill-valley margin, vertically displaces the pediments as much as 140 m. The orientation of the fault plane and the magnitude of lateral displacement, however, are not known. Neither the San Joaquin fault nor the O’Neill fault appears to displace the alluvium of San Luis Ranch or younger alluvium.