Big Blue Formation, San Joaquin Valley
The generally homoclinal eastern flank of the Diablo Range along the west side of the San Joaquin Basin of California (Fig. 6) is modified by a series of en echelon folds. One of these folds, the Coalinga anticline, is the southeast plunging extension of the Idria (or Joaquin Ridge) uplift, cored by a large serpentine diapir (Eckel and Myers, 1946). In Miocene time, basin margin shallow-marine and non-marine environments interfingered in the area of Coalinga anticline depositing a sequence of siliciclastic sediments. This lithological pattern was interrupted, when a major episode of diapiric movement extruded an enormous volume of serpentine foliate breccia, eroded the previously deposited sediments, and provided a source for sedimentary serpentinite in the same near-shore environments (Dickinson and Casey, 1976). These serpentinite deposits together make up the Big Blue Formation.
The initial development of the Diablo uplift of serpentinite and Franciscan rocks occurred in post middle Eocene time (Nilsen and others, 1974), changing the continental margin from an open shelf to a partially enclosed basin. Subsidence followed (Hackel, 1966). The resulting transgressive middle Eocene Domengine Formation is the first Franciscan-derived sediment in this area (Dickinson and others, 1979). It is conformably overlain by the siliceous and calcareous shales of the Kreyenhagen Formation, which are up to a thousand meters thick (Wilson, 1943). Through the Oligocene and early Miocene, uplifts and growth of Coalinga anticline resulted in numerous angular unconformities in the Vaqueros Formation. During the Saucesian the whole northern end of the basin was subjected to 300 to 600 meters of shoaling (Bandy and Arnal, 1969), resulting in a widespread unconformity above the Vaqueros, and exposing the Kreyenhagen Formation along the flanks of the Diablo uplift.