Middle Cenozoic Depositional, Tectonic, and Sea Level History of Southern San Joaquin Basin, California

The Eocene to lower Miocene fill of the southern San Joaquin basin contains three complete depositional sequences--the Tejon, San Emigdio, and Pleito--each of which corresponds to a formal formation. The Tejon sequence (lower to middle Eocene) is marine and incorporates nearshore, shelf, slope, and basinal deposits. The San Emigdio (upper Eocene to lower Oligocene) and Pleito (upper Oligocene) sequences intertongue eastward with alluvial-fan deposits of the Tecuya Formation. The lower part of the San Emigdio sequence was deposited by a westward-prograding Gilbert-type delta. The upper part of the San Emigdio sequence and lower part of the Pleito sequence were deposited by a system of shelf fan-deltas that prograded at least 10 km to the west. The middle and upper parts of the Pleito sequence were deposited by a slope fan-delta in relatively deep (hundreds of meters) water. Regional transgression during the early Eocene initiated deposition in the southern San Joaquin basin. The lower San Emigdio Gilbert-type delta prograded from the shelf edge during a lowstand in eustatic sea level at approximately 40 Ma. Relative highstand deposits in the San Emigdio and Pleito Formations consist of widespread progradational shallow-marine and nonmarine facies. The Eocene to early Miocene tectonic history of the southern San Joaquin basin included three distinct periods of increasingly intense activity. Rapid early to middle Eocene subsidence of the basin was associated with the emplacement of the Salinian block. Late Eocene to early Oligocene uplift and subsidence occurred in two discrete pulses, and a seismically(?) generated submarine-slumping episode triggered the change from shelf fan-deltas to slope fan-deltas. During the late Oligocene to early Miocene, the southern San Joaquin basin was disrupted by major uplift, proximal volcanism, and syndepositional faulting, as the Mendocino triple junction migrated past the study area, and San Andreas fault-related tectonism was initiated. In general, local tectonic events, rather than eustatic ea level events, seem to have exerted the predominant control on middle Cenozoic sedimentation in the southern San Joaquin basin.