IMPERATO, D. P., Institute for Crustal Studies, University of California, Santa Barbara, CA

ABSTRACT: Overview of Middle-Late Cenozoic Structure, Tectonics, and Sedimentation of Elk Hills and Vicinity

During the Cenozoic, the San Joaquin Valley (SJV) evolved from a forearc basin to a "remnant" basin associated with the transform plate margin. For most of this time, structural and depositional styles of the Elk Hills/southern Temblor Range area (EH/STR) differed from the northern Temblor Range/southern Diablo Range area (NTR/SDR), perhaps due to conditions inherited from subduction tectonics.

Subduction tectonics dominated the SJV until the middle Miocene. Uplift of the NTR/SDR, perhaps resulting from tectonic wedging of obducted Franciscan assemblage and west-vergent thrusting, was concurrent with deposition of east- and southeast-prograding delta/slope/fan sequences separated by basin-margin unconformities. During this time, the EH/STR area was dominated by "distal" turbidite deposition, perhaps concurrent with extension and normal faulting. Uplift, tectonic wedging, and thrusting, not evident in the EH/STR area, may have occurred west of the modern San Andreas fault.

A major regional angular unconformity at the base of the Relizian (middle Miocene) in the NTR/SDR may correspond with subduction of the Pacific/Farallon spreading ridge to the south and initiation of San Andreas transform tectonics. In the EH/STR area, the middle Miocene is represented by a continuous condensed shale sequence deposited in bathyal water depths. Local thinning of these intervals at Elk Hills suggests folding that began prior to Relizian time. Folds probably formed as fault-bend folds, where a horizontal detachment ramped upsection to a shallower level.

Coarse-grained sedimentation associated with the San Andreas fault did not occur in the EH/STR area until the late Miocene, when a thick sequence of marine sandstone and conglomerate was shed northeastward from a highland west of the San Andreas fault. Channelized turbidity currents were deflected around topographic highs formed by folds on the basin floor. Onlap of younger sediments indicates that the southern Temblor Range was uplifted concurrent with deposition of these units. In contrast, the NTR/SDR was dominated by deposition of a relatively thin sequence of siliceous shale. During the remainder of the late Miocene through the early Pliocene, the SJV was dominated by marine sedimentation in a structurally stable setting.

Renewed folding and faulting and establishment of nonmarine to shallow-marine conditions throughout the SJV began in the late Pliocene (about 4-3 Ma). Folding at Elk Hills may have resulted from renewed activity on the older Miocene fault ramp or from imbricate thrust stacking of fault ramps. Relatively minor strike-slip faulting east of the San Andreas fault occurred concurrent with renewed uplift of the Temblor Range. This latest deformation, which continues at present, may be related to a clockwise reorientation of Pacific plate motion relative to North America at about 3.5 Ma.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.