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The Etchegoin (late Miocene-early Pliocene) and San Joaquin (Pliocene) formations represent the final marine phase in the late Cenozoic filling of the San Joaquin Basin. During this time, the basin was subjected to both sea-level fluctuations (eustatic and local) and tectonic influences (movement on the San Andreas fault system; syndepositional growth of anticlines). Sequence and parasequence development in such an active-margin setting is complex. A detailed micropaleontologic analysis of these rocks yields insights into high-frequency environmental fluctuations and stratigraphic relationships. Cores from South Belridge, Cal Canal, Elk Hills, and Buena Vista Hills oil fields yield foraminiferal biofacies indicative of environments ranging from brackish marine to upper bathyal. Contin ous coring and approximately 3-m sampling in three wells allows comparison of biofacies fluctuations to parasequence development, as reflected on well logs. Within the Etchegoin Formation, environmental cycles occur at a frequency of 15-60 m (stratigraphic thickness) and range from inner neritic (shoreface) to outer neritic (mid-to-outer shelf). Parasequence flooding events are marked by influxes of Florilus cushmani and Nonionella miocenica, consistent with a water depth range of about 50 m for parasequence cycles. Environmental trends correlate well with coarsening-upward well log motifs. San Joaquin Formation biofacies patterns indicate fluctuations between brackish marine (lagoon, mud flat) and inner neritic (shoreface). Parasequences are difficult to define on logs without the aid o faunal data. Two major deepening events are recognized as genetic sequence stratigraphic boundaries and help demonstrate that, locally, the basal Etchegoin is laterally equivalent to the upper Reef Ridge Formation. Depositional sequence boundaries are, in the absence of seismic, much more difficult to recognize.
AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)