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Foraminifera and Paleoenvironments in the Etchegoin and Lower San Joaquin Formations, West-Central San Joaquin Valley, California

LAGOE, MARTIN B., JOHN A. TENISON, and ROBERT BUEHRING, University of Texas at Austin, Austin, TX

The Etchegoin and San Joaquin formations preserve a rich stratigraphic record of paleoenvironments, deposition, and tectonics

during the late Miocene-Pliocene development of the San Joaquin basin. The distribution of foraminifera within these formations can help constrain this record, which includes final filling of the basin, facies responses to sea level changes, and active movement on the San Andreas fault system. The distribution of foraminifera in core samples is analyzed from seven wells along the west-central San Joaquin basin--four from Buena Vista oil field, one from western Elk Hills oil field, and two from an area just south of South Belridge oil field. A model of modern, shallow- to marginal-marine foraminiferal biofacies is used to interpret the Etchegoin-San Joaquin faunal distributions. This modern model distinguishes marsh, tidal channel, intertidal, lagoonal, littoral, and shallow sublittora environments. Low-diversity biofacies predominate with the dominant taxa being Elphidium spp., Buccella spp., Ammonia beccarii, Elphidiella hannai, Buliminella elegantissima, Nonionella miocenica, and Florilus cushmani. Agglutinated taxa generally are rare or absent but occasionally are common in the lower Etchegoin. Changes in microfauna and lithology indicate a first-order pattern of basin filling, with maximum water depths in the lower Etchegoin Formation of approximately 50 m and fluctuating marginal marine/nonmarine conditions in the San Joaquin Formation. Paleobathymetric fluctuations are not pronounced in the Etchegoin Formation and do not exceed a few tens of meters. Subsidence appears to be in balance with sediment loading as reflected in this maintenance of a narrow range of s allow marine water depths throughout a thickness of up to 1400 m. Ongoing work calibrating this foraminiferal record to the lithologic and macrofossil records in addition to interpreted depositional systems within these formations will further define relationships between paleoenvironments, relative sea level, and tectonics.


AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)