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Geohistory Analysis of Neogene Point Arena Basin, California: Implications for Its Tectonic Evolution

Karen B. Loomis, James C. Ingle, Jr.

The Neogene Point Arena basin of northwestern California is located west of the San Andreas fault system and south of the Mendocino triple junction. Key units of the Point Arena sequence are exposed from Iversen Landing north to Point Arena, California, including the Oligocene-Miocene Iversen Basalt (23.8 Ma), the lower Miocene Skooner Gulch and Gallaway Formations, and the lower to mid-Miocene Point Arena Formation. Lithologic and thickness data, together with evidence of age and paleobathymetry from both onshore and offshore sequences in the Point Arena basin, were used in a geohistory analysis of basin development. The resulting geohistory diagram tracks depths of specific stratigraphic zones, variations in paleobathymetry, and patterns of subsidence and uplift during ate Paleogene through Neogene time. Geohistory analysis indicates that the late Paleogene margin was uplifted during approach of the Pacific-Farallon spreading ridge. Subsequently, a pulse of volcanism during latest Oligocene-Miocene signaled initial Neogene subsidence of the margin as marked by the Iversen Basalt. Subsidence likely involved both initial thermal subsidence as well as later transtensional deformation during the passage of the Mendocino triple junction and initiation of the San Andreas fault system. Rapid initial subsidence was accompanied by deposition of turbidites (Skooner Gulch and Gallaway Formations). The highly organic shales and petroliferous sands of the overlying Point Arena Formation indicate an abrupt cessation of turbidite deposition and a slower rate of basin subsidence during the middle Miocene. Episodes of warping from mid-Miocene through Holocene can be attributed to crustal flexing associated with wrench tectonism, with a major event bringing the Point Arena sequence above sea level during the late Pliocene-Pleistocene.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.