A THREE-DIMENSIONAL GEOLOGIC MAP OF THE SAN JOAQUIN BASIN, CALIFORNIA

The San Joaquin Basin (SJB) of California has bordered a succession of active tectonic environments since Jurassic time, and is filled with up to 10 km of Cretaceous through Recent-aged sedimentary deposits. The basin is bounded on the east by the Sierran magmatic arc complex and on the west by the Central Coast Ranges and the San Andreas Fault. The White Wolf and Stockton faults bound the basin on the south and north, respectively. A three-dimensional geologic map of the SJB is being compiled in the USGS National Oil and Gas Assessment project for a quantitative evaluation of oil and gas resources.

The 3-D map spans 370 x 135 km, oriented along the basin axis, and extends to ~17 km depth. Mapped surfaces include the Mesozoic crystalline basement; Cretaceous, Paleocene, Eocene, and Oligocene strata; several Miocene-aged units (which form both petroleum source and reservoir rocks) and Pliocene deposits. Data for these rock units are derived from well logs and seismic reflection lines. The 3-D geologic map shows that Mesozoic basement crops out on the basin’s eastern margin in the Sierra Nevada, then plunges westward to more than 16 km depth approaching the basin’s western edge. Although rocks of Jurassic and lower Cretaceous age crop out in the bordering Coast Ranges, these are not found beneath the SJB itself. Instead, upper Cretaceous rocks lie conformably on the basement surface and extend over the northern 2/3rds of the basin. The Eocene Kreyenhagen Formation and Domengine Sandstone form laterally continuous surfaces over much of the basin. Pliocene and Miocene-aged rocks dominate the southern half of the basin area. Differences in rock distribution may reflect tectonism, erosional events, and/or shifting sediment depocenters within the SJB. A series of northwest-southeast trending anticlines on the western margin of the basin extend from the ground surface to Eocene-aged strata, but do not involve Cretaceous-aged rocks, constraining the timing, duration, and style of folding.

The 3-D geologic map of the SJB is the foundation for USGS petroleum systems modeling and resource assessment. This map may also be useful for modeling other geologic processes, such as seismic wave propagation and tectonic strain accumulation on basin-bounding faults.