Detrital Zircons and Sandstone Petrofacies of the Sespe Formation, Southern California

C. D. Spafford¹, R. V. Ingersoll¹, and M. Grove^2
1University of California, Los Angeles, CA, [email protected], [email protected]
²Stanford University, Stanford, CA, [email protected]

The upper Eocene-lower Miocene, nonmarine Sespe Formation represents the last stage of forearc sedimentation in southern California. Detrital-zircon analysis and sandstone petrology determine provenance of the Sespe, which constrains mid-Tertiary drainage patterns in southern California. The results of this study suggest that the lower Sespe received material from mid-Cretaceous Peninsular Ranges Batholith, and Late Cretaceous and Precambrian rocks in the Mojave and Sonoran deserts; the upper Sespe received material dominantly from the Peninsular Ranges and recycled local sources. The waning extra-regional signal through time is consistent with documented extension and uplift associated with the end of Laramide flat-slab subduction. Discriminant analysis of sandstone divides the Sespe into three geographically defined petrofacies: Santa Ana-Santa Monica-Simi (SA-SM-SI), Pine Mountain (PM), and Santa Ynez (SY); each petrofacies has distinct sandstone and zircon characteristics. Detrital zircons from the SA-SM-SI petrofacies form the proximal part of the reconstructed basin and have the greatest extra-regional signal; PM and SY petrofacies restore father west and south, respectively, and contain more local and recycled material. Pine Mountain is derived mostly from local recycled sources, but with some input from the Mojave Desert. SY has a significant local signal from both the Peninsular Ranges and the adjacent Franciscan Complex, but also received some Mojave detritus. All three petrofacies have a smaller extra-regional signal in the upper Sespe than the lower Sespe.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009