Sandstone and Conglomerate Populations from the Late Cretaceous-Eocene Gualala Basin, CA

The German Rancho and Gualala Formations are composed of Late Cretaceous – Eocene deepwater sandstone and conglomerate, which have been interpreted to represent inner, middle, and outer fan deposits from bathyal (>2000 m) depths. Understanding the provenance history of these sediments will provide unique insight into regional paleogeography and paleodrainage patterns. Preliminary age spectra from detrital zircons from four sandstone samples (sample sizes between n=100 to n=124) from the Gualala and German Rancho Formations in the basin demonstrate that they share a provenance history with previously studied, interbedded conglomerates. However, the sandstone samples also include additional populations not observed in the conglomerates. Peaks between 95 and 105 Ma are present in all four sandstones. These peaks, consistent with the climax of volcanism in the Sierran arc, are represented in conglomerate clasts from the Stewarts Point Member and from the Eocene German Rancho Formation, but not in the Anchor Bay Member. Both ~100 Ma conglomerate populations have inferred provenance from the Salinian block, and it is likely that the sandstones were derived from the same source. A 152 Ma peak in ages from the sandstone from the upper Anchor Bay Member is consistent with previously reported ages from gabbroic conglomerate clasts, but is represented by only four zircons in a subsequent sandstone from the Paleocene German Rancho Formation. Sandstone from the Eocene German Rancho Formation has three major populations of zircons with peaks at 98 Ma, 168 Ma, and 250 Ma. Only one dated clast in Eocene German Rancho conglomerates has an age close to the 250 Ma peak, and none have ages close to the 168 Ma peak. This result shows that apparent provenance of deep-water clastics can be strongly influenced by clast size, and therefore depositional history.