Abstract:

Detrital zircon U/Pb ages and Pb isotope compositions of detrital feldspars from the Late Cretaceous–Eocene Gualala Basin in northern California indicate that the basin received sediments from the southern Sierra Nevada, the Mojave block, and the Salinian block. Mesozoic detrital-zircon age components and Pb isotope compositions of detrital feldspars from the basin are similar to U/Pb zircon ages and Pb isotope compositions from previously studied coeval conglomerate clasts. The detrital zircons and feldspars, however, also include additional age and Pb isotope components that are unobserved or poorly represented in the conglomerate-based provenance record. These new data suggest a more complex provenance history than that inferred from previously studied conglomerate clasts, and strongly imply a connection to the Mojave Desert. Provenance evolution of the sediments in the basin indicates that significant contribution of sediment from the Mojave Desert began during the Paleocene, when it is inferred that the drainage network expanded eastward from its previous extent. The presence of Mojave-derived Late Permian zircons, as well as feldspars that have high 208Pb/204Pb ratios, in the Eocene-age German Rancho Formation indicates that the basin remained in the vicinity of the Mojave Desert until the Eocene. This result demonstrates that the Gualala Basin had undergone very little northward motion at that time, and consequently requires that no significant northward translation took place prior to ∼ 50 Ma. The results of this study highlight the value of additional provenance constraints combined with age distributions when identifying sediment source terranes.

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