Abstract

Eocene conglomerates from the German Rancho Formation, currently exposed in the Gualala basin of coastal northern California, are compositionally distinct from both underlying Upper Cretaceous conglomerates and coeval conglomerates elsewhere in the California Coast Ranges. Zircon U/Pb ages as well as isotopic (O, Sr, Nd, Pb) and chemical characteristics of German Rancho Formation conglomerates indicate an origin in the central and western portions of the Cretaceous Cordilleran batholiths. The majority of clasts in the Eocene section are ca. 100 Ma hornblende- and biotite-bearing tonalites and granodiorites, a common rock type in the Cordilleran batholiths of the southwestern United States. A subordinate, but distinctive, clast type is made up of garnet-bearing trondhjemites that have Early Cretaceous U/Pb zircon ages and isotopic compositions that are characteristic of the western (oceanic) margin of the Cordilleran batholiths. The occurrence of igneous almandine-rich garnet and epidote in the clasts may suggest crystallization at high pressure. Conclusive provenance ties for the Eocene conglomerates at Gualala are elusive because the most distinctive clasts (i.e., garnet-bearing trondhjemites) lack currently exposed source bodies. Temporal changes in conglomerate clast provenance imply early Paleogene tectonism in the vicinity of the basin. The possibility of large-scale Paleogene translation of the basin or an Eocene sediment source in a passing exotic terrane appears unlikely, but cannot be ruled out. A western continuation of the currently exposed Salinian magmatic arc is our preferred source for the Eocene clast assemblage.

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