--> Abstract: Provenance Evolution of the Late Cretaceous - Paleogene Gualala Basin: Geochemical, Isotopic, and U/Pb Zircon Age Evidence from Conglomerate Clasts, by R. C. Schott and C. M. Johnson; #90935 (1998).

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Abstract: Provenance Evolution of the Late Cretaceous - Paleogene Gualala Basin: Geochemical, Isotopic, and U/Pb Zircon Age Evidence from Conglomerate Clasts

SCHOTT, RONALD C.; and C. M. JOHNSON, Dept. of Geology & Geophysics, Univ. of Wisconsin-Madison, Madison, WI 53706

Provenance constraints for Late Cretaceous-Paleogene sediments of the Gualala basin have important implications for both paleogeographic and paleotectonic reconstructions of the western margin of North America. Palinspastic reconstruction of Neogene strike-slip offsets restores the Gualala basin to a pre-Neogene (depositional?) position outboard of, and adjacent to portions of the Mesozoic Cordilleran batholithic belt currently exposed in the northern Salinian Block, southernmost Sierra Nevada, and western Mojave Desert. In such a position, the basin may have derived sediments from the adjacent (now dissected) Sierran- SalinianMojave magmatic arc or from colliding or passing exotic terranes. In contrast, paleomagnetic data from within the basin have been interpreted to suggest the Gualala basin is itself an exotic terrane.

Conglomerate clasts in the Upper Cretaceous section at Gualala suggest derivation from distinct, but juxtaposed source terranes. Granitic clasts yield U/Pb zircon ages that are dominantly mid-Cretaceous and have "continental" isotope signatures (87Sr/86Srinitial>0.706). Gabbroic clasts have "oceanic" isotope signatures (87Sr/86Srinitial 0.705) and Jurassic ages. The "continental" isotope signatures of the granites effectively rule out an exotic location for the Gualala basin by the initiation of Late Cretaceous sedimentation.

Eocene conglomerate clasts are dominantly tonalitic in composition, have mid-Cretaceous ages, and are isotopically similar to western batholithic rocks (87Sr/86Srinitial =0.704 to 0.706). A small but distinctive group of leucocratic, garnet-bearing trondhjemites have chemical and isotopic singatures which distinguish them from all other class groups. These lithologies appear to have a distinct source compared to the Upper Cretaceous conglomerates.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California