DO LATE CRETACEOUS-PALEOGENE SEDIMENTS OF THE GUALALA BASIN (CALIF.) RECORD THE COLLISION AND/OR PASSAGE OF OUTBOARD TERRANES?

SCHOTT, Ronald C., and JOHNSON, C.M., Dept. of Geology & Geophysics, Univ. of Wisconsin-Madison, 1215 W. Dayton St., Madison, WI 53706, rschott@geology.wisc.edu.

Provenance constraints for Late Cretaceous-Paleogene sediments of the Gualala basin, currently located in coastal, northern California, 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-Salinian-Mojave magmatic arc or, alternately, from colliding or passing exotic terranes. Maxson & Tikoff (Geology, 1996) have suggested that the “Baja BC” terrane may have shed detritus into the Gualala basin during the Late Cretaceous or Earliest Paleogene as it translated northward after colliding with North America at the latitude of present-day Baja California.

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 (90-105Ma) and have “continental” isotope signatures (87Sr/86Srinitial >0.706). Gabbroic clasts have “oceanic” isotope signatures (87Sr/86Srinitial = 0.703 to 0.705, eNd(t) = +6 to +9) and Jurassic ages (140-165Ma). In the southernmost Sierra Nevada “tail” mid-Cretaceous granitic rocks with 87Sr/86Srinitial > 0.706 are closely juxtaposed with “oceanic” gabbroic rocks of Jurassic age across the Pastoria fault. The “continental” isotope signatures of the granites, as well as the dearth of pre-Cretaceous granitoid clasts cast doubt on the possibility of a “Baja BC” provenance. 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). Although these lithologies appear to have a distinct source compared to the Upper Cretaceous conglomerates, and may be a better match to “Baja BC” type source rocks, their depositional age (~45Ma) effectively precludes this possibility. Moreover, similar rocks are common in the western Sierra Nevada.

We conclude that the conglomerate clasts of the Gualala basin are consistent with a provenance in the palinspastically restored Sierran-Salinian-Mojave magmatic arc, and neither require nor support the “Baja BC” hypothesis. However, it is noteworthy that the changes of provenance with time appear to be at odds with the temporally and spatially related Great Valley Sequence.