CONGLOMERATES OF THE LATE CRETACEOUS-PALEOGENE GUALALA BASIN, CALIFORNIA: CHANGING PROVENANCE AND IMPLICATIONS FOR CORDILLERAN PALEOGEOGRAPHY

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.

Currently located at the northern end of the displaced Salinian block of western California, conglomerates of the Gualala basin reflect Late Cretaceous to Eocene turbidite deposition at the western edge of North America. Palinspastic reconstruction of Neogene strike-slip fault offsets restores the basin to a depositional location adjacent to the southernmost Sierra Nevada.

Gabbroic cobbles with Late Jurassic U/Pb zircon ages (140-165Ma), and “oceanic” isotope compositions (87Sr/86Srinitial = 0.7026 to 0.7048, eNd(t) = +5.5 to +9.2) are abundant in Upper Cretaceous outcrops (30-60% of all cobbles) but sparse in the Eocene section. The remainder of cobbles in the Upper Cretaceous section are rhyolites, granites, and granodiorites, and have more “continental” isotope compositions (87Sr/86Srinitial = 0.7061 to 0.7090, eNd(t) = -6.6 to +1.3, 206Pb/204Pb = 18.91 to 19.76); U/Pb ages range from 93-120 Ma. We infer that Late Cretaceous deposition tapped distinctive oceanic (gabbroic) and central to eastern Cordilleran (granitic) terranes in subequal proportions. In contrast, Eocene conglomerates are dominated by tonalitic to granodioritic cobbles. U/Pb zircon ages of most cobbles are mid-Cretaceous (95-112 Ma), but isotope compositions of the most common cobble types are more restricted (87Sr/86Srinitial = 0.7056 to 0.7064, eNd(t) = -2.3 to +0.9, 206Pb/204Pb = 18.77 to 19.17). In general, Eocene cobbles suggest sediment derivation dominated by a western Cordilleran (tonalitic) source terrane.

A distinctive group of leucocratic, garnet-bearing tonalites and trondhjemites constitutes <5% of all Eocene cobbles, but have chemical and isotopic compositions which distinguish them from all other cobble groups. These cobbles have restricted 87Sr/86Srinitial (0.7034 to 0.7044) and 206Pb/204Pb (18.58 to 18.72), but diverse U/Pb zircon ages (108-140 Ma) and eNd(t) (+0.9 to +7.2). Potential sources of garnet-bearing lithologies of these ages and compositions are extremely uncommon in the Cordilleran batholiths of western North America, and are only found in the western-most (largely buried) Sierra Nevada. The temporal changes in source terranes tapped by the Gualala basin are markedly different than the temporally-related Great Valley Sequence.