PROVENANCE OF VOLCANIC AND VOLCANICLASTIC SUCCESSIONS IN THE CERRO TOLEDO FORMATION, PAJARITO PLATEAU, JEMEZ MOUNTAINS VOLCANIC FIELD, NEW MEXICO

Alternating sequences of primary volcanic and volcaniclastic units of the Cerro Toledo Formation preserved within Alamo Canyon, located on the Pajarito Plateau on the southeastern edge of the Jemez Mountains volcanic field have a different source than similar age deposits found in Pueblo Canyon located ~ 12 km to the north. These deposits document a dynamic interplay between eruptive activity and landscape development that took place between the two caldera forming eruptions of the Bandelier Tuff at 1.6 and 1.2 Ma. Field descriptions, geochemistry, and age dates are used to compare deposits in these two areas. Tephras from Alamo Canyon yield ⁴⁰Ar/³⁹Ar ages of 1.65 to1.56 Ma and appear to contain abundant xenocrystic sanidine derived from the underlying Otowi Member of the Bandelier Tuff. A tephra in the upper part of the Alamo Canyon exposures has a 1.42 ± 0.03 Ma ⁴⁰Ar/³⁹Ar sanidine age. Fluvial deposits in Alamo Canyon contain clasts of Paliza Canyon Formation andesite and dacite, Canovas Canyon and Bearhead rhyolite, and Rabbit Mountain obsidian. In contrast, the volcaniclastic deposits in Pueblo Canyon are dominated by sandstone composed of reworked tephras erupted from rhyolite domes in the Toledo embayment and lenses of conglomerate eroded from the Tschicoma Formation. The difference in provenance forms the basis for the recent update of the stratigraphic nomenclature for the Cerro Toledo Formation, separating the sedimentary deposits underlying the Pajarito Plateau into two members, the Alamo Canyon Member and the Pueblo Canyon Member. The primary ash-fall tephras, ignimbrites, and block-and-ash deposits are included in the Valle Toledo Member of the Cerro Toledo Formation. The tephras in the Alamo Canyon section generally have higher Zr, Th, Rb, and Cs values and lower Ce/Nd ratios compared to tephras in Pueblo Canyon, suggesting that the tephras in the two areas were derived from different eruptive centers.