REVISED AGE AND GEOLOGIC SIGNIFICANCE OF THE NORWOOD TUFF, MORGAN VALLEY, NORTHERN UTAH

The Norwood Tuff is an Eocene volcaniclastic package present in Ogden and Morgan valleys of northeastern Utah. The age of the Norwood Tuff was previously constrained using K-Ar and mammalian paleontology techniques, and was potentially correlated with the Keetley Volcanics to the south. However, previous ages are based on outdated techniques and are spatially and stratigraphically limited. In this project, we aim to constrain the age, extent, and thickness of the different Cenozoic volcanic-sedimentary units previously mapped as the Norwood Tuff. Preliminary U-Pb results from rocks previously mapped as Norwood Tuff yielded a 9 Ma crystallization age, indicating necessary reevaluation of the age of this unit in the area. We collected and analyzed a range of samples for zircon U-Pb geochronology within the type section in Norwood Canyon (n=8), and an equivalent package in Porterville to the west (n=11), covering ~8 km from west to east and at least four separate stratigraphic intervals. Zircon U-Pb ages were obtained using LA-ICP-MS at the Arizona Laserchron Center and a subset of additional grains were targeted for Hf isotopic and trace element analysis. We complement these analyses with detailed sedimentological descriptions of the units, thin section petrographic, and whole rock geochemical analysis. Preliminary results indicate that the unit previously mapped as the Norwood Tuff contains strata deposited during at least three separate volcanic intervals, between 42 and 40 Ma, 22 Ma, and ~9 Ma. Ongoing analyses include trace and rare earth elements in zircons to understand the source and petrology of the parent magmas. The sandstone petrography and detrital U-Pb data will be used to understand the provenance of the sedimentary deposits and elucidate patterns of subsidence and uplift, which, when combined with volcanic data, will allow for a reconstruction of the basin’s evolution over the past 50 Ma. These findings offer new perspectives on regional stratigraphy and the complex interplay of volcanic activity and basin development in northeastern Utah. This study compliments concurrent research being done to the north in Ogden Valley.