REVISITING THE NORWOOD TUFF: UNVEILING YELLOWSTONE HOTSPOT ASHES IN OGDEN VALLEY, NORTHERN UTAH

The Norwood Tuff is a sequence of ash-fall tuff, mudstone, and sandstone exposed in Ogden and Morgan valleys along the eastern side of the Wasatch Mountains in northern Utah. Previous studies dated nearby exposures of Norwood Tuff at around 38 Ma, and suggested its correlation with the Keetley Volcanics to the south. More recent unpublished zircon U-Pb ages from samples near Pineview Reservoir, in northern Ogden Valley, yielded Miocene ages, suggesting these rocks are instead part of a younger volcanic sequence. To determine the extent and thickness of the Miocene volcanic deposits, we targeted a relatively well-exposed transect in southern Ogden Valley, between Huntsville and Peterson, Utah, for new U-Pb geochronology and Hf isotopic and trace element geochemistry. Fifteen samples were collected covering an east-west distance of 5 km and 8 km north-south. These new data will help to constrain the age and geochemical signatures of the tuffs and correlate them to known Miocene eruptive events from the region (e.g., Yellowstone hotspot). We complement these analyses with whole rock geochemistry, thin section petrology, stratigraphic, and structural data to piece together the geological history of the Neogene sedimentary basin in Ogden Valley. Preliminary results show zircon ages as young as 8 Ma, which is coeval with the Idaho Falls eruptions of the Yellowstone Hotspot. The majority of igneous zircons between 8 and 10 Ma have epsilon Hf values between 0 and -11, with a few evolved outliers less than -11 epsilon, consistent with either reworking of evolved continental crust and/or enriched mantle material from the Yellowstone Hotspot Track. The Miocene strata are tilted, suggesting post–9 Ma deformation in Ogden Valley. These results demonstrate that currently mapped Eocene Norwood Tuff actually represents a broader range of depositional ages (Eocene–Miocene) in the Ogden Valley area. These findings will be used to refine the Neogene stratigraphy and structural evolution of the Ogden Valley area, as well as to understand possible links between hotspot-driven magmatism and the sedimentology of the basin.