Uncovering Yellowstone's Hidden Eruptive History Through Core-Based Geochronology

Abstract

In the late 1960s, the U.S. Geological Survey drilled a series of cores throughout Yellowstone National Park to investigate the subsurface beneath major hydrothermal basins. These cores were logged prior to development of the modern geologic map of the Yellowstone Plateau Volcanic Field (YPVF) and before most modern studies of the geochronology, geochemistry, and petrology of Yellowstone rhyolites. Consequently, some volcanic units may be misidentified in the logs and sedimentary intervals remain poorly characterized. This study revisits the Y-2 core from the Lower Geyser Basin using detailed core logging, petrography, geochemistry, and geochronology to refine the volcanic and sedimentary stratigraphy.Results indicate that several original stratigraphic interpretations are inconsistent with the geochronologic and petrologic data presented in this study. New zircon 238U-230Th and sanidine 40Ar/39Ar geochronology provide the first direct age constraints for volcanic units within the Y-2 core. These results demonstrate that portions of the original core log require revision as radiometric ages are inconsistent with the original core logging interpretations, including identification of a previously unrecognized lava flow. Geochronologic, geochemical, and petrographic evidence further identifies a rhyolitic interval that does not correlate cleanly with currently mapped post-Lava Creek Tuff eruptive units, suggesting the presence of a previously unrecognized rhyolitic unit within Lower Geyser Basin. Detailed investigation of sedimentary intervals identifies primary to near-primary ash deposits, glaciofluvial deposits, paleosols, and hydrothermal deposits that records the evolution of the Lower Geyser Basin from a proglacial lake, to glacial outwash from recession of Pinedale and Bull Lake glaciations, and development of the modern hydrothermal system. Legacy drill cores preserve volcanic and paleoenvironmental records not evident from surface exposures, and this study demonstrates the value of integrating modern analytical techniques with archived subsurface materials to refine the geologic history of Yellowstone.