REVEALING SUBSURFACE FLUID PATHS FOR THE HOT SPRINGS OF MIDWAY GEYSER BASIN IN YELLOWSTONE

Midway Geyser Basin in Yellowstone is home to the largest hot springs in the park, including Grand Prismatic, the third largest hot spring in the world. Among the massive blue alkaline springs are regions of pink acidic mudpots, rust-red circumneutral pools, green-orange acid marshes, and white cloudy springs. Midway Geyser Basin displays a massive diversity in hot spring geochemistry over a relatively small area. How do these hot springs coexist, and why do they appear where they do? Geologic structures in the subsurface like faults and impermeable rhyolites may control where fluid upwellings access the surface. Midway Geyser Basin sits between 3 rhyolite flows at the edge of the Mallard Lake Resurgent Dome; no mapped faults run through the region, but multiple large faults terminate under the thermal area, which is covered in up to tens of meters of young silica sinter. Traditional geologic and fault mapping is difficult in complex thermal regions, where volcanic bedrock and ice-cap glacial deposits are covered in sinter and altered to pinkish white clays by acidic fluids and gases. Mapping subtle to concealed faults and local stratigraphy is crucial in determining how hydrothermal fluid is reaching the hot springs. In this study, high-resolution LiDAR and field observations allow for the creation of a new geomorphic indicator index to analyze the subtle surficial features seen along fault paths in hot spring regions. Fine-scale field geologic mapping also reveals small outcrops of Lava Creek Tuff that when combined with data from the Y-5 borehole imply significant subsurface offsets up to 40m along previously unknown faults in the thermal area. Fifteen years of hot spring geochemical sampling was used to classify, map, and understand the fluid sourcing of the hot springs. Hot springs of various compositions appear along these faults, revealing how deep hydrothermal fluid and shallow meteoric fluid mix in the subsurface to produce the intense diversity in geochemistry seen in the area. Combining the geologic and geomorphic view of the region with the geochemistries of the hot springs gives a complete picture of how the complex subsurface plumbing of the Midway Geyser Basin operates, and how Yellowstone’s biggest hot spring and the region it is in may have formed and evolved.