HIDDEN STRUCTURES IN THE SUBSURFACE: UNDERSTANDING THE GEOLOGY, GEOMORPHOLOGY, AND HOT SPRING GEOCHEMISTRIES OF A COMPLEX THERMAL AREA

An individual thermal area of Yellowstone can host a complex array of hot springs, ranging from pink acidic mudpots to tomato soup colored circumneutral springs to deep blue alkaline pools. How do such divergent geochemistries exist in thermal regions no larger than a few square km? Geologic structures in the subsurface may provide an explanation— studies indicate that structures such as faults and fluid upwellings control hot spring distribution. Though the geology of Yellowstone National Park has been well mapped, Yellowstone’s thermal areas are often mapped simply as “hot spring deposits”. A combination of acid erosion and silica sinter deposition resurface the thermal area constantly, making traditional bedrock mapping difficult. Detailed fault traces mapped across rhyolite domes typically terminate under thermal areas. In this study, we map the geology of the Rabbit Creek Thermal Area in Midway Geyser Basin to understand the processes and structures that influence hot spring chemistry and thereby affect the energy supplies available to hot spring microbial life. Subtle geomorphic indicators on high-resolution LiDAR topographic maps, detailed field mapping, and almost 15 years of hot spring geochemical data are combined to interpret the subsurface and hypothesize about the evolution of the Rabbit Creek Thermal Area.

Faults and fractures from the Mallard Lake Resurgent Dome dissect the region, giving hydrothermal fluids surface access. Pit craters, bands of acid erosion, and hot spring alignments hint at where concealed faults may lie beneath sinter. Observations of Lava Creek Tuff outcrops combined with Y-5 borehole data imply significant offsets in the subsurface, with up to 35-40m displacements along N-S faults since the Pinedale Glaciation retreat at ~10ka. Additionally, layers of sinter may cap a shallow confined aquifer, allowing lateral transport of fluid to point outlets throughout the hydrothermal creek-precipitated sinter plain. Phase separation of an acidic vapor phase from an alkaline fluid phase may explain why massive alkaline pools and small acidic mudpots coexist only a meter apart in places. By unraveling the complex geology of the Rabbit Creek Thermal Area, we reveal the many different paths hydrothermal fluids take to get to the surface of the Yellowstone hydrothermal system.