USING SPRING DEPOSITS TO TRACK THE EVOLUTION OF FLUID PATHWAYS THROUGH FAULTS: LITTLE GRAND WASH FAULT AND SALT WASH GRABEN, UTAH, USA

Fault zones commonly act as pathways for mineralising fluids, and the spatial and temporal variability of fault zone permeability will control the rate and volumes of mineralising fluid flow. Dating of travertine mounds associated with two normal faults in central Utah has constrained the varying position of fluid flow to the surface along the faults through time. Modern and ancient travertine occurs at multiple locations along two faults near Green River, Utah: the Little Grand Wash fault and Salt Wash graben. Previous studies have shown that all of the travertine occurs in the footwall of the fault zones, consistent with ponding of southward directed regional groundwater flow in a three-way anticlinal trap. Geochemical and structural analyses have shown that CO2-rich groundwater is stored in a series of shallow sandstone reservoirs capped by impermeable caprock, and that the CO2-rich groundwater moves to the surface through fractures in the footwall damage zone of the faults.

Uranium series dating of multiple travertine mounds in both study areas constrains the timing of initial spring activity to ~110,000 ka. Multiple dates from a single spring deposit show that an individual mound has a life span of at least 10,000 years. The variation in ages between mounds along both faults shows that pathways for CO2 rich waters to the surface switch repeatedly through time. However, at least one location has three travertines with distinct ages that range over 40,000 years, suggesting that a single pathway can be re-used many times. These data allow us to constrain the spatial and temporal evolution of fluid flow pathways in the faults, and to estimate the rates and volumes of flow in individual pathways. The changes in flow pattern may be caused by the sealing of fault pathways by carbonate precipitation, causing new flow paths to be established along the route of least resistance to the surface. Alternatively, changes in the stress on the faults or changes in the hydrology of the region could have prompted switching of flow pathways. The observation that leakage of CO2 rich groundwater from a fault can last for tens of thousands of years has implications for geological storage of CO2.