THE INFLUENCE OF FAULT ZONE ARCHITECTURE ON REGIONAL FLUID FLOW IN A CO2-RICH SYSTEM: THE LITTLE GRAND WASH FAULT ZONE, UTAH

ABSTRACT

We examined the structure and hydrochemistry of the Little Grand Wash Fault in Green River, Utah, to better understand the effect of this clay-rich fault on the regional groundwater system. This normal fault cuts Jurassic sandstones, siltstones, and shales. A clay-rich gouge generated by this fault should form a barrier to the flow of fluids. However, CO2-charged springs and a geyser, which erupts from an exploration drill-hole that cuts across the fault, are localized along the fault. Mapping of the fault in the immediate area of the geyser shows 290 m of throw with a damage zone approximately 200 m wide. A network of fractures with calcite/aragonite mineralization is localized along the fault trace. Most are horizontal, though some vertical fractures cross cut the horizontal ones. Analysis of the veins matches the present day composition of the water showing that there is a long history of fault-associated fluid flow in this area. Preliminary stable isotope and chemical analyses of water samples collected from the geyser and near-by springs are supersaturated with respect to calcite and aragonite. These samples suggest that the source of the water is greater than one kilometer deep. Ultimately, this study will help us better understand the relationship between fluid flow, fault architecture, mineralization, and host rock permeabilities.