CREATING A GEOLOGIC MODEL FOR UNDERSTANDING THE RECHARGE OF A FRESHWATER-BRINE GROUNDWATER SYSTEM WITH HIGH-RESOLUTION SEISMIC REFLECTION IN PILOT VALLEY, UTAH

The Pilot Valley playa represents an 8 to 16 km wide and ~50 km long remnant of ancient Lake Bonneville, located just east of the Utah-Nevada border near Wendover, within the eastern Basin and Range Province. The playa corresponds to the upper surface of a closed basin that is inferred to be a strongly asymmetric graben delimited by two mountain ranges with heights exceeding 2700 m above sea level. Previous research has indicated that Pilot Valley is unusual in that the attitude and chemical evolution of the freshwater-brine interface is controlled by the hydrodynamic pressure of the shallow brine; however, the structural and stratigraphic controls on this interface have remained unknown due to lack of subsurface information. Preliminary common-depth-point high resolution seismic reflection profiles (10-ft group intervals; 12-fold; hammer P-wave source; 28-Hz phones) acquired by us last year near the base of the west-bounding Pilot Range provide good images of (1) the playa sediments prograding over the alluvial fans, (2) the steep alluvial fan sediments themselves, and (3) the boundary zone between the base of the alluvial fans and the playa sediments. This boundary zone coincides with numerous fresh-to-saline water springs that represents the expression of the dynamic fresh-saline water interaction. The reflection profiles also reveal coherent images of the sediments beyond this limit beneath the playa. Our first round of seismic acquisition has demonstrated the good feasibility of applying the seismic method to an acoustically challenging alluvial fan-playa environment. These preliminary results provide the beginnings of a geologic framework for a hydrodynamic model in which basin brines are stratigraphically “guided” as a wedge that overlies the freshwater system that is being recharged from the alluvial fan, which is in turn being buried by playa sediments. Further seismic acquisition will provide a comprehensive structural and stratigraphic model for the entire basin system.