A STRUCTURAL-HYDROGEOLOGIC MODEL OF A FRESHWATER-BRINE SYSTEM FROM INTEGRATING HIGH-RESOLUTION SEISMIC REFLECTION PROFILES AND HYDROGEOLOGY 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. We have acquired three common-depth-point high resolution seismic reflection profiles at strategic locations at the playa-alluvial fan border (3-m group interval; 24-fold; accelerated weight dropper with a ~45-kg hammer P-wave source; 28-Hz phones). Each profile is over 1.5 km long. Two are near the base of the west-bounding Pilot Range and provide good images of (1) the playa sediments prograding over the alluvial fans, (2) the steep alluvial fan sediments themselves, and (3) the vertical 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 represent the expression of the dynamic fresh-saline water interaction. The third profile covers the east side of the playa and will help to understand the groundwater recharge conditions plus the structure of the basin. The seismic acquisition to date has demonstrated the achievability of applying the seismic method to an acoustically challenging alluvial fan-playa environment. These results are providing 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. The freshwater component is being recharged from the alluvial fan, which is in turn being buried by playa sediments.