IMAGING THE MARGINS OF PLEISTOCENE LAKE DEPOSITS WITH HIGH-RESOLUTION SEISMIC REFLECTION IN THE EASTERN BASIN AND RANGE: PILOT VALLEY, UTAH (USA)

A vast area of the northeastern Great Basin of the western USA was inundated by a succession of Plio-Pleistocene lakes, including Lake Bonneville. Playa sediment deposition from these lakes onlapped onto pre-existing alluvial fans that blanketed the slope of adjacent mountain ranges to create prominent angular unconformities. Understanding these unconformities is useful for constraining interpretations for aquifer recharge and for local tectonics. The Pilot Valley playa, located just east of the Utah-Nevada border near Wendover, Utah, represents a remnant of these lakes. In order to investigate the interaction of lake sedimentation and alluvial fan development, high-resolution seismic profiles have been acquired in the area where alluvial fans meet playa sediments, basinward from the base of the bounding mountain ranges. The profiles reveal the stratigraphic relationships between Quaternary pluvial sediments as a shoreline depositional facies and the adjacent bounding fan deposits. On the western side of the basin, sub-horizontal playa sediments prograde over the inclined alluvial fan sediments, which were deposited along the relatively steep flanks of the Pilot Range. The boundary between the playa and fan sediments is marked by a prominent angular unconformity. Seismic images from the opposite (eastern) side of the basin (Silver Island and Crater Island Ranges) reveal a more heterogeneous structural and stratigraphic style, including down-to-the-basin normal faulting of shallow Paleozoic bedrock overlain by alluvial fan deposits, which are in turn on-lapped by a thin veneer of playa sediments. The new geophysical images, when integrated with available geologic data, aid in constraining how aquifers are locally recharged from an adjacent range and how aquifer units are vertically partitioned. The results also demonstrate the strong structural asymmetry of the range and playa system, consistent with a classic half-graben structure. Lastly, this study demonstrates the utility of the seismic method to provide high-resolution subsurface images in the geophysically challenging environment of alluvial fan-playa geology.