SEDIMENTATION BEYOND THE BIG LOST TROUGH AS AN ANALOG FOR SEDIMENTARY INTERBEDS IN THE VOLCANOSTRATIGRAPHY OF THE EAST SNAKE RIVER PLAIN

The alluvial, fluvial, lacustrine and eolian surface sediments of the Big Lost Trough have been studied and presented by several researchers as an analog for subsurface sediments in the volcanostratigraphy of the 1 to 2 km thick basalt flows on the East Snake River Plain (ESRP). These basalts host the Snake River Aquifer, the most productive basalt aquifer in the world whose recharge waters originate in the Northern Rockies surrounding the ESRP. Reconnaissance mapping of the distribution of sediments, estimated sediment thicknesses and identification of chronologically discongruous sediment infiltration into lava tubes and other collapse structures in areas adjacent to the Big Lost Trough, at the junction of the ESRP and neighboring Basin and Range valleys, along the length of the axial volcanic high from Shoshone to south of Terreton, and along the Snake River drainage from Idaho Falls to Wapi suggests a modified scenario of sedimentation as an analog for subsurface interbeds and intertrappean layers in the volcanostratigraphy of the ESRP - one which accounts for thin eolian veneers along the axial volcanic high and other vent-rich areas, the effect of fires on vegetation, eolian, playa and lacustrine environments and contrasts in sedimentation on Quaternary and Holocene basalts along the Snake River vs. the Big Lost Trough, Mud and Market Lakes and the Wood River Drainage. Sediment type and distribution in the subsurface has a direct impact on hydraulic conductivity in the basalt-hosted aquifer, where fine-grained lacustrine and eolian sediments act as significant aquitards in the heterogeneous flow patterns of the fractured basalts and discontinuous interbeds. Surface patterns of sedimentation beyond the Big Lost Trough help to explain certain flow patterns observed in the aquifer, including the so-called fast flow paths and low-transmissivity zones in and adjacent to the Big Lost Trough.