FILL AND DISSECTION WITHIN A VOLCANIC LANDSCAPE: THE SURFICIAL GEOLOGY OF REYNOLDS CREEK, IDAHO

Though the transient response to uplift and erosion have been extensively explored over the past few decades, few studies have focused on landscape response to constructional processes such as volcanic deposition. The Reynolds Creek Critical Zone Observatory (RC CZO), a 239 km² watershed in southwest Idaho, provides an environment to explore how multiple eruptive events shape the development of a watershed. Through the use of LiDAR, geochronology and field observation, we are creating a 1:35,000 surficial geologic map that captures the Miocene to present the evolution of the watershed. This surficial geologic map uses a 3 meter resolution LiDAR base map that will be published online via the Idaho geological survey website.

Most of the watershed is dominated by volcanic and granitic colluvium and their respective bedrock outcrops. The colluvium is disturbed in locations by Quaternary landslides that are responding to stream incision in Reynolds Creek and its tributaries. Strath terraces are created by incision of Reynolds Creek and its tributaries. These terraces are preserved 5 to 30 meters above the channel and the fill is characterized by volcanic cobble.

Lower Reynolds Creek is dominated by an intriguing lacustrine basin composed of diatomites, vitric tuff and fine clastic sediments. An arkosic alluvial sediment eroded from the granite bedrock is also present in the lower watershed and can be found lithified with cross-bedding or as coarse sediment. The lacustrine fill has since been dissected into a series of pediments topped with diverse gravels and cobbles sourced from the local bedrock units.

Geochronology will be used to further constrain timing of the lacustrine basin formation and its dissection. U/Pb dating of zircons within the vitric tuff will put this basin into context with nearby depocenters such as Poison Creek or the western Snake River Plain. Cosmogenic radionuclide dating of pediment gravels will help constrain when dissection begins. Optically stimulated luminescence dating of terraces will put the more recent incision of the basin into perspective with when Reynolds Creek established more direct connectivity to the Snake River.