USING ALLUVIAL TERRACE DEPOSITIONAL CHARACTERISTICS OF THE TEN MILE TERRACE NEAR BOISE, IDAHO TO UNDERSTAND PAST CHANGES IN CLIMATE AND DEPOSITIONAL HISTORY

This study examines the depositional history and paleoclimate of the Ten Mile River Terrace near Boise, Idaho. The Ten Mile Terrace (1.7 Ma) is the oldest of nine Boise River Terraces. The Ten Mile terrace covers ~ 83 km², with an overall length of ~ 19 km and a maximum width of ~5 km. It is located in the northwest corner of the Western Snake River Plain, between the Snake and Boise Rivers. We present initial observations and interpretations of analyses performed on a 30m deep x 240m long exposure of the Ten Mile Terrace. Prior work on the Ten Mile Terrace only focused on the top 2 meters. This study examines a 30-meter stratigraphic sequence of the Ten Mile Terrace to describe sequences of sand and gravel deposition, the paleoclimate and paleo-streamflow conditions of the terrace deposits and the possible sources of depositional material. High-resolution structure-from-motion drone imagery and samples collected along two transects indicate six major depositional units. This study investigates the timing and possible climate drivers of these depositional events. The 30-meter stratigraphic unit includes a range of clasts from silt, sand, and gravel, to pebbles and cobbles comprised of quartz, albite, anorthite and orthoclase feldspars, and muscovite. The gravel clasts are dominated by granitic rocks and porphyritic felsites from the Cretaceous Idaho Batholith Atlanta Lobe. Within these units, sediments fine upward. Methodology for this project includes: 1) a 4 meter contour map of the Boise River Valley, (2) hand textures of soils, (3) percent by weight soil particle distribution, (4) portable x-ray fluorescence (pXRF) and XRD (x-ray diffraction) geochemical analyses. The composition of the sediment clasts and absence of basaltic materials suggest the sediments are composed of recycled material sourced from the Atlanta Lobe of the Idaho Batholith in the Boise River Watershed. Further work may include 1) dating of the exposed deposit using OSL (Optically Stimulated Luminescence) and/or tephrochronology, 2) mapping of palo-channels and flow directions within the exposure, and 3) sampling and geochemical analyses of the alluvial fans located on the North side of the Boise River Valley to compare with the Ten Mile Terrace sediments.