SEDIMENTOLOGY, STRATIGRAPHY, GEOMORPHOLOGY AND PALEOCLIMATE IN MID-LATE PLEISTOCENE GHOST DUNE HOLLOWS, EASTERN SNAKE RIVER PLAIN, IDAHO

More than thirty, 5-10 m-deep, basalt-rimmed, barchan and barchanoid ridge-shaped topographic hollows, here termed ghost dunes, are distributed over ~40 km² of the arid to semi-arid, eastern Snake River Plain (ESRP) near Ashton, Idaho. Deposits in the ghost dunes provide a rare window into a mid-Late Pleistocene to Holocene proxy paleoclimate history that is largely obscured beneath widespread volcanic and sedimentary deposits of the ESRP. Initially postulated to have paleodune origins by U.S.G.S. geologists R.T. Holcomb and M.A. Kuntz in the 1970s, the sedimentology and stratigraphy in these unusual geomorphic features have been undocumented until now. We suggest the ghost dunes were once part of a more extensive active dune field akin to the modern St. Anthony dunes, a 20,000 km² concentration of SW-NE-oriented barchan, barchanoid-ridge and parabolic dunes <5 km to the west. Hand-auger data from three ghost dunes reveal >3 m of paleodune sand unconformably capped by ~2 m of finer-grained paludal and loessial deposits and local, basalt-clast rich colluvium. Paleodune sands are quartzo-feldspathic and basalt-lithic, massive to low- and high-angle cross-stratified, and contain inversely graded ripple cross-laminae. The upper meter of the paleodune deposits are phytoturbated and iron-stained indicating they were stabilized by vegetation following their partial deflationary removal. The thickest paleodune-sand deposits are located immediately upwind of columnar-jointed scarps in the Split Butte basaltic lava (ca. 62 ± 3 ka). Detrital zircon-provenance data suggest the mid-Late Pleistocene ghost dunes were derived primarily from the Yellowstone volcanic plateau whereas the Holocene St. Anthony dunes were derived primarily from Henry’s Fork of the Snake River alluvium and from Pleistocene Lake Terreton deposits. Air-photo interpretation and arcGIS analyses reveal that the ghost-dunes are oriented similarly to dunes in the St. Anthony dune field, suggesting comparable modern and paleowind patterns. Mottled orange-red colors and clay-cemented granular aggregates in the paludal, loessial and colluvial deposits that overlie paleodunes suggest at least two episodes of stabilization and soil development. Optically stimulated luminescence dating of ghost-dune deposits is ongoing.