Eolian Landforms and Deposits of the Eastern Snake River Plain, Idaho

The Eastern Snake River Plain (ESRP) is a northeast-trending, 300 kilometer-long depression underlain by Cenozoic volcanic rocks (Fig. 1). Well-known for many examples of volcanic landforms including basalt lava flows, cinder and tuff cones, fissure vents, and shield volcanoes, the ESRP also contains substantial eolian deposits of loess (wind-deposited sandy silt) and sand dunes. These deposits are important to people because they form the parent materials for the rich soils upon which much of Idaho's agricultural economy is based, including the "famous potatoes" mentioned on Idaho automobile license plates.

Topography and Drainage of the Eastern Snake River Plain

Topography and drainage in the ESRP reflect interactions of a mantle plume (an upwelling of abnormally hot rock) with crust of the North American tectonic plate. Beginning about 16 million years ago (Ma) and continuing to the present, the plate has moved progressively over the plume, causing uplift and rhyolitic caldera eruptions followed by subsidence and basaltic volcanism. As a result, the ESRP slopes to the southwest, away from the present location of the plume beneath the Yellowstone Plateau. The ESRP is divided into north and south segments by a cluster of large shield volcanoes, lava flows and rhyolitic domes. This topographic feature is called the axial volcanic high. Drainage in the northern Big Lost River-Mud Lake segment is internal, i.e. it does not have an outlet. Rivers that flow into this area sink into highly porous basalt lavas or form lakes. South of the axial volcanic high, the Snake River flows down the slope of the plain. The topography of the ESRP is favorably oriented to funnel the strong westerly winds typical of mid-latitude North America. This produces dominantly northeast-directed winds in the ESRP.

Pleistocene Glaciations and Eolian Deposits

Pleistocene glacial climates greatly influenced the development of ESRP eolian features. Although never glaciated, large amounts of sand and silt were brought onto the plain by meltwaters from glaciers in surrounding highlands. These deposits were reworked and transported by the strong winds characteristic of glacial periods. During the Pleistocene, a large ice sheet formed several times on the Yellowstone Plateau (Fig. 2). Two periods of glaciation have been recognized and dated at Yellowstone: the Pinedale glaciation between 14,000-25,000 years ago (14-25 ka); and the Bull Lake glaciation between about 140-150 ka. An intermediate glaciation between about 60-75 ka probably occurred but moraines are poorly preserved. Small alpine glaciers also formed on the higher mountains northwest of the plain during these glaciations.

Sources of Eolian Sediment

During glacial periods the Snake River was the principal meltwater channel of the Yellowstone ice sheet. The river was transformed from a narrow, largely single-channel meandering stream into a huge braided stream with a floodplain 10 to 30 kilometers wide. Deposits of the braided stream are dominated by gravels but considerable sand and silt are also present. Surface deposits are of Pinedale age. Older deposits are probably present in the subsurface. Today, the outwash deposits are an important source of gravel and sand, and also host aquifers tapped for regional drinking water supplies. A shallow series of lakes, of which Lake Terreton was the largest, formed in areas of internal drainage during glacial periods. These lakes filled with fine-grained sediments that were reworked into eolian deposits when the lakes dried up. Throughout the region, alluvial fan sedimentation increased during these times, also bringing fine-grained sediment onto the plain. At 17.4 ka, Lake Bonneville, the largest lake in the region, catastrophically drained onto the ESRP. Large amounts of sand and silt were deposited in the American Falls area by this event.

Loess

Loess is widespread across the ESRP, reflecting multiple sources of fine-grained sediment. South of the axial volcanic high, the major source was outwash deposits along the Snake River. North of the axial volcanic high, alluvial fans and outwash from drainages with alpine glaciers were likely the most important loess sources. During Pleistocene glaciations, the Snake River probably operated like modern high latitude rivers with high discharge and flooding during spring-early summer meltout, and low discharge during winters when the glacial outwash system was largely frozen. Silt and clay on vegetation-free braid plains were exposed during winter to strong drying winds and mobilized as loess. Surface deposits of ESRP loess are almost entirely of Pinedale age (Fig. 3). Stable areas preserve older loess deposits in the subsurface dating from Bull Lake and intermediate glacial times. Paleosols (ancient buried soils) cap the older loess. Dating with the optically stimulated luminescence (OSL) technique indicates that Pinedale loess near Idaho Falls ranges between about 16-25 ka, and accumulated at a rate of about 0.6 m/ka (meters per thousand years). Loess accumulation diminished greatly at about 14 ka with retreat of the Yellowstone ice sheet and reduction of outwash stream and alluvial fan discharge. OSL dating of fill-cut terrace sediments at Idaho Falls shows that the Snake River became incised at Idaho Falls between 14.4 - 12.6 ka, probably as a result of diminishing discharge. Stream incision greatly reduced the area of vegetation-free outwash deposits subject to deflation of fine sediments (Fig. 4). Loess covers all Pleistocene lava flow surfaces but is thin on 17.4 ka Bonneville Flood deposits and almost absent on a 6 ka lava flow.

Sand Dunes

Sand dunes are widespread over most of the ESRP. Transport of sand to form dunes largely occurred after cessation of loess deposition and continues to the present day. The largest dunes are in the northern end of the plain near St Anthony where sand from desiccated lake deposits is trapped by a large Pliocene-Pleistocene volcanic edifice (Fig. 5). Bonneville Flood deposits form the sand source for the field of parabolic and hairpin dunes that extends over 110 kilometers from near American Falls to near Idaho Falls (Fig. 4). OSL dating of ESRP dunes yields mostly Holocene ages that suggest regional droughts controlled periods of dune destabilization and movement. Today, severe dust and sand storms occur several times a year in the ESRP. These events occur at areas recently burned by wildfires and over plowed fields.

• Forman, S. L., J. Pierson, 2003, Formation of linear and parabolic dunes on the eastern Snake River plain, Idaho in the nineteenth century: Geomorphology, vol. 56, no. 1-2, pp.189-200.
  
• Forman, S.L., R.P. Smith, W.R. Hackett, J.A. Tullis, and P.A. McDaniel, 1993, Timing of late Quaternary glaciations in the western United States based on the age of loess on the eastern Snake River Plain, Idaho: Quaternary Research, v. 40, p. 30-37.
• Gianniny, G. L; G.D.Thackray, D.S. Kaufman, S.L. Forman, M.J. Sherbondy, D. Findeisen, 2002, Late Quaternary highstands in the Mud Lake and Big Lost Trough subbasins of Lake Terreton, Idaho: Special Paper--Geological Society of America, vol.353, pp.77-90.
• Pierce, K.L., M.A. Fosberg, W.E. Scott, G.C. Lewis, and S.M. Colman, 1982, Loess deposits of southeastern Idaho: Age and correlation of the upper two loess units, in Bill Bonnichsen and R. M. Breckenridge, eds., Cenozoic Geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26, p. 717-725.
• Pearce, H. R. and T. M. Rittenour, 2009, A record of drought and dune activation on the Snake River Plain in southeastern Idaho--results from testing a hypothesis regarding the potential Bonneville-flood source of the dune sand: Geological Society of America Abstracts with Programs, Vol. 41, No. 6, p. 48.
• Phillips, W. M., T.M. Rittenour, G. Hoffmann, 2009, OSL chronology of late Pleistocene glacial outwash and loess deposits near Idaho Falls, Idaho: Abstracts with Programs--Geological Society of America, vol. 41, no. 6, pp.12.
• Scott, W.E., 1982, Surficial geologic map of the eastern Snake River Plain and adjacent areas, 111° to 115° W., Idaho and Wyoming: U.S. Geological Survey Miscellaneous Investigation Series Map I-1372, scale 1:250,000.
• Tsukamoto, S., G. A. T. Duller, A.S. Murray, J-H. Choi, 2009, Introduction to the special issue on application of luminescence: Geomorphology, vol. 109, no. 1-2, pp.1.

• http://www.idahogeology.org/ [Idaho Geological Survey website; source for bedrock and surficial geologic maps]