The Pliocene/Pleistocene Evolution of the Western Snake River Plain Near Grand View Idaho

The Western Snake River Plain provides us with a unique opportunity to study the evolution of basaltic volcanism. In addition to studying the horizontal variation at the surface of the plain, erosion by the Snake River also allows the study of the vertical variation. With the aid of a USGS EDMAP grant we mapped the Dorsey Butte quadrangle and southern half of the Little Joe Butte quadrangle at a scale of 1 :24,000. The quadrangles are located within both the Ada and Elmore counties of southwestern Idaho, approximately 30 km west of Mountain Home and 2 km north of Grand View. The two maps will extend westward the eight 7.5 minute quadrangles previously mapped by Shervais et al. (2002). The result of this work will be an area encompassing 530 square miles centered around Mountain Home that have been geologically mapped at a scale of 1 :24,000. The geology of this area consists predominantly of basaltic volcanism, lake sedimentation, and quaternary faulting. Lava flows, shield volcanoes, and cinder cones dominate the landscape with a concentration of phreatomagmatic deposits along the cliffs of the Snake River. An eight-meter stratigraphic column was generated from a phreatomagmatic outcrop within the Basaltic Tuff and Breccia of Hayland School. The base of the unit consists of subaerial scoria fall deposits. Above the scoria deposits sedimentary material is present within cross-bedded and massive layers that may be the product of channel flow, subaqueous reworking of material, and/or subaqueous debris flow. The final stages of the Hayland School eruption involved a de-watering of the system resulting in an increase in the proportion of basaltic clasts and finally a basaltic lava flow. The periods of hydration and de-watering of the volcanic system are likely the product of damming of the ancestral Snake River and/or growth of the volcanic edifice above water level. Geochemical data for the basalts present within and adjacent to the Dorsey Butte and Little Joe Butte quadrangles support the presence of two dominate magma types. The older basalts have compositions consistent with Fe-rich tholeiites while the younger basalts are transitional between olivine tholeiites and alkali olivine basalts. The younger basalts have higher alkali, Mg, and Cr concentrations and higher Mg/Fe ratios. These findings are consistent with previous work by Vetter and Shervais (1992) and White et al. (2002)