2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 44
Presentation Time: 9:00 AM-6:00 PM


PHILLIPS, William M., Idaho Geological Survey, University of Idaho, 875 Perimeter Dr MS 3014, Moscow, ID 83844-3014 and WELHAN, John, Idaho Geological Survey, Idaho State University, MS 8072, Pocatello, ID 83209-8072, phillips@uidaho.edu

The Idaho Geological Survey is conducting geologic mapping along the Snake River in the Blackfoot-Idaho Falls-Rexburg area of the Eastern Snake River Plain (ESRP). Widespread loess deposits in the ESRP obscure many geological features, making conventional geologic mapping challenging. We use a freely available digital database of >26,000 water wells maintained by the Idaho Department of Water Resources to supplement surface observations. Key data consist of well location, scanned images of well logs, well depth, static water level, and well yield. Data quality is uneven, with relatively frequent mistakes in well locations and/or inadequate logs. The well records could be improved by equipping drillers with GPS receivers and providing rudimentary geological training. Well depth is seldom >150 m and in some locations averages <30 m. Except for the few wells with GPS coordinates, location precision is no better than 4 hectares, making it impossible to precisely identify well elevations. However, our study area is among the flattest in Idaho, and elevation uncertainties are mostly <3 m. Well logs seldom provide geological details with only the most prominent lithologic changes generally recorded. Reliably identified lithologies are basalt lava, cinders and scoria, loess, gravel and sand, clay, and pumice. Despite these limitations, the well records greatly improve geologic mapping in a region of poor surface exposure provided that well density is high and that careful error checking is conducted. The well logs are useful for tracing and correlating units such as lava flows, lake deposits, outwash gravels, and rhyolitic tuffs, and for estimating fault location and displacement. These units reflect the Miocene-Holocene evolution of the Snake River drainage and correspond to events such as damming or diversion of the river by lava flows and glaciation of the headwaters region. We use Geostatistical Analyst and other GIS tools within ArcMap to construct isopachs and cross-sections with the well record data.