OCR Text |
Show trend. The above inferences from subsurface data and geomorphology support the concept of Repenning and others ( 1995, USGS Bull. 2105) that much of the Glenns Ferry Formation was deposited since the diversion of the Snake River drainage into Hells Canyon perhaps 3.5 million years ago, and that fluvial systems extended across the plain to the outlet sometime between 2.1 and 1.6 million years ago. E- W seismic line and resistivity logs across the western Snake River Plain W + 1- % o - 3 * . i 20 km El Paso Richardson \ VS# 1 V Chevron- Halboucv JNJ#! Meridian Citv Gilbert Deltas _ Hewlett- Packard ^• S^^£^* r ffj^ m^ ^ s r-;-' - if { ST?! v. No Data E - A fj J 2000' ^ as? * • V - 2000' - 4000 known sand m aquifers mostly mud | | exaggerated dip scale Figure 1. East - west cross section of the western Snake River plain showing configuration of strata interpreted from seismic reflections and location of sand aquifer systems identified by drilling and geophysical logs. Resistivity logs are shown for 4 wells. Excursions of the log trace to the right indicates sand aquifers, and low values to the left indicate clayey muds of low permeability. Vertical scale is greatly exaggerated, so that the inclination of the strata is incorrectly shown as being steep, whereas in reality the inclination from horizontal ( dip) is everywhere less than 6 degrees. |