T32 Geological Mapping: Key to Successful Management of Water and Land Resources (Posters)

Sunday, 2 November 2003
8:00 AM - 12:00 PM

MAGNETOSTRATIGRAPHIC AND GEOLOGIC MAPPING OF THE COLUMBIA RIVER BASALT AQUIFER IN THE NORTHERN WILLAMETTE VALLEY

Paper number: T32-70
Program: Technical Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

In the northern Willamette Valley, Oregon, ground water development in the Miocene Columbia River Basalt (CRB) poses problems for resource management: storage is limited and the resource can become depleted from over-drafting. In cooperation with DOGAMI and USGS WRD, we have begun a multi-faceted mapping investigation of the CRB to better understand its internal stratigraphy and structure and their role in controlling aquifer recharge and water quality. The investigation includes 3-dimensional geologic mapping from surface exposures and bore holes, paleomagnetism, geochemical analyses, and aeromagnetic surveys. Paleomagnetic data, in combination with chemical analyses, have proven a useful tool for flow by flow mapping of the CRB. Over 1000 oriented samples (130 sites) for paleomagnetism have been collected in quarries, outcrops and the Beaverton ASR-3 bore hole (302 m). The CRB section in the northern Willamette Valley largely consists of the reversed-polarity (R) Wapshilla Ridge and Grouse Creek members, and the normal-polarity (N) Ortley, Winter Water and Sentinel Bluffs members of the Grande Ronde Basalt. Overlying the Grande Ronde are transitional and N-polarity flows of the Frenchman Springs member of the Wanapum Basalt. In the ASR-3 core four polarity intervals are found (R1-N2), and individual flows have been identified from their signature inclinations and chemistries. Inclinations are ~-70° for a Teepee Butte flow (R1), ~50° for an as yet unidentified flow (N1), ~-50° to ~-70° for Wapshilla Ridge flows (R2), ~-80° for Grouse Creek flows (R2), ~70° for Ortley flows (N2), ~20° for Winter Water flows (N2), and ~50° for Sentinal Bluffs flows (N2). Other samples from flows like the Basalt of Ginkgo (at surface exposures) have readily identifiable transitional directions (I=20°, D=135°). As an example, Wapshilla Ridge flows have been shown to be a productive aquifer on Parrett Mountain, and recent paleomagnetic study has confirmed the extent of this flow unit and its relationship to pronounced aeromagnetic lows traceable over a wide area. A 3-dimensional understanding of CRB geology in the northern Willamette basin appears possible and can be used to identify potential targets in the exploration for additional water resources.