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

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.