DETERMINING THE ORIGIN OF A COLUMBIA RIVER BASALT INTERBED AND RESULTING IMPLICATIONS FOR LOCAL DEPOSITIONAL PROCESSES DURING THE MIOCENE

The Columbia River Basalts extend from the inland northwest all the way to the Pacific Ocean, and in multiple locations host sedimentary interbeds of various origins. These sedimentary deposits are grouped into the Ellensburg Formation, and record local depositional processes occurring between major lava flows. Ellensburg Formation interbeds have been locally identified as lake deposits or paleosols. A possible layered paleosol of the Ellensburg Formation crops out north of Dayton, WA. Based on the ages of the bracketing Grande Ronde N2 and Frenchman Springs basalt flows, we estimate deposition occurred between 15.9 and 15.6 Ma. In this study we determined the chemical composition of each paleosol horizon using bulk chemistry via pXRF. By comparing ratios of mobile and immobile elements we calculated pedogenic changes in mass balance. We compare this element profile to a known paleosol profile in the Blue Mountains and a lake deposit in the stratigraphically equivalent Latah Formation near Spokane, WA.