STRATIGRAPHY AND STRUCTURE OF THE COLUMBIA RIVER BASALT GROUP (CRBG) BENEATH THE WALLA WALLA BASIN (WWB) OF WASHINGTON AND OREGON - SOME INITIAL FINDINGS

The triangular shaped WWB encompasses ~1230 km² in SE WA and NE OR. The basin developed at the intersection of the NW-trending Olympic-Wallowa Lineament and the NE-trending Blue Mountains. Previous studies mapped the CRBG around the margins of the WWB, but not beneath it where the sediment fill can be >240 m-thick. Cuttings from a number of deep water wells in the WWB in the last 7 years have provided new information on both the subsurface CRBG stratigraphy and structural geology. The Saddle Mtns. Basalt (SMB) in the WWB is represented by, from oldest to youngest, Umatilla Member, Buford Member, Ice Harbor Member (IHM), and a previously unidentified unit. This new unit, informally called the Walla Walla member (WWm), consists of up to 3 flows and overlies the 8.5 Ma IHM in the SE portion of the WWB. A 70 to 90 m-thick sedimentary interbed is often present between the IHM and WWm. Compositionally WWm flows do not resemble other SMB members. The 3 WWm flows can be subdivided into 2 units, with the lower unit having lower TiO₂ (2.00 – 2.20 wt.%), P₂ O₅ (0.30 – 0.40 wt%), and Cr (8 – 21 ppm) and higher MgO (3.50 – 3.75 wt. %) than the upper unit (TiO₂ 2.35 – 2.45 wt.%; P₂ O₅ 0.57 wt%; Cr 25 – 40 ppm; MgO 3.20 – 3.40 wt. %). The Wanapum Basalt beneath the WWB is represented by Eckler Mtn. Member, Lookingglass member, and Frenchman Springs Member (FSM). All units of the FSM are found in the WWB. Few wells have been drilled into the Grande Ronde Basalt, but those that have encounter a 120 m-thick Sentinel Bluffs Member and at least 50 m-thick Winter Water Member. Mapping of the top of the CRBG shows that this basin is very asymmetric and its deepest parts are in the western third of the basin. The top of the CRBG is cut by a series of NW-trending faults, most of them can be traced into the overlying sediments. Data suggests that the WWB began forming by at least late FSM time (~15.3 Ma). The WWB continued to develop along these pre-existing structural trends.