REEVALUATION OF THE THICK EOCENE-AGE COLUMBIA BASIN, SOUTH CENTRAL WASHINGTON – N OREGON, BASED ON A SYNTHESIS OF GEOLOGICAL AND GEOPHYSICAL DATA

The Columbia Embayment has a complex Late Cretaceous to Neogene history that we are studying with a synthesis of geological and geophysical data including newer data from the Paleogene WA Cascades and a regional evaluation of Paleogene history based on strike-slip fault reconstructions. The pre-Miocene basin is buried under 1-4 km of mid Miocene Columbia River basalt (CRB), but 6 deep oil and gas wells penetrated the pre-CRB strata. The well and geophysical data show a thick basin under the west-central Columbia basin with these layers top to bottom: thin post CRB sediments; 3-4.5 km CRB thickening into the basin; a 5-6 km pre-CRB central basin 60-80 km across (and elongate to N and NW) with a 3-4 km western margin and thinning to <1 km to the NE; lower crust of thinned Siletzia lithosphere and underplated Eocene basalt whose deep portion changed to eclogite during CRB loading. The pre-CRB basin was interpreted from wells to be Eocene strata projected from the Swauk-Roslyn and Chumstick basins exposed in the Cascades to the NW and N. We follow these past interpretations broadly with important updates from data since the deep wells were drilled in the 1980s to suggest this ancient Columbia basin history. (i) At ~85 – 60 Ma terranes move north away from the Klamath – Blue Mountains to form the initial Columbia Embayment in a California borderland like setting with large areas of thinned crust. (ii) 60 – 50 Ma the 4-5 km thick Swauk basin formed in the forearc position of the flat northern Farallon slab; a SE projection of the Swauk basin over thinned crust from the northward growing Embayment may underlie the Columbia basin. (iii) 50 – 45 Ma a southern pull-apart basin related to, but wider than, the 3-4 km thick (vertical dimension) Chumstick basin formed the upper part of the thickest core of the Columbia basin; geophysically defined basin edges correspond to southern projections of the Entiat and Leavenworth faults. (iv) 45 – 40 Ma a thinner Roslyn - Deadhorse Canyon Fms layer may comprise the uppermost part of the broader pre-CRB basin over the pull-apart basin and the older Swauk basin to the SW. (v) 17 – 16 Ma and younger CRB loads the Columbia basin and transforms the deep portion of the Eocene basalt to ecologite in the lower crust causing final basin subsidence and the thickest known CRB accumulates.