GEOLOGY OF THE CARSON AND MT DEFIANCE 7.5’ QUADRANGLES, COLUMBIA RIVER GORGE, OREGON AND WASHINGTON

The Carson and Mt. Defiance quadrangles lie astride the Columbia River in the central part of the Columbia River Gorge between the towns of Cascade Locks and Hood River, Oregon. Here the river has cut through the broad arch of the Cascade magmatic arc, exposing the uplifted 3 m.y.-old Bridal Veil Channel of the Columbia River, now at ~900 m elevation in the south wall of the Gorge. Uplifted hyaloclastites derived from low-potassium tholeiite (LKT) flows that entered the channel overlie more than a kilometer of Columbia River Basalt Group flows, which have filled earlier paleochannels of the river and its tributaries. At least 8 members of the Grande Ronde Basalt and its magnetozones R1, N1, R2, and N2 can be mapped through the Gorge, along with Wanapum Basalt flows and the Pomona Member of the Saddle Mountains Basalt. The CRBG section is tilted southeast by WSW-trending Yakima style folds and thrust faults, which apparently control the location of the river. The upper part of the CRBG is repeated by the northwest-verging Wygant thrust on the Oregon side. At Dog Mountain and Augspurger Mountain in Washington, more than 1 km of Grande Ronde Basalt overlies tuffaceous sediment and dips south towards the river. The area is intensely fractured and deformed by abundant faulting, sackungen, and mega-landslides of the Cascade landslide complex. Cascade basalt, basaltic andesite, and andesite shields with an aggregate thickness of ~600m have been constructed unconformably on top of the tilted CRBG in Oregon. Flows from Mt. Defiance (~520 ka) and Viento Ridge (1.1 Ma), now forming inverted topography, flowed north down ancestral tributaries into the Pleistocene gorge and are currently about 330 m above present river level. NNW-oriented faults forming the western margin of the Hood River graben offset the LKT flows and are Holocene active, with the timing of the latest earthquake similar to the nearby 1447 CE 15-km² Bonneville landslide that dammed the Columbia River. Some of the graben-bounding faults in OR project toward the damage zones on Dog Mountain but are poorly developed on the Washington side, where instead, far traveled LKT flows as young as 9 ka have flowed south to the river. The northward younging of LKT flows and graben bounding faults may represent the tip of a previously hypothesized northward propagating rift in the arc.