GEOLOGY OF THE TANNER BUTTE AND WAHTUM LAKE 7.5’ QUADRANGLES, COLUMBIA RIVER GORGE, OREGON

Geologic and structural mapping of the Tanner Butte and Wahtum Lake quadrangles reveal a dynamic history of late Cenozoic tectonics and volcanism that has driven deformation, uplift, and incision of the OR Cascade Range. The map area straddles the crest of the Cascade Range along the OR side of the Columbia River Gorge, including Columbia tributaries (Eagle, Tanner, and Herman creeks) and parts of the Bull Run and West Fork Hood rivers. Fieldwork is challenged by high relief (~1300 m), steep terrain, dense rainforest vegetation, recent forest fires, and limited access to the Hatfield Wilderness (~50% of map). Ar/Ar geochronology, XRF geochemistry, and paleomagnetic data constrain the timing, composition, and polarity of volcanic units. Interpretation of lidar and aeromagnetic data identify and characterize key contacts and structures. The ~65 km-wide Cascade arch uplifted the ~3-Ma Bridal Veil paleochannel of the Columbia River ~800 m, exposed just north of the map. This arching exposes Early Miocene volcaniclastic deposits of the Eagle Creek Fm where Tanner and Eagle creeks meet the Columbia. These deposits are unconformably overlain by up to ~700 m of Early-Middle Miocene Columbia River Basalt Group (CRBG), including previously unmapped flows in Tanner Creek of the ~16-Ma Frenchman Springs Member of the Wanapum Basalt and the ~11-Ma intracanyon Pomona Member of the Saddle Mountains Basalt. Up to ~300 m of Late Miocene-early Pliocene(?), andesite-dacite volcaniclastic deposits locally overlie CRBG in the southern map area. 300-500 m of Plio-Pleistocene mafic to intermediate lavas of the Cascade arc cap uplands with an eruptive pulse near Wahtum Lake ca. 1.2 Ma. Elevations of ancestral Tanner Creek alluvium and interbedded lavas indicate that at least 50% of incision occurred by ~1.2 Ma. Cirques and glacial till occupy high elevations. ENE-WSW Yakima fold belt structures transect the map, including the Bull Run syncline and Eagle Creek thrust. The thrust shows evidence for 350-400 m of south-up offset in CRBG and a hanging wall anticline with forelimb dips of ~40° NNW. Late Miocene volcaniclastic deposits appear less deformed, which may record the transition from Miocene NNW-vergent folding and thrust faulting to Pliocene extension-dominated transtension on the NNW-striking Mount Hood fault zone ~1 km east of the map.