GEOLOGIC MAPPING PROGRESS IN THE COLUMBIA GORGE, OREGON AND WASHINGTON

Compilation and ongoing mapping constrain the geologic history of the Columbia Gorge, where the Columbia River cuts the Cascade volcanic arc. Oldest exposed rocks are Oligocene to early Miocene volcaniclastic deposits, lava flows, and intrusions of the ancestral Cascade arc. Lavas of the Miocene Columbia River Basalt Group (CRBG) crossed the arc through a broad paleo-lowland whose northern margin lies within the western gorge. During and since CRBG emplacement, Yakima folds and northwest-striking dextral faults have shaped drainage networks. Late Miocene fluvial deposits of Blue Mountains provenance and volcaniclastic debris shed from andesite-dacite arc volcanism accumulated in The Dalles Basin in the eastern gorge. The ~5.2 Ma basalt of Tetherow Butte erupted from vents near Redmond, Oregon and followed an ancestral Deschutes River ~160 km into The Dalles Basin. The late Miocene and perhaps Pliocene Columbia River, marked by quartzite-bearing gravel, was north of the Columbia Hills, joining the present gorge near White Salmon.

Pliocene to early Pleistocene tholeiitic volcanism in Oregon coincides with initiation of the Hood River graben. Volcanism since 2 Ma has produced calc-alkaline basaltic andesite in the central and western gorge. Quaternary andesite shield volcanoes form Mt. Defiance and Larch Mtn. Since 1 Ma, tholeiitic basalt in the central gorge has erupted from vents mainly in Washington. Trachybasalt of the Simcoe volcanic field erupted in the far eastern gorge about 0.9 Ma.

Lava flows and hyaloclastite deposits trace former courses of the Columbia River and its tributaries and constrain timing and magnitude of post-Miocene uplift and incision. Distribution of Pliocene hyaloclastite indicates uplift as great as 900 m near the central gorge. Lava flows as old as 0.9 Ma and as young as ~9 ka that entered canyons of the Columbia River and its tributaries have bases within ~20 m of modern river levels.

Pleistocene to modern eolian deposits are widespread at both ends of the gorge. Late Pleistocene Missoula floods left deposits in valley bottoms and granitic erratics scattered as high as 345 m. Landslides and debris flow fans are common throughout the gorge, several of them historically active. Displaced Quaternary deposits suggest some faults in the gorge are still active.