PRELIMINARY GEOLOGIC MAP OF THE COLUMBIA RIVER GORGE NATIONAL SCENIC AREA, OREGON AND WASHINGTON

The Columbia River has carved a spectacular gorge where it crosses the Cascade volcanic arc, and much of the gorge is managed as the Columbia River Gorge National Scenic Area. Compilation of recent and ongoing mapping constrains the geologic history of the gorge. The oldest exposed rocks are Oligocene to early Miocene volcaniclastic deposits, lava flows, and intrusions of the ancestral Cascade volcanic arc. Voluminous lavas of the Miocene Columbia River Basalt Group (CRBG) crossed the arc by way of a broad paleo-lowland, the northern margin of which corresponds with the western gorge. During and since CRBG emplacement, Yakima folds and northwest striking strike-slip faults have influenced the distribution of lava flows. Late Miocene rivers draining the Blue Mountains left deposits in The Dalles Basin in the eastern gorge. Late Miocene arc volcanism south of the map area produced volcaniclastic rocks that accumulated in The Dalles Basin, where they are as thick as 300 m. The ~5.4 Ma basalt of Fulton Ridge followed an ancestral Deschutes River into the eastern gorge. Quartzite-bearing gravel north and west of the Columbia Hills records a late Miocene Columbia River course that entered the gorge near White Salmon.

A pulse of Pliocene low-K tholeiitic volcanism roughly coincides with initiation of the Hood River Graben. Volcanism continued into the Quaternary with dominantly mafic eruptions in the Simcoe, Boring, and Indian Heaven volcanic fields, Hood River graben, and elsewhere in the arc. Lava flows and hyaloclastite deposits record courses of the Columbia River and its tributaries and constrain timing 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 entered canyons of the Columbia River and its tributaries and have bases within ~20 m of modern river levels.

Pleistocene to modern eolian deposits, mostly derived from Columbia River sand and silt, are widespread at both ends of the gorge. Latest Pleistocene Missoula floods left deposits in valley bottoms and likely influenced the distribution of landslides. Landslides and debris flow fans are extensive throughout the gorge and have been historically active. Displaced Quaternary deposits suggest some faults in the gorge are still active.