LANDSCAPE EVOLUTION IN A FLOOD-BASALT PROVINCE: AN EXAMPLE FROM THE PACIFIC NORTHWEST

Dynamic interplay between the Columbia River flood-basalt eruptions and regional tectonics produced a diverse landscape between the Rocky Mountains and Pacific Ocean that ultimately controlled the evolution of the Columbia River system. The Columbia River basalt (CRB) erupted from long, linear fissures in the eastern WA and OR, and flowed down a westward slope through center of the province and through a lowland gap in the Cascade Range with a number of flows reaching ocean. Peak CRB activity occurred between 16.5 to 15 Ma, but then declined and became episodic ending about 5 Ma. During this time eastern WA and northeastern OR were part of a broad, elongated NE-SW basin with the deepest portion west of the North American craton. Regional N-S compression folded and faulted each new CRB flow giving rise to the Yakima fold belt. CRB eruptions during peak activity were large enough to bury the developing structural topography leaving a nearly featureless plain after each eruption. Flows produced during the waning period of CRB activity were smaller and only locally buried the evolving landscape. Initially the huge CRB flows completely obliterated the drainage system and forced the Columbia River to the margin of the province. Continuing subsidence after each CRB eruption pulled the Columbia River back toward the center of the province. As the frequency of CRB activity waned, the greater hiatus between CRB eruptions allowed the drainages to become better established in portions of the province. As the volume of CRB erupted declined, continued subsidence within the Columbia Basin pulled the Columbia River progressively to the east and CRB flows filled drainage channels forcing the paleo-Snake (Salmon-Clearwater) River to constantly modify its course. At 6 to 5 Ma, the Columbia River essentially occupied its modern-day course through the Columbia Plateau with slight modifications from Pleistocene cataclysmic floods.