ABSTRACT Miocene flood basalts of the Columbia River Basalt Group inundated eastern Washington, Oregon, and adjacent Idaho between 17 and 6 Ma. Some of the more voluminous flows followed the ancestral Columbia River across the Cascade arc, Puget-Willamette trough, and the Coast Range to the Pacific Ocean. We have used field mapping, chemistry, and paleomagnetic directions to trace individual flows and flow packages from the Columbia River Gorge westward into the Astoria Basin, where they form pillow palagonite complexes and mega-invasive bodies into older marine sedimentary rocks. Flows of the Grande Ronde, Wanapum, and Saddle Mountains Basalts all made it to the ocean; at least 33 flows are recognized in the western Columbia River Gorge, 50 in the Willamette Valley, 16 in the lower Columbia River Valley, and at least 12 on the Oregon side of the Astoria Basin. In the Astoria Basin, the basalt flows loaded and invaded the wet marine sediments, producing peperite breccias, soft sediment deformation, and complex invasive relations. Mega-invasive sills up to 500 m thick were emplaced into strata as old as Eocene, and invasive dikes up to 90 m thick can be traced continuously for 25 km near the basin margin. Mega-pillow complexes up to a kilometer thick are interpreted as the remains of lava deltas that prograded onto the shelf and a filled submarine canyon southeast of Astoria, possibly providing the hydraulic head for injection of invasive sills and dikes at depth.

Abstract Before the time of the Laramide orogeny, an active orogenic and magmatic system was more or less continuous along the continental margin of western North America and had long dominated the Cordileran geologic framework; Laramide events reflected a major break in that continuity and were unusual in several respects. Contractional Laramide orogenesis affected a very wide zone, with deformation and foreland uplift extending nearly to the middle of the continent. Laramide magmatism, too, although discontinuous along strike of the orogenic region, extended locally far eastward. These events may have occurred in response to rapid westward drift of the North American Plate and extreme flattening of the Farallon subduction zone (Dickinson and Snyder, 1978).