Abstract The Channeled Scabland of east-central Washington comprises a complex of anastomosing fluvial channels that were eroded by Pleistocene megaflooding into the basalt bedrock and overlying sediments of the Columbia Plateau and Columbia Basin regions of eastern Washington State, U.S.A. The cataclysmic flooding produced huge coulees (dry river courses), cataracts, streamlined loess hills, rock basins, butte-and-basin scabland, potholes, inner channels, broad gravel deposits, and immense gravel bars. Giant current ripples (fluvial dunes) developed in the coarse gravel bedload. In the 1920s, J Harlen Bretz established the cataclysmic flooding origin for the Channeled Scabland, and Joseph Thomas Pardee subsequently demonstrated that the megaflooding derived from the margins of the Cordilleran Ice Sheet, notably from ice-dammed glacial Lake Missoula, which had formed in western Montana and northern Idaho. More recent research, to be discussed on this field trip, has revealed the complexity of megaflooding and the details of its history. To understand the scabland one has to throw away textbook treatments of river work. —J. Hoover Mackin, as quoted in Bretz et al. (1956, p. 960)

Abstract The Middle Miocene Columbia River Basalt Group (CRBG) is the youngest and smallest continental flood basalt province on Earth, covering over 210,000 km 2 of Oregon, Washington, and Idaho and having a volume of 210,000 km 3 . A well-established regional stratigraphic framework built upon seven formations, and using physical and compositional characteristics of the flows, has allowed the areal extent and volume of the individual flows and groups of flows to be calculated and correlated with their respective dikes and vents. CRBG flows can be subdivided into either compound flows or sheet flows, and are marked by a set of well-defined physical features that originated during their emplacement and solidification. This field trip focuses on the Lewiston Basin, in southeastern Washington, western Idaho, and northeastern Oregon, which contains the Chief Joseph dike swarm, where classic features of both flows and dikes can be easily observed, as well as tectonic features typical of those found elsewhere in the flood basalt province.

We examined Grande Ronde Basalt lava flows from surface sections and boreholes throughout Washington, Oregon, and Idaho to determine chemical and physical properties that would allow the recognition and mapping of these flows on a regional scale. We estimate there are ~100 flows covering nearly 170,000 km 2 , with a total volume of ~150,400 km 3 , that were erupted over four polarity intervals (reverse 1, normal 1, reverse 2, and normal 2) in ~0.42 m.y. These flows are the largest known on Earth, with individual volumes ranging from ~100 km 3 to greater than 10,000 km 3 . Although all known Grande Ronde Basalt flows erupted in the eastern part of the Columbia River flood basalt province, the thickest and most complete sections (>3 km) occur in the central Columbia Basin. From the center of the basin, the number of flows decreases outward, resulting in a nearly complete stratigraphy in the interior and an abbreviated and variable stratigraphy along the margins. The areal extent of many flows suggests that the Chief Joseph dike swarm greatly expanded after Imnaha Basalt time, and now many dikes are buried beneath younger flows in the eastern part of the province. The Grande Ronde Basalt has a relatively uniform lithology with only a few distinctive flows. However, when compositions are combined with paleomagnetic polarity, lithology, and stratigraphic position, the Grande Ronde Basalt can be subdivided into at least 25 mappable units. Grande Ronde Basalt flows are siliceous, with typically SiO 2 >54 wt%, MgO contents ranging from ~2.5 to 6.5 wt%, and TiO 2 ranging from 1.6 to 2.8 wt%, with an enrichment in iron and incompatible elements relative to mid-ocean-ridge basalt. Although most Grande Ronde Basalt flows have homogeneous compositions, some are heterogeneous. Dikes that fed the heterogeneous flows show that the first composition erupted was not typical of the flow, but as the eruption progressed, the compositions gradually evolved to the bulk composition of flow. The average effusion rate was ~0.3 km 3 /yr, with basalt volume peaking during the R2 polarity with the eruption of the Wapshilla Ridge Member. Eruption and emplacement rates for the flows are controversial, but available data collected from the field suggest that many of the flows could have been emplaced in a few years to perhaps a decade.

The Columbia River system is one of the great river systems of North America, draining much of the Pacific Northwest, as well as parts of the western United States and British Columbia. The river system has had a long and complex history, slowly evolving over the past 17 m.y. The Columbia River and its tributaries have been shaped by flood basalt volcanism, Cascade volcanism, regional tectonism, and finally outburst floods from Glacial Lake Missoula. The most complex part of river development has been in the northern part, the Columbia Basin, where the Columbia River and its tributaries were controlled by a subsiding Columbia Basin with subtle anticlinal ridges and synclinal valleys superimposed on a flood basalt landscape. After negotiating this landscape, the course to the Pacific Ocean led through the Cascade Range via the Columbia Trans-Arc Lowland, an ancient crustal weakness zone that separates Washington and Oregon. The peak of flood basalt volcanism obliterated the river paths, but as flood basalt volcanism waned, the rivers were able to establish courses within the growing fold belt. As the folds grew larger, the major pathways of the rivers moved toward the center of the Columbia Basin where subsidence was greatest. The finishing touches to the river system, however, were added during the Pleistocene by the Missoula floods, which caused local repositioning of river channels.