THE COLUMBIA RIVER BASALT GROUP: A RETROSPECTIVE OF STRATIGRAPHIC FIELD STUDIES, LEADING TO CURRENT IDEAS ON FLOOD-BASALT GENESIS

The 16.7-5.5 Ma Columbia River Flood-Basalt Province (CRFBP) has long been viewed as one of the best exposed and most extensively studied flood basalt provinces in the world, a model for the study of other continental basalt fields worldwide. In this retrospective, we highlight several field studies, but note in particular the important contributions of Don Swanson, who helped to establish the way forward for future workers. Field studies on the CRFBP began in 1893, when USGS Director John Wesley Powell commissioned I.C. Russell to investigate the artesian resource potential of central Washington. Russell’s horseback reconnaissance study led to the recognition of a vast sequence of sheet flows that he called the Columbia lavas. In the first half of the 20^th century, work was largely local in scope, related to water resources and the construction of Federal Dams and irrigation projects. The recognition of regionally extensive flows led to early attempts at developing a regional stratigraphy in the 1950s and 1960s. In 1961, Aaron Waters produced the first regional map of the CRFBP. He introduced the term Columbia River Basalt Group (CRBG), which he subdivided into the older Picture Gorge Basalt and the younger Yakima Basalt. A more expansive stratigraphy evolved over the next two decades, mostly through regional mapping programs. During this time, flows and groups of flows were more accurately defined by their chemical and paleomagnetic characteristics, which helped to establish a fundamental stratigraphic nomenclature (Swanson et al., 1979, USGS Bull. 1457-G). Today, we recognize of seven formations, 42 formal and informal members, over 350 extensive sheet flows, and perhaps thousands of flow lobes. This evolving stratigraphy provides a number of stratigraphic constraints that genetic models must address, for example: (1) the recent addition of Steens Basalt as the oldest formation of the CRBG, thus extending the CRFBP into SE Oregon, (2) the rapid northward migration of successively younger units and their respective dikes during the main eruptive phase (16.7-15.6 Ma), and (3) the coeval initiation of flood basalt volcanism in SE Oregon, rhyolite volcanism at the western end of the Snake River Plain hotspot track, and the abrupt onset of Basin and Range crustal extension, all within a narrow age range of ~16.7 to ~16.5 Ma.