Flood basalts and ice age floods; repeated late Cenozoic cataclysms of southeastern Washington

Like nowhere else on Earth, repeated cataclysmic floods-first of molten lava, then of water from Ice Age floods-decimated southeastern Washington during the late Cenozoic. Beginning ca. 17 Ma, successive outpourings of Columbia River basalt spread for hundreds of kilometers from volcanic vents located in the southern and eastern Columbia Plateau. Up to 300 separate basalt flows have been identified, reaching cumulative thicknesses of 5 km in the Pasco Basin. With the close of basalt volcanism ca. 6 Ma, only a few million years elapsed before the Pacific Northwest succumbed to a new era of flooding. Outburst floods are associated with regular glacial cycles that have occurred periodically over the past 1-2 m.y. from one or more Pleistocene, ice-marginal lakes. During the last glacial cycle (15,000-20,000 calendar yr) alone, as many as 100 separate flood events, mostly from glacial Lake Missoula, are postulated. In the Channeled Scabland, after removing a blanket of loess, differential erosion through hundreds of meters of layered basalt with widely contrasting variations in fracture patterns and structure resulted in a unique assemblage of erosional landforms including multi-tiered cataract canyons, buttes, mesas, and rock basins. A number of depositional features, including huge flood bars blanketed with giant current ripples, as well as ice-rafted erratics and bergmounds, are also prevalent.