Long History of Pre‐Wisconsin, Ice Age Cataclysmic Floods: Evidence from Southeastern Washington State

Abstract

Cataclysmic Ice Age floods in the Pacific Northwest began as early as 1.5–2.5 Ma, on the basis of an evaluation of surface exposures and recent borehole studies within southeastern Washington. Field evidence suggests at least two episodes of pre‐Wisconsin (i.e., >130 ka) glacial‐outburst flooding. A Middle Pleistocene flood is identified by normal magnetic polarity, calcrete‐capped deposits that yield maximum Th/U age dates from 200 to >400 ka. The deposits with reversed polarity are correlated to Early Pleistocene (>780 ka) floods. While exposures of pre‐Wisconsin deposits are limited because of erosion and/or burial, the record of earlier Pleistocene flooding is preserved within giant flood bars. These bars show incremental growth, representing a composite from cataclysmic floods deposited intermittently through the Pleistocene. In one giant flood bar, up to 100 m thick, deposits interpreted as Matuyama age indicate that the bar had grown to half its present height by 780 ka. Furthermore, Matuyama‐age, reversed‐polarity flood deposits may be underlain by up to another 15 m of normally magnetized deposits at the base of the flood sequence. This normal‐polarity interval appears to be associated with Early Pleistocene cataclysmic floods, perhaps of Olduvai age (>1.77 Ma). Many of the features associated with cataclysmic floods, such as coulees, giant bars, and streamlined loess hills, may have been established during the Early Pleistocene and were only slightly modified by up to hundreds of subsequent flood episodes.