PNNL

Abstract

Cataclysmic ice-age floods in the Pacific Northwest began as early as 1.5-2.5 Ma, based on 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->400 ka. The deposits with reversed polarity are correlated to early-Pleistocene (>780 ka) cataclysmic flood episodes. Whie exposures of pre-Wisconsin flood 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, as much as 100 m thick, deposits interpreted as Matuyama age indicate 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 flood 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 only slightly modified by up to hundreds of subsequent flood episodes.