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Abstract: This study provides a detailed magnetostratigraphy of sediments composing the Cold Creek cataclysmic flood bar in the Pasco Basin, Washington. Our interpretation suggests onset of Missoula floods or similar events prior to 1.1 myr, later than previously suggested by Bjornstad et al. [Bjornstad, B.N., Fecht, K.R., Pluhar, C.J., 2001. Long history of pre-Wisconsin, Ice Age cataclysmic floods: evidence from southeastern Washington State. Journal of Geology 109 (6), 695–713]. Nonetheless these data suggest that Channeled Scabland features formed over a much longer timespan than commonly cited, that continental ice sheets of the early Pleistocene reached as far south as those of the late Pleistocene, and that similar physiography existed in eastern Washington and perhaps Montana to both generate and route Missoula-flood-like events. This study adds paleomagnetic polarity results from 213 new samples of silts and sands derived from nine new drill cores penetrating the Cold Creek cataclysmic flood bar to our previous database of 53 samples from four boreholes, resulting in a much more robust and detailed magnetostratigraphy. Rock magnetic studies on these sediments show pure magnetite to be the predominant remanence-carrying magnetic mineral, ruling out widespread remagnetization by secondary mineralization. The magnetostratigraphy at eastern Cold Creek bar is characterized by a normal polarity interval bracketed by reversed polarities. Equating the normal zone with the Jaramillo subchron (0.99–1.07 myr) affords the simplest correlation to the magnetic polarity timescale. Western Cold Creek bar was likely deposited during the Brunhes chron (0–0.78 myr) since it exhibits mainly normal polarities with only two thin reversed-polarity horizons that we interpret as magnetic excursions during the Brunhes. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]