CASE FOR >1 MILLION-YEAR-OLD, ICE-AGE MEGAFLOODS IN EASTERN WASHINGTON

A combination of stratigraphic, pedogenic, paleomagnetic and radiometric evidence suggests Ice Age megafloods in the Pacific Northwest began in the early Pleistocene going back a million years or more. Pre-Wisconsin, Ice-Age flood deposits are spread out across eastern Washington, often underlying one or more, Stage III-V calcic paleosols. Visible evidence for older pre-Wisconsin floods is scarce, however, due to subsequent burial and/or erosion.

At least six episodes for pre-Wisconsin outburst flooding have been identified based on flood-cut unconformities along with interbedded loess and flood deposits. Some older Palouse loess and flood deposits reveal a reversed magnetic polarity (i.e., >780 ka = >MIS 20). A detailed paleomagnetic study by Foley (1982) documented a strong and consistent reversed polarity on over 30 samples from the lower 2/3 of a thick loess sequence near Washtucna. Subsequent studies by McDonald and Busacca (1988), Baker et al. (1991), and Bjornstad et al. (2001) further documented reversely magnetized, interbedded loess and flood deposits of likely early-Pleistocene age.

The oldest Th/U radiometric age date on carbonate-coated basalt clasts in flood gravel is 800ka. Radiometric dates, along with a reversed magnetic polarity, corroborate an early Pleistocene age. Most telling for a maximum age is the Revere outcrop located in the heart of the Cheney-Palouse Scabland Tract. Here, flood gravel lies beneath a 5m-thick loess sequence that includes three, separate calcrete horizons. All three intervening loess layers between calcretes are of reversed magnetic polarity. If the uppermost reversed calcrete-loess bed is >780ka, then the underlying flood gravels must be >1Ma, considering the additional time required for the development of three separate calcrete layers. Furthermore, calcrete-capped flood deposits, deeply buried within Palouse loess suggest pre-Wisconsin megafloods were likely synchronous with early growth and development of the Palouse hills.