ANCIENT CATACLYSMIC FLOODS: ANCESTORS OF THE MISSOULA FLOODS IN THE PACIFIC NORTHWEST, USA

The Missoula Floods were catastrophic Ice Age floods that greatly shaped and fashioned the landscape of the Pacific Northwest through erosion and deposition of sediments from 15,000 – 18,000 calendar years ago. The idea of these floods was developed by J Harlen Bretz in the 1920’s and has been developing every since. Over 89 flood events have been recognized with 40 passing through Portland, Oregon on their way to the Pacific Ocean. The water originated from an ice dam breaking in the Pend Oreille Trench and liberating over 500 cubic miles of water held in Glacial Lake Missoula. The largest floods were the first ones, and they got succeedingly smaller as the ice sheet thinned at the ice dam.

Evidence has been found for older similar cataclysmic floods in the Pacific Northwest. In order to differentiate these from the earlier Missoula Floods, I would like to call them “Ancient Cataclysmic Floods”. These flood deposits are characterized by old soils within the sediments that many times have calcareous paleosols (Bk and K horizons). Sometimes the sediments have been laid down in quiet water environments and show reversed paleomagnetic orientation (giving them an age > 700,000 years BP). At least five paleosols have been found south of the Dalles, Oregon on Highway 97 by Cordero and Burns. A classic section has been described by Baker and Nummendal (1978) at Marengo, Washington, but they also mentioned similar ones at Revere, Macall, and Old Maid Coulee, Washington. Bruce Bjornstadt and others (1991) discovered old soils in flood deposits in the southwest Pasco Basin, Washington at Kiona Quarry, South Bombing Range Road, Leslie Road, and Oak Street. They also described reversed magnetic polarity sediments at Poplar Heights, Vernita Grade and Yakima Bluffs in the Pasco Basin, Washington. Gene Kiver and others (1991) also found similar flood deposits with old soils developed in them at Latah Creek, Malden Gravel Pit and Willow Creek, all near Spokane, Washington.