REVISITING ANCIENT CATACLYSMIC FLOOD DEPOSITS AT THE DALLES, OR

The Missoula Floods occurred in the Pacific Northwest between fifteen and eighteen thousand calendar years ago. At least forty flood events are documented in the Columbia Gorge. The Ancient Cataclysmic Floods, occurring from two million to twenty thousand years ago (Allen et al., 2009), have received less attention. A series of paleosols were studied in a roadcut first described by Cordero (1997) on Highway 197, three kilometers southeast of The Dalles, Oregon. Cordero (1997) described five Ancient Cataclysmic Paleosols in the section. We sampled his lower two paleosols and concluded there are five paleosols, not two, each representing a flood event. The ten horizons had a measured thickness of 725 cm. Each paleosol had a K/Bk couplet. Soil textures ranged from sand to sandy loam. Laboratory analyses determined the presence of Stage I to Stage III pedogenic calcium carbonate (CaCO₃) development (1.9%-33.95%). The pH of the samples from 8.22 to 8.74. Particle size analysis done on the four Bk horizons revealed sand grains of such size that only a fluvial origin is possible. Five flood events are represented in our study, bringing the total for the section to eight. The lowest horizon, Dalles #10, is massive and blocky, though only a Stage I carbonate, concluding that it is both likely cemented with silica, and the ancient surface all flood deposits formed upon. Estimated dates of formation of each paleosol with Stage II and III CaCO₃ development could be between twenty to fifty thousand years (Cordero, 1997). Paleomagnetic dates are forthcoming for six of the ten horizons, this should confirm both the ages of each paleosol as well as improve the chronology for the Ancient Cataclysmic Floods.