EARLY AND MIDDLE PLEISTOCENE CATACLYSMIC FLOOD DEPOSITS AT THE DALLES, OR

Forty or more Missoula Floods occurred in the Pacific Northwest during 15-18 ka, leaving deposits in the Columbia Gorge and elsewhere. Evidence has been growing for earlier similar episodes in this region. This includes flood deposits capped by very-well-developed paleosols, middle Pleistocene U-series disequilibrium dates on some flood sediments, and reversed-paleomagnetic-polarity flood sediments indicating early Pleistocene age. We document additional evidence for Ancient Cataclysmic Floods at a site three kilometers southeast of The Dalles, OR along Highway 197, first described by Cordero (1997). The site consists of eight Ancient Cataclysmic Flood sediment packages each capped by a paleosol. The sand to sandy loam of the soils exhibit Stage I to Stage III pedogenic calcium carbonate (CaCO3) development (1.9%-33.95%) with K/Bks couplets and pH from 8.22 to 8.74.

Paleomagnetic analyses on two dozen samples from the site reveal stable primary remanence with normal polarity in the top 5 flood sediment/paleosol pairs, and reversed polarity in unit 8. Preliminary results for units 6 and 7 are equivocal. Since the third unit from the top contains tephra of the Dibekulewe volcanic ash (0.5 Ma), we interpret the paleomagnetic reversal occurring between units 8 and 5 to be the Matuyama-Brunhes boundary (0.78 Ma). Low temperature and stepwise thermal demagnetization experiments indicate that most samples contain single through multi-domain magnetite as well as goethite, with primary magnetization held in single domain magnetite. Characteristic remanence directions yield a mean that is indistinguishable from paleo-north. Results of these analyses support the proposition that the samples carry a primary remanence acquired at the time of deposition. Future work will improve certainty of the Matuyama-Brunhes location within the stratigraphic section.