AN UNUSUAL SUCCESSION OF LATE-PLEISTOCENE, ICE AGE FLOOD RHYTHMITES, WHITE BLUFFS, WASHINGTON

Slackwater, megaflood rhythmites fill a two-mile-wide paleochannel exposed along the White Bluffs in south-central Washington. Most of the 120-m-tall bluffs are a steep erosional escarpment of clay-silt-dominated facies of the fluvial-lacustrine, Pliocene, Ringold Formation. An exception occurs near Locke Island, where either a paleo-tributary to the Columbia River or outburst floodwaters incised a 35-m-deep paleochannel into the Ringold Formation. The since-abandoned paleochannel depression served as a depocenter for the accumulation of 17 successive slackwater rhythmites, presumably representing as many cataclysmic flood events. The thickness and number of rhythmites increases toward the axis of the paleochannel. Since the 1970’s the rhythmites have been conveniently exposed along scarps of a massive, two-mile-wide landslide complex, which exactly conforms to the width of the paleochannel.

Rhythmite beds overlie sharp, erosional boundaries and are characterized by normally graded beds that transition upward from horizontally laminated, medium to coarse sand to climbing-ripple cross-laminated fine to very fine sand to coarse silt. The upper portions of many rhythmites consist of draped, wavy-laminated to massive fine silt that locally have been deformed by overlying coarse sediment. Rhythmite thicknesses (ave. = 2.6 m; max. = 6.3 m) are significantly greater than Ice Age flood rhythmites observed elsewhere, perhaps due the geometry of the embayment and higher sediment load for megafloods at this location.

Paleomagnetic, pedologic, and OSL-dating techniques indicate the rhythmites are of late-Pleistocene age. The normal-polarity succession is interrupted atop rhythmite #13 (from base) by a calcic paleosol cut and filled with four additional rhythmites. An OSL age on sediment just below the unconformity is 21 +/-2 cal ka, while the youngest of four rhythmites above the unconformity has an OSL age of 12 +/-2 cal ka. Disconformably overlying the rhythmites are many meters of Holocene dune sand and loess derived primarily from remobilized megaflood and Ringold Formation deposits. A maximum age from the base of the eolian unit, from OSL, is 3.3 +/-0.5 cal ka. Eolian deposits continue to aggrade atop the White Bluffs.