CHRONOLOGY OF MISSOULA FLOOD DEPOSITS AND THE DEMISE OF THE COYOTE CANYON MAMMOTH, BENTON COUNTY, WASHINGTON
The timing and relative flood stage of these temporary lakes remain poorly constrained. Here we report optically stimulated luminescence (OSL) ages for a sequence of at least seven flood beds found in a high (315 m asl) backwater portion of Lake Lewis, impacted by only the largest flood stages. The oldest flood deposits exposed at the Coyote Canyon Mammoth Site (CCMS) in Benton County, WA (46.1587°, -119.2648°) yielded an OSL age of 20.9 ± 2.6 ka, while the youngest yielded an OSL age of 16.8 ± 2.7 ka. OSL dates from a second section located at a similar elevation (~314 m asl) approximate 300 meters to the west-northwest (46.1592°, -119.2686°), yielded similar dates (17.4 ± 2.6 ka, 16.9 ± 3.2, and 16.3 ± 2.8 ka) for the Missoula flood deposits. OSL dating of loess deposits overlying the flood deposits at the mammoth site, yielded ages of 14.0 ± 2.3 ka and 10.9 ± 2.0 ka. Radiocarbon dating of the left humerus of a Columbian Mammoth (Mammuthus columbi) found within the middle of the sequence of flood deposits, yielded Accelerator Mass Spectrometry (AMS) conventional radiocarbon ages of about 14.4 ± 0.05 ka BP and 14.2 ± 0.06 ka BP with a pooled calibrated age of death of about 17.5 ± 0.2 ka cal BP.
These data suggest that at least seven Missoula floods occurring between 20.9 ka and 16.3 ka created a high-stand of Lake Lewis that exceeded 315 m asl and impacted the Coyote Canyon Mammoth Site. The relationship of flood rhythmites with the concentration of mammoth bones indicates that a flood at 17.5 ka led to the demise of the Coyote Canyon Mammoth.