LUMINESCENCE GEOCHRONOLOGY AND CORRELATIONS OF TERRACE DEPOSITS OF COLORADO RIVER TRIBUTARIES WITHIN THE GRAND CANYON AND GRAND WASH TROUGH, NORTHWESTERN ARIZONA

Terrace deposits within tributary drainages of the Grand Canyon and the Grand Wash Trough record incision rates and climatic influences on fluvial and hillslope processes. Previous age control for these deposits was limited, and some of our age control provides only minimum terrace ages through cosmogenic dating. In order to obtain depositional ages of the terrace deposits and thus have more direct dating of sedimentary-process changes, the Optically Stimulated Luminescence (OSL) technique was applied to three study sites in conjunction with U-series dating of travertine interbedded in the gravel fill at one of the sites. Samples were collected from tributary-steam terraces in the Furnace Flats reach of eastern Grand Canyon, the Travertine Grotto catchment in western Grand Canyon, and the northern Grand Wash Trough, immediately west of Grand Canyon. These sample sites extend over a 200-mile reach of the Colorado River corridor and provide potential correlation and inferences about tributary response to climate changes in this east-west transect.

Despite short transport distances, OSL results suggest that the samples are fairly well bleached and have produced ages of 50-34, 12-7 and 5-3.6 ka for the three lowest fill terraces (T3-T1) in eastern Grand Canyon tributaries, respectively. In western Grand Canyon, OSL and U-series ages were collected from a T3 deposit consisting of a basal unit of travertine-cemented fluvial gravels overlain by a sediment poor upper unit predominantly composed of travertine. OSL ages of 56-31 ka are consistent with U-series ages ranging from 54-47 ka in contemporaneously deposited travertine within the terrace gravels, supporting the reliability of both methods. These ages are similar to the T3 terrace fills in the eastern Grand Canyon tributaries. Within the Grand Wash Trough, a similar series of fill terraces are seen as in the Grand Canyon tributaries. However, these terraces have much thicker fills and bedrock straths are not seen, as is common in the Grand Canyon tributaries. OSL ages from these tributaries are still in progress. Overall, results suggest that tributary drainages along the east-west transect across Grand Canyon have similar terrace sequences and, pending final age analyses, appear to have similar timing of fluvial responses, consistent with a climate driver.