IMPLICATIONS OF NEW QUATERNARY STRATIGRAPHIC RESEARCH AND AGE CONTROL IN EASTERN GRAND CANYON FOR LANDSCAPE EVOLUTION AND LAVA-DAM LAKES

The tributary drainages and Colorado River corridor of eastern Grand Canyon contain a suite of well-preserved and exposed Quaternary hillslope and stream deposits that hold clues about landscape evolution of the area. It has been proposed that lakes created by Pleistocene lava dams in the Lava Falls/Toroweap reach of the Colorado River in Grand Canyon are responsible for these deposits. However, the stratigraphy, morphology, and age of the fill deposits in eastern Grand Canyon are not consistent with deposition in the proposed lakes. Extensive field mapping and reconnaissance have not yielded evidence of the lacustrine sediments expected to be associated with the proposed lava-dam lakes, but this is understandable as the high incision rate in the area causes the preservation time of surficial features to be short. Results from recent and ongoing research indicate that tectonics and drainage changes are the longer-term mechanisms controlling incision of the landscape, but local changes in sediment production and delivery, in response to climate cycles, are controlling the shorter-term formation and morphology of fluvial terraces in eastern Grand Canyon.

Ongoing research focused on resolving the Quaternary landscape history for eastern Grand Canyon has resulted in some intriguing conclusions. Ages obtained with U-series, cosmogenic surface-exposure, and optically stimulated luminescence dating indicate the fill deposits in the tributaries of eastern Grand Canyon are younger than previously thought, but the fill deposits along the mainstem of the Colorado River are older than previously thought. Signifying that robust aggradation/degradation cycles have occurred relatively recently in the tributary drainages and mainstem incision rates are lower than previously determined by others. Lastly, the stratigraphy and morphology of the fill deposits indicate that during at least a portion of individual aggradational episodes, the Colorado River and one or more of the tributary streams were simultaneously aggrading.