COLORADO RIVER CANYONS ARE CUT BY MID-LATE PLEISTOCENE TRANSIENT INCISION AND REFLECT LOCAL GEOLOGIC CONTROLS

In tectonically-quiescent landscapes, transmission of baselevel signals can be complicated by strong lithologic contrasts and by local geomorphic controls on topography. The Colorado River, traversing the tectonically-stable Canyonlands of the central Colorado Plateau, has extreme reach-scale contrasts in gradient and form, which present a world-class example of these local controls operating during transient incision. We map and date Colorado River terraces along a ~250 km stretch to investigate this Pleistocene incision history. Analysis of tributary profiles also reveals how baselevel change has propagated through variably resistant sedimentary strata.

Fluvial terraces are preserved from ~5 to 200 m above the river in the Glen Canyon and Meander Canyon reaches, downstream and upstream of the Cataract knickzone. Twenty-nine new luminescence and four new cosmogenic ages from these deposits range from ~50 to 750 ka. Results add to an emerging regional record marked by a ~1 Ma episode of quiescence and planation in the Middle Pleistocene followed by ~150 m of rapid, transient incision beginning ~300 ka and decelerating toward the modern. A goal that remains elusive is to measure the celerity, or difference in age of the onset, of this incision across the study area. Average incision rates since ~300 ka are ~0.5mm/yr in Meander Canyon but ~0.3mm/yr downstream in Glen Canyon, and this may reflect more youthful incision upstream or the effect of isostatic rebound. At a smaller scale, terraces in Meander Canyon record an upstream-propagating wave of rapid incision since ~100 ka.

The distribution and geometry of knickpoints in tributary catchments seem to mostly reflect strength differences in the layered stratigraphy and variable dip of the bedrock strata. Thus, the unsteady incision history documented by fluvial terraces in the region is obscured in the tributary profiles. The ultimate cause of a sharp increase in baselevel lowering just before ~300 ka on the Colorado River is an intriguing mystery that seemingly cannot be linked to climate or mantle-driven uplift. Our results indicate that local geology, lithologic controls, and unsteady, transient incision dominate a dynamic erosional evolution of the central Colorado Plateau.