Quantitative reconstruction of paleo-Colorado-River profiles to test river integration and uplift models

Abstract:

Spatial and temporal distribution of paleo Colorado River (CR) deposits form a primary dataset to better understand the evolution of a continental-scale river system and quantify regional uplift. We focus on the elevations of Bouse Formation, Bullhead Alluvium, and Chemehuevi Formation outcrops in the lower CR corridor taken from published maps and ongoing regional mapping efforts and the elevations of published and newly dated strath and fill terraces in Grand Canyon (GC). Our premise is that paleoprofile reconstruction can reveal overall incision magnitude, change in incision rates through time and space, fault dampened incision, and regional tilting due to differential epeirogenic uplift. Paleoprofile geometries were determined by projecting the elevation of dated CR deposits and bedrock straths onto a longitudinal valley profile using a semi-automated GIS routine. Base and top of deposits provide information about net bedrock incision and aggradation magnitude, respectively. In the lower CR corridor, the base of the ca. 4 Ma Bullhead Alluvium is subparallel to the modern CR, where not locally subsided, and projects to near sea level indicating little bedrock incision or surface uplift since 4 Ma. In GC paleoprofiles older than ca. 100 ka are above modern river level due to prolonged bedrock incision. Incision rates in western GC of ~100 m/Ma and 160 m/Ma in eastern GC necessitate ~400 m of differential block uplift across faults in the Lake Mead area and an additional ~240 m of epeirogenic mantle-driven surface uplift in eastern GC. Bedrock incision is locally dampened by fault-related folding associated with the Hurricane and Toroweap faults, but there is little regional uplift across these structures. Ongoing efforts focus on detailed geologic mapping and improved geochronology of defined paleoprofile segments to test depositional models for the Bouse, post-Bouse / pre-Bullhead uplift models, and further constrain differential uplift rates.