Geomorphic and geochronologic study in and near Grand Canyon: Testing landscape responses to climate change and exploring the paleo-longitudinal profile of the Colorado River

Utah State University, Logan UT

Investigators

Abstract

The response of landscapes to climate change and the incision of elevated terrain are fundamental yet incompletely understood issues in geomorphology. The Grand Canyon landscape contains an unusually well-preserved and exposed record of both of these greater processes. Ongoing work by our larger research group indicates that strong landscape responses to Milankovich-scale climate changes have been superimposed on longer-term incision rates of ~150 m/my in eastern Grand Canyon, and that variable incision rates along the length of Grand Canyon can be related to Quaternary faulting. Our detailed mapping and initial dating of the Quaternary stratigraphy in eastern Grand Canyon has resulted in a new conceptual model for dryland responses to climate that includes complex linkages and out-of-phase relations between hillslopes, tributary streams, and the mainstem Colorado River during middle-late Pleistocene climate changes. A picture is emerging of regional incision that is relatively active and spatially variable, and that exposes a rich record of landscape responses to climate change. We propose research that combines optically-stimulated luminescence, in situ cosmogenic nuclide, and U-series dating with detailed field-geomorphic research on the spectacular hillslope and alluvial stratigraphy at three key locations at the head, within, and at the terminous of Grand Canyon. Our fieldwork and geochronology in eastern Grand Canyon has led to two research emphases and related hypotheses we propose to address. First is local catchment responses to climate. Our working conceptual model of landscape responses to climate in eastern Grand Canyon (included in a manuscript recently submitted to GSA Bulletin) is more specific and detailed than previous models. It highlights the unique responses of drylands and is pertinent to the changes predicted with global warming. We propose to test and refine this model by investigating the predicted time-transgressive aggradation and incision moving down and up tributary streams, which we hypothesize is responsible for the unexpected antiphasing of tributary and mainstem Colorado River aggradation and incision. Our working hypothesis.that such antiphasing is caused on one hand by a lag time associated with the production of local sediment supply and the passing of that sedimentary signal down the catchment, and on the other hand by a delay in the progression of the signal of base-level fall up the catchment due to locally great sediment supply.is testable via detailed sedimentologic, stratigraphic, and especially geochronologic study. The second research emphasis is the bedrock incision of the Colorado River along its regional profile. We propose to add long-term incision rate determinations at two new study sites at key points along the length of the Colorado River to complement our existing data in eastern and western Grand Canyon. Comparison of the alluvial stratigraphy, deposit composition, and incision rates along this 450-km transect of the Colorado are basic to recording the river.s long-profile evolution and testing for hypothesized knickzones or waves of incision passing through the fluvial system in response to neotectonic forcing at the active Hurricane-Toroweap fault zone or much older drainage capture. The spectacular preservation and exposure of the complete hillslope-tributary-mainstem Quaternary stratigraphy at the three study sites, in combination with the use of three complimentary geochronologic methods, will elucidate the complex linkages between the local landscape and a continental-scale river and their responses to climatic and tectonic perturbations. In addition, this research will provide an enlightening comparison of the three dating methods, and will help complete the emerging understanding of landscape evolution of this famous area. Our involvement of researchers and students from diverse institutions, training of beginning scientists, and framework for communicating results to the public via the National Park Service insure that the proposed research will have broad impact.