High-resolution Holocene alluvial chronostratigraphy at archaeological sites in eastern Grand Canyon, Arizona

Understanding the nature of Colorado River deposits in Grand Canyon helps reveal how the river responds to changes in its Colorado Plateau tributaries and Rocky Mountain headwaters. This study focused on Holocene alluvial deposits associated with archaeological sites excavated near Ninemile Draw in Glen Canyon and at Tanner Bar in eastern Grand Canyon. Two previously-developed conceptual models of deposition were tested based on previous work. Previous researchers have suggested that Holocene alluvial deposits in Grand Canyon are a series of inset aggradational packages that correlate to valley fills and arroyo-cutting cycles in Colorado Plateau tributaries and are laterally consistent throughout the river corridor. An alternate hypothesis is that alluvial packages record paleoflood sequences along the Colorado River with no Holocene change in river grade. In this model, deposits are preserved more variably as a function of local hydrologic geometry, and they should be less correlatable.Detailed stratigraphic columns of terrace deposits and several stratigraphic panels of archaeological trenches, combined with facies interpretations, were used to reconstruct a high-resolution alluvial history at two locations. Optically stimulated luminescence (OSL) and radiocarbon dating methods were used at both locations with consistent results. At both sites, the sediment includes multiple depositional facies of mainstem and local-source material, and it consists of stratal packages bound by unconformities.These stratigraphic relations, combined with geochronology, lead to the interpretation that the alluvium is composed of six correlatable alluvial packages at overlapping heights above river level throughout the canyon. The four older packages include facies that imply aggradation throughout the river corridor, suggesting oscillations in river grade. The youngest two packages consist only of mainstem flood deposits. These packages suggest that preservation of deposits over the past &sim1 ky has not been driven by aggradation, although incision since &sim1 ky is possible. Comparison of the interpreted chronostratigraphy to climate records suggests that this large river's grade has not responded visibly to smaller century to millennial-scale climate oscillations. This work is the first to document that the alluvial record in Grand Canyon spans the entire Holocene, and conclusions support to both previous conceptual models of deposition.