LOESS CANYON STRATIGRAPHY AND MORPHOLOGY IN THE UPPER REPUBLICAN RIVER WATERSHED, CENTRAL GREAT PLAINS

Holocene cycles of erosion in the loess-mantled landscape of the upper Republican River have created a spectacular system of box canyons resulting in what is regionally known as the Arikaree Breaks. These canyons provide an excellent setting in which to (1) reconstruct the history (events and chronology) of Breaks development, and (2) document the manner in which late-Pleistocene sediments and underlying Cretaceous bedrock have controlled canyon morphology. Previous work on Holocene cut-and-fill cycles in this area has documented extensive erosion in the middle Holocene (Altithermal) that ceased 5 ka to 4 ka. Radiocarbon and OSL dating have been used to study the onset of aggradation recorded in terrace remnants in lower parts of canyons entering the Republican River. Remnants of a terrace (T-2) sediment package, including sequences of buried soils, preserved throughout the Breaks, has yielded a basal soil dated 3.2 ka to 2.9 ka, suggesting widespread system stability. Other periods of soil formation, recorded in T-2 terraces throughout canyons, occurred ~ 1.8, 1.5, and 1.2 ka. Entrenchment of the T-2 likely occurred during the Medieval Warm Period, ~ 1 ka. Resultant T-1 terraces are characterized by minimal pedogenic alteration and are cut by gullies of presumed proto-historic or historic origin. In addition to establishing a timeline for Holocene cut-and-fill cycles by dating terrace fills in lower reaches of the Breaks, new radiocarbon and OSL ages from loess units and intercalated paleosols provide a more replete record of regional chronology for the area. Morphology of the canyons is controlled by these loess units and paleosols and by the underlying bedrock. This stratigraphic control is especially evident in upper reaches of the Breaks, which are subject to concentrated erosion and removal of remnant terraces. Canyon development and the stratigraphic controls on morphology are being assessed with terrestrial laser scanning (TLS). TLS scans of headwall and sidewall locations are being correlated with stratigraphy to derive a model of canyon evolution. In sum, this research is being conducted in an effort to better understand landscape response to climate change, internal system adjustments, and stratigraphic control on loess canyon development and morphologic expression.