Floodplain aggradation and pedogenesis in a semiarid environment

Alluvial valley fills in the upper Republican River basin, southwestern Nebraska, provide evidence for processes of aggradation and pedogenesis on semiarid cut-and-fill floodplains. Sheetflow deposition on unchanneled valley floors represents an important mechanism of valley aggradation. Sheetflow deposits contain parallel to sub-parallel laminae and discontinuous lobate sedimentary structures. Rates of sediment deposition on cut-and-fill floodplains strongly determine the degree to which pedogenic features develop within aggrading alluvium. Three pedofacies common to semiarid alluvial deposits correspond with increasing aggradation rates: (1) cumulic soils; (2) multiple buried soils; and (3) no pedogenic features. Radiocarbon age determinations from alluvial fills indicate that floodplain aggradation greater than approximately 0.5 cm year−1 limits soil formation: this represents a threshold rate of pedogenic assimilation. Floodplain soils formed under aggradation rates lower than this exhibit a strong positive relationship between aggradation rate and total CaCO3 percent, and a negative relationship between aggradation rate and organic carbon percent. Recognizing the associations between pedogenic/sedimentologic features and floodplain aggradation rates can help interpret past rates of sediment transport, storage and deposition.