Distinct periods of fan aggradation and incision for tributary valleys of different sizes along the Bailong River, eastern margin of the Tibetan Plateau

Abstract

Understanding the mechanisms of fan incision/aggradation provides key insights into the dynamics of fan evolution and hazardous fan-forming processes. This paper focuses on the discrepancy in fan evolution for two nearby valleys of different catchment areas along the Bailong River. Specifically, we study fan evolution in the small-sized CJB valley (watershed area being 1.1 km2) using sedimentary analyses and 14C dating. Sedimentary logging of seven exposed profiles indicates that mudflows and debris flows are the primary fan-forming processes. Seven samples were taken from paleosols developed in mudflow sediments, and the humin fraction was extracted for 14C dating. These ages constrain the fan aggradation period to between 10 and 4.9 cal kyr BP, and then the incision period occurred after 4.9 cal kyr BP. As the mudflow sediments may contain organic matter from hillslope legacies, the fan aggradation period may be later than the 14C ages defined in this study. In any case, the time of fan incision/aggradation in CJB is younger than that of the GLP valley (watershed area being 20 km2) where fan aggradation occurred in 21.7–7 ka and incision occurred afterward. The fan aggradation period defined by the 14C ages in CJB is consistent with an alluvial fan of similar thickness in the southeastern Tibetan Plateau and two other fans along the Bailong River. This consistency may suggest a plausible climatic control on fan evolution for small-sized tributary valleys, while the inconsistency with the larger GLP valley may suggest different climate-response regimes for tributary valleys of different sizes. More research on similar types of alluvial fans and cross-validation of different dating methods is needed.