Comparative study of paleoflood slackwater deposits in the Yarlung Zangbo and Hanjiang Rivers, China

Abstract

Paleoflood slackwater deposits have recently been investigated in small to moderately sized rivers. However, our understanding of the characteristics of paleoflood slackwater deposits in large rivers remains limited, which poses a notable obstacle to accurately predicting the magnitude, frequency and force of extraordinary floods. To address this research gap, this study compares the characteristics of paleoflood slackwater deposits in the middle Yarlung Zangbo River (YZR) and the upper Hanjiang River in China. Paleoflood slackwater deposits and aeolian deposits (e.g. loess, paleosol S₀ and aeolian sand) were investigated and sampled from these locations. For the paleoflood slackwater deposits and aeolian deposits, sedimentology (e.g. grain size distribution, magnetic susceptibility and geochemical elements) was analyzed in the laboratory. The macroscopic features of the paleoflood slackwater deposits in the middle YZR valley are very similar to those of the upper Hanjiang River valley. The paleoflood slackwater deposits were dominated by sand and silt in the middle YZR and by silt and sand in the upper Hanjiang River; this implied that the paleoflood slackwater deposits were suspended sediments of floodwater from different source regions. Compared with the paleoflood slackwater deposits in the upper Hanjiang River and other large rivers in China (e.g. the Yellow and Yangtze Rivers), the paleoflood slackwater deposits contained more coarse particles in the middle YZR; this result may be closely related to the particular hydrogeologic and geomorphic conditions (e.g. rich sandy sediment, high riverbed slope and narrow valley) in the southeastern Tibetan Plateau. Compared with the paleoflood slackwater deposits in the upper Hanjiang River, the paleoflood slackwater deposits presented considerably worse sorting in the middle YZR; this may be closely related to the short transport distance in the middle YZR. The magnetic susceptibility values of the paleoflood slackwater deposits were relatively higher in the middle YZR and the upper Hanjiang River, suggesting that these slackwater deposits with minimal pedogenesis may contain a relatively high ferromagnetic mineral content because of the formation process of slackwater deposits. The sediment provenance of the paleoflood slackwater deposits may be closely related to that of the loess in the middle YZR valley, which implies that the sediment provenance of deposits of different genetic types may be greatly affected by the particular hydrologic–climatic and geomorphic conditions in the southeastern Tibetan Plateau.