Diverse floodplain deposits of reworked loess in a monsoon climate (Hanzhong Basin, central China)

Abstract

This paper describes an assemblage of diverse floodplain facies of reworked loess (facies b, c) in a Middle Pleistocene monsoonal setting of the Hanzhong Basin, central China. The vertical and lateral sedimentary sequences show changing energy conditions. Apart from the highest energy in the channel facies (facies a), a relatively high energy floodplain environment (facies b) prevailed in waterlogged conditions, with small, laterally migrating (sub)channels. Facies b generally interfingers with aggrading horizontal sheets of overbank deposits in alluvial pools and swamps in a floodplain with much lower energy (facies c), in which phases of stability (soil formation) occasionally interrupted overbank deposition. Reworked loess forms the main part of the floodplain deposits. The paleosols are considered to have been formed under low hydrodynamic conditions in an interglacial environment. These interglacial conditions follow the commonly assumed glacial conditions of channel facies a. The sedimentary successions in the floodplain show a recurrent composition and cyclicity between wet and dry floodplain sedimentation terminated by stability with soil formation. The cyclic rhythm of stacked high- and low-energy floodplain sediments is attributed to varied intensity of different hydrodynamic flooding events that may have been due to changing monsoonal rainfall or simple intrinsic fluvial behavior.

Prolonged Response of River Terrace Flooding to Climate Change

“…Although the upper coarsegrained deposits at the latter site represent an alluvial fan rather than a river channel, the overlying fine-grained deposits are similar. The third case represents a rich variety of floodplain deposits, superposed on channel gravels, which has been recently illustrated in detail from the middle Pleistocene terraces of the Hanjiang River, a major tributary of the Yangtze River, for instance, at Jinxing (Figure 1) [27,28] and neighbouring sites [29]. The final site at Silong is also from a Yellow River terrace, east of Lanzhou, described in detail by [30].…”

“…Typical floodplain deposits of the Hanjiang River are primarily composed of two facies [28]. The sedimentary structures in the relatively high-energy floodplain environment (facies b), e.g., trough crossbedding and occasionally finely laminated sands and silts, point to deposition in small, laterally migrating channels, bars, and sand sheets (Figure 4).…”

“…Figure 5. (A) Sedimentary succession of Section 2 at the Jinxing site (modified from[28]). (B) Grainsize distributions of selected samples (equally distributed between the two red stars in (A)).…”

Fluvial Responses to Late Quaternary Climate Change in a Humid and Semi-Humid Transitional Area: Insights from the Upper Huaihe River, Eastern China

“…Fluvial responses to climate change and other impact factors are one of the key issues in fluvial geomorphology [1][2][3][4][5][6][7][8][9]. Of these multiple impact factors, climate change has the capacity to control river behavior by directly and indirectly affecting river runoff and sediment supply [10][11][12][13].…”

Eurasian environmental dynamics and humans: interactions over different time scales

“…It should be noted that the sea surface temperature gradient, which is used as a proxy for the EAWM, had been partly affected by changes in the currents in marginal seas, which are linked with glacial sea level lowering (Huang et al, 2011). Vandenberghe et al (2021, this volume) describe a 20-m thick fluvial sediment sequence in the Hanzhong Basin, central China. It is composed of diverse sedimentary facies, from a gravelly, high energy channel (at the bottom of the sequence) to a relatively low energy floodplain, and was inferred to have developed in MIS 16–13.…”