Fluvial terrace formation and its impacts on early human settlement in the Hanzhong basin, Qinling Mountains, central China

Abstract

The Qinling Mountains (QLM) form the climatic boundary between the temperate north and subtropical south of China. Many important Paleolithic archaeological sites located on fluvial terraces in this area have been reported in recent decades. Abundant artifacts have been excavated in silt layers overlying fluvial gravels and coarse sands. These silt layers have thus far been interpreted as aeolian deposits. However, in principle they could also represent (in part) fluvial (floodplain) deposits, especially near the base of fine-grained sequences. Reconstruction of fluvial terrace formation is crucial for the correct interpretation of the environment of hominin occupation. In this paper, two sediment sequences from two Paleolithic sites, located on different terrace levels of the Hanjiang River in the Hanzhong basin, are studied mainly using grain-size and grain-shape analyses. In addition, grain-size distributions have been unraveled by applying end-member modelling to distinguish different sedimentary environments. The results show that three different units can be discriminated in each section. The lower unit, consisting of gravelly sand mixed with fine silt, is interpreted as shallow-channel-fill sediment deposited during the start of the transition from a channel to a floodplain environment. The middle unit comprises a fine-grained, gradually fining-upward sequence, representative a floodplain environment. At its base, it reflects a high-energy floodplain situation; at its top, the sequence is interpreted as a low-energy floodplain environment with aeolian input (settling in static water). The third, uppermost unit consists of aeolian loess interbedded with paleosol(s) and sediments that are interpreted as the results of episodic surface runoff. The gradual transition between the 3 units and the gradual fining upward trend of the middle unit indicates that there is no considerable age gap (no hiatus) between the fluvialand aeolian sedimentary environments. Stone artifacts have been found in all 3 units, with difference abundance, indicating that both the aeolian and floodplain depositional environments provided favorable living conditions. For the floodplain environment, the resources of water and raw materials (fluvial gravels) for tool making may have offered fundamental resources for hominin settlement.

Fluvial or aeolian? Unravelling the origin of the silty clayey sediment cover of terraces in the Hanzhong Basin (Qinling Mountains, central China)

“…Combined with the large width of the surfaces, this implies an important contribution to the transport of sediment by surface runoff. This interpretation is in agreement with Yang et al (2019) as their results also indicate episodic deposition of coarse sediment in aeolian deposits on terrace levels T2 (11 km to the east of the Baoshan section) and T4 (in the west of the Hanzhong Basin). They suggest that this component is deposited by temporary overland flowing water or, alternatively, that it reflects the presence of pedogenic nodules that were not dissolved during the pre-treatment process.…”

“…Although the fluvial deposits at the base of the Baoshan section (profile D) show a similar shape-size trend; profile E is characterized by overall significantly lower aspect ratios. Similar findings are reported in Yang et al (2019), where it is shown that fluvial deposits have slightly lower aspect ratios with relatively large variations compared with the loess deposits.…”

“…The characteristics of the remaining 20.5 m of the Baoshan section are similar to other clayey silt-dominated sequences overlying fluvial terrace deposits in the Hanzhong, Ankang, Yunxian, Lushi-and Luonan basins. These deposits are thus far interpreted as aeolian deposits, comprising loess-palaeosol sequences (Lu et al, 2011a(Lu et al, , 2011bZhang et al, 2012;Sun et al, 2012Sun et al, , 2013Sun et al, , 2014Sun et al, , 2015Sun et al, , 2017bFang et al, 2017;Yang et al, 2019).…”

“…(B) The Hanzhong Basin is located in between the Qinling mountains and the Daba Mountains. The location of the Baoshan section and sites studied by Yang et al (2019) are indicated with black dots. 2011a, 2011b; Zhang et al, 2012;Sun et al, 2013Sun et al, , 2014Sun et al, , 2017bFang et al, 2017).…”

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

“…This region has been studied mainly for its aeolian loesspaleosol sequences interbedded with slack-water deposits covering river terraces. From bottom to top, the stratigraphic sequence is composed of fluvial gravel, alluvial sand interbeds, and ultimately loess and soils (Lu et al, 2011(Lu et al, , 2017Zhou et al, 2016;Sun et al, 2017;Yang et al, 2019). Yang et al (2019) have shown the gradual floodplain development of the second and fourth terraces as a result of channel incision during a climate transition from cold to warm.…”