The Stages of Debris Flow Activity During the Quaternary in the Xiaojiang River Basin, China

This study aimed to predict the laws and trends of modern debris ow by examining the activity cycle of debris ow during the Quaternary and its relationship with environmental change. The timing of the debris ow activity period was determined using sediment dating data based on typical debris ow deposition proles selected from various terraces in the Xiaojiang River Basin, China. The relationships between debris ow and environmental change were compared and analyzed using neotectonic history, the deep-sea oxygen isotope δ 18 O curve, and the loess paleosol series. The results showed that the debris ow activities in the Xiaojiang River Basin could divided into eight activity cycles, namely 11.42 Ma, 0.985 Ma, 0.562–0.52 Ma, 0.218 Ma, 0.137 Ma, 14 Ka, 0.73 Ka, and since 3 Ka. The activity cycle of debris ow deposition coincided with the neotectonically active period. Neotectonic movement has controlled the cycle of large debris ow deposition in the Xiaojiang River Basin since the Quaternary. The sedimentary activity of debris ow was not completely consistent with a warm and humid climate, and the sedimentation activity of debris ow was the result of the combined effects of tectonic movement and climate change.


Introduction
The Xiaojiang River Basin in Yunnan Province, China is well known globally to be an area of debris ow, and has been referred to as the "natural museum of debris ow" (Peng 2019;Yu et al. 2017;Li 2019;Wei and Li 2012). There has been an expansion in debris ow activities in the Xiaojiang River Basin due to the in uence of human activities and the complexity of climatic conditions (Wang et al. 2002;Chen and Cai 1993). The study of the activity cycle of Quaternary debris ow is helpful for understanding the development of modern debris ow in the Xiaojiang River Basin and can provide the basis for mitigation of debris ow. The sedimentary pro le of the Xiaojiang River Basin provides a historical record of the development of debris ow during the Quaternary (Feng et al. 2006;Li et al. 2007). The history of development of debris ow has a far-reaching tectonic and environmental background (Panek et al. 2009;Shi et al. 1994;Li 2002). The activity cycle and developmental environment of ancient debris ow in the Xiaojiang River Basin can be studied based on the sedimentary pro le of Quaternary debris ow, thereby allowing the developmental conditions and the temporal regularity of debris ow activities to be identi ed. There have been past studies on the sedimentary pro le of the Xiaojiang River Basin to identify the activity cycle of ancient debris ow. Shi and Zhang (Shi et al. 1994) categorized debris ow in the Xiaojiang River Basin into ve distinct long-term periods according to the geomorphic location and lithofacies of debris ow deposits, as well as by the chemical characteristics of paleosols developed on the deposits and some absolute dating information. Li (1991) used absolute dating, geochemical analysis, and regional stratigraphic correlation to analyze the sedimentary pro les of Quaternary debris ow of the Xiaojiang River Basin, and proposed a preliminarily categorization of Quaternary debris ow into seven stages, namely 1.42-0.985 MaB.P., 0.61 MaB.P., 0.3-0.5 MaB.P., 0.14 MaB.P.-80,000 aB.P., 40,000-25,000 aB.P., 8,000-5,000 aB.P. and since 200 aB.P.. At present, the research on the Quaternary debris ow in the Xiaojiang River Basin mainly focuses on the stages of ancient debris ow, rarely on the environmental factors of the formation of ancient debris ow.
In addition, the research on the stages of Quaternary debris ow is not systematic. Therefore, the current study aimed deriving a more detailed and systematic staging of debris ow activities in the Xiaojiang River Basin. To achieve this, eld investigations were conducted and dating information was examined for a comparative analysis of typical debris ow sedimentary pro les and strata age in the Xiaojiang River Basin. On this basis, the correlations between the activities of debris ow and history of neotectonic movement (Shackleton and Opdyke 1973), the deep-sea oxygen isotope curve (Diviacco et al. 2006;Li 1990;Chen and Zhang 1989), and loess paleosol series (Jomelli et al. 2007;Ding and Yan 2007) were comparatively analyzed. The current study also discussed the relationship between debris ow activity, tectonic movement, and environmental change.

Materials And Method
The approach used to select the debris ow sediment pro le The Xiaojiang River Basin shows interspersed sedimentation by debris ow and geomorphological processes during the Quaternary, with clear records of neotectonic movement and climate change evident in different terraces and stratigraphic sedimentary pro les. Therefore, the activity cycle of debris ow was examined by studying the relationship between the sedimental layer of debris ow and the terrace landform and its geological age (Li et al. 2007).
Field investigations have shown that neotectonic movement in the Xiaojiang River Valley has resulted in the formation of a terrace (platform) landform showing seven clear stages, with the development of debris ow in each terrace. Tectonic movement has played an important role in the distribution of this terrace (Wang et al. 2007).
The debris ow deposits in different areas of the same terrace show different degrees of erosion, making it impossible to identify all sedimentary layers of all levels of terraces representing each active period of debris ow (Ni and Liu 2008). Therefore, the current study selected representative typical sedimentary pro les of debris ow from each terrace and from different locations of the same terrace with less erosion and representing a certain active period of debris ow.

Research method
The earlier a terrace is formed, the larger the difference in elevation between the terrace and the adjacent area. Representative sedimentary pro les of debris ow were selected in each terrace to analyze the process of debris ow activities in the Xiaojiang River Basin during the Quaternary. The typical sedimentary pro les of debris ow of each terrace were constructed on the same elevation coordinate map based on the sequence of terrace formation according to their relative differences in elevation. The sedimentary age was determined according to the dating information for the sedimentary layer of debris ow. The sedimentary layers of debris ow for different terraces and different periods were constructed on the same chronological coordinate map, and the debris ow activity cycle was categorized according to the sedimentary layers of debris ow and their ages in different terraces.
The principle of sedimentology states that sediment properties will change due to changes in the sedimentary environment or sedimentary conditions over a certain period, resulting in sedimentary discontinuity, thus forming a sedimentary cycle with a discontinuous interface (Kuang et al. 2002). Therefore, the environmental information provided by the sedimentary layer of debris ow in the Xiaojiang River Basin was analyzed through a comparison to a typical debris ow activity cycle provided by other paleoenvironmental records, such as the history of neotectonic movement, the loess paleosol series, and the deep-sea oxygen isotope δ 18 O curve. This allowed the corresponding relationships between the deposition of debris ow and other environmental changes to be identi ed and the activity cycle of debris ow to be predicted according to environmental change, thereby providing a basis for the prevention and control of debris ow.

Stages of debris ow in the Xiaojiang River Basin during the Quaternary
The characteristics and age of typical debris ow deposits in the different terraces in the Xiaojiang River Basin were obtained based on a eld investigation and sedimentary layer dating data. Figure 1 shows the elevation pro le and Fig. 2 shows the chronology of the sedimentary pro le of debris ow.
The highest terrace (T 6 or T 7 ) in the Xiaojiang River Basin is represented by platform sediments under the Duozhaogou platform near Jiangjia Gully (Fig. 1). The highest elevation within the pro le is 1,000 m. Two layers of extremely thick deposits of debris ow exist, which were formed during the Yuanmou and Jinshajiang movements, respectively. The two layers of debris ow facies are sandwiched in thick uvial and alluvial fan sediments (Kuang et al. 2002) and have an electron spin resonance (ESR) age of 1.42 MaB.P. (Li and Kuang 1995).
The T 5 terrace, situated 1,450-1,600 m above sea level is in the upper Jiguanshi platform pro le of the Xiaojiang River Basin. The relative elevation of the top surface of the terrace is 530 m and the sedimentary strata of the platform has a thickness of ~ 90 m. The lower part of the terrace comprises a mixture of debris ow and water rock ow, whereas the middle part of the terrace is comprised of a set of debris ow deposits with a thickness of 70 m, and the upper part of consists of a layer of red clay. The Shanyuan red soil has a development age of 0.985 MaB.P. (Kuang et al. 2002) and originates from the debris ow sediment of the Xiaojiang River Basin during the late-early Pleistocene.

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The T 4 terrace is in the lower part of the upper Jiguanshi platform pro le and has a thickness of 74 m.
The relative elevation of the top surface of the terrace is 320 m. The sediment of debris ow in this terrace is composed of multiple layers of suspended and graded mud particles with an ESR age of 0.562 MaB.P. (Kuang et al. 2002) .
The Nideping platform is the representative pro le of the T 3 terrace situated outside the mouth of Jiangjiagou Gully. The terrace shows an accumulation of a thick layer of debris ow. The relative elevation of the top surface of the terrace is 280 m and its total thickness is 106 m. The lithology of the debris ow can be categorized into three layers: (1) a lower part composed of loam and silt with a gravel layer; (2) a middle part comprising debris ow deposits with a thickness of 85 m, and; (3) an upper part comprising red clay, a loam layer, and a gravel layer with red clay and loam. The ESR dating of the platform is 0.218 MaB.P..
The Daduo platform is the representative pro le of the T 2 terrace and is situated opposite the Jiangjiagou gully on the left bank of the Xiaojiang River. The terrace has an accumulated elevation that is ~ 200 m higher than the Xiaojiang River. The total thickness of the sedimentary stratum is ~ 195 m. The platform shows alternating debris ow deposits and river deposits. The ESR dating of the terrace is ~ 14,000 aB.P..
The T 1 terrace forms two-or three-stage fan-shaped elds of debris ow along the gullies on both banks of the Xiaojiang River, directly covering the gravel layer of the river, with many reaching the riverbank. The Hejiabao pro le situated along the Xiaojiang River with a relative elevation of ~ 40 m and a total thickness of 22 m was regarded as a representative pro le. The lower part of the pro le comprises a brown clay layer with a thickness of ~ 4 m, whereas the upper part consists of a modern deposit of debris ow with an ESR age of 7,300 aB.P. (Li and Kuang 1995).
By the late Holocene (~ 3 KaB.P.), the in uence of human activities resulted in an expansion in the scale and scope of debris ow in the basin compared with that of the previous period. The eld investigation revealed the presence of sediments of modern debris ow on both sides of the Xiaojiang River.
The above analysis and Fig. 2 show that ancient debris ow during the Quaternary in the Xiaojiang Basin can be divided into seven active periods. As shown in Table 1, an additional period can be systematically added by consideration of the deposition of modern debris ow during the past 3,000 years. The relationship between the active cycle of debris ow and neotectonic movement Neotectonic movement resulted in surface uplift and folding or fracture of rock strata, which provided the slope conditions and material basis for the development of debris ow in the Xiaojiang River Basin. Figure 3 shows a strong conjugate relationship between the sedimentary layer debris ow during the of Quaternary and the intermittent uplift (Du et al. 1987) resulting from neotectonic movement, indicating that debris ow developed after each stage of neotectonic movement.
The active cycle of debris ow and the loess paleosol series in Luochuan The loess deposition period in the Luochuan loess series represents a period characterized by a decrease in temperature and precipitation, characteristic of a glacial period. The development of paleosol in the loess strata indicates a warm and humid climate, characteristic of an interglacial period or interglacial stages of the glacial period (Kuang 2003). Both the deep-sea δ 18 O and paleosol re ected a similar evolution of the Quaternary climate with alternating dry-cold and warm-wet cycles. Fig, 3 shows that the intensity of debris ow was relatively high in the Xiaojiang River Basin during the period of the formation of the loess sedimentary layer of the Luochuan loess paleosol series. Although debris ow was also deposited during the soil development period of the Luochuan loess paleosol series, the intensity of deposition was less than that during the loess formation period. The results showed that the deposition of debris ow occurred during the warm wet and cold dry periods in the Xiaojiang River Basin, with the deposition effect during the latter more obvious than that during the former.
The activity cycle of debris ow and the deep-sea oxygen isotope δ 18 O curve Since the deep-sea δ 18 O curve re ects climate change, the even and odd number stages (high and low δ 18 O values, respectively) represent the cold and warm periods, respectively (Li 1990). Figure 3 shows that debris ow deposits were found during both the even and odd stages of the deep-sea δ 18 O curve in the Xiaojiang River Basin. However, the intensity of debris ow deposition during the even phase of the deepsea δ 18 O curve exceeded that during the odd stage, which similarly indicated that the deposition of debris ow during the cold dry period was more obvious than that during the warm wet period.

Discussion
The Xiaojiang River Basin is located along the eastern margin of the Qinghai Tibet Plateau. The environmental evolution of this area during the Quaternary included neotectonic movement and climate change. The intermittent uplift of the surface of the basin due to neotectonic movement resulted in the formation of the material conditions required for the development of debris ow in the Xiaojiang River Basin.
Given the position of the Xiaojiang River Basin in the lower latitudes, the moist air ow along the surface of the Southern Ocean frequently moves northward into the basin during the early interglacial period and the during the northward expansion of the low latitude planetary wind system. A weather front is often formed when this wind mixes with cold air to the north of the Xiaojiang River Basin, causing abundant rainfall (Wang et al. 2007) and consequently frequent debris ow activities. The southwest monsoon is more intense and precipitation is more abundant during the summers of the interglacial period. The debris ow formed was dominated by rare ed or sediment-laden ow when the amount of surface material exceeded a certain threshold. Moreover, this period was characterized by large river ow rates, resulting in debris on both sides of the Valley being carried away by the river. Therefore, less debris ow deposits were formed in this period, as indicated by the period of weak debris ow activity in the debris ow deposits.
The westerly belt at 40 °N-60 °N moved southward with the advent of the glacial period. The westerly belt is divided into two north and south torrents on the west side of the Qinghai Tibet Plateau do to its effect as a physical barrier, and these two torrents exist throughout the year (Li 2006). The south branch westerly torrent affects the Xiaojiang River Basin given its position in 25 ° 45 'N-26 ° 35 ′ N. The westerly belt moved northward during the summers of the glacial period compared to that during the winter, and the westerly torrents forced around the southern branch of the Qinghai Tibet Plateau converged and weakened, but remained positioned near the Xiaojiang River Basin. The direct point of the sun moves northward during summer, and the Xiaojiang River Basin is located near the equator. This results in the surface temperature of the Xiaojiang River Basin heating up the surface air, thereby inducing the movement of moist air ow of the southwest monsoon in the Southern Ocean from south to north to the vicinity of the Xiaojiang River Basin. This air ow converges with the southward westerly torrents to form a front, which was the main process responsible for rainfall during the glacial period. Therefore, although precipitation persisted during the glacial period in the Xiaojiang River Basin, the amount of precipitation decreased. Viscous debris ow occurred under conditions of su cient loose material on the catchment surface and suitable slope conditions. Moreover, there was obvious deposition of debris ow on both banks of the Xiaojiang River due to the small runoff at this time,.
The Xiaojiang River Basin had strong debris ow activities during both the glacial and interglacial periods after the active period of neotectonic movement or as the end of neotectonic movement. Debris ow occurred after the formation of each terrace or after each tectonic period, regardless of whether the climate was that of the glacial or interglacial period, resulting in deposits of debris ow. The intermittent quiescence of neotectonic movement resulted in the exhaustion of a large quantity of loose materials, a gradual attening of the large surface relief, and the gradual disappearance of debris ow activity. Thus, a debris ow activity cycle in the Xiaojiang River Basin was completed until the occurrence of the next neotectonic movement. Therefore, neotectonic movement has controlled the activity cycle of debris ow in the Xiaojiang River Basin since the Quaternary.

Conclusions
(1) An examination of the typical sedimentary pro le of debris ow and dating data during the Quaternary in the Xiaojiang River Basin allowed the debris ow activity cycle in basin to be systematically divided into eight periods, i.e. (2) The activity cycle of debris ow deposition coincided with the neotectonically active period. The neotectonic movement provided slope conditions and the material basis for the development of debris ow. Debris ow activities occurred during both the glacial and interglacial periods after the neotectonically active period or the end of neotectonic movement. Neotectonic movement has controlled the activity cycle of large debris ow in Xiaojiang River Basin since the Quaternary.
(3) The deposition of debris ow during the glacial period was more obvious than that during the interglacial period in the Xiaojiang River Basin due to the in uence of climate factors. The sedimentation of debris ow was not completely consistent with the warm and humid climate of the basin. This anomaly was the result the combined effects of tectonic movement and climate change. Therefore, the effects of tectonic movement and climate change should be integrated when predicting long-term and large-scale debris ow. Figure 1 Typical relative elevation pro le of the Xiaojiang River Basin, China Comparison of debris ow activity in the Xiaojiang River Basin, China with the oxygen isotope curves, loess-paleosol series and neotectonic movement