Sedimentary facies and depositional processes of the Diexi Ancient Dammed Lake, Upper Minjiang River, China

Journal Pre-proof Sedimentary Facies and Depositional Processes of the Diexi Ancient Dammed Lake, Upper Minjiang River, China

Hui Xu, Jian Chen, Zhijiu Cui, Ruichen Chen PII:

S0037-0738(19)30236-2

DOI:

https://doi.org/10.1016/j.sedgeo.2019.105583

Reference:

SEDGEO 105583

To appear in:

Sedimentary Geology

Received date:

25 September 2019

Revised date:

20 December 2019

Accepted date:

21 December 2019

Please cite this article as: H. Xu, J. Chen, Z. Cui, et al., Sedimentary Facies and Depositional Processes of the Diexi Ancient Dammed Lake, Upper Minjiang River, China, Sedimentary Geology(2019), https://doi.org/10.1016/j.sedgeo.2019.105583

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Sedimentary Facies and Depositional Processes of the Diexi Ancient Dammed Lake, Upper Minjiang River, China

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Hui Xu a, Jian Chen a,*, Zhijiu Cui b, Ruichen Chen a

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Corresponding author. E-mail address: [email protected] (J. Chen). 1

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a.

The School of Engineering and Technology, China University of Geosciences Beijing, Beijing 100083, China

b.

The College of Urban and Environmental Sciences, Peking University, Beijing 100871, China

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Abstract Landslide events causing rivers to become dammed are common natural disasters

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in mountainous area. Based on the sedimentary facies analysis of 44 sections

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between proximal and distal parts of a dammed lake, this study reveals the

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sedimentary facies distribution of the Diexi Ancient Dammed Lake located in the

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upper Minjiang River, on the southeastern margin of the Tibet Plateau. Fluvial

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gravels and sand were deposited in the proximal part of the dammed lake. Rhythmically bedded silt and horizontally laminated silt to clay were deposited in

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the distal part of lake due to the deep and steady water conditions. After progradation of the fluvial system, flood sediments were deposited in the middle part of the lake. The flood carried a large amount of sediments into the lake, and the decelerating flow generated successively alternating layers of gravels and sand, followed by alternating layers of silt with low-angle cross-stratification, parallel bedding, and climbing ripples downstream. Gravel layers formed by four fluvial progradations were found based on the distribution of sedimentary facies in 44 sections. After the first fluvial progradation, lacustrine sediments at the highest point emerged from the water surface. The interval between the second and third fluvial 2

Journal Pre-proof progradation is relatively short, and only a terrace was formed, namely terrace Ⅵ. After the fourth fluvial progradation, terrace Ⅳ was formed. Although gravel layers formed by fluvial progradations corresponding to terraces Ⅱ, Ⅲ and Ⅴ were not found, it can be determined that a terrace level does not necessarily correspond to a period of fluvial progradation. Therefore, the analysis of sedimentary facies

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upstream of landslide dams can be used as a supplementary method for inferring the

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evolution of the dammed lake.

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Keywords: Sedimentary facies; Ancient dammed lake; Terraces; Evolutional

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process

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1. Introduction

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Throughout the world, mountain belts are known to host the key prerequisites for

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the formation of natural dams (Korup and Tweed, 2007). Various types of dams

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formed by natural processes, such as landslide dams, ice dams, and moraine dams, have been observed to widely exist (Lliboutry et al., 1977; Costa and Schuster, 1991; Liu, 1992; Korup, 2002). Among these types, most of them (about 80%) fail within one year of formation (Costa and Schuster, 1988; Ermini and Casagli, 2003). Only a small number of dams have the ability to last for hundreds or even thousands of years or longer (e.g., Deline and Orombelli, 2005; Korup et al., 2006; Scherler et al., 2014; Liu et al., 2018). For instance, landslide blockage of the Phung Chu (river) is reported to have been present for about 26 ka in southern Tibet (Chen et al., 2016). For these long-lived dammed lakes, various dating methods can be used to 3

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determine the life-span of lakes, such as OSL (optically stimulated luminescence) and 14C dating (e.g., Xu et al., 2015; Zhang et al., 2015; Chen et al., 2018). Environmental proxy indicators like sporo-pollen, carbon and oxygen isotopes and calcium carbonate from lacustrine sediments are valuable materials for paleoclimate reconstructions (e.g., Duan et al., 2002b; Rodrigues et al., 2002; Zhang et al., 2011).

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Fluvio-lacustrine sediments or sedimentological investigations are used to infer the

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evolution of dammed lakes (e.g. Wang et al., 2005, 2019; Chen et al., 2016). The

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evolution of dammed lakes, including their formation, duration, and breach,

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provides important information for geological hazard assessment of modern

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dammed lakes (e.g., Fan et al., 2014; Scherler et al., 2014). Moreover, lithologic facies and/or facies assemblages of one or several vertical sedimentary sequences

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are also used to define the relevant sedimentary processes and settings (Mángano et

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al., 1994; Yang et al., 2008; Hatano and Yoshida, 2017; Sancho et al., 2018).

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However, research on the systematic analysis of sedimentary facies of dammed lakes is rarely found in the literature. In this study, the sedimentary structures and grain sizes of 44 well-exposed outcrops from proximal to distal parts of the Diexi Ancient Dammed Lake were analyzed in detail in order to determine the sedimentary facies of the dammed lake.

The Diexi Ancient Dammed Lake is located in the upper Minjiang River, at the southeastern margin of the Tibetan Plateau (Figs. 1, 2). It was formed by a landslide blocking the Minjiang River. The lake had an average width of about 0.65 km and an area of about 20 km2, and the widest lake surface is at Jiaochao Village, reaching 4

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2 km (Wang, 2009). The dam of this landslide is approximately 3 km in length and 1 km in width, with a volume of more than 1.2×109 m3 (Ma et al., 2018) (Fig. 2). The dammed lake breached and caused a large outburst flood at about 27 ka BP (Ma et al., 2018), and the dam-break flood carried an abundance of diamicts from the dam which accumulated approximately 5 km downstream (Ma et al., 2018; Chen et

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al., 2019) (Fig. 2). The range of the dammed lake is from the dam to Yonghe

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Village (approximately 30 km distance), and the maximum thickness of the

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lacustrine sediments is greater than 200 m (Wang et al., 2005) (Fig. 2). Well-

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preserved soft-sediment deformation structures occur in lacustrine sediments, such

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as liquefied convolute deformation, water-escape structures, flame structures and so on, which are caused by earthquakes, slumps, and landslides (Wang et al., 2011).

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The goals of this study were: (1) to log the lithofacies based on sedimentary

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structures, grain size and strata boundaries; (2) to describe and interpret the

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distribution of sedimentary facies; and (3) to link the sedimentary facies with fluviolacustrine terraces to further analyze the evolution of the dammed lake.

2. Geological background The stratigraphic ages in the study area range from Devonian to Triassic, with only a small amount of Quaternary sediments located at the top of the ancient lacustrine sediments. The Triassic strata are the most widely distributed, and consist of low metamorphic clastic rocks, mainly sandstone and slate. The Permian and Carboniferous strata are marine carbonate deposits. The exposure areas of the 5

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Devonian strata are the smallest, and are mainly composed of sandstone and phyllite.

The Minjiang River is an important branch of the upper reaches of the Yangtze River (Fig. 1). It is located on the western edge of the Sichuan Basin, and originates from the southern foot of the Minshan Mountain at the junction of Sichuan Province

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and Gansu Province (Chen et al., 2006) (Fig. 1). The Minjiang River flows south,

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and then merges into the Yangtze River. The Upper Minjiang River (31°26′-33°16′

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N; 102°59′-104°14′ E) essentially acts as the transition zone of the Tibetan Plateau

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to the Sichuan Basin (Zhang et al., 2002).

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Discharge of the Upper Minjiang River is controlled by the southern branch of the

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westerlies and warm-wet southeastern monsoons (Chen et al., 2006). According to

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the Zipingpu Hydrological Station in the Upper Minjiang River, annual average runoff is 469 m3/s (Liang et al., 2008), and annual average sediment discharge is

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9.1253×106 t (Ding et al., 2013). The climate in this area is characterized by a plateau monsoon climate, with dry and cold winters, and humid and cool summers (Chen et al., 2006). The diurnal temperature variation in this area is large (Pang et al., 2008). The annual average rainfall ranges between 500 mm and 850 mm (Ding et al., 2013). The rainy season is from May to October, accounting for 85% of annual precipitation (Pang et al., 2008). The dry season is from November to April (Pang et al., 2008).

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Geomorphologically, the upper reaches of the Minjiang River show highland canyon landforms. These are high in the west, low in the east, with deep incisions in the valley, and major relative height differences (Chen et al., 2006). The valleys on both sides of the river are steep, and subject to debris flows, landslides and rockfall.

As detailed in Fig. 1B, the main active faults in the upper reaches of the Minjiang

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River include the Longmenshan Thrust Nappe Tectonic Belt, Minjiang Fault Zone, Xueshan Fault, and Huya Fault (Zhang et al., 2006). The Minjiang Fault Zone is a

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Holocene active fault trending north-south along the upper reaches of the Minjiang

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River. Historically, the Diexi Ms. 7.0 earthquake in 1713 and the Diexi Ms. 7.5

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earthquake in 1933, both occurred along this fault zone (Chen et al., 1994). The Songpan Earthquake Group (Ms. = 7.2, 6.7, and 7.2) in 1976 and the Jiuzhaigou

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3. Methodology

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Li et al., 2017).

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Earthquake (Ms. = 7.0) in 2017, both occurred on the Huya Fault (Chen et al., 1994;

In this study, the research was mainly focused on logging and classifying sediments in outcrops exposed in the entire Diexi Ancient Dammed Lake area, based on the sedimentary structures, grain size and overall lithofacies characteristics. There was a total of 44 well-exposed sections identified in the field (Fig. 3). The sediments in each section were classified into sedimentary facies. The grain size of gravels was measured using a tapeline, and the particle size of the fine grain materials was measured using a laser diffraction particle size analyzer 7

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(Mastersizer 3000) at the Institute of Geology and Geophysics of the Chinese Academy of Sciences. The grain sizes were categorized according to the UddenWentworth grain size scale (Udden, 1914; Wentworth, 1922). The sections on the east bank of the Minjiang River were selected to construct a longitudinal profile of the lake. The distribution of sedimentary facies in a longitudinal profile of the lake

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was analyzed. Then, combined with terraces in the distal part of the dammed lake,

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the evolution of the lake was inferred.

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4. Sedimentary facies

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On the basis of sedimentary structures, grain size and lithofacies characteristics,

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B, C, D, E, F, G, H, I, and J.

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sediments of 44 well-exposed sections in the lake area were classified into Facies A,

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4.1 Facies A: Clast-supported, inverse-graded gravel

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Description: This facies is 3 to 10 m thick, and laterally extends from 40 to 90 m. Clasts are composed of clast-supported, inverse-graded gravel. There is upward increase in clast grain size. Grain size of sediments in the lower part of the section ranges between 2 and 10 cm, with a maximum major axis of 20 cm. However, the clasts in upper section are 20 to 60 cm in length, with a maximum size up to 1 m (Fig. 4A). Gravels are angular to sub-angular and disordered in arrangement. The matrix is composed of poorly sorted sand and granules. Facies A is inverse-graded, without any internal bedding.

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Interpretation: Angular gravels indicate that the clasts have not been transported over long distances. The coarseness and poor sorting argue against selective deposition, and the boulders at the top reveal a high yield strength of the fluid. The inverse grading suggests that this clast fabric developed during flow rather than during deposition. In addition, the high concentration of clasts places flow in the

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inertial realm, which is beneficial to the development of inverse grading (Shultz,

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4.2 Facies B: Clast-supported, massive gravel

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1984). Facies A is interpreted as a clast-rich, high-strength debris flow.

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Description: This facies consists of clast-supported massive gravels. The matrix is

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poorly sorted, consisting of sand, silt, and clay. In accordance with clast roundness,

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Facies B is divided into subfacies B1 and B2. Subfacies B1 is 5.5 to 18 m thick and

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laterally extends from 10 to 100 m. Gravels are angular, with grain size ranging from 5 to 30 cm, and a maximum particle size of up to 70 cm (Fig. 4C). Subfacies

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B1 displays sharp erosive contact with the underlying fine-grained sediments (Fig. 4C). The lower part of this subfacies locally contains agglomerates composed of the underlying fine-grained sediments (Fig. 4C), with a maximum agglomerate size of 1.5 m. Significant deformation structures are observed in the upper part of the underlying fine layer. Curved depressions are observed in the underlying rhythmical layer (Fig. 4E). Subfacies B2 is 1 to 5 m in thickness, and laterally extends from 20 to 50 m. The grain size of gravel is mainly between 2 and 10 cm, with maximum grain size up to 15 cm. The gravel arrangement is disorderly, without preferred direction. The roundness of gravels is mainly sub-rounded to rounded (Fig. 4B). 9

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Both clast-supported and matrix-supported facies are unevenly distributed in subfacies B2, and the former is often dominant. The matrix is mainly composed of sand and silt. Subfacies B2 is generally massive, having sharp erosional contacts with underlying Facies E.

Interpretation: Clasts in subfacies B1 are much smaller than these in Facies A,

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so subfacies B1 represents the product of clast settling from debris flows with relatively low yield strength due to a relatively high content of water (Shultz, 1984).

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As the debris flow moves, underlying sediment along the path is scraped with great

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force, resulting in agglomerates of the underlying sediments occurring in the lower

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part of subfacies B1 (Fig. 4C). The weight of the debris flow deposits compressed the underlying unconsolidated sediments, causing significant deformation (Fig. 4E).

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Since the arrangement of the clasts in subfacies B2 is disordered, and lacking a

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preferred direction, the clasts lacked ability to move freely, either due to the higher

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flow viscosity or to rapid settling. Therefore, it is assumed that there was no time to develop a regular fabric (Collinson and Thompson, 1982). For these reasons , subfacies B2 may have been formed by the rapid accumulation of sediments during sudden flood conditions. In addition, the erosional contact between subfacies B2 and the underlying Facies E also illustrates sudden changes in the depositional environment.

4.3 Facies C: Clast-supported, horizontally stratified gravel

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Description: This facies consists of clast-supported pebbles and cobbles in which crude horizontal stratification is sometimes apparent (Fig. 4D). Facies C ranges in thickness from 0.3 to 3.8 m, and extends laterally from 5 to 70 m. The majority of clasts in Facies C are rounded, and grain size range is from 3 to 10 cm, with a maximum size of 30 cm. The matrix is mainly composed of sand and silt. Facies C

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is usually in the form of a lens and has erosive contacts with the underlying Facies

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E, as well as the underlying Facies I.

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Interpretation: Facies C is interpreted as lag deposits within the river channel

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when the water energy is at highest. Selective transport causes fine-grained

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sediments to be carried away, leaving gravels originating from upstream region and angular coarse material formed on eroded banks to form discontinuous lenticular

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bodies. Facies C is similar to facies Gh in Miall (1996).

4.4 Facies D: Matrix-supported, massive gravel

Description: Facies D has a thickness of 2.5 to 7.1 m, and a lateral extent of 30 to 91 m. This facies is comprised of poorly-sorted silty sand with pebbles to granules (Fig. 4D). The gravels are unevenly distributed in the massive silty sand with disordered internal arrangements. Facies D is found to generally occur at the top of the section, which is sometimes capped by loess, as well as overlying Facies E1.

Interpretation: Low to moderate concentration of clasts keeps the flow in a viscous state, so that grading is weak or absent. Facies D is massive due to the fact 11

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that deposition takes place by "cohesive freezing" (Lowe, 1982) caused by thinning and disappearance of the basal-shear zone in response to decreasing slope, thickness, or water content, or a combination of those factors (Shultz, 1984). Therefore, Facies D is interpreted as a debris flow with substantial plastic yield strength

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4.5 Facies E: Parallel bedded silty sand and sandy silt

Description: Facies E is laterally persistent, subhorizontally planar bedded grey

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silty sand and sandy silt units, which are divided into subfacies E1 and E2

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depending on whether or not clay clasts are present. Subfacies E1 (clay; 11.2%, silt;

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48.5%, sand; 40.3%) does not contain clay chips, and displays large thickness

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variations of 0.18 to 12 m. The sharpness of the laminae in subfacies E1 is related to

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variations in bed thickness. When subfacies E1 is thinner, the mm-scale laminae are straight and parallel to the bed plane (Fig. 5D). However, as the thickness of

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subfacies E1 increases, the thin and horizontal laminae become more dispersed. Subfacies E2 (clay; 9.7%, silt; 53.9%, sand; 36.4%) is characterized by aligned yellowish clay clasts or chips (Fig. 5A). These clay chips or mud clasts are intermittently and linearly arranged, and the long axis of the clasts is commonly oriented parallel to the lamina plane. The clasts are generally less than 3 cm in diameter, with the larger ones appearing to be well rounded, and the smaller ones are subrounded.

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Interpretation: Subfacies E1 is formed by the migration of a flat bed form in the upper flow regime. The phase is found to be the most stable in fine- to mediumgrained sandstone at flow rates of about 1 m/s and water depths of 0.25 to 0.5 m, but also occurs at shallower depths with lower flow rates (Miall, 1996). Subfacies E1 up to several meters in thickness may be deposited in a single dynamic event, such as

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flash floods (Miall, 1996). During this time, the water flow conditions may remain

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in a critical stage for hours. Therefore, the transition of laminae from sharp to

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indistinct may be related to higher sedimentation rates during floods. Coarser clasts

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are rarely observed in subfacies E1, and are emplaced by rolling in the sand traction

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carpet (Miall, 1996). However, the long axes of clay clasts in subfacies E2 are parallel to flow, which indicates that the clay clasts are not emplaced by rolling on

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the deposition surface. Moreover, the aligned clay chips in subfacies E2 indicate

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sheared layers exist at the bottom of the overpassing flow during the deposition of

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bedload (Xian et al., 2018). In addition, the color difference between clay clasts and sand indicates that the flow might be from outside of the dammed lake.

4.6 Facies F: Low-angle cross bedded sandy silt

Description: Facies F (clay; 10.7%, silt; 58.7%, sand; 30.6%) is composed of yellow and gray sandy silt units with cross bedding. The thickness of this facies is between 0.08 and 0.64 m, with a maximum thickness of 1.74 m. Upper and lower bounding surfaces are generally undulating, nonparallel, and sometimes intersected. 13

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The lateral extension of the set interfaces is parallel to the bed plane. Facies F displays tangentially downlapping foresets which dip between 10° and 20° (Fig. 5F), oriented in a consistent direction with the current flow of the Minjiang River.

Interpretation. The foresets of Facies F are formed by the migration of lowrelief dunes at the base of an overpassing turbulent flow (Zavala et al., 2006). Since

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dips of foresets in the facies are consistent, it indicates that the flow is unidirectional. The wavy set plane indicates interruption and a change to low-energy

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dominantly oscillatory flow during deposition (Midtgaard, 1996). In the flume

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experiments conducted by Leclair (2002), it was revealed that these dunes and the

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resulting stratification are the result of both bedload and suspended-load sediment

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transport, with an aggradation rate up to 0.014 mm/s.

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4.7 Facies G: Climbing-ripple laminated silt

Description: According to the preservation of stoss side laminae, as well as the ripple amplitude and laminae migration, Facies G was divided into four subfacies as follows: G1, G2, G3, and G4. These subfacies correspond to the type A, type B, type C, and sinusoidal ripple-drift cross-lamination, respectively, of Jopling and Walker (1968). Subfacies G1 (clay; 5.5%, silt; 66.3%, sand; 28.2%) is approximately 10 cm thick, and consists of climbing sets of leeside laminae. The stoss side laminae have been completely eroded, and leeside laminae are either concave-up or sigmoidal in shape. The wave ridges migrate in the direction of the 14

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current while growing upwards, forming a series of nearly parallel pseudo-interfaces that are inclined in the opposite direction to the current (Fig. 5D). Subfacies G2 (clay; 5.8%, silt; 75.4%, sand; 18.8) is about 12 cm thick and composed of climbing sets of leeside laminae. The complete preservation of relatively thick stoss side laminae is evident. Laminae can extend from one ripple to the next. As the wave

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ridges migrate downstream, the ripple amplitude of the entire coset changes but does

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not decrease upwards (Fig. 5B). Subfacies G2 is sometimes interbedded in Facies J.

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Subfacies G3 is approximately 30 cm thick, with a marked grading within cosets.

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The proportions of clay, silt and sand at the lower part of subfacies G3 are 4.5%,

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72.2%, and 23.3%, respectively, and those at the upper part are 9.1%, 88.2%, and 2.7%. The ripple amplitudes gradually decrease upwards (Fig. 5C), and may pass

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into horizontal lamination. However, with the exception of amplitudes decreasing

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upwards, all of the features are identical to these of subfacies G2. Subfacies G4

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(clay; 14.2%, silt; 77.7%, sand; 8.1%) is composed of superimposed undulating laminae with a slight displacement of the crests. The laminae can be traced from one ripple to the next, and the sediments on lee and stoss sides are equal in thickness. The thickness of subfacies G4 ranges between 2 and 10 cm (Fig. 5E), with a maximum thickness up to 1 m.

Interpretation: The type of climbing ripple lamination depends upon current velocity and composition and concentration of suspended sediments, and the primary controlling factor is the ratio of suspension load to traction load (Jopling and Walker, 1968). Subfacies G1 represents one end member in which bed traction 15

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is dominant, and subfacies G4 represents the other end member in which suspension load is dominant. Subfacies G2 lies between these two end members. Since there is no grading observed in the cosets of subfacies G1, G2 and G4, it is indicated that current and sediment parameters, especially the current velocity and ratio of deposition from suspension to bed load movement, are stable (Jopling and Walker,

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1968), suggesting that the deposition conditions probably remain stable for a period

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of time. However, the gradual upward decrease in amplitude and grading within

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cosets of subfacies G3 is probably due to the gradual weakening of the turbidity

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current (Walker, 1963).

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4.8 Facies H: Flaser, wavy and lenticular bedded silt

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Description: Facies H consists of an alternation of dark gray fine-grained layers

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(clay; 28.1%, silt; 70.0%, sand; 1.9%) and pale yellow coarse-grained layers (clay; 10.1%, silt; 78.6%, sand; 11.3%), forming intervals up to 11.4 m thick. The gray fine-grained layers (0.5 to 4 cm thick) are massive. The yellow coarse-grained layers (1 to 7 cm thick) display well-developed tangentially downlapping foresets which dip downstream. The flaser bedding shows gray layers are completely or partially preserved in the trough and ridges of yellow layers, respectively (Fig. 5I). In contrast, the yellow layers are enclosed within gray layers, forming lenticular bedding (Fig. 5G). Wavy bedding consists of an alternation of yellow and gray undulating successive layers (Fig. 5H). In the field, these three types of beddings 16

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often appear together, sometimes in a successive transition. Compared to other facies, Facies H appears in outcrops less frequently.

Interpretation: Facies H forms where both sand and mud are supplied, with variation in current velocities (Reineck and Singh, 1973). The foresets in the yellow layers indicate that these are deposited in the form of bed ripples during flow

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acceleration. However, the massive structures in the gray layers are suggestive of rapid deposition of suspended sediments (Collinson and Thompson, 1982) during

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flow deceleration. Furthermore, from flaser to wavy bedding, and then to lenticular

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bedding, it could be discerned that the deposition and preservation of fine-grained

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sediments progressively increase, and the hydrodynamic force becomes weaker. The dips of foresets in the coarse-grained layers of facies H are consistent, suggesting

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that the direction of the flow is unidirectional (Reineck, 1960).

4.9 Facies I: Horizontally laminated or massive silt and clayey silt

Description: Facies I (clay; 21.6%, silt; 75.4%, sand; 3.0%) consists of light gray and off-white clayey silt and silt units (Fig. 6B). The sedimentary structure is dominated by massive to horizontally laminated bedding. The upper bounding surface of Facies I is horizontal, and the lower bounding surface conforms to the geometry of the underlying layer (Fig. 5B). When the overlying layer consists of sand, Facies I may develop soft-sediment deformations. As shown in Fig. 6C, a layer of clayey silt with a flat bottom, intrudes upwards into a bed of fine sand, 17

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forming a flame structure. The dip direction of these structures is generally the same. Facies I has a maximum thickness of more than 200 m in Tuanjie Village (Wang et al., 2005). There are obvious deformations in Facies I in Yangliu Village. These deformations are divided into two categories. One is that the bedding surfaces were not deformed, but Facies I inclined as a whole, and the other is the bending

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Facies J often occurs in Facies I as a thin interlayer.

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deformation of the bedding surfaces, including synclinal bedding. Additionally,

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Interpretation: Facies I consists of fine sediments that was deposited in low

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energy settings. The absence of current and wave-induced structures, preservation of

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thin lamination and the fine grade of the sediments involved indicate deposition from suspension (Mángano et al., 1994). Massive structures in fine sediments can be

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caused by very uniform and possibly fairly rapid deposition, or can be caused by the

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lack of platy grains (Collinson and Thompson, 1982). Deformation structures in

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Facies I were caused by earthquakes (Wang et al., 2011).

4.10 Facies J: Rhythmically bedded silt

Description: Facies J is composed of regularly alternating light-yellow and darkgray laminae 1-10 mm thick which form couplets (Fig. 6A). The couplet thicknesses are irregular, ranging between 2 and 20 mm. The contacts between couplets, including between dark and light laminae of a couplet, are sharp (Fig. 6A). The light-yellow laminae have a mean grain size of 9.1 μm and are clay-enriched (clay; 18

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25.3%, silt; 69.0%, sand; 5.7%). The dark-gray laminae with a mean grain size of 13.2 μm contain a higher content of silt (clay; 16.0%, silt; 76.8%, sand; 7.2%). The organic matter content of the light-yellow and dark-gray laminae are 0.541% and 1.086% on average, respectively (Wang et al., 2010). Facies J ranges from 1.2 to 20 m in thickness, and is partly interbedded with 2 to 3 cm thick layers of subfacies G2

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or 12 to 20 cm thick layers of subfacies E1.

Interpretation: The regular alternation of light-yellow and dark-gray laminae

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may be result of rainfall intensity (Wang et al., 2010) or alternation between flood

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and non-flood periods. The coarse layer represents abundant rainfall or flood events

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in which flow carried coarser sediments and more organic matter into lake because of stronger flow power (Duan et al., 2002a). So coarse layers are dark in color.

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When there was little rainfall or it was in a calm period after the flood, the flow

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power was weaker. The flow carried finer sediments and less organic matter, and

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light fine layers were deposited (Duan et al., 2002a).

5. Rhythmicity

After careful study and analysis of the 44 sections in Fig. 8, two types of rhythmic sediments were observed. One is coarse-grained rhythmic sediments consisting of Facies B2/C and Facies E1, and the other is fine-grained rhythmic sediments consisting of Facies E, F, and G.

5.1 Coarse-grained rhythmic sediments

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Description: The coarse-grained rhythmic sediments are formed by repeated superposition of Facies B2/C and Facies E1. According to the thickness of facies, the coarse-grained rhythmic sediments are classified into: larger-scale rhythms (Fig. 7A) and smaller-scale rhythms (Fig. 6D). In larger-scale rhythms, each facies is thicker than 40 cm, and Facies B2/C has sharp erosive contacts with underlying

f

Facies E1. In smaller-scale rhythms, the thickness of facies is no more than 8 cm,

oo

and the boundary between Facies B2/C and Facies E1 is poorly distinguished,

pr

without erosive contacts. Smaller-scale rhythms are often interbedded in Facies E1

e-

of larger-scale rhythms.

Pr

Interpretation: Rhythmicity usefully can infer a repeated sedimentary signature that reflects both a presumed cyclicity in depositional timing and repetition of

al

process (Carling, 2013). Erosional contacts indicate two separate depositional

rn

events. The flood carries a large amount of sediments into the lake. During

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transport, as the speed of flow decreases, coarse gravels (Facies B2/C) are deposited first, and then are covered by fine sand (Facies E1). When the next flood arrives, the fine sand layer from the previous event will be scoured and eroded by the flood. Then the same sedimentary sequence will accumulate on the erosion surface. Therefore, a larger-scale rhythm may represent a single flood event. Non-erosion contacts may represent the rapid deposition of rhythmic layers, without significant intervening erosive episodes. Therefore, smaller-scale rhythms may be evidence of pulsing in a separate flood event, and represent variation in a single deposition event. 20

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5.2 Fine-grained rhythmic sediments

Description: Fine-grained rhythmic sediments are formed by cyclical superposition of Facies E, F, and G (Fig. 7B), with a maximum thickness of up to 12 m. The thickness of each facies mainly ranges between 5 and 20 cm, with a maximum thickness of no more than 30 cm. There is a gradual transition between

oo

f

facies, without erosive contacts.

Interpretation: Following flash floods, hyperpycnal flows originate when

pr

sediment-laden fluvial discharges enter standing, low-density lake water in the form

e-

of an underflow due to its excess density (Zavala et al., 2006, 2011). As transport

Pr

distance increases, the decelerating hyperpycnal flow under conditions of traction-

al

plus-fallout generates a sequence of sedimentary structures starting with low-angle

rn

cross-stratification (Facies F), followed by parallel bedding (Facies E), and ending with climbing ripples (Facies G) (Zavala et al., 2006). However, in the field

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observations, successive superposition of Facies F, E, and G is rarely seen, and one facies may be commonly missing. A cyclical occurrence of Facies F, E and G suggests fluctuation in velocities and/or fallout rates from the parent flow during deposition (Zavala et al., 2006).

6. Distribution of sedimentary facies Vertically, the fluvial deposits (Facies B2 and C) overlie fine-grained sediments (Facies E, F, G, H, I and J) and are covered by debris flows (Facies A, B1, and D), rockfalls, or loess. However, the majority of the sequences are observed to be 21

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incomplete, and some layers may be missing. In the longitudinal profile, fluvial sediments (Facies B2 and C) are mainly between Yonghe Village and Zhenping Village (Fig. 8). The thick Facies E1, coarse- and fine-grained rhythmic sediments (Facies B2/C and E1, Facies E, F, and G, respectively) are mainly between Zhenping Village and Jiefang Village (Fig. 8), and fine-grained rhythmic sediments

f

tend to be deposited downstream of coarse-grained rhythmic sediments. Between

oo

Jiefang Village and Taiping Village, rhythmically bedded silt (Facies J) is dominant,

pr

while flaser, wavy and lenticular bedded silt (Facies H) occurs less frequently (Fig.

e-

8). Between Taiping Village and Tuanjie Village, horizontally laminated or massive

Pr

silt and clayey silt (Facies I) is dominant (Fig. 8). In summary, from the proximal to distal parts of the lake, the variety of facies is smaller, and the grain size of

7.1 Terraces

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7. Discussion

al

sediments is finer (Fig. 8).

In Tuanjie Village, there are six levels of terraces of the Diexi Ancient Dammed Lake, as shown in Fig. 9B. The terraces consist of three divisions (Fig. 6E): horizontally laminated silt and clayey silt (Facies I) in the basal part, fluvial gravels and sand (Facies B2 and C) in the middle part, and cultivated soil in the upper part. Tuanjie Village is located in the distal part of lake (Fig. 3). If gravels and sand in the terrace are deposited by the Minjiang River, then this means that the Minjiang River has prograded into the distal part of the lake, and the dam has been completely 22

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breached. In this case, the fluvial regime is completely re-established, so it is impossible to form terraces in Tuanjie Village. In addition, the largest flat planes of gravels in terraces tend to incline towards upstream of the tributary, which further proves that these gravels and sand are tributary sediments, not Minjiang River sediments.

oo

f

During the ‘high lake-period’, relatively deep and stable water conditions at the distal part of lake favor deposition of fine-grained sediments such as Facies I. As the

pr

dam breaks out, the lake surface descends. Due to progradation of the tributary that

e-

used to join the river, coarser sediments like gravels and sand (Facies B2 and C)

Pr

carried as bedload accumulate on the lacustrine sediments (Facies I). Finer sediments like silt and clay transported in suspension will accumulate on gravels

al

and sand, forming cultivated soil under the influence of weathering and climate.

rn

After the lake surface descending, a new base level of erosion is formed. Due to the

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decline in base level, the river begins to downcut. When the lake surface descends again, fluvio-lacustrine sediments previously deposited could remain stranded as terraces, with terraces being higher than the new riverbed. A series of such terraces and their sediment associations could therefore provide useful evidence for the evolutionary history of the dammed lake. Six levels of terraces in Tuanjie Village records six progradations of tributary, i.e., six declines of lake surface, during the history of the dammed lake from the highest water level to its complete disappearance.

7.2 Progradation of Minjiang River 23

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Between Yonghe Village and Zhenping Village, Facies B2 and C are dominant, with interbedded thin layers of Facies J seen occasionally (Section 1 in Fig. 8B). This means that the fluvial sedimentary environment is dominated, with short periods of still water conditions of the lake. After the lake surface has descended, Minjiang River progrades. When sediment-laden fluvial discharges enter the

f

standing lake during floods, coarse sediments quickly accumulated. If the period

oo

between the two floods is short, then larger-scale coarse-grained rhythms will be

pr

formed. Under the influence of flow pulses in a single flood event, small-scale

e-

coarse-grained rhythms are sometimes interbedded in a large-scaled coarse-grained

Pr

rhythm. As transport distance increases, the flow gradually decelerates. Fine-grained sediments begin to settle to form fine-grained rhythms. Sections 4-14 in Fig. 8B are

al

formed by sediments carried by multiple floods entering the lake. According to the

rn

altitudes of Facies B2 and C in Sections 4-14, it can be seen that the lake surface has

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fallen and the river is gradually prograding. When the river progrades into Jiefang Village, Facies J in the deep water area begins to slowly emerge from the lake, and coarse sediments of Facies B2 and C formed by the river are directly deposited on Facies J. Evidence of the last progradation of the fluvial system that can be found in the field is the gravel layer (Facies B2 and C) covering Facies J in Section 27 (Fig. 8B). The thick layer of Facies E1 in Section 34 (Fig. 8B) is the last flood sediment found in the field. Although there are also Facies E1 and fine-grained rhythmic sediments in the lower part of Section 37 (Fig. 8B), Facies E1 in Section 34 and 37 is not formed in the same period. There are soft-sediment deformations in Facies I in 24

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both Section 34 and 37, but in Section 34, fine-grained sediments of Facies I do not intrude into the overlying Facies E1, and the bedding surfaces of Facies E1 are horizontal and do not deform. This indicates that Facies E1 is deposited after softsediment deformation. However, in Section 37, fine-grained sediments of Facies I intruded into the overlying Facies E1 to form a flame structure, which indicates that

f

Facies E1 is deposited before deformation. In addition, there is no erosive contact

oo

between Facies E1 and overlying Facies J in Section 37, indicating subaqueous

pr

deposition. Facies E1 is overlain by rockfall deposits in Section 34, indicating

e-

subaerial deposition. These differences all indicate that Facies E1 in Section 34

Pr

represents flood deposits during dying out of the dammed lake. Between Yangliu Village and Tuanjie Village, thick layers of Facies I are dominant, indicating that

al

Segment 5 should belong to the deep-water area of the lake (Fig. 8B). According to

rn

the positions of Facies B2 and C in the existing sections, the lake surfaces after

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progradation can be identified (Fig. 9A).

7.3 Relationship between terraces and progradation of the Minjiang River

The elevation of lake surface closely links the development of the terrace with progradation of the fluvial system, so that the development of terraces is roughly consistent with the progradation of the Minjiang River in terms of time and elevation. However, there is not a one-to-one correspondence between each progradation of fluvial system and each formation of terraces. If the period among several fluvial progradations is relatively short, then only one terrace may be formed. 25

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Fig. 9 compares elevations of lake surface based on terraces and fluvial progradations. The top elevation of lacustrine sediments above terrace Ⅵ is 2357.29 m (Wang, 2009). The elevation of Facies J in Section 1 is 2396 m, but the actual water surface should be higher than 2396 m. Therefore, it can be inferred that the initial elevation after the formation of the dammed lake is approximately 2400 m.

f

The lake's elevation after the first fluvial progradation is about 2350 m. At this time,

oo

the topmost lacustrine sediments (2357.29 m) have already emerged from the water

pr

surface. The elevation of lake surface after the second and third progradation of

e-

fluvial system is about 2330 m and 2310 m, respectively. The elevation of the lake surface after the third fluvial progradation is approximately equal to that of terrace

Pr

Ⅵ. A terrace formed after two fluvial progradations, probably due to the short

al

period between these events. The elevation of the lake surface after the last

rn

progradation of fluvial system is about 2220 m, and is roughly equal to the elevation

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of terrace Ⅳ. We did not find gravel layers due to fluvial progradation corresponding to terraces Ⅱ, Ⅲ, and Ⅴ in the field, which may have been covered by later rockfall deposits. However, according to the relationship between terraces and fluvial progradation, it is known that there are at least three progradations of the Minjiang River in addition to the four mentioned above.

When the water level of the dammed lake is highest (2400 m), the shore region is located between Yonghe Village and Zhenping Village. When terrace Ⅵ is formed, the elevation of the lake surface is about 2310 m, and the shore region is between

26

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7.4 General evolution of Diexi Ancient Dammed Lake Zhang et al. (2009) dated sediments at the bottom of dammed lake by AMS 14C dating, and the results show it started at 40.5 ka B.P. Wang et al. (2012) carried out

oo

f

AMS 14C dating on drill cores and concluded that the dammed lake was formed about 30,000 years ago. OSL (optical stimulated luminescence) dating of Facies E1

pr

in Section 5 is 29.1 ± 5.0 ka (Guo, 2018). According to the history of progradation

e-

of fluvial system, the formation of the dammed lake should be earlier than the first

Pr

fluvial progradation, so the dammed lake should be formed at about 40 ka B.P. OSL

al

and 14C dating of soft-sediment layers from Shawan Village indicate that intense

rn

earthquakes occurred during the period 25-20 ka B.P (Wang et al., 2011). Ma et al. (2018) carried out OSL dating on the outburst deposits downstream, and believed

Jo u

that the dammed lake breached and caused a large outburst flood at about 27 ka B.P. Dam breach, paleo-earthquakes and the first fluvial progradation occurred around the same period. Therefore, it can be inferred that between 30 ka B.P. and 20 ka B.P., due to dam failure caused by the ancient earthquakes, the lake level descended and the river prograded for the first time, which caused a large outburst flood. At about 10 ka B.P., the dammed lake began to die out in stages, forming six levels of terraces in Tuanjie Village (Wang, 2009).

8. Conclusions 27

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The sediments on the upstream of the dam are distributed regularly. Vertically, the sequence is fine-grained silt and clay (Facies E, F, G, H, I and J), fluvial gravels and sand (Facies B2 and C), and debris flows (Facies A, B1 and D) from bottom to top. The top of the sequence is sometimes covered with rockfall deposits or loess. In the longitudinal profile, fluvial sediments (Facies B2 and C) are mainly

f

between Yonghe Village and Zhenping Village. Flood sediments after fluvial

oo

progradation are dominant between Zhenping Village and Jiefang Village. A large

pr

volume of sediments was carried by floods to enter the standing lake. As transport

e-

distance increased, the decelerating flow generated successively alternating layers

Pr

of gravels (Facies B2 and C) and sand (Facies E1), followed by alternating layers of silt with low-angle cross-stratification (Facies F), parallel bedding (Facies E),

al

and climbing ripples (Facies G). Rhythmically bedded silt (Facies J) is dominant

rn

between Jiefang Village and Taiping Village. The deep-water area of the lake is

Jo u

between Taiping Village and Tuanjie Village, and horizontally laminated or massive silt and clayey silt (Facies I) is dominant. The general feature is that grain size of the sediment is finer, and facies become more uniform from the proximal to distal parts of the lake.

The elevation of the lake surface controlled the development of terraces and progradation of the fluvial system. Therefore, these are closely related in time and elevation. In the field, gravel layers formed by four progradations of the fluvial system were found. Comparing the elevation of lake surface based on terraces and fluvial progradation, it can be inferred that the initial elevation of lake surface was 28

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2400 m when the dammed lake was first formed. At this time, the end of the lake is in Yonghe Village. After the first fluvial progradation, the lake surface was lowered by about 50 m, and lacustrine sediments at the top elevation of 2375.29 m emerged from the water surface. After the second fluvial progradation, the lake surface dropped by 20 m. After the third fluvial progradation, the lake surface

f

dropped again by about 20 m. Terrace Ⅵ formed after these two fluvial

oo

progadations, probably due to the short period between these events. The elevation

pr

of the lake after the fourth progradation of the fluvial system is 2220 m, which

e-

corresponds to terrace Ⅳ. If the evolution stages of dammed lake are judged based

Pr

only on the times of fluvial progradation from the field, then it would be incorrect to believe that the evolution of dammed lake has gone through five stages. If the

al

times of fluvial progradation are judged based only on the number of terraces at the

rn

distal part of the lake, then one case may be ignored, i.e., the interval between

Jo u

multiple progradations is short, and only a single terrace may be formed. Therefore, when studying the entire evolution of the dammed lake, attention should be paid not only to the fluvio-lacustrine terraces formed by the tributaries, but also to the sediments formed by progradation of the fluvial system upstream of the terraces.

Acknowledgements This research study was supported by the National Natural Science Foundation of China (Grants nos. 41571012 and 41230743), and the National Key R&D Program of China (Grant no. 2018YFC1505003). We are greatly indebted to Dr. J.X. Ma 29

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and Dr. S.E. Wu for sampling and field assistance. We sincerely thank Professor Jasper Knight and the two anonymous reviewers for their constructive comments and advice for the substantially improved manuscript.

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Zavala, C., Arcuri, M., Di Meglio, M., Diaz, H.G., Contreras, C., 2011. A genetic facies tract for the analysis of sustained hyperpycnal flow deposits. In: Slatt, R.M., Zavala,

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C. (Eds.), Sediment Transfer from Shelf to Deep Water-Revisiting the Delivery System. AAPG Studies in Geology 61, pp. 31-51.

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Zavala, C., Ponce, J.J., Arcuri, M., Drittanti, D., Freije, H., Asensio, M., 2006. Ancient

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lacustrine hyperpycnites: A depositional model from a case study in the Rayoso

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Formation (Cretaceous) of west-central Argentina. Journal of Sedimentary

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Research 76, 41-59.

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Zhang, H.P., Yang, N., Zhang, Y.Q., Meng, H., 2006. Geomorphology of the Minjiang

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drainage system (Sichuan, China) and its structural mplications. Quaternary Sciences 26, 126-135 (in Chinese, with English Abstr.).

Zhang, J.P., Ye, Y.Q., Fan, H., 2002. Studies on Grassland Resource and Rational Utilization in the Upper Reaches of Minjiang River. Journal of Mountain Science 20, 343-347 (in Chinese, with English Abstr.).

Zhang, Y., Zhu, L.D., Yang, W.G., Luo, H., Jiang, L., He, D.F., Liu, J., 2009. High resolution rapid climate change records of lacustrine deposits of Diexi Basin in the eastern margin of Qinghai-Tibet Plateau, 40-30 ka BP. Earth Science Frontiers 16, 91-98 (in Chinese, with English Abstr.). 38

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Zhang, Y.S., Zhao, X.T., Lan, H.X., Xiong, T.Y., 2011. A Pleistocene landslide-dammed lake, Jinsha River, Yunnan, China. Quaternary International 233, 72-80.

Zhang, Y.Z., Huang C.C., Pang, J.L., Zhou, Y.L., Shang, R.Q., Zhou, Q., Guo, Y.Q., Liu, T., Hu, G.M., 2015. OSL dating of the massive landslide-damming event in the Jishixia Gorge, on the upper Yellow River, NE Tibetan Plateau. The Holocene 25,

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745-757.

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List of figures: Fig. 1. (A) Location of Fig. 1B. (B) Geological map of the Minjiang drainage basin showing location of Fig.2 (after Zhang et al., 2006).

Fig. 2. Distribution of Diexi Ancient Dammed Lake (after Ma et al., 2018). Shanghaizi

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in 1933, and Diexi old town are submerged in Xiahaizi Lake.

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Lake and Xiahaizi Lake are modern barrier lakes induced by the Ms. 7.5 Diexi earthquake

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Fig. 3. The map of the study area showing the location of 44 sections. AA’ is the location of

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Fig. 9B.

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Fig. 4. Field photographs showing sedimentary facies. (A) Clast-supported, inverse-grained gravels (Facies A). (B) Clast-supported, massive rounded gravel (Facies B2), having

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erosional contact with underlying Facies E1. (C) Clast-supported, massive angular gravels

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(Facies B1) which contain agglomerates of underling Facies J. (D) A thin silt layer is

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interbedded between matrix-supported, massive gravels (Facies D) and clast-supported, horizontally stratified gravel (Facies C). (E) Significant deformation structures are observed in the upper part of Facies J because of compression of Facies B1.

Fig. 5. Field photographs showing sedimentary facies. (A) Parallel bedded silt to sand with clay chips (Facies E2). (B) Superposition of parallel bedding (Facies E1), low-angle crossstratification (Facies F), horizontal laminae (Facies I) and climbing ripples (Facies G). The lower bounding surface of Facies I conforms to the geometry of the underlying layer. (C) Climbing-ripple laminated silt with ripple amplitudes gradually decreasing upward (Facies G3). (D) Climbing-ripple laminated silt (Facies G1). The wave ridges migrate in the 40

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direction of the current while growing upwards, forming a series of nearly parallel pseudointerfaces. Facies G1 has sharp contact with underlying parallel bedded silty sand and sandy silt (Facies E1). Laminae in Facies E1 are mm-scale, straight and parallel, probably because of lower sedimentation rates. (E) Climbing-ripple laminated silt with superimposed undulating laminae (Facies G4). (F) Low-angle cross bedded sandy silt (Facies F)

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displaying tangentially downlapping foresets. (G) Lenticular bedding showing yellow layers

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are wrapped in gray layers (Facies H). (H) Wavy bedding consisting of an alternation of

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yellow and gray undulating successive layers (Facies H). (I) Flaser bedding showing gray

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layers are mainly preserved in the trough of yellow layers (Facies H).

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Fig. 6. (A) Rhythmically bedded silt composed of regularly alternating light-yellow and dark-gray laminae. (B) Horizontally laminated or massive silt and clayed silt (Facies I). (C)

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The layer of clayey silt with a flat bottom, intrudes upward into fine sand (D) Smaller-scale

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coarse-grained rhythms composed of regularly alternating parallel bedded sand (Facies E1)

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and clast-supported, massive rounded gravel (Facies B2). (E) Terrace Ⅲ, fluvial gravels (Facies C) has erosional contact with underlying lacustrine deposits (Facies I).

Fig. 7. (A) Section 11, showing larger-scale coarse-grained rhythms composed of regularly alternating clast-supported, massive rounded gravel (Facies B2) and parallel bedded sand (Facies E). (B) Section 9, showing debris flows (Facies B1) overlying fine-grained rhythms formed by cyclical superposition of low-angle cross-stratifications (Facies F), climbing ripples (Facies G), and parallel bedding (Facies E).

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Fig. 8. (A) Longitudinal profile of dammed-lake showing locations of sections on east bank of river except Sections 31, 35 and 40. (B) Lithofacies column of the sections shown in Fig. 8A. Segment 1 is shore region when the lake level is the highest (2400 m). After the first fluvial progradation, flood sediments are deposited in Segment 2. After the second fluvial progradation, flood sediments are deposited in Segment 3. After the third fluvial

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progradation, flood sediments are deposited in Segment 4, and Facies J in the deep-water

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area begins to slowly emerge from lake. The last fluvial progradation is in Segment 5, and

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flood sediments are deposited on Facies I in the deep-water area.

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Fig. 9. Comparison of lake surface elevations based on fluvial progradation (Fig. 9A) and

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terraces (Fig. 9B, after Wang., 2009, see Fig. 3 for the location). Terraces Ⅴ and Ⅳ were formed between 8496 ~8255a B.P. and 4880 ~ 4741a B.P., respectively, and terrace Ⅲ was

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formed at 3428 ± 82a B.P. (Wang et al., 2012). The highest water level of the dammed lake

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is 2400 m. After the first progradation, the topmost lacustrine sediments (2357.29 m) have

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already emerged from the water surface. After the second fluvial progradation, the lake surface dropped by 20 m. After the third fluvial progradation, the lake surface dropped by about 20 m. Terrace Ⅵ was formed after these two fluvial progadations. After the fourth fluvial progradation, elevation of the lake surface is 2220 m, and terrace Ⅳ was formed.

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1. Logging the lithofacies of 44 sections from the proximal to distal parts of the Diexi Ancient Dammed Lake.

2. Describing and interpreting distribution of sedimentary facies in the longitudinal profile.

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3. Linking sedimentary facies with fluvio-lacustrine terraces to further analyze the

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evolution of dammed lake.

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

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Figure 8

Figure 9