# Episodic rainfall events characterise complex sediment deposition in a fault-bounded sag pond in Northwest China

Chris Gouramanis, Stephen Chua, Marie Etchebes, Yann Klinger, Xiwei Xu, Gao Mingxing, Adam D. Switzer, Gary Hancock, Paul Tapponnier

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Chris Gouramanis, Stephen Chua, Marie Etchebes, Yann Klinger, Xiwei Xu, et al.. Episodic rainfall events characterise complex sediment deposition in a fault-bounded sag pond in Northwest China. Geomorphology, 2025, 470, 10.1016/j geomorph.2024.109538. insu-05080932

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# Episodic rainfall events characterise complex sediment deposition in a fault-bounded sag pond in Northwest China

Chris Gouramanis<sup>a, *, 1</sup>, Stephen Chua<sup>b, c, 1</sup>, Marie Etchebes<sup>d</sup>, Yann Klinger<sup>e</sup>, Xiwei Xu<sup>f</sup>, Gao Mingxing<sup>g</sup>, Adam D. Switzer<sup>b, c</sup>, Gary Hancock<sup>h</sup>, Paul Tapponnier<sup>b, 2</sup>

## ARTICLE INFO

**Keywords:**
Earthquakes
Storms
Colluvial wedges
Talus cones
Grain size analysis

## ABSTRACT

Fault-bounded sag pond sediment records are commonly found to be excellent archives of palaeoearthquakes and more rarely, they can provide robust evidence of local environment and climate change. We examined the sedimentary and geomorphological record of the Aksay Pond within the Karachingar Valley along the southern margin of the Altai Mountain Range in northwestern China. This pond is associated to a shutter ridge resulting from cumulative deformation associated with successive earthquakes along the Fuyun Fault, including the most recent 11 August 1931 $M_w$7.9 earthquake. However, detailed chronology based on $^{14}$C Accelerator Mass Spectrometry, $^{210}$Pb and $^{137}$Cs dating suggests that only deformation related to the 1931 earthquake have been preserved at the pond site. A small wedge of sediments comprising cobble and gravel sized clasts are separated by two palaeosoil deposits suggesting that very minor sediment could accumulate prior to 1931. In the northern part of the pond, sediment was deposited predominantly from a colluvial fan that has periodically been mobilised from the steep mountain slopes to the east. In the southern part of the pond we find that the dry climate of the region has experienced periodic storms that have resulted in the deposition of 11 fining upwards packages formed by remobilised and deposited sand to mud-sized sediments within the pond. Associated with these fining upward cycles are two colluvial wedges that emanate from the fault scarp. The upper wedge is dated at approximately 1986 CE and coincides with a high precipitation event recorded at the Fuyun Meteorological Station in 1986. The middle wedge may be related to an earlier high precipitation event, but due to challenges in obtaining accurate ages, cannot be verified. Alternatively, these two colluvial wedges could coincide with minor ca. $M_w$5 earthquakes in the region. Importantly, these wedges are not associated with large scale rupturing similar to the 1931 earthquake. Based on the sedimentary and geomorphological evidence of the Aksay Pond, we propose a model for the sequential formation of sag ponds highlighting the influence of both climatic and tectonics processes.

## 1. Introduction

Determining the long-term seismological history of a region from geological archives is a fundamental requirement to evaluate the future

hazard related to seismological activity in a region. This can be achieved through individual or combined analysis of geomorphic features (e.g. Klinger et al., 2011; Wang et al., 2024), archaeological sites (e.g. Forlin et al., 2024), subsurface geophysical profiling (e.g. Schnellmann et al.,

<footnote><sup>a</sup> Research School of Earth Sciences, The Australian National University, 2600, Australia</footnote>

<footnote><sup>b</sup> Earth Observatory of Singapore, Nanyang Technological University, 639798, Singapore</footnote>

<footnote><sup>c</sup> The Asian School of the Environment, Nanyang Technological University, 639798, Singapore</footnote>

<footnote><sup>d</sup> ELIS, Clapiers, 34830, France</footnote>

<footnote><sup>e</sup> Université Paris Cité, Institut de Physique du Globe de Paris, CNRS, Paris, France</footnote>

<footnote><sup>f</sup> School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China</footnote>

<footnote><sup>g</sup> School of Geology and Mining Engineering, Xinjiang University, Urumqi, 830047, China</footnote>

<footnote><sup>h</sup> Formerly of the Commonwealth Scientific and Industrial Research Organisation, Acton, ACT 2601, Australia</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: Chris.Gouramanis@anu.edu.au (C. Gouramanis).</footnote>

<footnote n="1">¹ Chris Gouramanis and Stephen Chua are to be considered as joint first authors on this paper.</footnote>

<footnote n="2">² Deceased</footnote>
2007), excavations, such as trenching (e.g. Audemard et al., 1999; Chen et al., 2022; Lienkaemper et al., 2002; Liu-Zeng et al., 2015; Sieh et al., 2012; Wang et al., 2021), coring (e.g. Hubert-Ferrari et al., 2012; Kirby et al., 2024), or through geophysical or tectonic modelling (e.g. Tapponnier and Molnar, 1976). These methods are frequently used in concert to derive detailed seismological studies beyond the limitations of individual methods.

Lacustrine and coastal environments have been extensively studied to reconstruct palaeoseismological activity (e.g. Ben-Menahem, 1976; Fan et al., 2020; Lienkaemper et al., 2002; Moernaut, 2020; Sabatier et al., 2022; Sims, 1973; Sieh et al., 1989; Bektaş et al., 2024). Deep (e.g. Moernaut, 2020; Sabatier et al., 2022; Lu et al., 2020; Brothers et al., 2024) and shallow (e.g. Fan et al., 2020; Howarth et al., 2018) lakes in seismically active regions can preserve seismites – sedimentological records formed by the redistribution of sediments within the lake basin from seismic activity that can inform on regional seismic activity. Other lacustrine systems, such as fault-bounded ponds (e.g. sag ponds) or pull-apart basins, can preserve the direct activity of faults, such as fault traces that show the spatial offsets of the stratigraphy caused by faulting (e.g. Nelson, 1992; Pavlides et al., 2006; Daëron et al., 2005; Daëron et al., 2007), fault reactivation or recurrence history (e.g. Fan et al., 2020; Hubert-Ferrari et al., 2012; Sieh et al., 1989; Toké et al., 2011; Lu et al., 2020), and environnemental change (e.g. Fan et al., 2020; Hubert-Ferrari et al., 2012; Kirby et al., 2024). Due to this wealth of potential information, small fault-bounded ponds are ideal locations to examine local and regional palaeoseismological activity, as well as palaeoenvironmental and palaeoclimatological change.

Fault-bounded ponds often occur within valleys that have been shaped by the fault and bounded by steep slopes or translocated shutter ridges (e.g. Wang et al., 2024; Burbank and Anderson, 2012). The slopes can contribute sediments to the lakes either through dry or wet landslides, sheetwash following precipitation, or from collapse of bounding fault scarps (e.g. Nelson, 1992; Burbank and Anderson, 2012). Within the valleys, permanent or ephemeral streams can contribute sediment when flooding overtops the banks of the ponds (e.g. Kirby et al., 2024; Toké et al., 2011). Aeolian redistribution of sediments from near and far-field sources also contribute to lake sediments, as do chemical

precipitates, and biological material deposited within and from outside the lake (e.g. Kirby et al., 2024, Toké et al., 2011). Water can pool on the sediments, evaporate or infiltrate to contribute to groundwater table. During and after deposition, the sediments can be significantly modified through subsequent faulting and cracking (e.g. Audemard et al., 1999; Lienkaemper et al., 2002; Toké et al., 2011), deformation, including liquefaction from subsequent faulting (e.g. Pavlides et al., 2006; Sims, 1973), erosion and or deflation (e.g. Toké et al., 2011), desiccation (e.g. Toké et al., 2011) and/or bioturbation (e.g. Burbank and Anderson, 2012). As such, sedimentary sequences within fault-bounded ponds are complex, potentially recording climatic, ecological and geophysical processes within and external to the lake basin that must be deconvolved to distinguish the timing and contribution of each process (e.g. Fan et al., 2021; Fan et al., 2020; Kirby et al., 2024).

In this study, we present a very-high resolution record of complex sedimentary stratigraphy preserved in a fault-bounded sag pond at the southern foot of Altai Mountains in Xinjiang Province, northwestern China (ca. 1670 masl, Fig. 1). The purpose of the study is to elucidate the stratigraphic evolution of the pond and demonstrate the stochastic nature of sedimentation in this pond from geophysical and climatic events. This will provide a fine-scale assessment of climate events, geophysical hazards and sedimentological processes that are not discernible from other records.

## 2. Geomorphology and regional tectonic setting

The Altay Mountains span 2525 km along the borders of northern China, western Mongolia, eastern Kazakhstan and southern Russia and has a mean elevation of 1500 to 1750 m above sea level. Within the narrow Aksay Valley, between the Karachingar Mountain and the Altai Mountains, we targeted a 60 m-long, 15 m-wide fault-bounded sag pond (Aksay Pond: 46°43′14.10″N, 89°55′44.53″E). The pond is dammed to the south by a prominent shutter ridge (Fig. 2), and by a steep, 1 to 7 m-high, 70° northeast-dipping fault scarp that bounds the east side of the shutter ridge and west side of the pond (Fig. 2d, 3).

The fault scarp maps the surface expression and deformation caused by activity on the Fuyun Fault. The Fuyun Fault is one of several active,

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** a) Regional map of central Asia showing the location of the major climate systems, black dashed line is the northern extent of the Asian monsoon system (Chen et al., 2015; Xiao et al., 2004) and dashed box area is the map boundary of Fig. 1b. b) 100 km² region around the Aksay Pond (red star) showing the fault trace of the Fuyun Fault (red dashed line) and other active faults (black lines), historical earthquake epicentres (black circles with date of rupture and moment magnitudes with high confidence and blue circles with low confidence - data from the International Seismological Centre-Global Earthquake Model (ISC-GEM) Global Instrumental Earthquake Catalogue (Di Giacomo et al., 2018; Storchak et al., 2013; Storchak et al., 2015)), Fuyun Meteorological Station (FMS, green circle), KYS and DEN tree ring records (Chen et al., 2016; Chen et al., 2014; Davi et al., 2009) and Bulgan Gol tree ring record (Davi et al., 2010), Yileimu Lake (Fan et al., 2021; Fan et al., 2020). The satellite image courtesy of Google Earth Pro v. 7.3.4.8248, Image data: Landsat/Copernicus.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** a) and b) LiDAR images of the Karachingar Valley with Karachingar Mountain to the west and Altai Range to east and showing Aksay Pond, the colluvial fan, fault trace and the offset ridge, white arrows signify direction of sediment movement; and, c), d) and e) photos showing the geomorphological setting of Aksay Pond from late September 2012 with very thin, patchy and impermanent snow cover; a) depicting topography in masl, and the four topographic profiles (P1–4) in Fig. 3; b) same as a) but with a perspective from the southwest and showing the locations of Trench A (TA) and Trench B (TB); c) view west from the slope of the Altai Mountains showing the offset of the fault scarp, offset ridge and Aksay Pond and freeze-thaw soil creep along slopes; d) and e) view east from the slope of the Karachingar Mountain showing the fault scarp, and, in d) the colluvial fan (note cars for scale in lower left); and in e) Aksay Pond and the southern tail of the colluvial fan and locations of Trench A and Trench B.</figcaption></figure>

north and northwest striking, right-lateral strike-slip faults bordering or cutting the eastern Altay Mountains (Fig. 1b). Together with larger, conjugate left-lateral faults in Mongolia (e.g. Tapponnier and Molnar, 1979), they absorb the last few millimetres of India/Asia convergence (e.g. Calais et al., 2003) left between Tibet and Siberia. On 11 August 1931, the Fuyun Fault generated a large $M_w$ 7.9 earthquake, with an average horizontal coseismic slip of 6 m (e.g. Klinger et al., 2011; Liang et al., 2021). Reconstructing the displacement of the shutter ridge from our LiDAR derived Digital Elevation Model yields cumulative horizontal and vertical offsets of 24 and 7 m, respectively. Based on the repetitive 6 m horizontal coseismic offsets measured on the fault to the south and north (e.g. Klinger et al., 2011, Liang et al., 2021), we inferred that the total displacement of the shutter ridge might correspond to 4 seismic events.

Aksay Pond is approximately 16 km south of the 1931 Fuyun earthquake epicentre (46°51'29"N, 89°53'17"E; International Seismological Centre–Global Earthquake Model (ISC-GEN) Catalogue (Di Giacono et al., 2018; Storchak et al., 2013; Storchak et al., 2015)). The ISC-GEN catalogue identifies 11 earthquakes with a $M_w$ > 5 recorded between 1917 and 2011 within 100 km of Aksay Pond (Fig. 1). Of these earthquakes, the ISC-GEN catalogue identifies the 11/8/1931 $M_w$7.9

Fuyun Fault Earthquake (red star Fig. 1b) and the 15/12/1980 $M_w$5.5, 24/4/1986 $M_w$5 and the 18/9/1987 $M_w$5.3 earthquakes (black dots Fig. 1b) as having high spatial, depth and magnitude confidence. The remaining seven earthquakes in the region from the ISC-GEN catalogue have low confidence (blue dots Fig. 1b).

Today the Aksay Valley is dry. Where the valley opens onto the Jungar Basin, a large alluvial fan has been deposited. Alluvial fans south and west of Karachingar Mountain still show surface rupture traces associated with the last Fuyun earthquake and the thrust fault beneath the Karachingar Mountain (Klinger et al., 2011), implying that fluvial activity in the region has been limited since at least August 1931. A small channel <0.5 m deep and up to 2 m wide snakes down the valley, but is truncated by the well-used dirt track. The slopes in the valley range from 40° to 60°, and at the north of the Aksay Pond, a colluvial fan has formed.

### 3. Modern climate of the southern Altai

The climate in the Altai Mountains is dominated by variation in the Northern Hemisphere Westerlies with no influence from the Asian Monsoon (Chen et al., 2015; Xiao et al., 2004). As such, precipitation in
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Topographic profiles across the pond. P1 to P3 are west to east oriented profiles across the colluvial fan and pond, P4 is a south to north oriented profile across the pond and southern margin of the colluvial fan (locations are found in fig. 2). Red line in P2 and P3 shows the height in metres above mean sea level (masl) of the base of the pond in trenches A and B, respectively.</figcaption></figure>

the region is sourced primarily from the North Atlantic after traversing much of the Eurasian continent (e.g. An, 2000; Wei and Gasse, 1999). In Xinjiang Province, direct rainfall and snowmelt events are more frequent in the high mountains compared to low-lying plains and basins (Schiemann et al., 2008; Shi et al., 2007). The low-lying plains and basins are subject to flooding when high precipitation or rapid snowmelt events occur (Liang et al., 2008; Tang et al., 2013; Zhang et al., 2012).

The city of Fuyun, located approximately 40 km northwest of our study site on the plains of the northern Xinjiang Province, has a climate

characterised by short, cool summers and long, severe winters (Fig. 4), with a warming and wetting trend observed since ca. 1987 CE (e.g. Shi et al., 2007; Xu et al., 2015; Yuan-An et al., 2013).

The mean annual temperature is 3° ± 15.2 °C with a mean annual precipitation of 167 ± 53 mm and snowfall typically occurring from October to March (e.g. Tang et al., 2013). The coldest month is January (average temperature of -20 °C) and the warmest month is July (average temperature of 21.3 °C). The wetter months span from April to August, with July being the wettest month (mean precipitation of 23 ±
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** a) Mean monthly precipitation (mm, pale blue bars), mean monthly temperature (°C, black line), mean monthly maximum temperature (°C, red line) and mean monthly minimum temperature (°C, blue line), spanning June 1961 to December 1990. b) Daily precipitation (mm) spanning 1st January 1961 to 31st December 2012. Both datasets come from the Fuyun Meteorological Station (World Meteorological Station: 51087 at 46°58′48″N, 89°31′12″E, elevation 803 m above sea level [masl] and accessed from KNMI Climate Explorer https://climexp.knmi.nl/getstations.cgi, on 18/12/2021).</figcaption></figure>

19 mm) and another peak in November (mean precipitation of 19 ± 12 mm) due to snowfall (e.g. Shi et al., 2007). The dry months are September and October, followed by December to March, with February the driest month (~7 ± 5 mm; Zhu et al., 2015). Daily precipitation fluctuates from as much as 40 mm per day to prolonged periods of no rainfall (Fig. 4b). Annually, Fuyun has snow cover for 120 days with an average thickness of 60 cm with a maximum thickness of 1–2 m in the high mountain regions (Liang et al., 2008). At the time the field work was conducted in late September 2012, snow was present on the slopes within the valley (Figs. 2c-e) but rapidly melted and infiltrated the sediments with no overland flow observed. The well-developed freeze-thaw soil creep along the slopes within the Karaxingar Valley (Fig. 2c) indicates that snow melt infiltration is prevalent.

# 4. Methods

## 4.1. Trenching across the Aksay pond

To explore the past climatological and sedimentological history of the Aksay Pond, we excavated two main trenches, A and B (≈ 20 m-long, ≈ 3 m-deep, perpendicular to the fault-trace) across the northern and

southern tips of the Aksay Pond, respectively (Fig. 2c, e). The trench depths were limited by the local groundwater table. Due to logistical constraints, we could not excavate a north-south trending trench connecting trenches A and B. To enable detail stratigraphic logging, the walls of both trenches were photographed (using a Canon EOS 40D camera) and scanned using terrestrial LiDAR. Upon visual inspection, the northern and southern walls of Trench A did not differ significantly, nor did the northern and southern walls of Trench B. Consequently, only the northern wall of Trench A (Fig. 5) and the southern wall of Trench B (Fig. 6) will be discussed in detail.

## 4.2. Pond sediment description and sampling

Here we describe the thickest sequences and lateral variability of sediments from the northern wall of Trench A (Fig. 5) and the southern wall of Trench B (Fig. 6). Sediments in Trench A are coarser (individual clasts up to 5 cm) than those in Trench B.

Representative samples were collected from Trench A (Fig. 5), but the grains were too coarse for $^{210}$Pb and $^{137}$Cs analysis. By contrast, in Trench B, we collected 1 cm-thick slices (≈ 50 g) at 1-cm intervals, of the finer sediments (i.e. grainsize <2 mm) of the thickest section in the
<figure><img src="image_7.png" /><figcaption>Fig. 5. Cross-section of Trench A (northern wall); a) photo of the trench wall; and b) interpretive image of the trench wall showing facies relationships and the collection location of the ¹⁴C AMS dating samples.</figcaption></figure>

pond. The laboratory analysis of these samples was performed at the Asian School of the Environment, Nanyang Technological University.

### 4.3. Loss-on-Ignition (LOI)

The organic and carbonate content in the collected sediments were determined using Heiri et al. (2001)'s method for loss-on-ignition (LOI). Heating the sample to different temperatures indicates the weight percent of water content (105 °C, LOI₁₀₅), organic content (550 °C, LOI₅₅₀) and carbonate content (950 °C, LOI₉₅₀). The remainder of the sediment is the clastic component. LOI₁₀₅ will not be discussed further as it is unclear how much moisture was lost from the time of sampling to the time of analysis.

### 4.4. Sediment grain size analysis

Approximately 5 g of each sediment sample was pre-treated with 10 v/v% hydrochloric acid (HCl) to remove carbonate. Upon the completion of the reaction, the sample was washed with distilled water and centrifuged at 2500 rpm for 8 min and the supernatant decanted. Subsequently, each sample was treated (up to 3 times) with 15 v/v% hydrogen peroxide (H₂O₂) to remove organic matter and disassociate clays, and once the reaction was completed, the sample was centrifuged and the supernatant decanted. Following the chemical treatment, particle size analysis was performed using the Malvern Mastersizer 2000 where samples were initially sonicated for 60 s and three replicates were averaged (Blott et al., 2004). Where the relative standard deviation of the mean grain size values exceeded 5 %, the sample was re-analysed. The Malvern Mastersizer produces 100 binned grain size measurements from 0.02 µm to 2 mm providing detailed information on the sediment grain size distributions for comparison (raw data available in

Supplementary Information).

### 4.5. Chronology

#### 4.5.1. ²¹⁰Pb and ¹³⁷Cs dating

Twelve fine-sediment samples (~10 g) were selected from the southern wall of Trench B (Fig. 6) and sent for ²¹⁰Pb and ¹³⁷Cs dating at the Institute of Geological and Nuclear Sciences Limited (GNS) Science Cosmogenic Isotope and Radiochemistry Laboratory in New Zealand. The samples were selected close to sedimentary contacts, within units that laterally correlate to the sediment wedges near the scarp face and intermediate layers to refine the ¹⁴C chronology (Fig. 6).

Activities of ²¹⁰Pb, ¹³⁷Cs and ²²⁶Ra were analysed on high-resolution, low-background well-type Germanium Gamma Spectrometer using 46 keV (²¹⁰Pb), 186 keV (²²⁶Ra) and 661 keV (¹³⁷Cs), and the detector was calibrated for ²¹⁰Pb and ²²⁶Ra using a pitchblende standard in quartz (GNS, pers. comm).

#### 4.5.2. Radiocarbon dating

Thirty-two charcoal, wood fragments, and plant fibres were collected from different horizons from the trenches in Aksay Pond and analysed at the Scottish Universities Environmental Research Centre (SUERC) for ¹⁴C Accelerated Mass Spectrometry (AMS) radiocarbon dating. The uncalibrated ¹⁴C ages that predate 1950 were calibrated using IntCal20 (Reimer et al., 2020). Uncalibrated ¹⁴C dates that post-date 1950 were calibrated using the post-bomb fractionated carbon (F¹⁴C) and calibrated using Hua et al. (2021) using the Northern Hemisphere Zone 1 (NHZ1) post-bomb calibration sets to obtain accurate dates on the post-1950 material.
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** As in Fig. 5 but for Trench B (southern wall). The black dashed box in b) shows the location where the sediment, $^{210}$Pb and $^{137}$Cs samples were collected (see Figs. 7 and 10).</figcaption></figure>

# 5. Results and interpretation

## 5.1. Stratigraphy

Detailed field observations of Trench A (northern wall, Fig. 5) and Trench B (southern wall, Fig. 6) show several distinct facies.

## 5.2. Trench A

The stratigraphy of the northern wall (Fig. 5) is also compared with the southern wall of Trench A (Supp Info Fig. 1). The two walls are two metres apart.

### 5.2.1. Facies A

#### 5.2.1.1. Description.
The base of this unit is not observed in the trench and the upper contact of this unit occurs at 144 cm below ground surface (bgs) in the centre of the trench but curves upward towards the modern ground surface to the east and west (Fig. 5). The unit consists of angular, often platy but also equidimensional boulders and cobbles supported by a matrix of clay and silt. The rock clasts are randomly oriented, but occasionally dip towards the centre of the pond to the west where there has been some uplift from the fault, and to the east where the mountain slopes downhill (Fig. 5).

#### 5.2.1.2. Interpretation.
This unit is the weathering profile of the underlying metamorphosed and foliated Late Devonian volcanic rocks. Some of this sediment unit may have been eroded and transported down the adjacent mountain slope during precipitation or minor slope failure events, or freeze-thaw processes. The curvature associated with this unit is due in part to the fault scarp to the east and parallels and becomes the mountain slope to the west.

### 5.2.2. Facies B

#### 5.2.2.1. Description.
An approximately 10 cm-thick, matrix-supported, brown/black muddy organic-rich layer (based on qualitative observations) drapes the upper surface of Facies A (Fig. 5). It contains large angular cobble and boulders derived from the weathering of the underlying rocks that are aligned parallel to the bedding surface and increasing in prevalence towards the top of the layer. Roots of plants extend from the upper boundary into this unit. Occasional large cracks that are infilled by sediments from Facies D are preserved (Figs. 5 and 6), and these are inferred to be co-seismic. This unit contains abundant charcoal and decomposing macro-organic fragments (i.e. leaves, roots and stems). This organic-rich layer is thickest in the central part of the pond where it is ca. 30-cm thick near a large boulder in the centre of the base of the ponded sediments, but locally thins to <1-cm thick above large boulders or disappears beneath overlying boulders (Figs. 5 and 6). To the east, the unit parallels the upward curvature of the pond draping the upper surface of Facies A. To the west this unit terminates abruptly against ca. 30-cm thick steeply dipping and aligned cobble and boulder clasts of Facies A to the west of the centre of the pond. Further west, the unit resumes as a large (ca. 50 cm diameter) accumulation that sits conformably between Facies A sediments to the east, west and below (Figs. 5 and 6). The upper part of this accumulation drapes the upper contact of Facies A extending west to the modern ground surface. Facies B also drapes Facies C1 in the centre of the pond and extends approximately one metre merging with the lower organic unit to the east (Fig. 5).

#### 5.2.2.2. Interpretation.
This unit represents the A-soil horizon where organic material is mixed with weathered bedrock clasts to form an upper soil. This soil drapes Facies A and maps the base of the pond, becoming part of the mountain slope to the east and following the fault scarp to the west (Fig. 5). In the centre of the pond of the northern wall,
the basal soil terminates against a small fault scarp (Fig. 5) containing aligned angular pebbles with a steep downward dip. A horizontally-bedded upper soil overlies a wedge of Facies C1 and merges with the lower soil. This feature is not seen in the southern wall, suggesting only a localised or small-scale feature. The upper surface of this soil profile was the former ground surface prior to the deposition of Unit D.

### 5.2.3. Facies C1

#### 5.2.3.1. Description.
Triangular cross section of large, angular clasts supported in a sandy matrix that tapers to a beige sand below the merging of the two soil (Facies B) units towards the east (Fig. 5). The clasts are randomly oriented. The thickest section of this unit occurs in the west where it abuts the steeply dipping angular clasts of Facies A.

#### 5.2.3.2. Interpretation.
This unit was formed by slope failure consisting of sediments and clasts from the up-thrown section of Facies A at the base of the pond (e.g. Nelson, 1992). As this unit is matrix supported, the wedge was likely formed subaqueously (e.g. Nemec and Kazanci, 1999). The soil that forms above this wedge accumulated after the wedge was developed. This unit is not observed in the southern wall of Trench A suggesting a localised mobilisation of sediments forming this wedge.

### 5.2.4. Facies D

#### 5.2.4.1. Description.
Basal gravelly coarse to medium sand grading normally to very fine sand beds, that are often parallel laminated or have

thin, weakly developed ripple cross beds dipping east. Occasionally in the southern wall of Trench A, the sand and mud have sharp contacts. Facies D drapes Facies B and the lowest layer is also the thickest (13 cm) in the centre of the pond and thins laterally above the soil (Fig. 5). Several layers of Facies D are found interbedded with the Facies E pond deposits mapped in Trench A often forming two and three stacked fining upwards sequences. Towards the top of the section in Trench A, sand layers extend from the fault scarp into the pond and form layers that pinch out towards the centre of the pond (Fig. 5). These sand layers are also slightly concave downwards indicating episodic deposition with layers draping older deposits (Fig. 5). Occasionally the lower laminated beds often terminate abruptly against larger cobbles and upper laminations drape the cobbles and form discontinuous lenses that extend up to two metres horizontally and are <10 cm thick (Fig. 5).

In the southern wall of Trench A, the sandy layers are more numerous and range from a beige, medium sand to a dark, chocolate-brown mud. The layers are continuous across the pond lying horizontal, or their upper and lower contacts are slightly undulating, often being sandier closer to the edge of the pond and becoming muddy towards the centre. The layers often show minor erosional bases with flame and pillow structures and convoluted laminations where sand layers overly mud.

In one location in the western section of the southern wall of Trench A, a large angular boulder aligned parallel to the dipping surface of Facies A disrupts the sandy-muddy layers and the weak laminae bifurcate against the leading edge of the boulder. Here the lower laminae bend beneath the boulder and laminae above the leading edge terminate

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** a) Trench A northern wall, showing two fining upwards sequences. Arrows show fining direction, top of trench is to the top of the photo, scale bar = 2 cm. Note sharp upper and lower contacts. b) Trench A northern wall showing complex bedding associated with Facies D (finer laminated sediments) and Facies E (coarse sediments). L = parallel laminations, R = ripple cross beds, TL = terminated laminae, DL = draping laminae, Ra = charcoal radiocarbon sample, CSG = clast-supported gravel, MSG = matrix-supported gravel. Scale bar = 5 cm. c) Trench A southern wall showing complex bedding associated with Facies D (finer laminated sediments) and Facies E (coarse sediments). P = pillow structures of brown mud in tan very fine sand, CB = complex bedding, PGL = pinching out gravel lens, CSG = clast-supported gravel showing slight imbrication dipping into photo, MSG = matrix-supported gravel. Scale bar = 5 cm (Note this image was taken after extensive cleaning of the trench wall at this location). d) Trench A southern wall showing boulder (B) that has slid down the fault scarp and the leading edge of the boulder as separated a lamination with part extending above and below the boulder. Younger sediments onlap (O) the boulder. Scale bar = 5 cm.</figcaption></figure>
on the upper surface of the boulder (Fig. 7). Units above this sandy-muddy layer onlap with the boulder.

**5.2.4.2. Interpretation.** Facies D is formed by distal and waning sheet flooded sediments (Facies E) deposited in the pond similar to flood deposits on a river floodplain, but formed from direct precipitation onto the surrounding slopes. Toké et al. (2011) inferred similar units from Parkfield, USA, as waning stages of historical floods. However, the lack of evidence of recent fluvial activity in the Karachingar Valley (i.e. no evidence of flow structures) suggests that down-valley fluvial activity has been minimal in the recent history of the valley, with little evidence for sufficient water depths or flow conditions to generate sufficient overbank flow into the sag pond. The fining upward sequences, flame and pillow structures and convoluted laminations reflect subaqueous sedimentation with an influx of sand waning to mud as background pond sedimentation resumed after rainfall or snowmelt sheetflow from the slopes floods the sag pond (e.g. Ventra et al., 2013). Minor freeze-thaw slip structures occur on the slopes. Large boulders slid down the slope face becoming embedded in the fine sediments and disrupted bedding and later sediments onlap these boulders (e.g. Nelson, 1992; Wasson, 1979). The increased number and larger lateral extent of Facies D units in the southern wall of Trench A indicates a significant decrease in gravitational energy from the source of the sediments at the northern end of the pond, albeit in a distance of two metres. It is inferred that the sediments of Facies D washed down the mountain slope to form the fan and entered the pond from its northern margin.

### 5.2.5. Facies E

**5.2.5.1. Description.** Facies E shows the greatest variability of all of the facies preserved in Aksay Pond and is only found in the northern and southern walls of Trench A. The onset of Facies E is distinct in Trench A, consisting of lenticular to undulating bodies of angular to very angular platy to equidimensional pebbles and cobbles (Fig. 5). The lenticular bodies can extend horizontally up to three metres, whereas the undulating layers can extend up to five metres across the pond (Fig. 5). Both lenticular and undulating bodies are <30 cm thick. The lenticular bodies pinch out sharply and can be separated from other lenses by thin units of Facies D. Both lenticular and undulating bodies have the coarsest material in the centre and both show lower inverse grading and upper normal grading. The lower contacts are sharp and non-erosive, often preserving the basal clasts impregnating the lower finer sands and muds of Facies D (Fig. 7). The upper contacts are also sharp but occasionally and locally grade into Facies D (Fig. 7). The individual units are either wholly clast supported forming an open framework or clast supported with interstitial silty sands, the latter being the most common. A weak imbrication is also present in some of the units with clasts dipping to the south.

**5.2.5.2. Interpretation.** This facies is formed by mass movement of sediment down slope and into the northern margin of the Aksay Pond (Fig. 2a,b). The sediment originated from the slope of the eastern mountain, and concentrated as sediment debris flows down the gully forming a colluvial fan at the base of the mountain (Fig. 2a,b). The morphology of the colluvial fan is partially controlled by the Fuyun Fault scarp that prevented sediments to move beyond the scarp to the west. The remainder of the sediments are funnelled to the south and north to form an elongate apron bounded to the east by the mountain and to the west by the fault scarp (Fig. 2a,b). The southern margin of the apron flows into the pond as a series of both open framework, grain supported deposits caused by dry grain flow, and deposits of clast supported with interstitial silty sands caused by sheet-flow (e.g. Nemec and Kazanci, 1999; Robustelli et al., 2005; Wasson, 1979) (Fig. 2a,b; Fig. 5). Imbricate clasts signify the contribution of water to downslope movement (e.g. Brancaleoni and Shnaider, 2021; Nelson, 1992). The top of the soil

(Facies B) is slightly higher in Trench A (i.e. 1664.8 masl, Fig. 3b) than in Trench B (1664.4 masl, Fig. 3c) and this would have promoted the horizontal movement of sediment to the south, however, the energy of the inflowing water and sediment decreases rapidly resulting in the coarse clasts to be localised in the north of the pond. Two metres to the south of the northern wall (Trench A Southern Wall), the increase in the lateral extent and number of fining upward sand sequences (Facies D) attests to the decreased clast mobility to the south within the pond.

As most of the Facies E units have silty sands infilling the cobble interstices, this implies that the downslope movement is predominantly by subaqueous sheet flow with entrapment of fine sediments between the larger clasts (e.g. Nelson, 1992; Nemec and Kazanci, 1999; Robustelli et al., 2005; Wasson, 1979). These sheetflow deposits likely formed from localised heavy precipitation events upon the eastern mountainside. Grain supported deposits may be related to local seismic events, but as these cannot be dated accurately and this remains untested.

### 5.3. Trench B

There was very little difference between the northern (Supp Info Fig. 2) and southern wall (Fig. 6) of Trench B. The southern wall was selected for detailed examination as it did not have was not affected by large cracks formed during excavation and also exposed the most distal sediments of the colluvial fan (Fig. 6).

Trench B preserves facies similar to those in Trench A, with Facies A, B and D present in very similar stratigraphic positions. Facies E is missing from Trench B, testament to the distal location of the trench from the feeder colluvial fan, and hence, input of very coarse clastic material. Compared to Trench A, Facies A in Trench B is finer-grained and matrix supported with large angular coarse clasts paralleling the slope of the adjacent topography representing a hillslope deposit. Facies D dominates the pond sedimentation with a series of fining upwards sequences ranging in grain size from sand to mud.

Facies C1 occurs in Trench B, but has very different characteristics and is hereby described as C2, although the interpretation of the formation of both C1 and C2 are similar.

#### 5.3.1. Facies C2

**5.3.1.1. Description.** This facies is restricted to the western edge of the pond and consists of two weakly arcuate to wedge-shaped deposits that follow the curving, east-dipping beds of Facies D (Fig. 6). Large, angular clasts (up to boulders) are found further into the pond and away from the fault scarp and a tail of smaller (pebble and cobble) angular clasts parallel the underlying bedding of the muddy deposits pinch out against the uplifted Facies A. The tails of these clasts are matrix supported with sand and mud derived from the weathered bedrock (Facies A). Beneath some of the largest clasts, the bedding of the underlying muds has been partially distorted. The upper wedge is stratigraphically coeval with a fining upward sequence preserved between 50 and 60 cm bgs in Facies D in the centre of the pond (Fig. 6). The largest clast has laminated sediment bifurcating around its leading edge with upper laminations onlapping the clast. The lower wedge coincides with a fining upward sequence between 109 and 116 cm bgs in the centre of the pond (Fig. 6).

**5.3.1.2. Interpretation.** These colluvial wedges consist of downhill-moved large clasts emanating from the uplifted scarp and potentially formed as the scarp was eroded during rainfall events prior to consolidation of the sediments. Fan et al. (2020)'s earthquake catalogue for the Fuyun region records 11 earthquakes occurring within 100 km of Aksay Pond that may have contributed to the formation of the lower and upper wedges. The wedge sediment matrix suggests deposition while the pond contained standing water. This differs from the slow freeze-thaw soil creep apparent on the eastern slope of the pond (Fig. 2).
### 5.3.2. Facies D

#### 5.3.2.1. Description. As for Facies D in Trench A.

**5.3.2.2. Interpretation.** Facies D in Trench B is interpreted as the distal sediment from the flows that formed Facies E in Trench A. The fining upward sequences attest to the episodic influx of sediment to the pond and the presence of vertical mud cracks with no lateral offset, and root traces terminating at the upper surface indicates periodic surface drying on at least six distinct levels (c.f. Toké et al., 2011).

### 5.4. Sedimentology

The fine sediments in Trench B contain a significant organic component (5 % to 18 %). From 1.35 to 1.90 mbgs the pond sediments contain 8 to 18 % organic material, from 0.8 to 1.35 mbgs, the pond sediments contain less organic material (5 to 10 %) and from 0.8 mbgs to the ground surface the organic content increases to 10 to 15 % (Fig. 8).

The carbonate content in the pond ranges from 0.9 % to 2.7 % with low variability and will not be considered further (Fig. 8). The remaining sediments comprise the clastic component.

The particle size analysis shows significant variation in grain size throughout the sediment profile in Trench B (Fig. 8). The sediment influx into the pond is punctuated by several fining upward sequences with coarsest sediments occurring at 0.09 mbgs (19 % sand), 0.41 mbgs (25 % sand), 0.96 mbgs (22 % sand), and from 1.57 to 1.72 mbgs (up to 32 % sand at 1.7 mbgs). The sediments are poorly to very poorly (2.56 to 7.97) sorted, high, positive skewness and at certain horizons, have extremely high leptokurtic distributions (Fig. 8).

### 5.5. $^{137}$Cs and $^{210}$Pb chronologies

The $^{137}$Cs curve (Fig. 9, Table 1) records relatively invariant $^{137}$Cs activities (10.9 to 17.4 Bq.kg$^{-1}$) down to 90 cm bgs, and below 90 cm bgs a steady increase occurs to a maximum $^{137}$Cs activity of 65.2 Bq.kg$^{-1}$ at 190 cm bgs. There is no observable 1986 $^{137}$Cs activity peak, in

agreement with reports that the Chernobyl fallout was not significant in China (Lan et al., 2020; Zhang, 2005).

The $^{137}$Cs curve does not record an activity peak that would correspond to the 1963–64 bomb-spike (Lan et al., 2020; Goodsite et al., 2001). However, rapidly declining $^{137}$Cs values from the base of the sequence indicates that basal sediment was deposited during or shortly after the period of maximum $^{137}$Cs fallout preceding the nuclear bomb test ban in 1963. Hence, from $^{137}$Cs test results alone, the entire sediment record in Trench B was likely deposited from the early-mid 1960s onwards, and is <50 years old at its base. The mean sediment accumulation rate over this period for the 200 cm profile therefore is >4 cm yr$^{-1}$. However, due to the episodic deposition of sediment as recorded in the sequentially fining upward layers in Trench B, sedimentation rates were likely higher and later waned. It is likely that sediment hiatuses also occurred when the pond dried and some sediment may have been deflated.

The $^{226}$Ra activity corresponds to the supported $^{210}$Pb ($^{210}$Pb$_{Sup}$) activity in the sediments, and is relatively constant throughout the profile, ranging from 34 to 47 Bq.kg$^{-1}$ (Fig. 9 and Table 1). The excess $^{210}$Pb ($^{210}$Pb$_{Ex}$) activity, given by the difference between the total and supported $^{210}$Pb activities, ranges from a maximum of 198 Bq.kg$^{-1}$ in the uppermost layer (10 cm bgs) to 64 Bq.kg$^{-1}$ at 90 cm bgs. Excess $^{210}$Pb values vary between 64 and 120 Bq.kg$^{-1}$ in the depth range 40 to 150 cm bgs, and are non-monotonic with depth. The absence of a monotonically-decreasing $^{210}$Pb$_{Ex}$ profile precludes strict application of the Constant Initial Concentration (CIC) approach to $^{210}$Pb geochronology (Appleby and Oldfield, 1992). Likewise, the Constant Rate of Supply (CRS), also called the constant $^{210}$Pb flux algorithm, cannot be directly applied because $^{210}$Pb$_{Ex}$ has not decayed to zero. It is also likely that most of the $^{210}$Pb$_{Ex}$ delivered to the pond comes from sporadic erosion of hillslopes in the pond catchment, thus invalidating the assumption of constant $^{210}$Pb flux. This erosion and delivery process is discussed later in this paper and is evidenced by the series of progressive fining sequences which are believed to arise from the delivery and deposition of sediment during major rainfall events. Hillslope erosion as the dominant source of $^{210}$Pb$_{Ex}$ favours the use of the CIC algorithm for chronological determination, and although CIC is strictly not applicable

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Profile of the sediment column from Trench B southern wall sampled at 1 cm increments. Samples were measured for the organic and carbonate contents, and grain size parameters including the median (d=0.5, red), 10th (d=0.1, dark blue) and 90th (d=0.9, green) percentiles, standard deviation, skewness and kurtosis. The grain size parameters are combined into a size fraction percentage heat map that shows the 11 fining upward sequences (white dashed lines) and the distributions of each sediment sample.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Fig. 9. $^{137}$Cs activity relative to depth in Trench B sediments, increasing towards the 1963–64 bomb-spike, note error bars are smaller than symbol size. b) $^{210}$Pb activity relative to depth in Trench B sediments, blue dots show the Supported $^{210}$Pb activity, red dots the Excess $^{210}$Pb activity and green dots the percentage of clay within each analysed sample. Note error bars for Supported $^{210}$Pb are smaller than symbol size.</figcaption></figure>

<table><caption>**Table 1**
Alpha spectrometry of $^{137}$Cs and $^{210}$Pb and dates of each sample.</caption><thead><tr><th rowspan="2">Sample ID</th><th>Depth</th><th colspan="2">$^{137}$Cs</th><th colspan="2">$^{210}$Pb (Bq.kg<sup>-1</sup>)</th><th colspan="2">$^{226}$Ra (Bq.kg<sup>-1</sup>)</th><th colspan="2">Excess $^{210}$Pb</th></tr><tr><th>(cm)</th><th>(Bq.kg<sup>-1</sup>)</th><th></th><th>(Total $^{210}$Pb)</th><th></th><th>(Supported $^{210}$Pb)</th><th></th><th>(Bq.kg<sup>-1</sup>)</th><th></th></tr></thead><tbody><tr><td>FY1</td><td>10</td><td>17.6</td><td>±</td><td>0.7</td><td>243</td><td>±</td><td>22</td><td>45</td><td>±</td><td>4</td><td>198</td><td>±</td><td>22</td></tr><tr><td>FY2</td><td>20</td><td>14.4</td><td>±</td><td>0.6</td><td>173</td><td>±</td><td>21</td><td>37</td><td>±</td><td>3</td><td>136</td><td>±</td><td>21</td></tr><tr><td>FY3</td><td>35</td><td>14.5</td><td>±</td><td>0.8</td><td>129</td><td>±</td><td>13</td><td>47</td><td>±</td><td>4</td><td>82</td><td>±</td><td>14</td></tr><tr><td>FY4</td><td>45</td><td>10.9</td><td>±</td><td>0.6</td><td>106</td><td>±</td><td>15</td><td>41</td><td>±</td><td>4</td><td>65</td><td>±</td><td>15</td></tr><tr><td>FY5</td><td>60</td><td>17.4</td><td>±</td><td>0.9</td><td>159</td><td>±</td><td>15</td><td>37</td><td>±</td><td>4</td><td>122</td><td>±</td><td>16</td></tr><tr><td>FY6</td><td>75</td><td>16.9</td><td>±</td><td>0.7</td><td>119</td><td>±</td><td>17</td><td>43</td><td>±</td><td>4</td><td>76</td><td>±</td><td>17</td></tr><tr><td>FY7</td><td>90</td><td>17.2</td><td>±</td><td>0.9</td><td>106</td><td>±</td><td>11</td><td>42</td><td>±</td><td>4</td><td>64</td><td>±</td><td>12</td></tr><tr><td>FY8</td><td>100</td><td>21.2</td><td>±</td><td>0.8</td><td>165</td><td>±</td><td>6</td><td>42</td><td>±</td><td>4</td><td>122</td><td>±</td><td>8</td></tr><tr><td>FY9</td><td>125</td><td>29.5</td><td>±</td><td>1.0</td><td>115</td><td>±</td><td>12</td><td>46</td><td>±</td><td>5</td><td>69</td><td>±</td><td>13</td></tr><tr><td>FY10</td><td>150</td><td>43.0</td><td>±</td><td>1.0</td><td>127</td><td>±</td><td>6</td><td>45</td><td>±</td><td>4</td><td>81</td><td>±</td><td>7</td></tr><tr><td>FY11</td><td>175</td><td>52.0</td><td>±</td><td>0.9</td><td>198</td><td>±</td><td>20</td><td>34</td><td>±</td><td>3</td><td>164</td><td>±</td><td>20</td></tr><tr><td>FY12</td><td>190</td><td>65.2</td><td>±</td><td>1.1</td><td>172</td><td>±</td><td>21</td><td>37</td><td>±</td><td>4</td><td>135</td><td>±</td><td>21</td></tr></tbody></table>

in Aksay Pond due to the aforementioned non-monotonic nature of the sediment profile, we can use CIC approach in a general sense to estimate the range in sediment ages over certain depth intervals. The CIC algorithm gives the age of sediment (t) at depth x as:

$$t = -1/\lambda[\ln(C_x/C_0)] \quad (1)$$

where $C_0$ and $C_x$ are $^{210}$Pb$_{Ex}$ activities at the surface and depth x, and $\lambda$ (yr$^{-1}$) is the $^{210}$Pb decay constant (ln(2)/half-life = 0.693/22.3 y). $C_0$ is not known exactly, but if we exponentially extrapolate the increasing trend of the uppermost three $^{210}$Pb$_{Ex}$ measurements, a value of about 250 Bq/kg is obtained. Thus Eq. 1 gives sediment ages of between 25 and 45 years for the depth range 40–150 cm. These ages are consistent with a post-1964 date determined from $^{137}$Cs profile. They are also consistent

with the charcoal radiocarbon ages at 100 and 115 cm (28 and 43 years), especially given the fact that there may have been a delay between charcoal formation and its delivery to the pond.

Despite being the stratigraphically-lowest samples analysed for $^{210}$Pb$_{Ex}$, the activities at 175 and 190 cm have $^{210}$Pb$_{Ex}$ activities well above younger sediment in the 40–150 cm depth range and appear anomalous. The anomalous lower dates may be related to particle size, since these sediments have the highest fine sediment (clay) component (17 % and 21 %).

The $^{210}$Pb$_{Ex}$ activities from 150 cm bgs to the surface of the pond provide some context to the sediment age and sources. The $^{210}$Pb$_{Ex}$ activities between 150 and 35 cm bgs show no relationship with depth and oscillate between 60 and 120 Bq/kg. We suggest that the sediments
entrained in this section had a similar erosion source (hillslope erosion) and that the oscillations are related to the fining-upward sequences representing individual erosion and deposition events. The relatively constant range of $^{210}$Pb$_{EX}$ values indicates that these events occurred over a period of time equal to or shorter than the $^{210}$Pb half-life (one to two decades).

## 5.6. Radiocarbon ages

Of the 32 $^{14}$C dates obtained from the trenches (Table 2), we consider that 28 dates are anomalously old, representing charcoal and wood/ plant fragments that grew and burned prior to being captured in the pond sediments. Many (21) of the $^{14}$C dates span recent times when the radiocarbon calibration curve plateaus, giving these calibrated ages a large uncertainty range. In addition, these 28 $^{14}$C dates do not match the $^{210}$Pb and $^{137}$Cs age profiles that suggest that the sediments were deposited since the 1931 earthquake. The four modern $^{14}$C dates coincide with inferred $^{137}$Cs and $^{210}$Pb dates and are used to complement the interpretation of the stratigraphy (Table 2). The post-1950 calibration provides chronological constraint for the maximum ages for the middle (ca. 1970 to 1972 CE) and upper (ca. 1959 to 1985 CE) wedges. Due to stratigraphic positioning, the upper wedge must be younger than ca. 1970 CE.

In summary, three geochronological methods ($^{210}$Pb, $^{137}$Cs and radiocarbon) indicate that the entire pond sequence was <50 years old at the time of collection. From the ages and the sedimentary record, we do not record any past large earthquake in those trenches as most of the sediment were deposited after to 1931.

## 6. Discussion

The fault-bounded Aksay Pond preserves a record of sediment accumulation that began after 1963. This date of the initiation of sediment accumulation is at odds with the tectonic history recorded in the pond and elsewhere.

The 11 August 1931 Fuyun Fault earthquake caused 6 m of horizontal displacement south of the Karachingar Mountain (Liang et al., 2021; Klinger et al., 2011). This movement is inferred to have continued into the valley between Karachingar Mountain and the Altai Range causing displacement of the shutter ridge. The shutter ridge corresponds to cumulative displacement of approximately 24 m (Fig. 2) from its original position, which suggests approximately four earthquake if we assume that locally past displacements were similar to the 6 m average offset measured from stream offsets along other parts of the Fuyun Fault for the 1931 event (Klinger et al., 2011; Liang et al., 2021). The lack of pre-1931 sediments in the pond indicates that movement of the shutter ridge did not create sufficient accommodation space for sediments to accumulate, or that earlier sediments were not preserved.

Klinger et al. (2011) nor Liang et al. (2021) record any vertical displacement of the Fuyun Fault south and north of the Aksay Pond. At Aksay Pond notable vertical displacement is observed with normal faulting generating 7 m of vertical displacement of the upper surface of the shutter ridge from its original position and presence of bedrock on the western side of the fault trace (Fig. 5). If we assume that within the valley, four earthquake cycles have occurred, we calculate that 1.75 m of vertical offset per earthquake occurs at Aksay Pond. This is similar to the findings of Rockwell and Klinger (2023) who recorded 1 m of vertical movement along 6 m of horizontal offset along the Imperial Fault in California. Burbank and Anderson (2012) suggest that on average the vertical to horizontal displacement in pull-apart basins has 1:4 ratio.

At the base of Trench A, a sediment wedge composed of cobbles and boulders (Facies C1) occurs between two palaeosoils (Facies B). The wedge pinches to the east and is bounded to the west by small normal fault with approximate 0.5 m of vertical offset (Fig. 5). Unfortunately, the wedge and palaeosoils could not be dated and we infer this wedge to have formed during a pre-1931 earthquake. No older sediments are observed in Trench B. There is no obvious mechanism for the removal of older sediment from the pond. It is unlikely flooding eroded earlier sediments as the water would flow southwards within the valley and would have deflected around the pond by the offset shutter ridge (Fig. 2). Any flow into the pond would need to scour out existing sediments, not deposit any new sediments, and to overtop the shutter ridge or the scarp at the toe of the shutter ridge. As there is no evidence of erosion at the base of the profile in Trench B, or of the scarp at the toe of the shutter ridge, and no evidence of flow structures within the valley, we conclude that flooding within the valley was likely insufficient to have removed sediment from the pond. Although large fans emanate from the mouth of the valley onto the floor of the Junggar Basin, these are all breached by the 1931 earthquake scarp and at least three previous earthquakes (Klinger et al., 2011, Liang et al., 2021), suggesting that the fan surfaces are substantially older.

Based on the preserved sedimentological history of Aksay Pond, we propose that the sag pond required several strike slip earthquakes to form sufficient subsidence and accommodation space for sedimentary archives to form (Fig. 10). The first and second earthquakes at Aksay Pond did not produce sufficient accommodation space for sediments to pond. Sufficient offset and initiation of sedimentation occurred after the pre-1931 earthquake (Klinger et al., 2011). Initial sedimentation occurred at the northern margin of the small embryonic pond as a sediment wedge that is overlain by a palaeosoil horizon (Fig. 5). The 1931 earthquake resulted in increased subsidence and north-south elongation of the depression allowing a larger zone of sediment

<table><caption>**Table 2**

Samples and the results of the modern $^{14}$C AMS dates from Trench A (northern wall) and Trench B (northern and southern walls).</caption><thead><tr><th></th><th>Sample Location<sup>1</sup></th><th>Name</th><th>Depth (cm)</th><th>Material analysed</th><th>Stratigraphic description</th><th>Lab code</th><th><sup>14</sup>C data (Cal BP)</th><th>δ<sup>13</sup>C relative to VPDB</th><th>Radioactivity (F<sup>14</sup>C)</th><th>Calibrated 2 s modern age (CE)</th><th>Median age (CE)</th></tr></thead><tbody><tr><td>Fig. 5: Trench A - North</td><td>5</td><td>FU12-C14-12</td><td>62</td><td>Charcoal</td><td>Thin fining upwards layer</td><td>SUERC-48682 (GU31567)</td><td>Post 1950</td><td>-25.7 ‰</td><td>1.2951 ± 0.0036</td><td>1962–1980</td><td>1979</td></tr><tr><td>Fig. 6: Trench B - South</td><td>12</td><td>FU12-C14-51</td><td>100</td><td>Charcoal</td><td>Base of fining upward unit above soil contact</td><td>SUERC-53136 (GU34157)</td><td>Post 1950</td><td>-26.7 ‰</td><td>1.1754 ± 0.0034</td><td>1959–1989</td><td>1988</td></tr><tr><td></td><td>21</td><td>FU12-C14-81</td><td>50</td><td>Charcoal</td><td>Upper Wedge</td><td>SUERC-48692 (GU31575)</td><td>Post 1950</td><td>-21.2 ‰</td><td>1.2178 ± 0.0038</td><td>1959–1985</td><td>1984</td></tr><tr><td></td><td>22</td><td>FU12-C14-94</td><td>110</td><td>Charcoal</td><td>Middle Wedge</td><td>SUERC-48696 (GU31577)</td><td>Post 1950</td><td>-26.2 ‰</td><td>1.5340 ± 0.0048</td><td>1970–1972</td><td>1970</td></tr></tbody></table>

<footnote n="1">¹ Other numbers refer to the location of other $^{14}$C samples that are to be discussed elsewhere.</footnote>
<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Conceptual model showing the progressive formation of a fault-bounded sag pond and shutter ridge from the displacement of a hill caused by sequential earthquakes within a valley. Dashed lines represent idealised hill and shutter ridge topographic contours, thin solid black lines are idealised topographic contours of the sag pond depression, thick black line shows the fault trace, hatching shows the degree of vertical displacement and the grey area shows sediment depocentres within the evolving pond.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Comparison of the sediment grain size heat map (from Fig. 8) that shows the 11 distinct fining-upward sequences (white dashed lines) preserved in the southern wall of Trench B in Aksay Pond, with daily precipitation data from Fuyun Meteorological Station (from Fig. 4b) and identifying >20 mm/day events (red circles), >30 mm/day events (blue circles) and > 40 mm/day (purple circle). Also shown is the annual precipitation (mm, black line) from Fuyun Meteorological Station compared to the Z-score derived from the composite tree ring records from the Fuyun region (thick red line; Chen et al., 2014).</figcaption></figure>
accumulation.

By 1963, sufficient vertical movement had occurred along the fault to develop accommodation space for water to pond and sediments to start accumulating. No pre-1963 sediments are found within the pond and no evidence of erosion exists on the surface of the buried palaeosol, so sediments have not been actively removed. Radiocarbon dates indicate that the palaeosol accumulated over the last approximately 2000 years (Table 2).

Of note are the two very coarse-grained colluvial wedges observed in Facies C2 of Trench B that were formed by the collapse of the 1931 Fuyun Fault scarp. These collapse structures are analogous to scarp collapse structures observed in other fault-bounded ponds (e.g. Lienkaemper et al., 2002; Nelson, 1992; Swan et al., 1980). Unlike other colluvial wedges that have helped inform on major seismic activity (e.g. Sieh et al., 1989), the colluvial wedges in Aksay Pond do not correlate with any major strike slip earthquakes along the Fuyun Fault (Klinger et al., 2011, Liang et al., 2021). The upper wedge is ¹⁴C AMS dated to have occurred between ca. 1970 and 1985 and ¹³⁷Cs dated at ca. 1986. The ISC-GEM catalogue records a Mw5 earthquake on 24/4/1986 and a Mw5.3 earthquake on 18/9/1987 that may have caused this wedge. Based on the available earthquake history, it is difficult to identify a nearby earthquake source to have formed the lowest wedge, and it may have been caused by the Mw5.5. 15/12/1980 earthquake, localised storm, or a minor landslide event.

The sedimentological archive recovered from the Aksay Pond (Fig. 11) preserves a complex record of episodic precipitation and subsequent drying. The presence of fine grained sediments in fault-bounded ponds due to typical interseismic accumulation is common in fault-bounded ponds (e.g. Fan et al., 2021; Fan et al., 2020; Lienkaemper et al., 2002; Sieh et al., 1989; Swan et al., 1980). In the north of the pond, coarse-grained clast-supported downslope sheetwash deposits form a talus cone with individual beds formed by subsequent high precipitation events (Facies E; Fig. 5). The coarse units subsequently thin and fine towards the south. The southern wall of Trench A preserves more discrete beds of talus material (Facies E) intercalated with silty muds (Facies D) higher in the sequence. The lack of coarse talus material in Trench B indicates that talus material does not contribute to the sediments to the south. The sediments of Facies D record at least 11 distinct fining upward sequences indicative of sediment settling through the water column following distinct precipitation events after ca. 1963 (Fig. 8).

The Fuyun Meteorological Station (FMS) is 40 km away from the Aksay Pond. Daily precipitation data from FMS records numerous high precipitation events between January 1961 and 2012, with 20 events above 20 mm/day (red circles Fig. 11), three events exceeding 30 mm/day (blue circles Fig. 11) and one event exceeding 40 mm/day (purple circles Fig. 11). Only the ¹³⁷Cs-dated fining upward sequence that contains the upper wedge corresponds to a high precipitation event in Fuyun in 1986 (Fig. 11). After 1986, 15 > 20 mm/day events are recorded at FMS, but only two fining upward sequences in Aksay Pond (Fig. 11). Between 1963 and 1986, FMS recorded four >20 mm/day events whereas Aksay Pond preserves seven fining upward sequences (Fig. 11). The dissimilarity between the FMS recorded high precipitation events and the Aksay Pond sediment record, indicates that high precipitation events in the region are highly localised. This finding demonstrates the difficulty in reconstructing past high precipitation events in the region. This is in contrast with the very good correlation between the annual precipitation recorded at FMS and the regional composite tree ring data (Chen et al., 2014). The tree ring data captures the annual precipitation signal, but fails to recognise whether the precipitation signal is due to extreme or high precipitation events, or to prolonged wet conditions.

From our analysis of the discontinuous precipitation record from the Aksay Pond, we provide a more nuanced interpretation of precipitation in the Altai Range that closely corresponds to the continuous, but time-averaged tree-ring precipitation records available. We suggest that

coupling high-resolution sedimentary records that document episodic rainfall events with continuous tree-ring records in the region may add further detail to reconstructing the nature of the precipitation and storminess.

## 7. Conclusion

The fault-bounded Aksay Pond located 40 km south of Fuyun in northwestern China provides a high-resolution sedimentary record of past precipitation events. The pond formed after the Mw7.9, 11th August 1931 Fuyun earthquake, which caused 6 m of horizontal displacement in the valley that separates the Karachingar push-up mountain from the Altai Range.

The sag pond likely formed sequentially over several >Mw7 events that offset hill that forms a shutter ridge and generated subsidence for the formation of accommodation space and deposition and preservation of sediments. As the shutter ridge was displaced, the pond sequentially grew parallel to the fault and by 1963 had subsided sufficiently to allow sediments to accumulate.

Subsequent to the formation of the pond, downslope sheet-wash caused by episodic heavy precipitation events brought gravelly sediments into the northern margin of the pond and formed a colluvial fan.

In the southern section of the pond, these sheet-washed sediments were much finer grained (sand to mud) and formed sequences of fining upward layers. The pond periodically dried and allowed mud cracks to form in the southern margin of the pond. It is likely that minor earthquakes (<Mw5.5) or localised high precipitation events also caused weakening of the coarse material of the fault scarp resulting in wedges of coarse sediment flowing into the pond in ca. 1986, and possibly earlier. This suggests that care be taken in interpreting these features as evidence of large-scale rupturing events elsewhere. The fining upward deposits indicate that rainfall events are more episodic rather than sustained wet periods that have been inferred from nearby tree ring records.

## CRediT authorship contribution statement

**Chris Gouramanis:** Writing – review & editing, Writing – original draft, Visualization, Supervision, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Stephen Chua:** Writing – review & editing, Writing – original draft, Validation, Methodology, Investigation, Formal analysis. **Marie Etchebes:** Writing – review & editing, Visualization, Project administration, Methodology, Investigation, Formal analysis, Conceptualization. **Yann Klinger:** Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Conceptualization. **Xiwei Xu:** Writing – review & editing, Project administration, Conceptualization. **Gao Mingxing:** Writing – review & editing, Methodology, Investigation. **Adam D. Switzer:** Writing – review & editing. **Gary Hancock:** Writing – review & editing, Validation, Methodology. **Paul Tapponnier:** Writing – review & editing, Project administration, Methodology, Funding acquisition, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

We would like to thank the Singapore's Ministry of Education for funding this research under Grant No: R-109-000-248-115 awarded to C. G. S.C.C.W. would like to thank Dr. Aron Meltzner and Prof. Ray Weldon for discussions on early data interpretation, C.G. would also like to thank to Prof. Bob Wasson for helpful discussions throughout. Y.K was partly
supported by Agence Nationale de la Recherche (ANR-05-CATT-0006).

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j geomorph.2024.109538.

### Data availability

All data used is available as part of this submission

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