Sakuma et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-021-00427-6(2021) 8:44

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open Access

Relationship between tectonism anddesertification inferred from provenanceand lithofacies changes in the Cenozoic
terrestrial sequence of the southwestern
Tarim Basin
Aki Sakuma1* , Ryuji Tada2,3, Tomohiro Yoshida1, Hitoshi Hasegawa4, Naomi Sugiura1, Akinori Karasuda1,
Ke Wang3,5 and Hongbo Zheng3,6

Abstract
The modern-day Tarim Basin is covered almost entirely by the Taklimakan sand desert and is one of the most arid
regions in the world. Unraveling the aridification history of the desert is important for understanding global climatechanges during the Cenozoic, yet the timing and mechanisms driving its formation remain controversial. One of
the leading hypotheses is that the uplift of the Pamir, located to the west of the Tarim Basin, blocked the intrusionof moist air and induced the aridification in the Tarim Basin. In this study, we explore the linkage between the upliftof the Pamir and the desertification in the Tarim Basin from the late Eocene to the middle Miocene in the Aertashi
section, which is located at the southwestern edge of the Tarim Basin and offers the longest sedimentary record
with a reliable age model. Provenance changes in fluvial deposits along the Aertashi section were examined using
electron spin resonance (ESR) signal intensity and crystallinity index (CI) of quartz in the sand fraction of fluvial
sandstones and clast counting based on the identification of clast types by thin section observation to identify
timings of tectonic events in the Pamir, from which clastic materials were supplied to the Aertashi section by rivers.
Our results suggest that major provenance changes in the drainage of the paleo-Yarkand river delivering clasts to
the Aertashi section occurred at ca. 27, 20, and 15 Ma. These timings are mostly consistent with those observed inprevious provenance studies in the Aertashi section and probably reflect tectonic events in the Pamir. On the other
hand, according to the previous studies, the first occurrence of sand dune deposits indicates that the Tarim Basinwas relatively arid after ca. 34 Ma. Hence, our result does not support the hypothesis that the initial aridification in
the Tarim Basin was triggered by the uplift of the Pamir and the resultant blocking of moisture supply from theParatethys Sea, although the subsequent intensification of tectonic events at ca. 27 Ma in the Pamir might havecaused aridification indicated by the initiation of loess deposition.
Keywords: Tarim Basin, Pamir, ESR signal intensity, Desertification, Aertashi section, Provenance* Correspondence: a.sakuma@eps.s.u-tokyo.ac.jp
1
Department of Earth and Planetary Science, Graduate School of Science,
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Full list of author information is available at the end of the article© The Author(s). 2021 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License,
which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give
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1 Introduction
The Tarim Basin, located on the north side of the Tibetan Plateau (Fig. 1), is one of the most arid places in
the world (National Center for Atmospheric Research
Staff, 2018). The majority of the basin is covered by sanddunes that characterize the Taklimakan desert. The wadi
and dry lakes located around the Taklimakan desert areimportant sources of aeolian dust and the emitted dust
is considered to have influenced the Cenozoic climate
throughout the northern hemisphere (Uno et al. 2009;
Isozaki et al. 2020). It has been suggested that the dust
particles, which can act as ice nuclei in high-altitude
clouds, affect the global radiation budget by stimulating
cirrus cloud formation, apart from influencing marine
ecosystems by supplying nutrients to the ocean (Unoet al. 2009). Accordingly, it is essential to unravel thehistory of the desert. However, the time and manner ofdesertification remains controversial (Sun et al. 2009;
Zheng et al. 2015; Heermance et al. 2018). Several causes
are proposed to explain of the Cenozoic aridification ofinland Asia such as the stepwise global cooling, the retreat of the Paratethys Sea, and the uplift of the Tibetan
Plateau and surrounding mountains (e.g., Dupont-Nivet
et al. 2007; Ramstein et al. 1997; Manabe and Broccoli
1990). In the Tarim Basin, the uplift of the Pamir andconsequent blocking of the intrusion of moist air fromthe Paratethys Sea is proposed as the cause of the aridification, yet this hypothesis has been inadequately tested(Sun and Liu 2018).

One of the reasons for the difficulty in examining thelinkage between the aridification in the Tarim Basin andthe tectonic movement in the Pamir is the large uncertainties in the estimated ages of the two events. Ages oftectonic events in the Pamir have been estimated in several ways, such as from timings of provenance changes
in the Cenozoic sequence along the southwestern margin of the Tarim Basin (e.g., Cao et al. 2014; Blayney
et al. 2016), through the reconstruction of the thermal
history of rocks exposed in the Pamir by using thermochronological methods such as biotite and muscovite
Ar/Ar dating and zircon and apatite (U-Th-Sm)/He dating (e.g., Sobel and Dumitru 1997), and through the calculation of the subsidence rate from the geologicalsubsurface structure on the basis of seismic reflection
data and geological mapping (e.g., Jiang and Li 2014;
Chapman et al. 2017). These studies have suggested thatmajor tectonic events occurred at different times,
namely from the late Oligocene to the early Miocene, atthe middle Miocene, and from the late Miocene to theearly Pliocene (e.g., Sobel et al. 2013; Rutte et al. 2017).
On the other hand, the timing of the desertification has
been estimated from the appearance of aeolian deposits
in Cenozoic terrestrial sequences of the Tarim Basin(Zheng et al. 2006, 2010, 2015; Sun et al. 2009; Tada
et al. 2010; Sun et al. 2011; Sun and Windley 2015;
Heermance et al. 2018). Because of the difficulty in constructing age models of terrestrial sequences, the estimated age of the initial aridification event in the Tarim

Sakuma et al. Progress in Earth and Planetary ScienceFig. 1 Tectonic setting in this study. a Location of the Pamir (Google Earth). b Simplified tectonic setting of the Tarim Basin and surrounding
areas (modified after Cowgill 2010). c Geological map of the eastern Pamir (after Robinson et al. 2007; Cowgill 2010; Cao et al. 2015)

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Basin ranges widely from the late Oligocene to the Pliocene (Sun et al. 2009; Tada et al. 2010; Sun et al. 2011;
Sun and Windley 2015; Zheng et al. 2015; Heermanceet al. 2018). Furthermore, the estimated ages of both aridification events of the Tarim Basin and tectonic eventsof the Pamir have errors of several million years typically, and the order of events is yet to be shown. Examining the timing of the two events within the same
sedimentary sequence would be an ideal way to avoid
such a correlation problem.
The provenance of fluvial deposits, whose drainage includes active tectonic belts such as the Pamir, is commonly used to specify the timing of the tectonic events(e.g., Najman 2006; Cao et al. 2014, 2015). To elucidate
the evolution of the Pamir, previous studies have presented provenance data such as U-Pb zircon age distributions, detrital apatite fission track ages, and Sm/Nd
isotope ratios of sediments from Cenozoic sequences
along the western margin of the Tarim Basin (Sobel andDumitru 1997; Bershaw et al. 2012; Cao et al. 2014,2015; Blayney et al. 2016; Clift et al. 2017). Althoughthese studies showed that provenance changes occurredfrom the late Oligocene to the middle Miocene approximately at the same time as the onset of the deposition of
thick conglomerate layers at the western edge of theTarim Basin, exact ages have not been well constrained
because of the low sampling resolution resulting fromthe time-consuming nature of the analyses used in previous studies. For the examination of the relationship between the aridification of the Tarim Basin and the upliftof the Pamir, it would be effective to investigate provenance changes in terrestrial sequences that have also preserved the evidence of aridification in the Tarim Basinby a new simple method which facilitates the analysis ofmany samples in a short time. In this study, we used theelectron spin resonance (ESR) signal intensity and theCrystallinity Index (CI) of quartz for investigating theprovenance changes. The ESR signal intensity correlateswith the total natural radiation dose of quartz, which increases with age (Toyoda and Hattori 2000), and the CIof quartz depends on the formation speed andtemperature of the quartz crystals (Murata and Norman
1976). They are expected to be suitable proxies for theinvestigation of the provenance changes of fluvial
deposits.
In this study, we examined provenance changes in thelate Eocene to early Miocene fluvial sequence of the
Aertashi section, located in the southwestern margin of
the Tarim Basin, to detect changes in the drainage basin,from which we inferred timings of tectonic events. The
ESR signal intensity and CI of quartz were analyzed forthe 64–500 μm fraction of the fluvial sandstone, which
was assumed to be transported by the paleo-Yarkand
river. In addition, we determined the grain compositionof fluvial sandstones by clast counting using thin sections. Finally, we compared the timings of the tectonic
events in the Pamir estimated from provenance changesof fluvial sediments with those of intensified aridification
obtained on the basis of the occurrence of aeolian deposits in previous studies to examine the linkage between the uplift of the Pamir and the desertification in
the Tarim Basin.

Sakuma et al. Progress in Earth and Planetary Science

2 Geological setting
2.1 Controversies of aridification in inland Asia

To explain the Cenozoic aridification of inland Asia andthe formation mechanisms of the Taklimakan desert,
three hypotheses have been proposed. The first hypothesis is that stepwise global cooling since the middle Eocene (probably caused by the decrease in theatmospheric pCO2) affected the regional climate(Dupont-Nivet et al. 2007; Bosboom et al. 2014b). Climate modeling studies showed that the change in atmospheric pCO2 altered the atmospheric heat distribution
and hydrological cycle, leading to less precipitation in inland Asia (Zoura et al. 2019). The second hypothesis isthat the westward retreat of the Paratethys Sea during
the late Eocene decreased moisture supply to inland
Asia, causing aridification (Ramstein et al. 1997; Bougeois et al. 2018; Kaya et al. 2019). The retreat of theParatethys Sea could have been caused by both decreasein the global eustatic sea level and change in geography
due to the uplift of the Tibetan Plateau (e.g., Bosboom
et al. 2014b). The last hypothesis is that uplift of the Tibetan Plateau and the surrounding areas of the Tarim
Basin, caused by the collision between the Eurasian continent and the Indian sub-continent, blocked moisturesupply from the Paratethys sea and/or the Indian Ocean(Manabe and Broccoli 1990; Kutzbuch et al. 1993; Anet al. 2001; Sun et al. 2017; Li et al. 2018; Sun and Liu
2018). In addition to the blocking effect, high mountain
ranges cause the formation of stationary wave troughs,
which also leads to the aridification in inland Asia (Manabe and Broccoli 1990). The uplift of the Pamir is especially significant among the mountains surrounding theTarim Basin in driving the aridification, owing to its rain
shadow effects blocking moisture from the west (Sunand Liu 2018). The Pamir gained high elevation during
the late Cenozoic (Burtman and Molnar 1993) and thishigh elevation might have blocked the intrusion of the
low-level westerlies into the Tarim Basin, leading to thearidification in the basin (Bosboom et al. 2014a; Sunet al. 2017). Although these hypotheses have been tested
to some extent, a consensus is yet to be reached. In thisstudy, we focused on third hypothesis and specificallytested the relationship between the desertification in the
Tarim Basin and the uplift of the Pamir.

Sakuma et al. Progress in Earth and Planetary Science(2021) 8:44

2.2 Tectonic setting of the studied area

The Tarim Basin is located to the north side of the Tibetan Plateau and surrounded by the Pamir in the west,
Kunlun Mountains in the south, Altyn Mountains in thesoutheast, and Tianshan Mountains in the north (Fig.
1a). The Cenozoic uplift of these mountains has been associated with the India-Asia collision that started at ~ 60
Ma (e.g., Sobel and Dumitru 1997; DeCelles et al. 2014;
Kapp and Decelles 2019).
Fluvial sediments deposited along the southwestern
margin of the Tarim Basin are interpreted to have been
supplied mainly from the Pamir (Zheng et al. 2010; Caoet al. 2015; Blayney et al. 2016). The Pamir terranes forma large arcuate mountain belt bounded by the Tarim
Basin on its east with the Kashgar-Yecheng Transfer
System (KYTS), by the Tajik Basin on its west with theDarvaz fault, and by the Alai Valley on its north with theMain Pamir Thrust (Cowgill 2010) (Fig. 1b). The Pamiris divided into four east-west trending terranes separated
by fault systems. From north to south these terranes are
the North Pamir, Central Pamir, South Pamir, andKohistan-Ladakh terranes, respectively. They were accreted during the late Paleozoic-Mesozoic, and eachterrane has distinct geologic features (rock types, age,metamorphic grade, etc.) (Cowgill 2010; Robinson et al.
2007, 2012). Although studying the tectonic evolution of
the Pamir is important for both understanding the continental collision process and examining its relationshipwith climatic change in the surrounding areas, when andhow the tectonic events of the Pamir occurred during
the Cenozoic continues to be debated (e.g., Sobel andDumitru 1997; Blayney et al. 2016; Cowgill 2010; Caoet al. 2014; Chen et al. 2018; Wei et al. 2018). Two hypotheses have been proposed for the timing and natureof the tectonic event of the Pamir (e.g., Sobel and Dumitru 1997; Blayney et al. 2016; Cowgill 2010; Cao et al.
2014; Chen et al. 2018; Wei et al. 2018); one is that the
Pamir salient was in line with the Kunlun Mountains before Oligocene times and started to move northward by~ 300 km since ~ 25 Ma (Cowgill 2010; Sobel et al. 2013;
Blayney et al. 2016) and the other is that there existed
an antecedent of the Pamir before the Cenozoic and its
displacement relative to the Tarim Basin during the
Cenozoic was only tens of kilometers (Chen et al. 2018;
Wei et al. 2018; Li et al. 2020). Although the uplift history of the Pamir is controversial, previous studies haveshown that there were several intervals during the
Cenozoic when tectonic movements in the eastern partof the Pamir were active (e.g., Sobel et al. 2013). Early
metamorphism in the South Pamir and its propagation
to the Central Pamir during the late Eocene to the early
Oligocene have been observed using Lu-Hf geochronology, U-Pb rutile thermochronology, and garnet thermometry, and these observations suggest that the

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formation of the Pamir deep crust started in late Eocene
or before (Smit et al. 2014). From the late Oligocene tothe early Miocene, the dextral KYTS has been suggested
to have been active on the basis of the similar coolingages observed over ~ 250 km long belt at the southwestern edge of the Tarim Basin (Sobel and Dumitru 1997).
Rapid exhumation of the Northern Pamir and Tianshan
Mountains located to the north of the Pamir and thetransition from thickening to exhumation of the domes
located in the Central Pamir has also been reported during this time period, indicating the northward migrationof the Pamir (Amidon and Hynek 2010; Bande et al.
2015; Stearns et al. 2015). In the late Miocene, the rapid
exhumation of high-grade schists and gneisses in the
footwall of the southern Kongur Shan fault in the eastern part of the Central Pamir is considered to be related
to the initiation of east-west extension (Robinson et al.
2007). The initiation of the subduction of the Indian slab
has been estimated to be at about 8 Ma based on the
tomography (Negredo et al. 2007). A thermochronological data set collected along the Yarkand River showed
the cessation of the KYTS movement after 3–5 Ma(Sobel et al. 2011).
2.3 Stratigraphy of the Aertashi section

The Aertashi section (37° 58′ N, 76° 33′–76° 34′ E) islocated in the southwestern margin of the Tarim Basin(Fig. 1). The Cenozoic sedimentary sequence of the Aertashi section comprises four formations and one group,
namely the Wulagen Formation, Bashibulake Formation,
Wuqia Group, Artux Formation, and Xiyu Formation inascending order (Zheng et al. 2015) (Fig. 2). The lithology of each formation is briefly summarized below on
the basis of the field observations of this study, andpartly, the summary is complemented by a description
based on previous studies (Zheng et al. 2010, 2015; Bosboom et al. 2011; 2014a, b; Blayney et al. 2016, 2019;
Wei et al. 2018).
The Wulagen Formation mainly consists of green
mudstone intercalated with greenish-gray limestone richin bivalve fossils and green fine sandstone. The thicknessof this formation is ca. 80 m in the studied section. The
depositional environment of this formation has beeninterpreted as nearshore to shallow marine on the basisof the fossil assemblages of bivalves, ostracods, calcareous nannofossils, and foraminifera (Bosboom et al.
2011; 2014a, b).
The Bashibulake Formation conformably overlies the
Wulagen Formation with a sharp contact and its boundary is easily recognizable by a change from green to redcolor (Bosboom et al. 2011). The formation is characterized by alternations of decimeter thick, dominantly
reddish-brown mudstone and red fine sandstone andsubdivided into three units. In the lower unit of this

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Fig. 2 Photographs showing the typical lithology of each formation. a Green mudstone with shell fossils of the Wulagen Formation. The shellheight is ~ 0.3 m. b Reddish-brown mudstone containing gypsum veins of the Bashibulake Formation. Chicken-wire nodules of gypsum are also
found in mudstones. c Aeolian sandstone showing high-angle cross-beddings at the bottom of the Wuqia Group. d Conglomerate of the lower
part of the Artux Formation. The conglomerate is matrix-supported by fine sandstone and pebbles are sub-rounded to sub-angular and poorlysorted. e Alteration of orange and yellow fine sandstone of the upper part of the Artux Formation. f Thick conglomerate and intercalated lightyellowish gray fine sandstone of the Xiyu Formation. The conglomerate comprises pebbles to cobbles of various rock types including basement
rocks and volcanic rocks

formation, red fine sandstone often shows parallel laminations or trough cross laminations, which disappear upward. Desiccation cracks occasionally appear at the topof the mudstone layers and chicken wire gypsum isfound within the mudstone in the middle unit of the formation. The upper unit of the formation dominantly
comprises mudstone that sometimes shows pedogenic
slickenside and red fine sandstone showing reverse grading. The thickness of this formation is ca. 850 m. The
depositional environment is interpreted to have changed
from a nearshore lake, playa to a flood plain (Blayney
et al. 2019; Zheng et al. 2010, 2015).
The Wuqia Group overlies the Bashibulake Formation
unconformably (Zheng et al. 2015; Blayney et al. 2016,2019) and is subdivided into three units characterized byalternations of red sandstone and reddish-brown mudstone. The lower unit of this group comprises 0.8 to 1 m

thick well-sorted red fine sandstone beds that show highangle trough cross bedding, with rare reddish brown
mudstone intercalations. Sandstones with large-scalecross beds of the lower unit are interpreted as theaeolian sand dune deposit (Zheng et al. 2010; Blayney
et al. 2019). The thickness of the lower unit is ca. 150 m.
The middle unit comprises red fine sandstone intercalated with reddish-orange fine sandstone and brown
mudstone. The reddish-orange fine sandstone shows anerosional contact with the red fine sandstone at its base,suggesting that the reddish-orange fine sandstone wasformed under the influence of relatively strong current.
The thickness of the middle unit is ca. 400 m. The upper
unit mainly comprises red fine sandstone and brown
mudstone that are occasionally intercalated with yellow
siltstone and gray fine sandstone with cross lamina. The
sandstone beds show thickening in the upward direction

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through the upper unit. The thickness of the upper unit
is ca. 550 m. The sedimentary environment of the middle and upper units has been interpreted as a meandering river system (Zheng et al. 2010, 2015; Blayney et al.
2019). The total thickness of the group is ca. 1100 m.
The Artux Formation conformably overlies the Wuqia
Group, and its base is defined as the first appearance ofpebble conglomerate. The formation is dominated by alternations of pink, orange, and yellow sandstone andred, reddish brown, and brown mudstone with pebble
conglomerate intercalations. The formation is divided
into two units on the basis of their lithology. The base of
the lower unit is defined by the first appearance of a
granule to pebble conglomerate bed which contains
large limestone clasts up to 0.5 m in diameter (Zheng
et al. 2010, 2015; Blayney et al. 2019). The lower unit
comprises pinkish fine sandstone interbedded with
granular to pebbly conglomerate and mudstone. Matrixsupported conglomerates occasionally show reverse
grading which is a typical feature of debris flow deposits
in alluvial fans. Furthermore, granule lenses and crossbeddings are found within sandstone layers. These features suggest that this unit is a distal fan deposit (Zheng
et al. 2010; Blayney et al. 2019). The thickness of thelower unit is ca. 700 m. The upper unit is characterizedby alternations of orange to yellow fine sandstone withripple marks and reddish-brown mudstone with rareintercalation of granule layers. The granule layers occasionally erode the underlying layers in the upper part of
the upper unit. Channels filled with trough cross-bedded
sands with some floating pebbles are observed in thisunit (Blayney et al. 2019). The thickness of the upper
unit is ca. 1050 m. The depositional environment has
been interpreted as a distal alluvial fan with abundantcrevasse and flood deposits (Zheng et al. 2010; Blayney
et al. 2019). The total thickness of this formation is ca.
1750 m.
The Xiyu Formation overlies the Artux Formation
with a sharp flat contact. It is dominated by dark grayconglomerate that consist of poorly sorted, well-rounded
pebbles to boulders, including basement rocks and volcanic rocks with a clast-supported fabric and rare
medium sandstone intercalations with cross bedding(Blayney et al. 2019). From the sudden appearance of theconglomerate, the depositional environment of this formation has been interpreted as the middle to proximal
alluvial fan (Zheng et al. 2010, 2015; Blayney et al. 2019;
Wei et al. 2018). The thickness of this formation exceeds
5000 m.

magnetostratigraphy (Yin et al. 2002; Bosboom et al.
2011; 2014a, b; Zheng et al. 2015; Wei et al. 2018). Bosboom et al. (2011, 2014a, b) estimated the age of the
Wulagen Formation at several sections in the western
Tarim Basin to determine the timing of the retreat of
the Paratethys Sea. They identified foraminifera, ostracods, bivalves, calcareous nannofossils, and dinoflagellate
cysts in the Wulagen Formation and concluded that theformation belongs to Zone CP14 (GTS2012) of calcareous nannofossil zonation on the basis of calcareous
nannofossils and dinocyst assemblages (Bosboom et al.
2011, 2014a, b). Zheng et al. (2015) performed the datingof the ash layer from the upper part of the Xiyu Formation using 40Ar/39Ar dating of biotite grains and U-Pb
dating of zircon grains, and the determined ages of thebiotite and zircon grains were 11.49 ± 0.34 and 11.18 ±0.13 Ma, respectively. Since these ages were close toeach other, they concluded that the eruption occurred
around 11 Ma and the ash was supplied instantaneously
to the Aertashi section. Later, on the basis of petrographic, geochronological, and whole-rock geochemical
evidence, Wei et al. (2018) found that the volcanic tuff
was likely to have been derived from the Dunkeldik volcanic complex. Bosboom et al. (2014b), Zheng et al.
(2015), and Blayney et al. (2019) conducted magnetostratigraphic studies at the Aertashi section and constructedmagnetostratigraphy-based age models independently.
All of these studies showed similar age models covering
the interval from the late Eocene to the middle Miocene,with the maximum difference between these models being less than 0.5 million years. However, the samplingroutes of Bosboom et al. (2014b) and Blayney et al.
(2019) were partly different from the route of thecurrent study, while that of Zheng et al. (2015) was identical. Therefore, in this study, we adopted the age modelof Zheng et al. (2015). The Bashibulake Fm., Wuqia Gr.,and Artux Fm. roughly correspond to the time intervalsof the late Eocene, Oligocene, and early Miocene (Zheng
et al. 2015), respectively.

Sakuma et al. Progress in Earth and Planetary Science

2.4 Age model of the Aertashi section

Three types of data are available to constrain the age
model of the Aertashi section: biostratigraphy, U-Pb zircon
chronology
ofvolcanic
material,
and

2.5 Previous provenance study in the Aertashi section

Blayney et al. (2016, 2019) showed the temporal changes
in the detrital zircon age distribution and mudstone SmNd ratio in the Aertashi section and interpreted that theprovenance of fluvial sediments shifted four times. At ~25 Ma, the negative shift of εNd values showed the firstprovenance change. Second shift at the bottom of the
Xiyu Fm (~ 14 Ma) is marked by the sudden increase ofzircons supplied from the North Pamir terrane inaddition to the zircons from the Western Kunlun Mountains. The < 11 Ma sample shows resemblance to themodern sample suggesting that the third shift of theprovenance occurred in the middle of the Xiyu Fm. The
detrital zircons in the modern Yarkand River draining in(2021) 8:44

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the Aertashi area indicate the increase in supply fromSouth Pamir, suggesting that the fourth provenancechange occurred after 11 Ma (Blayney et al. 2016).

sample of soda lime glass beads (UB-911L, UNITIKA
Ltd.). The Mie model was used for the determining grain
size distributions from raw diffraction patterns. The
values of 1.590 and 1.0 were used for the real and imaginary parts of the complex refractive index, respectively (Nagashima et al. 2016).
All of the fluvial sandstone samples showed multimodal grain size distributions. Grain size distribution of
selected representative samples are shown in Fig. 3 andall data are presented in Supplementary Table 2. In general, the multimodal distribution of fluvial sandstonecomprises several log normal distributions that represent
the transportation of sets of particles through different
mechanisms such as suspension, saltation, and bottom
traction (e.g., Ashley 1978; Sun et al. 2002). Thus, numerical partitioning into subcomponents of log normal
distributions was conducted using the GRG Non-linear
solution tool of MS Excel, which is installed by default.
All of our samples showed three modes, and the squared
sum of residual errors when we decomposed into threesubcomponents was acceptable for all the samples. The
median diameters of these three subpopulations were in
the ranges 1.3–18.8, 6.4–118, and 38.8–301 μm and theaverages of these median diameters were 7.1, 41.4, and115 μm, respectively. In modern Taklimakan desert, it
has been shown that fine silt particles of the dry rivers
and dry lakes along the outer margin of the alluvial fansand of the dry lakes in the Tarim Basin have beeneroded and transported by surface wind, resulting in theparticles contaminating the river sand in the southwestern margin of the Tarim Basin (Isozaki et al. 2020). The
coarsest fraction, which roughly corresponds to the 64–
500 μm (fine to medium sand) fraction, is coarser than
the subpopulation representing the eolian dust in modern Taklimakan desert and it is considered as representing particles derived from exposed host rocks in thedrainage basin and transported by bottom traction in
the river (Isozaki et al. 2020). Therefore, in this study,
the 64–500 μm fraction was extracted from a bulk sample by sieving and used for the ESR and CI analyses toavoid the effect of contamination by aeolian dust
particles.

Sakuma et al. Progress in Earth and Planetary Science

3 Samples and methods
3.1 Samples for analyses

Approximately 200 g of samples were collected throughout the Aertashi section at stratigraphic intervals of ~ 50m during field surveys conducted in 2007 and 2010. For
the reconstruction of the river drainage evolution andthe determination of the effect of tectonics, fifty-nine
fluvial fine sandstone samples were selected for ESR and
CI analyses to determine the provenance of quartz
grains. Twenty-seven sandstone and conglomerate samples were selected for thin section observations to identify rock types exposed in drainage basin. The sandstone
samples covered the entire sequence from the base of
the Bashibulake Formation to the base of the Xiyu Formation. The sampling location and stratigraphic positionof each sample are presented in Supplementary Table 1.
3.2 Pre-treatment of samples for ESR and CI analyses

Prior to ESR and CI analyses, carbonate, iron and manganese oxides, and organic matter were removed fromthe samples through chemical treatments to isolate detrital components and to remove elements that interferewith the E1′ center signal (Tada et al. 2000). Approximately 2 g of disaggregated samples were treated with
40 ml of 20 vol% acetic acid solution at 40 °C for 2 h toremove carbonate. To remove iron and manganese oxides, the samples were treated with 40 ml of 0.3 M sodium citrate solution, 5 ml of 1.0 M sodium bicarbonate
solution, and 1.5 g of sodium dithionite at 80 °C. Organic matter was removed by treating the samples with
40 ml of 10 vol% hydrogen peroxide solution at 60 °C
for 10 h.
Grain size distributions of all the samples subject tothe ESR and CI analyses were measured to characterize
the fraction used for the analyses, since the fluvial sandstone was contaminated with aeolian dust. The grain
size measurement was conducted at room temperatureusing a laser diffraction particle size analyzer (Malvern
Panalytical, Mastersizer 2000) at the University ofTokyo. Approximately 50 mg of a homogeneous aliquotof each pre-treated sample was emptied into theanalyzer filled with deionized water and circulated in aclosed transport circuit. Each sample was ultrasonicatedfor 5 min before the measurement and stirred at 2000
rpm throughout the measurement to disperse the particles. For each sample, the measurement was repeatedfive times and the result was averaged. The measurement was conducted for the diameter range 0.02–2000
μm. The reproducibility was better than ± 1.0 μm for the
median diameter in five measurements of a standard

3.3 ESR and CI analyses

The ESR signal intensity and CI of quartz in 64–500 μm
fraction of fluvial sandstone in the Aertashi section wasmeasured to examine provenance changes. Generally,quartz is the most abundant mineral in sandstones, and
it is resistant to physical and chemical weathering. Furthermore, its specific gravity (= density) is close to thatof bulk sediments. Consequently, it is less influenced byweathering and sorting during transportation and best
represents bulk sediments. Thus, it is an ideal material
for use in provenance analysis.

Sakuma et al. Progress in Earth and Planetary Science(2021) 8:44

Page 8 of 21

Fig. 3 Typical grain size distributions of representative samples. Measured data is indicated in red, and the three identified sub-components areindicated in blue, yellow, and green. The 64–500 μm fraction used for the ESR and CI analyses in this study corresponds to thecoarsest sub-population

The number of lattice defects with unpaired electronsof minerals can be evaluated by the ESR signal intensityof the E1′ center (Toyoda 1992). The E1′ center ofquartz is an unpaired electron in single silicon sp3 orbit
oriented along a bond direction into an oxygen vacancy(Feigl et al. 1974) and used to estimate the amount ofoxygen vacancies in quartz, which increases with the ageof quartz grains (Toyoda and Hattori 2000). Namely,
high (low) value of the ESR signal intensity of quartz
suggests that quartz grains were supplied from old(young) host rocks. To measure the ESR signal intensityof quartz, first, the 64–500 μm fractions of pretreatedsamples were irradiated with gamma ray (total dose of
2.5 kGy) using a 60Co source at Takasaki Advanced Radiation Research Institute, National Institutes for
Quantum and Radiological Science and Technology tocreate hole centers. Then, approximately 0.1 g of irradiated samples is heated at 300 °C for 15 min to convert
the oxygen vacancies to E1′ center. The ESR signal intensity was measured at room temperature with X-band
ESR spectrometer (JEOL, JES-FA100) at the Universityof Tokyo under 0.01 mW of microwave power, 0.1 mT

magnetic field modulation (100 kHz), 5 mT scan range,
2 min scan time, and 0.03 s time constant. The ESR signal intensity is expressed in spin units (1 spin unit =1.3 × 1015 spins/g) (Toyoda and Naruse 2002).
The quartz content (QC) was measured by the internal standard method (Klug and Alexander 1974)
with silicon powder (Wako Pure Chemical Industries,Ltd.) as an internal standard, using PANalytical X’Pert
PRO X-ray diffractometer (XRD) at the University ofTokyo with Cu target. The measurement condition
was set as the tube voltage of 45 kV, the tube currentof 40 mA, the slit of 1°, scanning interval of 20 to 30°2θ, the time per step(s) of 6.35, and scan speed of
0.334°/s. The error of the quartz content estimation isbetter than ± 8.5 wt%.
The CI of quartz defined by Murata and Norman(1976) was calculated from the X-ray diffraction data.
The CI measurement was also conducted using the same
XRD as is used for the QC measurement. The measurement condition was set as the tube voltage of 45 kV, thetube current of 40 mA, the slit of 1°, scanning interval of
65 to 75 °2θ, the time per step(s) of 22.86, and scan

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speed of 0.02321°/s. The error of the CI for the three repeated measurements is better than ± 0.25.

and the standard deviation of 3.4, respectively. The ESR signal intensity fluctuates between 5.4 and 11.8 from the 0 m
level at the base of the Bashibulake Fm. to the 1500 m levelin the middle part of the Wuqia Gr. At the 1500 m level thatcoincides with the first appearance of yellow siltstone, whichis interpreted as aeolian loess in the Kekeya section (Zheng
et al. 2015), the ESR signal intensity abruptly increases to
15.6. The ESR signal intensity ranges between 13.0 and 18.7
from the 1500 m level to the 2200 m level in the lower partof the Artux Fm. Between the 2200 m and 3000 m levels in
the middle part of the Artux Fm., the ESR signal intensity
fluctuates significantly between 7.4 and 18.5. Between the3000 m and 3600 m levels in the upper part of the Artux
Fm., the ESR signal intensity decreases again and fluctuateswith smaller amplitude between 9.0 and 12.6. At the 3600 m
level at the base of the Xiyu Fm., the ESR signal intensity
suddenly decreases to 4.0.
The temporal change in the CI of quartz in the 64–
500 μm fraction is shown in Fig. 4b. The CI ranges between 7.15 and 8.44 with the average of 8.05 and thestandard deviation of 0.27. The CI shows a low value between 7.15 and 8.06 from the 0 m level to the 800 m

Sakuma et al. Progress in Earth and Planetary Science

3.4 Observation of thin sections

Thin section observations of fluvial sandstone and conglomerate obtained from the Aertashi section were performed to determine the exposed rock types in thedrainage basin. Thirty-eight samples were impregnatedwith epoxy resins Devcon ET-300 and Petropoxy 154,and thin sections of the impregnated samples were prepared. The grain components of all the thin section samples are observed to identify types of clasts. Seven
samples selected to cover the entire sequence werepoint-counted at a step of 0.5 mm. At least 500 points,
excluding the matrix and pore space, were counted forevery sample.

4 Results
4.1 Results of the ESR signal intensity and the CI analyses

The temporal change in the ESR signal intensity of quartz inthe 64–500 μm fraction is shown in Fig. 4a. The ESR signal
intensity ranges from 4.0 to 18.7, with the average of 10.9

Fig. 4 Temporal changes in a the ESR signal intensity and b CI of the Aertashi section. Red lines between columnar section of ours and Zheng
et al. (2015) indicate the correlated layers and black lines indicates stage boundaries in this study

(2021) 8:44

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level in the Bashibulake Fm. From the boundary between
the Bashibulake Fm. and the Wuqia Gr. at the 800 m
level, the CI starts to increase gradually up to the levelof 1500 m at which the CI reaches approximately 8.2.
Above the level of 1500 m, the CI fluctuates between
7.98 and 8.44.
On the basis of the temporal trend in the ESR signal
intensity and CI, the temporal variation in provenances
can be divided into six stages, from stages 1 to 6 in theascending order (Fig. 4).
Stage 1 (ca. 39–34Ma) corresponds to the Bashibulake
Fm. whose stratigraphic interval ranges from the 0 m
level to the 800 m level. In this stage, the ESR signal intensity shows a relatively low value, ranging from 5.4 to
11.2 with an average of 8.5 and a standard deviation of
1.8, and the CI shows low values ranging from 7.15 to8.06 with an average of 7.74 and a standard deviation of
0.26.
Stage 2 (ca. 34–27 Ma) corresponds to the lower to
middle part of the Wuqia Gr, ranging from 800 to 1500m in stratigraphic level. Because of the probable hiatus
at the boundary between stages 1 and 2, the actual
boundary age cannot be constrained between 37 and 34
Ma. We tentatively defined the stage boundary as 34 Ma
when the ESR signal intensity and CI started to change.
The boundary between stages 1 and 2 is defined by thestart of a long-term gradual increase in the CI, which coincides with the unconformable boundary between theBashibulake Fm. and the Wuqia Gr. In stage 2, the ESR
signal intensity remains low values ranging from 6.7 to
11.8 with an average of 9.5 and a standard deviation of
1.5, while the CI gradually increases from 7.83 to 8.26
with an average of 8.02 and a standard deviation of 0.13.
Stage 3 (ca. 27–20 Ma) corresponds to the upper partof the Wuqia Gr. and the lower part of the Artux Fm.
and ranges from 1500 to 2200 m in stratigraphic level.
The boundary between stages 2 and 3 is marked by an
abrupt increase in the ESR signal intensity at ca. 1500 m,
which coincides with the first appearance of the yellow
siltstone interpreted as loess (Zheng et al. 2015). In stage
3, the ESR signal intensity shows a high value ranging
from 13.0 to 18.7 with an average of 15.5 and a standard
deviation of 1.9, while the CI stops increasing and shows
relatively high and stable values between 8.04 and 8.39
with an average value of 8.25 and a standard deviation of
0.11.
Stage 4 (ca. 20–17 Ma) corresponds to the middle partof the Artux Fm. from 2200 to 3000 m in stratigraphic
level. At the boundary between stages 3 and 4, the ESR
signal intensity starts to oscillate with a large amplitude,
which ranges between 7.4 and 18.5 and has an averagevalue of 12.4 and a standard deviation of 4.5, while the
CI remains stable ranging between 8.04 and 8.41 with anaverage value of 8.23 and a standard deviation of 0.14.

Stage 5 (ca. 17–15 Ma) corresponds to the upper partof the Artux Fm., from 3000 to 3600 m in stratigraphic
level. The boundary between stages 4 and 5 is defined by
the ESR signal intensity beginning to have stable values.
In stage 5, the ESR signal intensity remains stable andhas moderate values ranging from 9.0 to 12.6 with anaverage value of 11.5 and a standard deviation of 1.1.
The CI also maintains stable high values ranging from8.01 to 8.44 with an average value of 8.23 and a standard
deviation of 9.14.
Stage 6 (ca. < 15 Ma) corresponds to the lower part of
the Xiyu Fm. above 3600 m in stratigraphic level. At theboundary between stages 5 and 6, the abrupt decrease in
the ESR signal intensity occurs. In stage 6, the ESR signal intensity shows a low value of 4.0, while the CI is7.98, which is similar to the values in the underlying
stage 5. Although only one sample was analyzed in thisstage, other proxies such as lithic compositions obtainedin this study and zircon age distributions presented byBlayney et al. (2016) support a significant provenancechange at the boundary between stages 5 and 6 (details
will be discussed in Section 5).

Sakuma et al. Progress in Earth and Planetary Science

4.2 Thin section observations

Before point counting, grain components such as detrital
minerals and lithic fragments in samples were identified
from 38 thin sections, to examine the rock types exposedin the source areas. Characteristics of each type of clast
are described below. The grain composition was determined from the point counting result and is described
below as per total grains.
Quartz is the most dominant mineral in all the samples examined. Three types of quartz grains are identified: monocrystalline, polycrystalline, and chalcedonic.
Monocrystalline quartz is the most dominant type ofquartz in all the samples examined, and its maximum
and minimum content were 61.9 and 24.9%, respectively,
with the average of 51.0%; it accounts for more than
two-thirds of quartz grains. About half of the monocrystalline quartz grains shows wavy extinction. Polycrystalline quartz is also commonly found in the studied
samples, and its maximum and minimum contents are
10.3% and 3.6%, respectively, with the average of 6.0%. It
shows unclear crystal boundary and contains various
crystal sizes ranging from silt size to coarse sand size.
Colorless chalcedonic quartz without impurities is rarely
found.
Feldspar is the second most dominant detrital mineraland accounts for a relatively small percentage, with the
maximum and minimum contents are 17.8% and 6.8%,
respectively, with the average of 11.6%. Three types offeldspar grains are identified: orthoclase, plagioclase, andmicrocline. Orthoclase grains are far more abundant(average 10.3%) than plagioclase (average 0.8%) and(2021) 8:44

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microcline (average 0.5%) grains and their relative ratios
vary slightly among samples.
Accessory detrital minerals such as micas, mafic minerals, and opaque minerals are found in small amounts(average: 3.4%). Biotite is the most common accessory
detrital mineral and is especially abundant in the Xiyu
Fm. Biotite grains in this formation are relatively large(~ 500 μm). Other accessory detrital mineral grains are
silt sized and weathered, which rendered their identification difficult.
Lithic fragments are classified into six groups: (1) limestone lithics, (2) mudstone lithics, (3) sandstone lithics,(4) chert lithics, (5) volcanic lithics, and (6) metamorphic
lithics (Fig. 5). Plutonic lithic fragment is not observedin the thin sections, possibly because the grain size ofsamples is smaller than the size of crystals in plutonic
rocks.
Limestone lithics account for 1.3–17.0% with the average of 7.5%. They mostly comprise micritic calcite withsome detrital quartz grains (Fig. 5a). Fossil imprints suchas foraminifera are occasionally observed in limestonelithics. Furthermore, limestone lithic fragments are indented by other clastic grains and show concave contacts because of pressure solution.
Mudstone lithics account for 0.7–17.6% with the average of 6.3%. Four types of mudstone lithic fragments arefound. Siltstone lithic comprises silt-sized grains ofquartz and feldspar along with a small amount of thematrix composed of clay minerals, iron oxides, and otherclay-sized particles (Fig. 6a). Siliceous siltstone lithic
consists of silt-sized quartz grains with microcrystalline
quartz cement (Fig. 6b). The sorting of siliceous siltstone
is good. Claystone lithic consists mostly of clay-size clay
minerals, iron oxides and other minerals with rare siltsize quartz grains (Fig. 6c). Siliceous claystone lithic
mostly comprises silt-sized detrital quartz grains cemented with microcrystalline quartz along with minor ironoxides and opaque minerals (Fig. 6d).
Sandstone lithics accounts for 0~4.8% with an averageof 1.4%. Some samples do not contain sandstone lithics
because of their small grain sizes. Four types of sandstone lithic fragments are identified. Quartzo-feldspathic

Sakuma et al. Progress in Earth and Planetary ScienceFig. 5 Optical microscope photographs of various types of lithic fragments in the Aertashi section (all photographs were taken under crossed
nicols). a Micritic limestone, b Sandstone, c Chert, d Volcanic rock, e Meta-quartzite, and f Quartz schist fragments

Sakuma et al. Progress in Earth and Planetary Science(2021) 8:44

Page 12 of 21

Fig. 6 Optical microscope photographs of clastic rock fragments in the Aertashi section (all photos are taken under crossed nicols). a Siltstone, b
Siliceous siltstone, c Claystone, d Siliceous claystone, e Quartzo-feldspathic sandstone, and f Orthoquartzite fragments

sandstone lithic dominantly comprises quartz withminor feldspar (Fig. 6e). It is poorly sorted, but thematrix is rare. Ortho-quartzite lithic consists of wellsorted and rounded single crystal quartz grains (Fig. 6f).
Calcite cement is observed in a small amount. The grain
contacts are generally concave-convex and sutured. Siliceous sandstone lithics mostly comprises quartz grains
with minor lithic fragments and feldspar. Their microcrystalline quartz cement content is ~ 30%. Siliceous
sandstone lithics are often subjected to weathering, andtherefore, they contain secondary minerals. Quartz
wacke lithic consists of quartz grains with an abundant(~ 50%) matrix of clay minerals and other clay-sized particles (Fig. 5b).
Chert lithic is less common compared with other sedimentary rock lithics and its content ranges from 0.9 to
5.2%, with the average of 2.4%. Its microcrystalline
quartz cement content exceeds more than 95%, and it
contains rare clay-sized particles of other detrital minerals such as clay minerals and opaque minerals (Fig.

5c). Quartz crystals are less than 64 μm and the grainboundary appears irregular, although it is not clear because of small crystal size.
Volcanic lithics are rare (< 3%) and found mostly in
the Xiyu Fm. They mainly contain a few tabular or
needle-shaped euhedral phenocrysts in an opaquegroundmass (Fig. 5d). Blayney et al. (2016) reported thatin sandstones from the Wuqia Gr. to the Artux Fm. in
the Aertashi section, the volcanic fragment content
exceeded 50%. However, their observation is not consistent with our observation, namely volcanic fragments are
rarely found in samples extracted from the Bashiblake
Fm. to the Artux Fm. Gravels in the conglomerate of theArtux Fm. dominantly comprises limestone and sandstone with minor igneous and metamorphic rocks (Blayney et al. 2019). Thus, it is unlikely that only sandstone
samples contain large amounts of volcanic rockfragments.
Metamorphic lithics include schist and meta-quartzite.
Meta-quartzite lithic (average 5.2%) is more abundant(2021) 8:44

Page 13 of 21than schist lithic (average 0.3%) (Fig. 5e). Schist lithics
include quartz schist, pelitic schist, and psammitic schist
lithics (Fig. 5f). Some of the meta-quartzite lithics are
equigranular and have a straight grain boundary, whileothers comprise quartz grains of various size and show aunclear and wavy boundary, which are likely to berecrystallized.
Authigenic minerals occur mostly as cement that fills
intergranular space, and they include calcite, iron oxides,
and silica. Sparry calcite cement is the most abundantand iron oxide cement is the second most abundant.

and CI values of quartz grains as well as from a comparison with the composition of fluvial sandstones. To show
the difference between different stages, a diagram thatshows the ESR signal intensity vs. the CI relationship ispresented in Fig. 8.
In stage 1, relatively large variations in the CI indicate
multiple provenance sources. Considering the change inthe sedimentary environment from a lagoon through aplaya lake to a floodplain in the Bashibulake Fm. (Zheng
et al. 2010), the site might not have been under the influence of a single stable river system. Clasts might havebeen occasionally transported along shore by coastal
currents during this stage. The grain composition of fluvial sandstone suggests that fragments of sandstone,
mudstone, limestone, chert, and metamorphic rocks
were supplied from its drainage. On the other hand, no
igneous rock fragments were observed in this stage. The
ESR signal intensity of quartz lying between 5.4 and 11.2
suggests that the source rocks are probably Paleozoic inage (Toyoda and Hattori 2000), whereas the U-Pb age ofzircon grains contained in fluvial sandstone ranges from
Jurassic to Archean (Blayney et al. 2016). No modernriver sediments having relatively large drainage basin in
the eastern part of the Pamir show zircon age peaks
around 1.9 Ga and 300 Ma, which are observed for samples taken from the Bashibulake Fm. (Blayney et al.
2016), suggesting that at least some of the detrital grainsin the Bashibulake Fm. were supplied from rocks thatare not the main provenance of the modern river sediment. Moreover, the absence of volcanic lithic fragments
in stage 1 sediments suggests that volcanic rocks were
not exposed in the drainage basin and the river head did
not reach the exposed Triassic volcanic rocks in the
North Pamir terrane. Hence, the basin infill of Mesozoic~Paleozoic sandstone, mudstone, and limestone exposed along the western edge of the Tarim Basinbetween the KYTS and the Aertashi section is the likely
source of the fluvial sediment in the Aertashi section(Fig. 9). The age distribution of zircon grains in thebasement rock of the southwestern Tarim Basin shows apeak around 1.9 Ga (Xu et al. 2013) and the modern
Kangkuole River sand derived from the Jurassic sedimentary rock exposed along the southwestern edge of
the Tarim Basin shows the peak of the zircon age distribution to be around 300 Ma (Rittner et al. 2016). Although these areas are deeply eroded now and not themain source of the river sand of the modern Yarkand
River (Blayney et al. 2016; Clift et al. 2017), these areas
might have been the sources of zircon grains which cannot be explained by the supply from the Pamir.
In stage 2, the gradual increase in the CI suggests anincrease in the relative contribution of a large amount ofcrystallized quartz. On the other hand, the low andstable values of the ESR signal intensity and the clast

Sakuma et al. Progress in Earth and Planetary Science

4.3 Temporal changes in grain composition of sandstone

Temporal changes in the grain composition based on
the point counting result are shown in Fig. 7. All thesamples have less than 15% of matrix and are classified
as lithic arenite which is consistent with a previous studyof Blayney et al. (2016) (Fig. 7a). To identify changes inthe types of source rocks, the Q-F-L plot is shown in
Fig. 7a and the change in lithic composition is shown in
Fig. 7b.
Sandstone samples from the Bashibulake Fm. to theArtux Fm. show relatively similar grain composition.
They mainly comprise monocrystalline quartz withminor feldspar, lithic fragments, and rare accessory minerals. Common types of lithic fragments are siliciclastic
rocks, limestone, chert, and metamorphic rocks. Only
one sample from the Artux Fm. (AT07-52cgl) contains asmall amount of volcanic rock fragments. Two samples
from the lower part of the Wuqia Gr. (AT10-21 andAT10-24) show a high percentage (> 10%) of limestone
fragments. The sample from the lower part of the Artux
Fm. (AT07-52cgl) contains a larger amount of sandstone
fragments, probably because of the larger grain size ofthe sandstone. The sample from the upper part of theArtux Fm. (AT07-61) contains relatively high amountsof accessory minerals (5.7%), especially opaque minerals(3.6%), which is more than twice those of other samples
from the strata below the Artux Fm. On the other hand,the sample from the Xiyu Fm. (AT10-49a) show distinctly different composition compared with samples
from other formations. It contains less detrital quartz(24.9%) and more lithic fragments (54.8 %), which include 5.0% of volcanic lithic. Moreover, the percentageof accessory minerals (11.8 %) is more than twice that ofother samples.

5 Discussion
5.1 Sources of coarse detrital materials in fluvial
sandstone

As described in the previous section, the temporal variation in the provenance of the coarse detrital materials
was divided into 6 stages. In the following, the sourcesof each stage will be discussed on the basis of the ESR

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Page 14 of 21

Fig. 7 Changes in grain composition. a Quartz-feldspar-lithic plot. b Lithic plot showing the ratio of sedimentary rocks, volcanic rocks, andmetamorphic rocks. c Changes in grain compositions of fluvial sandstone in the Aertashi section. d Changes in lithic fragments in the
Aertashi section

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Page 15 of 21

Fig. 8 ESR vs. CI plot. The relationship between the ESR signal intensity and CI of quartz for the fine sand fraction of fluvial sandstone in the
Aertashi section. The stages are divided on the basis of the temporal variations of the ESR signal intensity and CI

composition being similar to stage 1 suggest that theprovenance did not change significantly throughout
stages 1 and 2. These observations imply that the river
gradually eroded and started to supply more crystallized
rocks of the same rock types.
At the beginning of the stage 3, the abrupt increase in
the ESR signal intensity and the relatively high CI suggest an increased contribution of source rocks containing older quartz grains. The lithic composition suggeststhat the rock types did not change significantly between
stages 2 and 3, but weakly metamorphosed rocks such as
meta-quartzite and polycrystalline quartz became dominant in addition to siliciclastic sedimentary rocks andlimestones while igneous rocks and highly metamorphosed rocks were nearly absent in the source area. According to Blayney et al. (2019), Sm-Nd isotope values of
mudstone also showed a sudden negative shift aroundthe base of stage 3, indicating the appearance of morecrustal source rocks. The change in the Sm-Nd isotope
values in the study of Blayney et al. (2016) appears to bedelayed relative to the change in the ESR signal intensity
in the current study. However, the exact timing of thechange is uncertain because the sampling interval of
Blayney et al. (2016) was large. We guess that the timelag between the timings of changes in these proxies reflects the difference in grain size of the material analyzed(i.e., mudstones were used for the Sm-Nd analysis andsandstones were used for the ESR signal intensity analysis). It has been shown that in a single river sediment
sample, particles with different sizes could have different
sources (Saito et al. 2017). Paleozoic (meta)sedimentaryrocks exposed in the North Pamir terrane are the oldest
sedimentary rocks in the northeastern Pamir (Cowgill
2010) and are likely to be the source of rocks containing
older quartz grains with a high ESR signal intensity characteristic of stage 3 (Fig. 9). While the ESR signal intensity changed considerably between stages 2 and 3, thetypes of lithic fragments, the CI, and the zircon age distribution did not change significantly (Blayney et al.
2016). Zircon grains are formed in igneous rocks andthen eroded, transported, and incorporated into sediments. Age distributions of zircon grains in fluvial sediments reflect age distributions of igneous rocks indrainage basin, and the zircon age distribution in sedimentary rocks has multiple peaks in a wide range when
zircon grains originate from multiple sources. In theeastern part of the Pamir where the main source of thepaleo-Yarkand River is assumed to be located, the zircon
ages in sedimentary rocks show a multimodal distribution (Blayney et al. 2016; Clift et al. 2017). It is conceivable to mask the provenance change between stages 2
and 3.

Sakuma et al. Progress in Earth and Planetary Science(2021) 8:44

Fig. 9 Acquired drainage of the paleo-Yarkand River during each stage

Page 16 of 21

Sakuma et al. Progress in Earth and Planetary Science(2021) 8:44

In stage 4, the ESR signal intensity vs. CI plots fall
into two groups, one is similar to those of stage 3
and the other is similar to those of stage 5 (Fig. 8).
Thus, oscillations in the ESR signal intensity in stage
4 are interpreted as reflecting repeated switching ofprovenance between the two distinctly different
sources characteristic of stages 3 and 5 during thisperiod.
In stage 5, the stable and moderate ESR signal intensity and the stable high CI values suggest that the provenance is stable and corresponds to one of the twosources of stage 4 (Fig. 9). Furthermore, the accessory
minerals such as biotite and mafic minerals increase
compared with earlier stages and schist fragments begin
to appear for the first time in the studied sequence, suggesting the appearance of igneous and metamorphic
source rocks containing more accessory minerals. Thisin turn implies that the paleo-Yarkand River acquired
new drainage in the North Pamir where these rocks are
exposed (Cowgill 2010). However, assemblages of othergrains and lithic fragments do not change significantly,
suggesting that the main source area is similar to that in
the earlier stages.
In stage 6, the ESR signal intensity and grain composition indicate that the source rocks of fluvial sedimentschanged drastically compared with those of stage 5.
The low ESR signal intensity value suggests that the
average age of quartz grains in the source rocks decreased, probably corresponding to the Mesozoic or
younger (Toyoda and Hattori 2000). According to thethin section observations, the grain composition of thefluvial sandstone of the Xiyu Fm. changed significantly
from that of the Artux Fm., showing a wider variety oflithic fragments such as volcanics and schists, apartfrom sandstones, limestones, and a small amount of
weakly metamorphosed rocks in the Artux Fm. Thissuggests that the paleo-Yarkand River acquired the newdrainage in the Pamir where additional types of hostrocks including volcanic rocks and schists were exposed. Moreover, the percentage of biotite increases
abruptly at the beginning of stage 6, indicating that the
detrital supply of acidic igneous rock fragments containing large biotite crystals started in stage 6. In thearea to the east of the KYTS, acidic igneous rocks are
not exposed. Therefore, it is expected that the riverdrainage reached the area near Kongur Shan fault
where Mesozoic igneous rocks are exposed (Fig. 9).
This is consistent with the zircon age distribution pattern of sandstone in stage 6 showing distinctive peaksof early Mesozoic, which are attributed to igneousrocks exposed in the North Pamir terrane on the basisof their resemblance to those in modern river sediments supplied from the North Pamir terrane (Blayney
et al. 2016; Wei et al. 2018).

Page 17 of 21

5.2 Causes of provenance changes

The transition from stage 1 to stage 2 at ca. 34 Ma ismarked by a relatively small change in the CI and might
have reflected the appearance of the steady area of thedrainage basin. Since the area of the drainage basin does
not appear to have changed significantly as suggested by
the little change in the ESR signal intensity, grain composition, and the age distribution of zircon grains (Blayney et al. 2016), a sudden significant tectonic movement
is unlikely to have occurred in the drainage of the paleoYarkand River between stages 1 and 2. The beginning of
the increase in the CI at the base of stage 2 might be attributed to gradual unroofing and the consequent increase in the supply of more crystallized rocks in thedrainage. Rapid global cooling that occurred around 34
Ma at the Eocene-Oligocene boundary and the associated sea level drop could have changed the hydrology ininland Asia (Dupont-Nivet et al. 2007; Li et al. 2018;
Zoura et al. 2019) and this might have led to the establishment of paleo-Yarkand River draining stably within
the area to the west of the KYTS and possible hiatus(Bosboom et al. 2014a, b). Although there are few studies on the climatic change in the Pamir during the
Eocene-Oligocene transition (EOT), it is unlikely thatthe gradual intensification of precipitation occurred atthat time considering the aridification in inland Asia atthat time (e.g., Dupont-Nivet et al. 2007). On the other
hand, it has been reported that deformation of the metamorphic domes in the Central and South Pamir started
in the Eocene and the metamorphism prograded northward during late Eocene to Oligocene (Smit et al. 2014;
Stearns et al. 2015). Thus, we suggest that the gradualuplift of the Pamir, rather than increase in precipitation,is more likely to be the cause for unroofing and the consequent increase in the CI during stage 2.
At the boundary between stage 2 and stage 3 (ca. 27Ma), host rocks containing older quartz grains of metasedimentary rock origin derived from the eastern part of
the North Pamir terrane suddenly appeared, suggestingthat the paleo-Yarkand River acquired a new drainage
basin with new source rocks in a short period. There are
two possible reasons for the river to supply fragments ofolder source rocks. One is that the river acquired a new
drainage basin upstream, where older meta-sedimentaryrocks were exposed. The other is that the underlying
older strata were newly exposed in the same drainage
basin because of the uplift and subsequent rapid erosion
within the same drainage basin. The latter possibility isless plausible because the Mesozoic-Paleozoic strata to
the east of the Pamir are relatively thick and the dip
angle is very large (Cowgill 2010). Therefore, we prefer
the first possibility that the river head of the paleoYarkand River extended into the North Pamir terraneand the river acquired a new drainage basin at the(2021) 8:44

Page 18 of 21

beginning of stage 3, which is likely to have resulted
from the uplift of the eastern North Pamir terrane associated with the fault movement surrounding the North
Pamir (Sobel and Dumitru 1997; Bershaw et al. 2012;
Cao et al. 2015). The appearance of the conglomerate
lagging behind the sudden increase in the ESR signal intensity suggests that tectonic movement first occurred
near the river head in the North Pamir terrane andpropagated toward the depositional site of the Aertashi
section. In the surrounding areas of the Pamir, evidence
for tectonic events was examined using other methods
such as geothermometer and seismic refraction, and theevidence suggested the existence of tectonic activity near
the Pamir during the late Oligocene to the early Miocene (e.g., Sobel et al. 2013; Jiang and Li 2014). Thermal
modeling based on the (U-Th)/He dating of zircon andapatite showed that the exhumation rate in the North
Pamir accelerated during the early Miocene (Amidonand Hynek 2010). In the Tian Shan Mountains locatedto the north of the Pamir, a rapid exhumation event during the early Miocene was suggested on the basis of
apatite fission track data (Bande et al. 2015). Bande et al.
(2015) further suggested that the north-south trend
shortening occurred in the area surrounding the Pamirat ca. 25 Ma because of the activation of the TalasFergana faults, one of the largest faults in the area.
In stage 4, the onset of the fluctuation of provenancebetween two sources suggests that the drainage basin expanded in the upper stream of the paleo-Yarkand Riverand started to provide younger clastics at ca. 20 Ma. The
tectonic activity such as the initiation of the fault movement in the eastern Pamir is likely to have occurred
around ca. 20 Ma. In fact, Sobel and Dumitru (1997)
dated clasts of fluvial sediment along the piedmont of
the Western Kunlun Shan by apatite fission track andfound that cooling ages were roughly 20 Ma, which wasinterpreted to be associated with the KYTS activity.
A steady drainage basin was probably established at ca.
17 Ma, judging from the stable values of the ESR signal
intensity and CI in stage 5. This stage is interpreted tohave been an interval of less intense tectonic activity.
The largest change in the drainage occurred at theboundary between stages 5 and 6 (ca. 15 Ma), and it wascharacterized by significant changes in the ESR signal intensity, grain composition, and lithology as well as U-Pb
age distribution of zircon grains and Sm-Nd isotope
values (Blayney et al. 2016). Deposition of the thick pebble conglomerate of the Xiyu Fm. suggests that the Aertashi section was located in the middle of the alluvial fan
and that the mountain range of the Pamir came closer
to the section. This, in turn, strongly suggests that thedeposition of the Xiyu Fm. started when a wide area in
the North Pamir terrane, including the new drainage inthe western part where Mesozoic igneous rocks were

exposed, was uplifted (e.g., Blayney et al. 2016; Wei et al.
2018; Zheng et al. 2006). Acquisition of the new sourceis expected to have been caused by the uplift of theNorth Pamir.

Sakuma et al. Progress in Earth and Planetary Science

5.3 Relationship between uplift of the Pamir andaridification in the Tarim Basin

In previous studies, the timing of the aridification in the
Tarim Basin was estimated on the basis of the first occurrence of sand dune or loess deposit in Cenozoic sequences (Sun et al. 2009; Zheng et al. 2015; Heermanceet al. 2018). In the Aertashi section, the deposition ofsand dune sediments started from the bottom of the
Wuqia Gr. at ca. 34 Ma (Blayney et al. 2019), indicating
relatively dry condition with poor vegetation. On theother hand, the result of this study suggests that theprovenance of fluvial sediments did not change significantly during stages 1 and 2 (ca. 39–34 Ma), indicatingthat significant tectonic events that affected the drainageof the paleo-Yarkand River did not occur in the Pamir in
association with the aridification in the Tarim Basin. In
other words, the initial aridification in the Tarim Basinoccurred before significant tectonic activity started in
the Pamir. The sudden appearance of the aeolian dunesand is likely to be related to the global cooling at theEOT. In fact, it has been reported that the rapid climatechange at the EOT affected the climate and vegetationin the other parts of inland Asia (Sun et al., 2014;
Dupont-Nivet et al. 2007), and it is conceivable that theclimate in the Tarim Basin was also affected. Gradualsubsidence could have changed the position of the Aertashi section relative to the Pamir and its depositional
environment (Blayney et al. 2019).
In the Kekeya section, which is also located at thesouthwestern edge of the Tarim Basin, aeolian silt deposition started in the Artux Fm. at ~ 26.7–22.6 Ma andloess deposition continued through the Artux and Xiyu
Fm. (Tada et al. 2010 with the new age model; Zheng
et al. 2015), although there is little clear evidence ofloess deposition in the Aertashi section because of thecontinuous presence of the persistent river system. From
this evidence, it is interpreted that the Tarim Basin has
been continuously under arid conditions at least since
the deposition of the Artux Fm. (Zheng et al. 2010,2015). The timing of the onset of loess deposition in the
Kekeya section is close to the boundary between stages 2
and 3 at ca. 27 Ma, which is much later than the initialdeposition of sand dune deposit in the Aertashi section(Zheng et al. 2015). Approximately 27 Ma is close to thetiming of the provenance change of fluvial sediments in
the Aertashi section between stages 2 and 3. Thus, thetectonic event in the North Pamir that occurred at theboundary between stages 2 and 3 might have been associated with the intensification of the dry conditions in(2021) 8:44

Page 19 of 21

the Tarim Basin. It is also worth to note that the active
tectonic uplift in the North Pamir is expected to havesupplied a large amount of the detrital materials to theTarim basin, which should have acted as a source ofaeolian dust and desert sand (Zheng 2016; Isozaki et al.
2020). In fact, onset of loess deposition occurred at thelatest Oligocene not only in the Tarim Basin but also in
the western Chinese Loess Plateau and Junggar Basin(Sun et al. 2010; Qiang et al. 2011). Further research of
the paleo-environment in the Tarim Basin is necessary
for a more detailed discussion on the linkage between
the aridification in the Tarim Basin and the uplift of the
Pamir.

Pamir and the aridification in the Tarim Basin more precisely in future studies.

Sakuma et al. Progress in Earth and Planetary Science

6 Conclusions
ESR signal intensity and CI analyses of detrital quartz
and thin section observations were performed for fluvial
sandstones obtained from the Aertashi section at thesouthwestern edge of the Tarim Basin to examine provenance changes in the drainage basin of the paleoYarkand River. The result suggests that the provenanceof fluvial sediments at the Aertashi section is marked by
five times shift approximately at 34 Ma, 27 Ma, 20 Ma,
17 Ma, and 15 Ma, and on the basis of this observation,
we divided the sequence into six provenance stages.
Thin section observations of the fluvial sandstones
showed that the rocks exposed in the drainage basin
were predominantly sandstone, mudstone, limestone,
and chert throughout the sequence. The contribution ofmetamorphic rocks and volcanic rocks started to increase in stage 6. A sudden increase in the ratio of volcanic lithic fragments and biotite at the boundary
between stages 5 and 6 at ca. 15 Ma suggests the expansion of the drainage basin and acquisition of new source
rocks such as volcanic rocks and granitic plutonic rocks
that were exposed in the North Pamir terrane. The results of this study and a previous provenance study(Blayney et al. 2016) indicate that major tectonic eventsin the NE Pamir occurred at ca. 27 Ma, ca. 20 Ma, andca. 15 Ma. On the other hand, the first appearance ofdesert dunes at ca. 34 Ma in the Aertashi section (Zheng
et al. 2010; Blayney et al. 2019) precedes the major tectonic events in the NE Pamir unraveled in this study.
Thus, the climate in the Tarim Basin was already arid
before the major tectonic events in the NE Pamir. However, the subsequent uplift of the NE Pamir at 27 Ma
may have intensified the aridification because it is observed to be coeval with the initiation of loess depositionin the Kekeya section (Tada et al. 2010). Testing the arid
conditions in the Tarim Basin by methods such as
chemical index of alteration (CIA), clay mineral, andpollen analyses can help us to examine the relationshipbetween the timing of the tectonic movements in the

Abbreviations
ESR: Electron spin resonance; CI: Crystallinity Index; KYTS: Kasugar-Yechengtransfer system; Fm.: Formation; Gr.: Group; EOT: Eocene-Oligocene transition

7 Supplementary Information
The online version contains supplementary material available at https://doi.
org/10.1186/s40645-021-00427-6.
Additional file 1.
Additional file 2.
Acknowledgements
We thank the staff at the Takasaki Advanced Radiation Research Institute,
National Institute for Quantum and Radiological Science and Technology(QST) for the assistance with γ-ray irradiation.
Authors’ contributions
RT proposed the topic, conceived, and designed the study. AS carried out
the experiment, analyzed the data, and wrote the manuscript. TY, HS, NS, AK,
KW, and HZ collaborated with the corresponding author in the constructionof manuscript. All authors read and approved the final manuscript.
Funding
This work was jointly supported by the National Natural Science Foundationof China (NSFC) (U1902208, 41991323, and 4188101) to the Strategic Priority
Research Program of Chinese Academy of Sciences (XDB26020301), and theSecond Tibetan Plateau Scientific Expedition and Research (2019QZKK0704)
to Hongbo Zheng
Availability of data and materials
The datasets analyzed during the current study are available from thecorresponding author on reasonable request.

Declaration
Competing interests
The authors declare that they have no competing interest.
Author details
1
Department of Earth and Planetary Science, Graduate School of Science,
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
2
Institute for Geo-Cosmology, Chiba Institute of Technology, 2-17-1
Tsudanuma, Narashino, Chiba 275-0016, Japan. 3Yunnan Key Laboratory of
Earth System Science, Yunnan University, Kunming 650500, People’s Republicof China. 4Department of Global Environment and Disaster Prevention,
Faculty of Science and Technology, Kochi University, Akebono-cho 2-5-1,
Kochi 780-8520, Japan. 5Department of Geology and Paleontology, National
Museum of Nature and Science, 4-1-1, Amakubo, Tsukuba, Ibaraki 305-0005,
Japan. 6School of Earth and Environmental Sciences, The University of
Queensland, Brisbane, Queensland 4072, Australia.
Received: 15 July 2020 Accepted: 12 May 2021

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