Climatic and Tectonic Evolution in the North Qaidam Since the Cenozoic: Evidence from Sedimentology and Mineralogy

Home , Qaidam Basin, Tanggula Mountains

Journal of Earth Science, Vol. 24, No. 3, p. 314–327, June 2013 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-013-0332-3

Climatic and Tectonic Evolution in the North Qaidam since the Cenozoic: Evidence from Sedimentology and Mineralogy

Chaowen Wang (王朝文), Hanlie Hong (洪汉烈) State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China; Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China Zhaohui Li* (李朝晖) Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China; Geosciences Department, University of Wisconsin-Parkside, Kenosha WI 53141-2000, USA Guojun Liang (梁国军), Jin Xie (谢瑾), Bowen Song (宋博文) State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China; Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China Eping Song (宋鄂平), Kexin Zhang (张克信) Geological Survey of China University of Geosciences, Wuhan 430074, China; State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China

ABSTRACT: Clay mineralogy and bulk mineral composition of Tertiary sediments in Qaidam were investigated using X-ray diffraction (XRD) and scanning electron microscopy in order to better understand regional climate change resulting from uplift of the Northeast Tibetan Plateau. Climate change in Qaidam since ~53.5 Ma could be divided into four stages: a warm and seasonally arid climate between ~53.5 and 40 Ma, a cold and arid climate from ~40 to 26 Ma, a warm and humid climate between ~26 and 13.5 Ma, and a much colder and arid climate from ~13.5 to 2.5 Ma, respectively. The illite crystallinity and sedimentary facies suggested that uplift events took place around >52–50, ~40–38, ~26–15, ~10–8, and <5 Ma in the Qaidam region, respectively. The climate in This study was supported by the China Geological Survey (No. Qaidam Basin could have been controlled by global 1212011121261), the National Natural Science Foundation of climate prior to 13.5 Ma. As the Tibetan Plateau China (Nos. 41272053 and 41072030), Specialized Research reached a significant elevation by ~13.5 Ma, and Fund for the Doctoral Program of Higher Education of China the climate cycles of the East Asian monsoon might (No. 20110145110001) and the Independent Research Project add additional influence. Foundation of State Key Laboratory of Biogeology and Envi- KEY WORDS: clay mineral, illite crystallinity, ronmental Geology, China University of Geosciences, Wuhan paleoclimate, Qaidam Basin, Tibetan Plateau. (No. GBL11307). *Corresponding author: [email protected] INTRODUCTION © China University of Geosciences and Springer-Verlag Berlin The uplift of the Tibetan Plateau played an impor- Heidelberg 2013 tant role in Cenozoic climate, such as enhancing Asian monsoon systems, creating widespread aridification, and Manuscript received October 18, 2012. increasing erosion since the onset of India-Asia plate Manuscript accepted December 7, 2012. collision during the Cenozoic (Dupont-Nivet et al., 2008; Climatic and Tectonic Evolution in the North Qaidam since the Cenozoic: Evidence from Sedimentology and Mineralogy 315

Sun and Wang, 2005; An et al., 2001; Raymo and Rud- mentary rates, and magnetic susceptibility (Lu and diman, 1992). With a rapid uplift in the Late Cenozoic, Xiong, 2009; Fang et al., 2007; Sun et al., 2005). How- more studies have been focused on climate changes as- ever, many of these studies either lack the entire cover- sociated with the uplift (Wang et al., 2008; DeCelles et age of the Cenozoic sediment sequence (Lu and Xiong, al., 2007; Rowley and Currie, 2006; Molnar, 2004; An 2009; Rieser et al., 2009; Fang et al., 2007; Sun et al., et al., 2001). 2005), or did not have precise age control (Wang X M et Previous hypotheses on climate change in this re- al., 2007; Wang J et al., 1999). gion have been supported by various tectonic and cli- In a recent study, the sediment sequence has been mate models and have been mainly attributed to two as- divided into 5 stratigraphic realms with 13 stratigraphic pects: global and regional (Dupont-Nivet et al., 2008; subrealms from analyses of 98 remnant basins based on Sun and Zhang, 2008; Harris, 2006). Global long-term the data of 1 : 250 000 geological mapping with the de- evolution of climate has been reconstructed using com- pression of Qaidam Basin began at 53.5 Ma marked by posite high-resolution deep-sea oxygen isotope records rudaceous deposits (Zhang et al., 2010a). In addition, a (Zachos et al., 2001). However, the research docu- few short sedimentary sequences with precise age con- mented on regional long-term evolution of climate has trol have been documented in recent geological surveys been lacking. Thus, it has been difficult to combine data in Qaidam, such as the profiles of Dahonggou (Lu and from deep-sea sediments with terrestrial deposits to bet- Xiong, 2009), Lulehe (Zhang, 2006), and Huaitoutala ter understand whether climate change in the region was (Fang et al., 2007) (Figs. 1a and 1b). caused by the uplift of the Tibetan Plateau or by global In this study, we characterized the mineralogy of a climate change. The main obstacle has been to find a composite section (S26, S27) about 6 500 m thick in single continuous sequence of deposits from land cov- Qaidam Basin (Fig. 1c) and detailed lithology and sedi- ering the entire Cenozoic (Sun and Zhang, 2008). mentary thickness was compared among Dahonggou, In response to the collision between India and Lulehe, and Huaitoutala profiles to obtain a precise age Eurasia plates, Qaidam Basin accumulated nearly control (Fig. 2). It was expected that the results could ~12 000 m of entirely Cenozoic fluviolacustrine sedi- provide new insights into tectonic-driven climate change ments continuously since 53.5 Ma (Fang et al., 2007). and/or regional climate change coinciding with the The strata provide us with an opportunity to investigate global climate change. long-term climate evolution in the region. Among various factors, evaporite deposits and fossils of salinity- GEOLOGIC SETTING AND TERTIARY tolerant invertebrates have been used in reconstructing STRATIGRAPHY long-term terrestrial climate change. In addition, the Qaidam Basin is one of the largest intermountain quartz/feldspar (Q/F) ratio has been a traditional proxy basins northeast of the Tibetan Plateau (Fig. 1a). Tec- of weathering intensity for detrital sediments (Mikesell tonically, it is encompassed by a high topographic relief et al., 2004; Kuhn and Diekmann, 2002). Illite crystal- including the Kunlun, Altyn Tagh, and Qilian moun- linity has also served as an appropriate proxy for inter- tains. Active thrusting tectonics and erosion made the preting the climate and tectonic evolution (Chamley, sediments of Qaidam Basin sensitive to the effects of 1989; Singer, 1984). Thus, these parameters have been uplift of the northern Tibetan Plateau and the environ- frequently studied for Cenozoic and Quaternary sedi- mental evolution since the Cenozoic, thus providing the ments (Hong et al., 2010a; Li et al., 2010; Dupont-Nivet sediments with a great potential for understanding the et al., 2007; Wang and Miao, 2006; Mikesell et al., 2004; relationships between regional and global climate Kuhn and Diekmann, 2002). changes (Rieser et al., 2009; Dupont-Nivet et al., 2008, The sediments of Qaidam Basin have been previ- 2007; Zhang et al., 2008; Molnar, 2004). The Cenozoic ous studied for its pollen sequence (Wang X M et al., sedimentary sequence of Qaidam Basin are divided into 2007; Wang J et al., 1999), structure (Yin et al., 2008, Lulehe, Ganchaigou, Youshashan, and Shizigou forma- 2002), C and O isotopes (Rieser et al., 2009), clay min- tions from bottom up and are mainly made of ﬂuvial erals (Hong et al., 2010a), magnetostratigraphy, sedi- sandstones and conglomerates and lacustrine 316 Chaowen Wang, Hanlie Hong, Zhaohui Li, Guojun Liang, Jin Xie, Bowen Song, Eping Song and Kexin Zhang

80o 90o 100o E 92o 96o E

Mountain

o Altyn Tagh fault Tianshan Mountains Qilian-Nanshan Strike-slip fault

40 N Tarim Basin Pamirs Hongsanhan City or town Karakoram fault (b) thrust bet West Kunlun EastQaidam Kunlun Basin fault Altyn Tagh fault Qiman Tagh Lulehe Lake o

Studied area 38 N Qaidam Basin Daqaidam Tanggula Mountains Southern Qinian Mountain Bangonghu-Nujiang structure Eastern Kunlun MountainDahonggou Gangetic Plain

o Dabsan depression Huaitoutala Delingha Qinghai-Tibet 30 Plateau 0 100 km Golmud

(a) (b) o

94o 96oE

38 N Juhongtu (c) N2 Q

31 N1 EN Zongwulong Shan Q E1-2 Xiao A Qaidam Delingha Qaidam Basin B S27 Q Olonbuluk Shan A' S26 Q B' Tuosu 0 4km Hu

E1-2 Paleocene-Eocene N2 Pliocene Fault

EN31Oligocene-Miocene Q Quaternary Lake and river

o N1 Miocene Pre-Tertiary B A Profile

37.5 Figure 1. (a) Generalized structure and the location of the Qaidam Basin, the rectangle indicates the outline of Fig. 1b; (b) sketch map of the Qaidam Basin showing the locations of studied sections that are mentioned in the text (after Lu and Xiong, 2009); (c) generalized structural and geology map showing the locations of the study area (modified after Yin et al., 2008). mudstones (see Fig. 2 for detailed lithological descrip- western margin was mainly lacustrine, fluvial, flood tion and Fig. 3a for full view of the landscape). The plains, and underwater alluvial fan facies, while to the initial depression started in Early Eocene because of east lacustrine deposits were the major facies in Qaidam Himalayan-Tibetan orogenic shortening (Yin, 2010). Basin in addition to a number of other basins, such as As such, piedmont braided fluvial and alluvial facies Delingha, Qilian, and Jiuquan basins (Zhang et al., 2010a, with coarse-grained clasts even boulders (Fig. 3b) b). During Pliocene, the depocenter migrated eastward began to deposit along the marginal basin since 53.5 and the basin started to shrink and dry out. Although the Ma. Lacustrine deposits were mainly distributed in the center was made of shore-shallow lake facies, the edge center of the basin at that time. During the Late of Qaidam Basin formed several alluvial and alluvial fan Eocene–Oligocene, the depression continued, result- facies due to the uplift of the surrounding mountains ing in expansion of lacustrine delta and lacustrine (Yin et al., 2002). deposits along the Altyn and Qilian mountains. Layers Based on petrologic and lithologic data, four dis- of gypsum and halite were occasionally deposited in tinct lithofacies were recognized in the Tertiary sedi- the mudstone (Fig. 3c). In Miocene, the lake expanded ments (Fig. 2). They are, from the bottom upwards, eastwards (Duan and Hu, 2001) and reached its fluvial, alluvial fan, lacustrine-lacustrine delta, and maximum extension as the climate became more fluvial. These have been used to identify the sedimen- humid (Rieser et al., 2009; Wang et al., 1999). The tary environment and uplift phases. Climatic and Tectonic Evolution in the North Qaidam since the Cenozoic: Evidence from Sedimentology and Mineralogy 317

Figure 2. Lithology and sedimentary faces of the study profiles with totally magnetic age control. Analytical samples by SEM (scanning electron microscope) have been marked throughout the profile.

EXPERIMENTAL METHODS air-dried and ground to less than 0.074 mm. The bulk XRD Analyses mineral composition was identified by X-ray diffrac- Representative fresh samples were collected tion (XRD) using a Panalytical X’ Pert PRO DY2198 along the whole sections of 26 and 27 according to the diffractometer with Ni-filtered Cu Kα radiation (35 lithological characteristics of detrital composition, kV, 35 mA). grain size, and color. A total of 319 samples were col- The minerals calcite, quartz, gypsum, K-feldspar, lected mostly from mudstone, siltstone, and silty plagioclase and halite were identiﬁed using 3.03 Å sandstone. For conglomerates the fine materials in (104), 4.26 Å (100), 7.60 Å (020), 3.23 Å (002), 3.18 cements were chosen to represent the contents of the Å (040), and 2.82 Å (200) reflections, respectively, bulk samples. and the volumetric content (vol.%) of each component A fraction of each sample (about 200 g) was was determined using the formula (Hong et al., 2010a) 318 Chaowen Wang, Hanlie Hong, Zhaohui Li, Guojun Liang, Jin Xie, Bowen Song, Eping Song and Kexin Zhang

Figure 3. Field photographs of profile in Qaidam Basin. (a) A full view of the landscape, along with the old river channel; (b) jumbled boulder and angular conglomerate with indicate it has undergone rapid accumulation, S27-19-1, 1 196 m thickness (the hammer is 30 cm long); (c) gypsum layer in the mudstone, S27-135-3, 2 300 m thickness (unit of the scale bar in centimeter).

1 V % = revealed by the XRD patterns (Fig. 4). However, their i I KKKK ii()+++1 " n−1 + n relative abundance varied greatly. Quartz and feldspar KI I I I ii11 n− n are mainly terrigenous components. Their presence in where the I and K were the intensity (as expressed by the sediments can be recognized as weathering prod- peak height) and the intensity ratio of the characteris- ucts of widespread occurrences of various sedimentary, tic peak of the ith mineral to the intensity of (012) metamorphic, and volcanic rocks on the surrounding reﬂection of corundum at 3.48 Å. The K values were mountains. They are the dominant minerals through- 3.58, 1.14, 3.00, 2.56, 2.25, and 6.29, for calcite, out the whole profile with a content between 0−35% quartz, gypsum, K-feldspar, plagioclase, and halite, and 0−38%, respectively. respectively. Calcite is also present in nearly the entire profile, The illite crystallinity was calculated and ranging from 0 to 70% with an average of 16%. expressed as the full width at half-height of its (001) Dolomite was not considered because of its small reflection, measured from the XRD pattern of the quantity (not detected by XRD) and was occasionally air-dried oriented clay samples. present. The contents of gypsum and halite fluctuated SEM Observation frequently in the Eocene Lulehe Formation, ranging Selected samples were platinum-coated for scan- between 0−38% and 0−37%, respectively (Fig. 5). ning electron microscope (SEM) observation of min- During the Late Eocene to Late Oligocene, their con- eral texture and morphology. A JSM-5610 SEM was tents decreased abruptly at the lower Xiaganchaigou used with an accelerating voltage of 20 kV and a beam Formation. They are not detected in the middle of current of 1–3 nA. Shangganchaigou Formation, until Mid-Miocene at the upper Xiayoushanshan Formation, after which their RESULTS AND DISCUSSION contents generally increased with frequent fluctuations XRD Analyses in the range of 0−29% and 0−16%, respectively. Quartz, K-feldspar, plagioclase, calcite, gypsum, Illite crystallinity varied from 0.2 to 0.68 along halite, and clay minerals were the major minerals as Climatic and Tectonic Evolution in the North Qaidam since the Cenozoic: Evidence from Sedimentology and Mineralogy 319

Q&I book-like assemblages and exhibited well developed outline of source mineral, indicating strong chemical

3.33 weathering. The basal (001) plane of the clay minerals Shizigou Fm. Pl was well developed, but some of which were turned S27-512-1 (a) Kf Q Cc3.18 over, which may be referred to strong overland runoff I Gy Ch Ch Q Q 3.24 Ha 4.25 Ha (Fig. 7c).

3.03

7.6

9.9 I

7.08

14.2

1.82

1.54

2.81

4.9

1.99 In addition to detrital minerals, authigenic miner- (b) als, such as halite and gypsum, were also present in Youshashan Fm. S27-311-1 the void spaces among quartz and feldspar particles. Fibrous crystals were observed at 4 659 and 5 686 m. (c) The individual particle was ~0.03 μm wide and 1−5 Ganchaigou Fm. μm long and arranged in different directions (Fig. 7d). S27-101-1 These straight fibrous grains are the characteristic (d) textures of palygorskite (Hong et al., 2007).

Lulehe Fm. S26-22-1 Origin of Minerals as Climate Indicators The morphology of detrital minerals including quartz, feldspar, and clay minerals in the assemblages 5 10 15 20 25 30 35 40 45 50 55 60 65 would provide information on degrees of weathering Figure 4. The XRD patterns of representative bulk and transport. The non-clay minerals usually exhibit samples (d-value in Å). (a), (b), (c), and (d) repre- rounded characteristic and clays exhibit irregular and sent the samples from the Lulehe Fm., the rounded outlines, indicating extensive abrasion in the Ganchaigou Fm., the Youshashan Fm., and the course of transport. Shizigou Fm., respectively. Ch. Chlorite; I. illite; In comparison to feldspar, quartz is more resis- Gy. gypsum; Q. quartz; Kf. K-feldspar; Pl. plagio- tant to weathering. Thus, the Q/F ratio could be used clase; Cc. calcite; Ha. halite; Fm.. formation. to indicate the maturity of the sediments and thus, the degree of weathering and climate change (Kuhn and the profile (Fig. 6), exhibiting a generally decreasing Diekmann, 2002). Detrital materials from trend from bottom to upper with notable periodic warm-humid regions normally would exhibit higher changes. For instance, the illite crystallinity showed Q/F ratio in comparison to those from deserts and ni- low values in five intervals at the upper Lulehe, lower val regions (Kuhn and Diekmann, 2002). Xiaganchaigou, Shangganchaigou to Xiayoushashan, Halite and gypsum were the evaporite minerals middle Shangyoushashan, and upper Shizigou forma- that deposited when the lake shrank or when evapora- tions, respectively. tion exceeded precipitation with a supply of solutes through groundwater and overland runoff SEM Observations (Dupont-Nivet et al., 2007). Generally, climate The overall texture of the Qaidam sediments is cooling/drying and warming would both favor the loose. Clay particles occurred as discrete poorly crys- deposition of halite and gypsum via a decrease in tallized clasts filling in the interstitial space among fresh water supply and increase water evaporation. As larger detrital particles (Fig. 7a). The rounded outline halite has a larger solubility than gypsum, its deposi- and small platy thickness with loose texture at 1 331 tion would be even more affected by cooling/drying or m suggested intense dissolution during weathering warming climate. Thus, a change in abundance of hal- (Fig. 7b). However, clay minerals in some samples ite and gypsum in Qaidam Basin along the profile exhibited hair-shaped outline in the bottom of the would signal climate change in different stages. Lulehe Formation, indicating intense physical weath- Illite is a common detrital mineral derived from ering. The clay particles at 3 201 m occurred as weathering of aluminum silicate minerals, such as 320 Chaowen Wang, Hanlie Hong, Zhaohui Li, Guojun Liang, Jin Xie, Bowen Song, Eping Song and Kexin Zhang

and

and and arid arid

arid

and

Cold Cold

stage

Warm

humid

seasonal

Evolution

0.4

26 Ma 40 Ma

0.3

13.5 Ma

Unconfromity

0.2

Halite

0.1

0.4

Eroded area

0.3

0.2

Gypsum

Mudstone

0.1

0.8

0.6

0.4

Silty mudstone

Calcite

0.2

0.4

Muddy slitstone

0.3

0.2

Bulk minerals content (vol.%)

Plagioclase

Slitstone

0.1

0.4

0.3

Sandstone

0.2

K-feldspar

0.1

0.75

0.55

Gravel-bearing sandstone

Quartz

0.35

0.15 Lithology

Conglomerate

(m) Thickness 0

500

6000 5500 5000 4500 4000 3500 3000 2500 2000 1500 1000

ta unit Stra. Z Fm. SZG YSFm. SYSS Fm. XYSS Fm. XGCG L Fm. LLH GGFm. SGCG

loeeN Pliocene E Eocene icn N Miocene lgcn E Oligocene 2 1 1-2 3

pc (Ma) Epoch

53.47 22 395.3 8.1 15.3 33.9 35.5 43.8 23 2.5

Figure 5. Bulk mineral contents of the sediments showing the occurrence of evaporated minerals. LLH. Lulehe; XGCG. Xiaganchaigou; SGCG. Shangganchaigou; XYSS. Xiayoushashan; SYSS. Shangyoushashan; SZG. Shizigou Formation; Fm.. Formation.

Climatic and Tectonic Evolution in the North Qaidam since the Cenozoic: Evidence from Sedimentology and Mineralogy 321

-1

warming

Oi-1 glaciation

intensity

Mid-Miocene

Mi-1 glaciation

ice-sheet appear

Late Oligocene

Small-ephemeral

climate optimum

Asian moonsoons

E. Antarctic ice-sheet

W.Antarctic ice-sheet

Unconfromity

O (SMOW)

Eeocene

δ‰

climate optimum

(Zachos et al., 2001)

Late Paleocene

thermal maximum

60 40

Eroded area

and

and arid

arid

and

and arid

Cold

stage

Cold

Warm

humid

seasonal

Evolution

15 18

13 13.5

age

Event

Mudstone

stage

Uplift

0.4

Silty mudstone

0.3

Illite crystallinity

0.2 0.3 0.4 0.5 0.6 0.7

Muddy slitstone

0.2

0.1

Slitstone

Quartz/feldspar

Sandstone

0.4

0.2

0.6 0.8

Gypsum disappear

Gravel-bearing sandstone

0.2 0.4

Gypsum+Halite

and

2.5

8.1

35.5

43.8

15.3

ge (Ma)

53.47

Conglomerate

a LLH Fm. SZG Fm.

SYSS Fm. XGCG Fm. XYSS Fm. SGCG Fm.

Strat. unit

Lithology (m)

0 Thickness

500

6 500 6 000 5 500 1 000 2 000 1 500 3 000 2 500 4 000 3 500 5 000 4 500

Epoch Pliocene Eocene Oligocene Miocene

Figure 6. Comparison among contents of gypsum+halite, quartz/feldspar ratio, illite crystallinity, and oxygen isotopes (Zachos et al., 2001) data of marine sediments. A, B, C, D, E for the phases of uplift. a. Zhang, 2005; b. Sun et al., 2005; c. Lu and Xiong, 2009; d. Fang et al., 2007. LLH. Lulehe; XGCG. Xiaganchaigou; SGCG. Shangganchaigou; XYSS. Xiayoushashan; SYSS. Shangyoushashan; SZG. Shizigou.

322 Chaowen Wang, Hanlie Hong, Zhaohui Li, Guojun Liang, Jin Xie, Bowen Song, Eping Song and Kexin Zhang

Figure 7. Representative SEM photographs indicate the rock texture, and clay mineral morphology. (a) Clay mineral filling the interstitial spaces among larger detrital particles, sample S27-31-1, 1 331 m thickness; (b) poorly developed clay plates with rounded outline or ragged edges, S27-31-1, 1 331 m thickness; (c) clay particles occur as book-like assemblages and twistied planes, S27-218-1, 3 201 m thickness; (d) authigenic palyorskites show fibrous texture, and are arranged in different directions, S27-489-1, 5 686 m thickness. feldspar and mica, and is formed under weak alkaline an important role in illite crystallinity. condition in dry and cold climate with weak leaching Overall, the illite crystallinity in Qaidam sedi- (Grim, 1968). Illite crystallinity is an important proxy ments exhibited a gradual decreasing trend from bottom when accounting for the cold-dry period climatic and to upper portion, suggesting that digenesis did not exert tectonic evolution (Singer, 1984). High rainfall and significant effects on illite, which could be proved by temperature are considered to be conducive towards a the poorly cemented structure and abundance of illite. strong hydrolyzation of the mineral, resulting in an The Qaidam Basin, however, has undergone “opening” of the structure (high crystallinity value) multi-stages of climatic evolution and tectonic activities (Chamley, 1989; Singer, 1984). When the condition since Cenozoic (Rieser et al., 2009; Yin et al., 2002; changes to low temperature and dryness, its crystallin- Wang et al., 1999). As the illite crystallinity displays ity is preserved (Chamley, 1989; Singer, 1984). In multi-stages evolution in Qaidam sediments, it may re- diagenesis, illitization of smectite will result in an in- flect the continental formation environment and tec- crease in the relative proportion of illite in the illite- tonic activities associated with each stage. smectite mixed layer clay minerals together with an increase in ordering of layer type distribution and an in- Climatic Changes and Tectonic Uplift in Qaidam crease in illite crystallinity as the burial depth increased Basin as Indicated by the Mineralogical Study (Lanson et al., 2009; Drits et al., 2002; Warr and Rice, Investigation of the evolution of tectonic lithofa- 1994). Rapid uplift of surrounding high lands also plays cies, paleogeography, and sedimentary facies suggested Climatic and Tectonic Evolution in the North Qaidam since the Cenozoic: Evidence from Sedimentology and Mineralogy 323 that Qaidam Basin started to accumulate since 53.5 Ma since ~50 Ma, which controlled the formation and evo- (Zhang et al., 2010a). Paleocurrent direction study and lution of the Qaidam Basin (Yin et al., 2008, 2002). distribution regularity of clay minerals showed that Therefore, the activity of Altyn Tagh fault and northern sediments in the northeastern Qaidam Basin came from thrust system in Qaidam Basin played an important role north and were the possible weathering products of the in the uplift and denudation of the northern mountains south Qilian area (Song et al., 2010; Wang et al., 1997). and produced large amounts of coarse clasts along the Based on the halite and gypsum contents, the Q/F North Qaidam Basin. ratio, and the illite crystallinity, the climate can be di- After the warm and seasonal arid period, cold and vided into four stages from extreme warm and arid to arid period followed during ~40−26 Ma. Unlike the cold and arid, and then followed by warm and humid to former period, halite displayed a continuous presence. cold and arid. Furthermore, the uplift events could in- The gypsum disappeared in the Qaidam Basin at ap- volve in five distinct phases according to the lithology proximate 38 Ma predating that in the Xining Basin at and decreased illite crystallinity. the Eocene-Oligocene boundary (Dupont-Nivet et al., During ~53.5−40 Ma, a high illite crystallinity was 2007). The illite crystallinity together with the Q/F ratio determined together with occasional presence of gyp- showed a decreasing trend. This climate condition sum and halite and low Q/F ratio (Figs. 5 and 6), indi- could also be demonstrated by the increase in clays, cating a warm and seasonal arid environment. Intense chlorite and illite, sporopollen, pine and xerophytes and dissolution of clay minerals was observed in SEM ob- decrease in oxygen isotopes (Hong et al., 2010b; Rieser servation (Fig. 7b). Smectite is the dominant clay min- et al., 2009; Wang et al., 1999). This cold and arid cli- eral assemblage during that period (Hong et al., 2010b). mate period was attributed to global cooling and Pine and xerophytes exhibited stable low sporopollen northward drifting of Qaidam Basin (Rieser et al., 2009; values in the Lulehe Formation (Wang et al., 1999). Wang et al., 1999). However, uplift of Qinghai-Tibetan These evidences provide a potential support for the in- should not be neglected in the climate evolution of the terpretation of climate evolution in the region. The clay region. The uplift was demonstrated from the index of the Lulehe Formation demonstrated that the conglomerate-dominant lithology, a decrease in illite climate of this period was mainly controlled by global crystallinity and an increase in Q/F ratio during ~40−38 climate, which was on an Early Eocene climate opti- Ma. In addition, the paleoaltimetry of Late Eocene mum and last to a high temperature during that period reached an elevation of >4 km (DeCelles et al., 2007; (Wang C W et al., 2011; Zachos et al., 2001). Rowley and Currie, 2006) and the Qiangtang terrane A short period marked by low illite crystallinity also reached a similar elevation at ~39 Ma (Wang et al., and low Q/F ratio during ~52−50 Ma possibly indicated 2008). Farther north, the Xining-Lanzhou Basin un- a tectonic uplift event. Field observation showed the derwent intense rotation, which provided the evidence presence of up to 300 m thick large boulder and angular for the deformation during Paleocene–Eocene (Dupont- conglomerates in the Lulehe Formation (Fig. 3b). The Nivet et al., 2004). Qilian Mountains experienced a presence of abundant chlorite and illite accompanying rapid exhumation cooling event at about 40 Ma (Jolivet an increase in plagioclase and abrupt decrease in et al., 2001). Furthermore, the sporopollen data from K-feldspar resulted in a rapid accumulation of sedi- the nearby Xining Basin also showed a cold and arid ments and a source change in north Qaidam sediments climate confirmed by the appearance of conifer at ~38.3 caused by the uplift (Hong et al., 2010b). This stage of Ma and was attributed to the regional uplift (Dupont- tectonic uplift might be earlier due to the presence of Nivet et al., 2008). Although each evidences was sug- boulder and conglomerate below the product of the gestive, combination of them, including data from this erosion stratum. Quick accumulation and lack of ce- study, would build a persuasive argument that the mentation was the important cause leading to extensive Qinghai-Tibetan was significantly uplifted and eroded erosion in this section. Due to the influence of India- during 40−38 Ma. Asia collision, an early stage Altyn Tagh fault resulted The halite+gypsum abruptly disappeared during in the development of the thrust system in the north ~26−13.5 Ma, suggesting a warm and humid climate. 324 Chaowen Wang, Hanlie Hong, Zhaohui Li, Guojun Liang, Jin Xie, Bowen Song, Eping Song and Kexin Zhang

The Q/F ratio also showed an increasing trend com- is getting much colder and arid. Palygorskite, a typical pared to the former stage, suggesting an enhancement clay mineral formed under arid-semiarid climatic con- in chemical weathering and breakdown of feldspar. ditions, was frequently found over this period as re- SEM photographs showed book-like assemblages of vealed by SEM observation (Fig. 7d). The occurrence clay particles, confirming the process (Fig. 7c). The of palygorskite was in good agreement with the pres- climate event in this period agreed well with a global ence of gypsum and halite, indicating a cold/arid cli- warming period during ~26−15 Ma (Zachos et al., mate. The illite crystallinity showed a similar result 2001). In fact, an increase in illite crystallinity during with a consistent low value. Notably, the halite+ 18−15 Ma compared to ~26−18 Ma might suggest a gypsum exhibited three episodes of high values at 13, response to the Mid-Miocene climatic optimum ~9–8, <5 Ma, respectively, consistent with the aridity (Wright et al., 1992). In Qaidam Basin, pollen curves and East Asian monsoon intensity (Sun and Wang, showed a similar result marked by a maximum in 2005). The illite crystallinity showed a short period of abundance of Pinus and a minimum in abundance of low values during ~10−8 Ma, suggesting a tectonic up- xerophytes at ca. 15 Ma (Wang et al., 1999). Even at a lift event. Other evidences suggested that the uplift of higher latitude, the pollen also recorded this global the Tibetan Plateau at the Himalaya-Gangdese reached warming event at Tianshan Range by the lowest abun- a significant height and gravitationally induced tension dance of conifer and highest abundances of Betula and was relieved and triggered the formation of the exten- thermophilous broadleaved trees during 18−15 Ma (Sun sion fault basin along north-south at ~10 Ma (Harrison and Zhang, 2008). et al., 1992). Most of the lakes north of the plateau be- The illite crystallinity on the other hand showed an gan to dry out and common conglomerates of inconsistent result in comparison with halite+gypsum subaqueous fan, braided river, alluvia fan started to ac- and Q/F ratio, which could probably be attributed to cumulate in the period (Zhang et al., 2010a, b). Fur- tectonic uplift. This could be confirmed from field ob- thermore, thermochronological records showed that the servation that the Qaidam sediments presented Gangdese, West Kunlun, Altyn Tagh, north-east pla- multi-layers subaqueous fan conglomerate in mudstone teau and east plateau were all intensely uplifted and since ~26 Ma, suggesting an intense denudation at the eroded at ~8 Ma (Clark et al., 2010; Zhang et al., 2008; northeastern Qaidam region. Simultaneously, Indian Zheng et al., 2006). Uplift during Pliocene was the Plate commenced its northward underthrusting at ~26 most wholesale in the Qinghai-Tibetan since ~5 Ma. In Ma (Chung et al., 2005), which might serve as a pivotal the Qaidam Basin, the enhancement of aridification was control to the formation of the East Kunlun thrust fault marked by an increase in minerals precipitation and at ~26.6 Ma in the Southeast Qaidam region, and the Q/F ratio. However, as the climate was cold and arid latter made the thrust fault system transfixion in the enough and could not affect the structure of the illite East Qaidam (Jiang et al., 2008). Apatite and zircon under this climate condition, minimal change in illite fission-track thermochronology, sedimentary sequence, crystallinity was observed. This stage of uplift triggered and depositional facies also demonstrated that the the dying out of widespread lakes and reduced their northeastern margin of the Tibetan Plateau underwent sizes into small basins which reached their greatest re- dramatically tectonic uplift and exhumation during lief as marked by the widespread conglomerates at the 23−21 Ma (Wang G C et al., 2011). In the eastern Ti- margin of the plateau at 3.5 Ma (Zhang et al., 2010a, b). betan Plateau, rapid uplift and exhumation was recorded during 26−15 Ma (Wang G C et al., 2011). CONCLUSION Therefore, the low illite crystallinity and presence of Clay mineralogy and bulk mineral composition conglomerate may reflect a regional activity of a thrust of the Qaidam Basin sediments were investigated for fault system in Qaidam Basin, which responded to the their potential use as indicators of their formation, and uplift of the Tibetan Plateau. thus, as proxies for revealing information related to The halite+gypsum content increased progres- regional climate changes and uplift of the northeastern sively during ~13.5−2.5 Ma, suggesting that the climate Tibetan Plateau. In Qaidam Basin, the presence of Climatic and Tectonic Evolution in the North Qaidam since the Cenozoic: Evidence from Sedimentology and Mineralogy 325 gypsum and halite, the Q/F ratio, and morphology of leogene Clockwise Tectonic Rotation of the the minerals indicated a warm and seasonal arid envi- Xining-Lanzhou Region, Northeastern Tibetan Plateau. ronment in ~53.5−40 Ma, a cold and arid climate in Journal of Geophysical Research, 109: B04401, ~40−26 Ma, a warm and humid climate in ~26− doi:10.1029/2003JB002620 13.5 Ma, and a much cold and arid climate in Dupont-Nivet, G., Krijgsman, W., Langereis, C. G., et al., 2007. ~13.5−2.5 Ma, respectively. On the other hand, the il- Tibetan Plateau Aridification Linked to Global Cooling at lite crystallinity, together with the lithology and the Eocene–Oligocene Transition. Nature, 445: 635–638, changes of sedimentary facies, suggested a five phase doi:10.1038/nature05516 uplift in ~52−50, ~40−38, ~26−15, ~10−8, <5 Ma in Dupont-Nivet, G., Hoorn, C., Konert, M., 2008. Tibetan Uplift the Qaidam region, respectively. Prior to the Eocene–Oligocene Climate Transition: Evi- dence from Pollen Analysis of the Xining Basin. Geology, ACKNOWLEDGMENTS 36(12): 987–990, doi:10.1130/G25063A.1 The authors wish to thank Dr. J S Yu for the Fang, X. M., Zhang, W. L., Meng, Q. Q., et al., 2007. XRD analysis, Dr. S B Mu for the SEM analysis, and High-Resolution Magnetostratigraphy of the Neogene other team members for the field work, and G. Jock Huaitoutala Section in the Eastern Qaidam Basin on the Churchman for the thoughtful suggestion. NE Tibetan Plateau, Qinghai Province, China and Its Im- plication on Tectonic Uplift of the NE Tibetan Plateau. REFERENCES CITED Earth and Planetary Science Letters, 258(1–2): 293–306, An, Z. S., Kutzbach, J. E., Prell, W. 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