Climate-driven environmental change in the Zhada basin, southwestern Tibetan Plateau

Joel Saylor* Peter DeCelles Jay Quade Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA

ABSTRACT man, 1992; Molnar et al., 1993; France-Lanord tion the direct link between uplift and environ- and Derry, 1994; Ruddiman et al., 1997; An mental change on the Tibetan Plateau. The Zhada basin is a large Neogene et al., 2001; Abe et al., 2005; Molnar, 2005). The environmental effects of tectonics and extensional sag basin in the Tethyan Hima- Uplift is also thought to have directly driven climate change can best be addressed in basins laya of southwestern Tibet. In this paper environmental change on the Tibetan Plateau that contain all of the proxies mentioned above: we examine environmental changes in the (e.g., Liu, 1981a; Zhang et al., 1981; Zhu et pollen, leaf fossils, mammal fossils, and carbon- Zhada basin using sequence stratigraphy, al., 2004; Wang et al., 2006). However, recent ates used in stable isotope studies. A case in isotope stratigraphy, and lithostratigraphy. work suggests that global climate change point is the Zhada basin in southwestern Tibet. Sequence stratigraphy reveals a long-term drives climate and environmental change on However, the lack of a coherent, comprehensive tectonic signal in the formation and fi lling of the Tibetan Plateau (e.g., Dupont-Nivet et al., basin analysis integrating all the paleoenviron- the Zhada basin, as well as higher-frequency 2007). Moreover, uplift histories of the Tibetan mental proxies has hampered efforts to untangle cycles, which we attribute to Milankovitch Plateau based on faunal or fl oral associations the climatic and tectonic signals in the Zhada forcing. The record of Milankovitch cycles differ signifi cantly from those based on stable record. The Zhada Formation is described as in the Zhada basin implies that global cli- isotope and other quantitative paleoelevation both upward fi ning (Zhang et al., 1981; Zhou mate drove lake and wetland expansion and studies. Paleofl oral assemblages from Pleisto- et al., 2000; Li and Zhou, 2001b) and capped contraction in the southern Tibetan Pla- cene deposits on the Tibetan Plateau are simi- by boulder conglomerates (Zhu et al., 2004; teau from the Late Miocene to the Pleisto- lar to modern fl oral assemblages at low eleva- Zhu et al., 2007). There is similarly little con- cene. Sequence stratigraphy shows that the tions (e.g., Axelrod, 1981; Xu, 1981; Zhang et sensus regarding the basin’s tectonic origin. Zhada basin evolved from an overfi lled to al., 1981; Li and Zhou, 2001a, 2001b; Meng et The Zhada basin is presented as having devel- underfi lled basin, but continued evolution al., 2004; Molnar, 2005; Wang et al., 2006) and oped in the hanging wall of the low-angle South was truncated by an abrupt return to fl uvial are used to argue for plateau uplift of at least Tibetan detachment system or as a half-graben conditions. Isotope stratigraphy shows dis- 1 km since the Late Miocene. A similar argu- produced in response to arc-normal extension tinct drying cycles, particularly during times ment is based on the abundance of mammal (Wang et al., 2004; S.F. Wang et al., 2008a). It when the basin was underfi lled. megafauna on the Tibetan Plateau in the Late is also proposed to be a fl exural basin respond- A long-term environmental change Miocene–Pliocene and their relative paucity ing to arc-perpendicular compression (Zhou observed in the Zhada basin involves a now (e.g., Cao et al., 1981; Zhang et al., 1981; et al., 2000). The presence of capping boulder decrease in abundance of arboreal pollen Li and Li, 1990; Meng et al., 2004; Y. Wang et conglomerates has led to the suggestion that the in favor of nonarboreal pollen. The simi- al., 2008a). In contrast, other lines of evidence basin was recently uplifted (Zhu et al., 2004). larity between the long-term environmen- indicate that the southern Tibetan Plateau has Until recently, the Zhada basin was understood tal changes in the Zhada basin and those been at high elevations since at least the Mid- to have been at low elevations until as late as observed elsewhere on and around the dle Miocene (Garzione et al., 2000a; Rowley the Pleistocene (e.g., Zhang et al., 1981; Zhou et Tibetan Plateau suggests that those changes et al., 2001; Spicer et al., 2003; Currie et al., al., 2000; Li and Zhou, 2001a; Zhu et al., 2004). are due to global or regional climate change 2005; Saylor et al., 2009) and central Tibetan In a recent paper (Saylor et al., 2009) we rather than solely the result of uplift of the Plateau since at least the Oligocene (Cyr et al., documented the chronostratigraphy and stable Tibetan Plateau. 2005; Graham et al., 2005; Rowley and Cur- isotope record of the Zhada basin. Here we pro- rie, 2006; DeCelles et al., 2007; Dupont-Nivet vide basin-wide lithologic and sequence strati- INTRODUCTION et al., 2008). These paleoelevation studies also graphic correlations, frequency analysis of the show that uplift predated widespread Late record of environmental change, and a detailed Uplift of the Tibetan Plateau has long been Miocene climate change (see Molnar, 2005, isotope stratigraphy. Our results suggest that viewed as a major forcing factor in regional and for a summary of evidence for Late Miocene global climate change, possibly in conjunc- global climate change (e.g., Raymo and Ruddi- climate change). These studies call into ques- tion with regional climate change, controlled

*Present address: Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78712-0254, USA.

Geosphere; April 2010; v. 6; no. 2; p. 74–92; doi: 10.1130/GES00507.1; 12 fi gures; 1 table; 2 supplemental tables.

74 For permission to copy, contact [email protected] © 2010 Geological Society of America

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80°85° 90° 95° 100° 105° Quaternary Alluvium A 40° Abbreviations: 50550000 kmkm JSZ : Jinsha Suture Zone Zada Basin Fill t faul Tar imm gh BSZ : Bangong Suture Zone Kailas Conglomerate Ta BasBasinin ltyn ISZ : Indus Suture Zone A QaQQaidama damdadam Mesozoic Tethyan rocks BasBBasinn MFT : Main Frontal thrust 35° SonSoSSonpan-Ganzioonnpanpapana -GGaGanzinzz TTeTerraneerraneerrar anene KF : Karakoram fault Paleozoic Tethyan rocks KFKF JSZJJSSZSZ QiaQiQQiangtangangtgtangaanng TTeTerraneerrarrr anene Higher Himalayan rocks BSZ Gangdese Batholith ThTThisssS StudyStuttuddy Legend: 30° Thrust fault LLhLhaLhasahaasasa TerTTeTerraneere ranaane EZ Trace of measured section ISZSZS Detachment/normal fault H imalayan Thrust Belt North-south transect MFTT Strike-slip fault Great Counter Suture zone Northwest-southeast transect thrust

B Leo Pargil Detachment Great Counter thrust Q Ayi Shan 1NWZ 2NWZ ZhadaZ 3NWZ Basin 32°00 Qusum Detachment ? 2NZ Karakoram Fault NRW System 1NZ

NRE EZ 3NZ Indus-Yalu Suture Zone Guga 31°20 SZ SEZ Great Counter thrust China India

S outh 30°40 11000000 kmkm Lake T Lake ib M e Pulan a ta i n Basin n D e Gurla Mandhata 79°00 C tac e hm 7728 m n en tr t al Thrust 30°00 India

80°00 Nepal

81°00

Figure 1. (A) Elevation, shaded relief, and generalized tectonic map of the Himalayan-Tibetan orogenic system showing the location of the Zhada basin relative to major structures. (B) Generalized geologic map of the Zhada region. Modifi ed from mapping by Cheng and Xu (1987), Murphy et al. (2000, 2002), and mapping by M. Murphy (2005, 2006, 2007, personal commun.).

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environmental variability in the southwestern After deposition, the basin was incised to base- be physically traced (Saylor, 2008). Magneto- Tibetan Plateau during the Late Miocene– ment by the Sutlej River, exposing the complete stratigraphy linking the South Zhada, South- Pleistocene. The data also point to the possi- thickness of the Zhada Formation. The best east Zhada, and East Zhada sections provides bility of establishing a high-resolution climate estimate for the age of the Zhada Formation is additional constraints. A fi nal independent

record for this high-elevation basin extending between ca. 9.2 and after 1 Ma, based on ver- constraint is the switch from exclusively C3 to

from the Pleistocene to the Miocene. tebrate fossils and magnetostratigraphy (Fig. 2) mixed C3 and C4 vegetation that is observed (Lourens et al., 2004; S.F. Wang et al., 2008b; between 130 and 230 m in the South Zhada REGIONAL GEOLOGICAL SETTING Saylor et al., 2009). section and at ~300 m in the East Zhada sec-

tion (Saylor et al., 2009). The expansion of C4 The Zhada basin is the largest late Cenozoic METHODS vegetation is observed across the Indian sub- sedimentary basin in the Himalaya. It is located continent and southern Tibet ca. 7 Ma (Quade just north of the high Himalayan ridge crest in Sedimentology et al., 1989, 1995; France-Lanord and Derry, the western part of the orogen (~32°N, 82°E; 1994; Garzione et al., 2000a; Ojha et al., 2000; Fig. 1A). The basin is at least 150 km long and We measured 14 stratigraphic sections span- Wang et al., 2006). 60 km wide, and the current outcrop extent of ning the basin extent from the Zhada county seat the basin fi ll is at least 9000 km2 (Fig. 1B). in the southeast to the Leo Pargil Range front Frequency Analysis of The Zhada basin is located in a zone of in the northwest (Fig. 1B). Sections were mea- Zhada Formation Cycles active arc-parallel extension (Ni and Barazangi, sured at centimeter scale. 1985; Zhang et al., 2000; Murphy et al., 2002; The sedimentological record of the Zhada Thiede et al., 2006; Valli et al., 2007; Murphy Correlations Formation archives the cyclical expansion and et al., 2009). It is bounded by the South Tibetan contraction of a large paleolake. Frequency detachment system to the southwest, the Indus The geomorphic surface that caps the Zhada analysis was conducted by spectral analysis suture to the northeast, and the Leo Pargil and Formation is correlative across the basin, and and also by calculation of the average duration Gurla Mandhata gneiss domes to the northwest provides the datum for sequence stratigraphic of cycles. In order to apply spectral analysis to and southeast, respectively (Fig. 1B). The role and lithologic correlation. Correlations are this record, a waveform was created by assign- of each of these structures in the development of based on major stratigraphic members that can ing numerical values to each of the depositional the Zhada basin is an area of ongoing research. The South Tibetan detachment system is a series of north-dipping, low-angle, top-to-the-north Global polarity normal faults that place low-grade metasedi- Age time Polarity (Ma) scale Chrons mentary rocks of the Tethyan sequence on 0 high-grade gneisses and granites of the Greater 1n Himalayan sequence. Along strike, both to the 1 east and west, ages for movement on the South Tibetan detachment system range from 21 to South Zhada Section 2n 12 Ma (Hodges et al., 1992, 1996; Noble and 800 800 2 Searle, 1995; Searle et al., 1997; Murphy and Harrison, 1999; Searle and Godin, 2003; Cottle 700 700 3 2An et al., 2007). To the northeast of the Zhada basin, 600 600 the Oligocene–Miocene Great Counter thrust, a 4 south-dipping, top-to-the-north thrust system, 500 500 3n cuts the Indus suture (e.g., Gansser, 1964; Yin et 5 al., 1999; Murphy and Yin, 2003). Exhumation of the Leo Pargil and Gurla Mandhata gneiss 400 400 6 domes (Fig. 1B) by normal faulting began 9– 3An 10 Ma (Zhang et al., 2000; Murphy et al., 2002; 300 300 Thiede et al., 2006) and continues today. C3/C4 Transition7 3Bn The Zhada Formation occupies the Zhada 200 200 Stratigraphic height (m) Stratigraphic 4n basin and consists of >800 m of fl uvial, lacus- 8 trine, eolian, and alluvial fan deposits. The sedi- 100 100 4An mentary basin fi ll is undisturbed and forms an 9 angular or buttress unconformity with under- CS SS Cgm -90 -45 0 45 90 lying Tethyan sequence strata that were previ- VGP latitude (°) 10 ously shortened in the Himalayan fold-thrust 5n belt (Saylor, 2008). The Zhada Formation is capped by a geomorphic surface that extends across the basin and is interpreted as a paleode- Figure 2. South Zhada lithologic section and associated magnetostrati- positional plain that marks the maximum extent graphic section and correlation to the geomagnetic polarity time scale of sediment aggradation prior to integration (GPTS) of Lourens et al. (2004). VGP—virtual geomagnetic pole; C— of the modern Sutlej River drainage network. claystone; S—siltstone; SS—sandstone; Cgm—conglomerate.

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environments as follows: 5—fl uvial and alluvial ski et al., 1993; Garzione et al., 2000b; Poage structures (Supplemental Table 22). Unless fan associations; 4—supralittoral associations; and Chamberlain, 2001; Rowley et al., 2001; otherwise indicated, all deposits are laterally 3—littoral associations; 2 or 1—profundal Rowley and Garzione, 2007). Freshwater gas- continuous for hundreds of meters to several associations, based on the presence or absence tropods precipitate shells with oxygen isotopic kilometers. Only abbreviated descriptions and δ18 of terrestrial clastic or plant material, respec- ratios ( Occ [Shell carbonate oxygen isotope interpretations are presented here (for details, tively. Depositional environments in the South ratio]) in equilibrium with ambient water, depen- see Saylor, 2008). Zhada measured section were identifi ed at 0.5 m dent on the temperature-dependent fractionation increments or where the depositional environ- factor (Fritz and Poplawski, 1974; Leng et al., Depositional Cycles in ment changed. The series was converted from 1999) between aragonite and water. The δ13C the Zhada Formation δ13 the depth domain to the time domain by linear values of gastropod shells ( Ccc) are controlled interpolation between magnetostratigraphic by the δ13C value of dissolved inorganic carbon Deposits in the Zhada Formation occur in two δ13 tie points, justifi ed by the generally linear ( CDIC) in the ambient water (Lemeille et al., types of cycles that mark periods of lake or wet- subsidence/sediment accumulation rates (Say- 1983; Bonadonna et al., 1999; Leng et al., 1999). land expansion and contraction. The bulk of a δ13 lor, 2008). The assumption of a linear sediment The CDIC value is controlled primarily by the typical type A cycle (Figs. 4A and 5A) consists accumulation rate likely breaks down at short residence time of water and secondarily by fac- of a 1–10-m-thick unit of fl uvial or alluvial fan time scales, implying that interpretation of tors including the local vegetation and substrate. sandstone or conglomerate (lithofacies associa- δ13 cycles <100 k.y. must await a more fi nely tuned The CDIC value of surface water is increased tion F1 or rarely A1–A4) with an erosional base, basin chronology. The result is a clipped wave- by photosynthesis or equilibration with the no grain-size trend, and a capping, upward- form with uneven sample spacing and temporal atmosphere (Talbot, 1990; Li and Ku, 1997). fi ning sandstone bed (lithofacies association resolution better than 4 k.y. (Fig. 3; Supple- Particularly in productive lakes, increased water F2 or occasionally S1). This is overlain by an 1 δ13 mental Table 1 ). Progradation of basin margin residence time increases the CDIC value. Thus, organic-rich, fi ne-grained unit that contains con- δ13 δ18 depositional environments leads to waveform both Ccc and Occ values of gastropod shells voluted bedding (lithofacies association F3). saturation and loss of resolution at ages younger are useful in reconstructing paleohydrologic An idealized type B cycle (Figs. 4B, 5B, and than 3.3 Ma. In order to evaluate the effect of and paleoenvironmental conditions (e.g., Abell 5C) is characterized by an upward-coarsening this saturation on spectral analysis, both the and Williams, 1989; Purton and Brasier, 1997; succession of, in ascending order, fossil-rich 5.23–2.581 Ma and the 5.23–3.3 Ma intervals Hailemichael et al., 2002; Smith et al., 2004). siltstone (lithofacies association L1), lami- were analyzed (labeled “Entire Series” and Fossil gastropod shell fragments and intact nated or massive siltstone or sandy turbidites “Short Series,” respectively, in Fig. 3). shells were collected from fl uvial, marshy, and (lithofacies association P1–P2), rippled and The Lomb-Scargle Fourier transform method lacustrine intervals from the lower ~650 m in 2 cross-stratifi ed sandstone (lithofacies associa- was applied using the SPECTRUM program, measured sections. Shells were powdered and tion L2), sandstone containing planar, trough, which allows analysis of unevenly spaced time homogenized prior to analysis. To check for or climbing-ripple cross-stratifi cation (lithofa- series without interpolation (Schulz and Statteg- preservation of biogenic aragonite, 12 repre- cies association F2 or S1–S3), and conglom- ger, 1997). We conducted univariate autospec- sentative gastropod samples from fl uvial, lacus- erate beds (lithofacies associations A1–A4 or tral analysis (Welch method) to determine the trine, and marshy intervals were powdered and F1). The uppermost sandstone beds have both dominant frequencies in the record. We also analyzed using the University of Arizona’s D8 erosional and gradational basal surfaces. The conducted harmonic analysis using Siegel’s test Advance Bruker X-ray powder diffractometer basal surface of the capping conglomeratic unit to discriminate periodic components from noise (Saylor et al., 2009). is either erosional or marked by soft-sediment δ18 δ13 in the Zhada record. Cross-spectral analysis was We measured Occ and Ccc values using an deformation. Organic-rich convoluted siltstone used to determine the coherence between the automated carbonate preparation device (KIEL- (lithofacies association F3) can occur at any Zhada record and the record of summer insola- III) coupled to a gas-ratio mass spectrometer point within the capping sandstone or conglom- tion for 65°N (Laskar et al., 2004). (Finnigan MAT 252). Powdered samples were erate succession. In all cases, the boundary reacted with dehydrated phosphoric acid under between the fl uvial or alluvial fan association Stable Isotopes vacuum at 70 °C. The isotope ratio measure- and littoral or profundal association is abrupt, ment is calibrated based on repeated measure- while the transition from the profundal asso- Stable isotopes of oxygen and carbon ments of NBS-19 and NBS-18 and precision is ciation to the fl uvial or alluvial fan association [expressed as δ18O and δ13C in units ‰, respec- ±0.1‰ for δ18O and ±0.06‰ for δ13C (1σ). is gradational, indicating a rapid transgression tively, and referenced to Vienna Peedee belem- followed by gradual progradation. Parts of both nite (VPDB) or Vienna standard mean ocean RESULTS type A and type B cycles may be missing from water (VSMOW)] are sensitive indicators of the idealized version depicted in Figure 4. hydrologic conditions. The principal controls on Sedimentology δ18 δ18 surface water O ( Osw) values in southern Correlations Tibet are increasing elevation (which decreases We identify 14 lithofacies associations and δ18 Osw values) and evaporation (which increases fi ve depositional-environment associations Type A and type B cycles stack in predictable δ18 Osw values) (Dansgaard, 1954, 1964; Rozan- based on lithology, texture, and sedimentary patterns within a larger sequence stratigraphic

1Supplemental Table 1. Word document containing data used in frequency analysis. If you are viewing the PDF of this paper or reading it offl ine, please visit http:// dx.doi.org/10.1130/GES00507.S1 or the full-text article on www.gsapubs.org to view Supplemental Table 1.

2Supplemental Table 2. Word document containing the lithofacies association codes, descriptions, and interpretations. If you are viewing the PDF of this paper or reading it offl ine, please visit http://dx.doi.org/10.1130/GES00507.S1 or the full-text article on www.gsapubs.org to view Supplemental Table 2.

Geosphere, April 2010 77

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/2/74/3338188/74.pdf by guest on 23 September 2021 Saylor et al. South Zhada (SZ) Record of summer insolation Legend lithologic section Depo-codes at 65°N (Laskar et al., 2004) Mud cracks 2.5 End leg 10 at Mr Convoluted bedding/ N 31˚ 22.910’ Mh/Sr E 79˚ 45.075’ Gcmi soft sediment deformation 4299 ± 8 m 620 St Gcmi Hummocky cross- Gmm St Mc stratification Mh St Mc Oscillatory current ripples Gcm St 600 Mh/Gcm St/Gcmi Mh/Sh Unidirectional ripples Gmt Mc St Mc St Mr/Mh Erosional surface St Mc 580 Sc Gastropods n=19 St/Gcmi

St Sc Plants/Plant fragments St Mh Sc Hcs St 3 Bivalves Sc 560 Mc/St St Sh St n=10 Sr Ostracods Sc Gcmi Start Sc Root Traces leg 10 at Gcmi N 31˚ 540 Sh n=28 St Fish Skeletons/Fragments 23.112’ Sh St E 79˚ 44.981’ Mh Gmt Terrestrial Mammal Fossils 4197 ± 6 m Gcmi End leg 9520 Climbing ripples GPS Sr unavailable Mh Sc St Shell fragments Mh Gmm Gcmi St Sf Number of paleocurrent Sr Mh n = 17 Sh measurements 500 Sr Ml Sh 3.5 Paleocurrent direction from Mh Sf trough cross beds Sr St Mh Ml 480 St Paleocurrent direction from Mh Sr imbricated clasts Mh Sc Paleocurrent direction from Sr Mh Mh/Sr groundwater tubes 460n = 19 St Gmm Mh Sc n = 17 St Mh Mr/Mc St Mh Sr Sf 440 St Mh Sh Mh

Sh Short Series St Entire Series Hcs

St 4 Age (Ma) Stratigraphic height (m) 420 Gct P Mr St Mr St St Sr Figure 3. The synthetic wave Mh St Ml St form constructed for spectral Mh Sh Start leg 9 at 400 Mh Mh analysis. Depositional codes N 31˚ 24.158’ St Sr E 79˚ 45.442’ Mh relate to lithofacies associations 4057 ± 6 m Sh Mh (5—alluvial fan and fl uvial asso- End leg 8 at St N 31˚ 380 Mr/Sr ciations; 4—supralittoral asso- 24.449’ Ml E 79˚ 45.342’ Sr ciations; 3—littoral associations; 4057 ± 13 m St Mh Sr 2 or 1—profundal associations, 360 St n = 16 St/Gct Mh 4.5 based on the presence or absence St Sf St Sh of terrigenous clastic or biologic Start leg 8 at St N 31˚ 24.584’ 340 Sr material). At ages younger than n = 13 Mr E 79˚ 45.371’ Mh Ml 4001 ± 9 m Mh St 3.3 Ma, the waveform saturates End leg 7 at Mh N 31˚ 25.280’ Ml at values of 5 due to the infi ll- E 79˚ 320 St Ml/St 44.916’ Sr ing of the Zhada paleolake and 4001 ± 10 m Start leg 7 at Mh N 31˚ Ml St the progradation of lake-margin 25.275’ n = 14 Ml St E 79˚ Sr Ml depositional environments. Simi- 44.989’ 300 Mh 3966 ± 6 m Ml St Mh larly, the inability to distinguish End leg 6 at St Mh Ml N 31˚ 25.281’ Sr Mh fl uctuations in water level dur- E 79˚ 44.986’ Sr/Sc 5 3966 ± 7 m Mh 280 Sr/Sc ing times of profundal or alluvial Start leg 6 at Mh/Sr N 31˚ 25.507’ Mh fan and/or fl uvial sedimentation Ml E 79˚ 45.118’ St Mh/Sh 3933 ± 8 m St Mh results in clipping of the wave- Ml End leg 5 at 260 Sh/St N 31˚ 26.121’ Mh form. The record of insolation St E 79˚ 45.421’ Mh 3929 ± 10 m variation (Laskar et al., 2004) Ml 1 2 3 4 5 -60 -40 -20 0 20 40 60 Sh 0 0.02 0.04 0.06 Mh St Depo-code 240 St/Sh Variation in is provided for comparison. Mh Eccentricity Lake/wetland 2 Ml Insolation (W/m ) GPS—global positioning system; Mh expansion

Ml C—claystone; S—siltstone; SS— St (Stronger 220 Mh monsoon) Ml sandstone; Cgm—conglomerate. Start leg 5 Ml St N 31˚ 26.260’ Mh Mh/Sh Sf E 79˚ 45.143’ Mh Sf 3875 ± 7 m Mh/Ml Sh CS SS Cgm

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hierarchy (Fig. 6). Because of the diffi culty margin facies and occasionally nondeposition 2HST2 is the second-order highstand systems of establishing a hierarchy of continental of a lithofacies association. It is on the basis tract second from the base of the Zhada For- sequences based on sequence duration as deter- of these minor unconformities and associated mation). Second-order lowstand systems tracts mined in marine sequences (e.g., Vail et al., shifts in depositional environments that we (2LST) are characterized by type A cycles 1991), we follow Catuneanu (2006) and estab- defi ne sequences of all orders. arranged in a retrogradational (landward step- lish a unique hierarchy for the Zhada basin. At the fi nest scale, 56 type A and type B ping of depositional settings resulting in an The Zhada Formation does not have signifi cant cycles are present in the Zhada Formation increase in lake and/or wetland area) stacking intraformational unconformities that might (Fig. 7). Four second-order sequences are pattern (Fig. 6). They are fl uvially dominated represent extended periods of nondeposition evident above the cycles described above. and become increasingly marshy upsection. or extensive subaerial exposure and erosion. Nomenclature used identifi es sequence order, Second-order transgressive (fl ooding) surfaces However, it does have Waltherian unconformi- systems tract or bounding surface, and strati- (2TS) are identifi ed by an abrupt transition to ties that represent rapid progradation of basin graphic position from lowest to highest (hence thick, profundal claystone (Fig. 8). Modern

A Type A cycle B Type B cycle

Fluvial or Alluvial fan Association Supra-littoral Association Sub-aerial exposure Fluvial or Alluvial fan Association (Panel III.) { Supra-littoral Association

Gradual progadation (Panel II. - III.)

1 - 10 m Littoral Association Fluvial or Alluvial fan Association

Fluvial or alluvial fan ~ 10 m Profundal deposition deposition (Panel II.) (Panel I.) Low energy transgression drowns Profundal Association lake-margin semi-aquatic Littoral Association grasses (Panel I.) Exposure or sediment Fluvial or Alluvial fan Association bypass (Panel III.) Wetland deposition (Panel II.) Sandstone or conglomerate Papery laminated siltstone or claystone Massive or laminated siltstone or claystone Massive, laminated or rippled siltstone

C Depositional Setting Panels Alluvial fan submerged lake Possible locations of margin grasses Type A cycles Paleo-Sutlej River I. II. Zhada Co III. (lake) Type B cycle Marshy wetlands D Lake level, water influx, and lake water δ18O Depo-setting Panels: III I II III I II Progradation FloodingProgradation Flooding

Greater lake area

Net + water influx- Greater δ18O values Time (t)

Figure 4. (A, B) Idealized forms of sequence types A and B. (C) Interpreted depositional environments. (D) Simplifi ed representation of the relationship between lake level, water, and sediment fl ux and lake δ18O values. The simplifi cations involve the assumption that end-member infl ux and effl ux δ18O values are invariant and that effl ux via evaporation is proportional to lake area. Vertical gray boxes indicate the time of retrogradation (sediment infl ux < water infl ux) associated with fl ooding. The black star denotes the tem- poral location of Kungyu Co within the systems tracts at the time of sampling (25 July 2006). The legend for sedimentary structures is found in Figure 10.

Geosphere, April 2010 79

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Tibetan lakes are typically broad and shallow, (2HST) are characterized by type B cycles and 2TST2. Tract 1HST occurs between the and occur on low-relief plains. The lateral con- arranged in prograding (basinward stepping) most widespread maximum fl ooding surface tinuity of depositional units implies that these or aggrading (no stepping pattern) stack- and the top of the Zhada Formation and is conditions also existed during deposition of ing patterns. At the coarsest scale, the entire composed of 2HST2, 2LST3–2HST3, and the Zhada Formation. When transgression Zhada Formation can be seen as a fi rst-order 2LST4–2HST4. occurred, it would have quickly fl ooded the sequence (approximately third-order sequence Type A and type B cycles can be corre- depositional plain, resulting in rapid retrogra- of Vail et al., 1977). Tract 1LST is below the lated from stratigraphic sections spanning the dation. As a result, second-order transgressive fi rst major lacustrine transgression and is entire thickness of the Zhada Formation (South systems tracts (2TST) are thin. They are char- composed of 2LST1 and 2TST1. Tract 1TST Zhada and Guga sections) toward the basin acterized by type B cycles arranged in a ret- occurs between the fi rst major lacustrine trans- margins. Sediment accumulation was great- rogradational stacking pattern and are capped gression and the most widespread profundal est in the region of the South Zhada and Guga by widespread profundal lacustrine sedimen- lacustrine sedimentation (maximum fl ooding sections. However, the maximum thicknesses tation. Second-order highstand systems tracts surface) and is composed of 2HST1, 2LST2, of fi ne-grained material were deposited to the

A TypeTypype A cyclescyclc ese F1F1 ErEErosion/Sedimentossioon/n/SeSedid mementnt bbypassypy asass F3F3 FlFFloodlooood F22 F1F1 ErEErosion/Sedimentrososioon/n/SeSedidimementnt bbypassypypasss

B

C Type B cycles Flood Prograde Flood

Prograde

Flood S1S1 Prograde L2 P1P Flood L11

Figure 5. (A) Type A cycles. Cliff is ~15 m high. (B) Photomosaic of typical progradational sequences in the lacus- trine portion of the Zhada Formation (Nl). Each cycle in B is ~ 10 m. However, the focus of photo B is to show the lateral continuity of the Zhada deposits. (C) Type B cycles. Lowermost cycle is ~9 m high. Lower slope-forming interval represents upward-coarsening profundal to littoral mudstones and siltstones (P1, L1, L2), which are capped by cliff-forming littoral or supra-littoral sandstones (L2 or S1).

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northwest of there, in the region of the Namru Frequency Analysis of entire series and the 5.23–3.3 interval indicates Road West section. The implication is that, Zhada Formation Cycles statistically signifi cant peaks at 91.7 k.y. though relative subsidence was greatest in the at the 95% confi dence level and at 22.4 k.y. at the region of the South Zhada and Guga sections, The best time control based on magneto- 85% confi dence level (Fig. 9A). Harmonic anal- these were also close to the source of coarse- stratigraphy in the Zhada basin is between ysis of the entire series reveals peaks at 91.7 ± grained material (identifi ed in Saylor, 2008, chrons 2An (2.581 Ma) and 3n (5.23 Ma) (Lou- 2, 126 ± 4, 140 ± 4, 221 ± 12, 379 ± 40, 662 ± as both the Kailash region to the north of the rens et al., 2004). There are 28 type B cycles 287, and 1330 ± 2000 k.y. at the 99% confi dence basin and also the mountain ranges immedi- within this interval, each with an average dura- level (Fig. 9B). However, in the analysis of the ately surrounding the basin). tion of 95 k.y. Spectral analysis of both the 5.23–3.3 Ma interval all of these peaks, except for

S.E. Zhada S. Zhada 1MFS

Figure 6. Portion of Figure 7 showing

1TS C S SS Cgm detailed parasequence scale correla- tions. See Figures 7 and 10 for leg- end. 1TA—fi rst-order transgressive (fl ooding) surface; 1MFS—fi rst-order maximum fl ooding surface.

: Type A Cycles

: Type B Cycles

Tethyan fold-and-thrust CS SS belt (basement)

Geosphere, April 2010 81

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/6/2/74/3338188/74.pdf by guest on 23 September 2021 Saylor et al. 2TS2 2TS1 2TS4 2SB4 2MFS2 2SB2 2MFS1 2MFS4 2MFS3 & 2TS3 2SB3 S CS S Cgm S. Zhada S. 6km S CS S Cgm East Zhada 6km S CS S Cgm Namru Road East Tethyan fold-and-thrust belt (basement) fold-and-thrust Tethyan S CS S Cgm 1 N. Zhada 1 N. St St Ml Ml St St Ml Ml Ml St Sf Sf Sh 6km 25km ). Basin-wide lithostratigraphic and sequence stratigraphic correlations. (A) North-south transect. ). Basin-wide lithostratigraphic and sequence stratigraphic correlations. S CS S Cgm 2 N. Zhada 2 N. Legend continued on following page Alluvial fan lithofacies assoc. lithofacies Alluvial fan Sequence stratigraphic surfaces and surfaces Sequence stratigraphic (first & second order) sequence number Profundal lithofacies assoc. Profundal lithofacies assoc. lithofacies Littoral assoc. lithofacies Supra-littoral assoc. Fluvial lithofacies boundaries Parasequence boundariesSystem tract (first & second order) interval and System tract (first & second order) sequence number Independent constraints 50 m vertical 25km Paleo-depositional Paleo-depositional plain (datum) Figure 7 ( Figure #LST# #TS# S C S S Cgm A 3 N. Zhada 3 N.

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2 N.W. Zhada 2 N.W.

nt Fault nt e ; assoc.—association. Paleo-depositional plain (datum) Paleo-depositional S CS S Cgm 3 N.W. Zhada 3 N.W. ?

?

usum Detachm usum Q ooding) surface; MFS—maximum fl ? ? S CS S Cgm ? S Namru Road West Namru Road 2TS1 2SB4 2SB3 2TS3 2TS4 CS S Cgm 2SB2 2TS2 2MFS1 2MFS2 2MFS4 2MFS3 2TST1 2LST1 2TST3 2TST2 2LST2 2HST1 2LST3 2HST2 2TST4 2HST4 2LST4 2HST3 S CS S Cgm Guga S Tethyan fold-and-thrust belt (basement) fold-and-thrust Tethyan CS S Cgm 20km 10km 35km 10km 40km 10km 8km 8km S CS S Cgm S.E. ZhadaS.E. Zhada S. Southeast ). (B) Southeast-northwest transect. See Figure 1B for locations of transects. TS—transgressive (fl TS—transgressive locations of transects. 1B for ). (B) Southeast-northwest transect. See Figure 1HST 1MFS 1TST 1TS 1LST B continued Overfilled Underfilled Balance filled sequence boundary; HST—highstand systems tract; LST—lowstand systems tract; assoc.—association; TST—transgressive systems tract TST—transgressive sequence boundary; HST—highstand systems tract; LST—lowstand assoc.—association; Figure 7 ( Figure

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the 379 and 91.7 k.y. peaks, are suppressed (Fig. 9C). This indicates that the suppressed peaks are likely the result of red noise due to wave- PP11 form saturation at ages younger than 3.3 Ma. -Both the entire series and the shorter interval 22TS2TS2 pass Siegel’s test, indicating that the record is not SS1/F21/F2 the result of white noise. A random time series of similar length showed no statistically signifi cant peaks and did not pass Siegel’s test. Coherence analysis of the shorter interval also reveals peaks at 80 ± 13, 26 ± 1, and 18.9 ± 0.6 k.y. (Fig. 9D).

Stable Isotopes

The X-ray diffraction analyses from 11 of 12 samples yielded only aragonite peaks (Saylor et al., 2009); the 12th sample was too small to δ18 Figure 8. Second-order transgressive surface showing the abrupt yield results. The Occ values of samples that transition from lithofacies association S1 and F2 sandstone to lithofa- we analyzed using X-ray diffraction ranged cies P1 profundal claystone. Homo sapiens (circled in red) for scale. from −20.3‰ to +0.2‰ (VPDB).

0 Eccentricity Obliquity Precession Precession 0.08 (100,000 yrs)(41,000 yrs) (23,000 yrs)(19,500 yrs) BW 0.07 379 ± 40 kyrs 1.33 -5 ± 2 Myrs 0.06 662 ± -10 287 91.7 ± 2 kyrs 0.05 kyrs 126 ± 4 kyrs 221 -15 0.04 ± 12 kyrs 140 ± 0.03 4 kyrs Relative Power -20 99% CL 95% CL 85% CL 0.02

Log relative power -25 Spectral background 0.01 y = 0 0.003x.003x2 -0461x-0461x - 88.833.833 R2 = 0.879 -30 0 0 1020304050607080 0 2 4 6 8 101214161820 ABFrequency (1/Ma) Frequency (1/Ma) 0.12 0.6

0.1 0.5

0.4 0.08

0.3 0.06

0.2 Relative Power Coherence False alarm 0.04 99% CL 0.1 0.02 0 0 102030405060 0 0 2 4 6 8 10 12 14 16 18 20 D Frequency (1/Ma) C Frequncy (1/Ma)

Figure 9. (A) Power spectrum of the entire interval (5.23–2.581 Ma). The spectrum has peaks at ~100 k.y. at the 95% confi dence level (CL) and ~23 k.y. at the 85% confi dence level. Y axis is log (base 10) of relative power. (B) Harmonic analysis of the entire interval reveals dominant peaks at 379 and 91.7 k.y., but also has signifi cant red noise. (C) Harmonic analysis of the interval 5.23–3.3 Ma reveals the same dominant peaks, but red noise peaks are signifi cantly suppressed. (D) Coherence analysis reveals a peak at 91 k.y. Vertical error bars indicate the 95% confi dence interval.

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Clearly identifi able trends in multiple cycles faces; Van Wagoner et al., 1988b). Parasequences Type A parasequences occur at the base were found only in the densely sampled South are typically thin (<20 m) and correspondingly of the Zhada Formation sequences (Fig. 6). Zhada section, particularly in the 250–470 m short-lived (~100 k.y.). We conclude that facies Marshy deposits become more prominent com- interval of the South Zhada section where we are controlled primarily by lake or wetland ponents of type A parasequences higher in the focus our discussion. (For analysis of the entire expansion and contraction, which are related by sequences, consistent with the general retrogra- δ13 data set, see Saylor et al., 2009.) The Ccc val- the interplay of sedimentation and base-level dational stacking pattern. The upward-fi ning ues of gastropods in this interval range from change at the shoreline. This is most evident in textural trend, retrogradational stacking pattern, δ18 –3.3‰ to +2.1‰ (VPDB), and Occ values are type B parasequences where fl ooding surfaces and location at the base of the Zhada Formation from −13.7‰ to +0.7‰ (VPDB; Table 1). are easily identifi able as the sharp basal contact of sequences suggest that type A parasequences There are 17 type B cycles within the 250– the fi ne-grained, fossil-rich, often papery interval represent onset of lacustrine transgression. 470 m interval of the South Zhada section (Fig. capping a coarser-grained unit (Fig. 4B). In type The associated rise in the water table resulted 10). Of those, eight had suffi cient sampling den- A parasequences fl ooding is probably recorded in increased marshy conditions, although the δ18 sities that trends in Occ values should be evi- by the transition from the fl uvial association to system was still dominated by fl uvial processes dent. Five cycles show a clear trend of increas- marshy deposits of the supra-littoral association (e.g., Bohacs et al., 2000). δ18 ing Occ values with stratigraphic height above rather than by an abrupt surface as in type B Type B parasequences occur in the middle the cycle boundary (Fig. 10). One additional parasequences (Fig. 4A). However, type A para- to upper Zhada Formation (Fig. 6) and coarsen cycle shows a similar, but muted, trend (Fig. sequences have clearly identifi able erosive sur- upward from a profundal lacustrine lithofacies 10). The fi nal two cycles do not show any trend faces that can be correlated to subaerial exposure association to a supralittoral or fl uvial lithofa- δ18 in Occ values (Fig. 10). surfaces in type B parasequences (Figs. 4, 5, and cies association. Thus, they represent progra- 6). Thus, the maximum regressive surface in both dational parasequences in a lacustrine setting. INTERPRETATION OF ZHADA type A and type B parasequences is defi ned as The persistence of these cycles to the top of FORMATION CYCLES the erosional surface at the base of the coarsest- the Zhada Formation indicates that lacustrine grained interval, if an erosional surface is present, conditions prevailed until the onset of incision Zhada Formation type A and type B cycles are or at the base of the lowest sandy interval show- by the modern Sutlej River, despite prograda- best interpreted as parasequences (a conformable ing signs of unidirectional traction transport if no tion causing replacement of the fi ne-grained succession of beds separated by fl ooding sur- erosional surface is present. littoral or supra-littoral deposits by basin- margin alluvial fans. Zhada Formation cycles obey Walther’s Law. TABLE 1. STABLE ISOTOPE DATA Within individual cycles, facies that are super- Stratigraphic height δ13C δ 18O posed occurred side by side spatially (e.g., Mid- Sample name (m) (‰, VPDB) (‰, VPDB) dleton, 1973; Posamentier and Allen, 1999). 1SZ12 309.65 –0.9 –3.9 1SZ13 310.65 –3.0 –12.5 This is consistent with sequence stratigraphy 1SZ13.8 311.45 –1.3 –3.2 theory (Van Wagoner et al., 1988b) but contrasts 1SZ18 315.65 2.1 –1.8 with reports of non-Waltherian cycles from the 1SZ24 321.65 –0.2 –5.4 1SZ24.1 321.75 –0.5 –4.6 Green River Formation and underfi lled lacus- 1SZ27.9 325.55 0.0 –2.8 trine basins in the Qaidam basin and Death Val- 1SZ32 329.65 0.5 –1.5 ley (Yang et al., 1995; Lowenstein et al., 1998; 2SZ43 376.1 0.9 –1.8 2SZ43.1 376.2 –0.4 –6.8 Pietras and Carroll, 2006). 2SZ47 380.1 –0.6 –1.9 2SZ51.5 384.6 –0.2 –2.4 DISCUSSION 2SZ51.5AD0.5 384.6 –0.6 –0.6 2SZ51.5AD10 384.6 0.2 –1.2 2SZ51.5AD11 384.6 0.2 –2.1 Sequence Stratigraphic and 2SZ51.5AD12 384.6 0.1 –2.2 Lithostratigraphic Correlations 2SZ51.5AD13 384.6 0.3 –2.1 2SZ51.5AD14 384.6 0.2 –2.5 2SZ51.5AD15 384.6 0.8 –0.8 The overfi lled, balanced-fi lled, and under- 2SZ51.5AD3 384.6 –0.4 –2.2 fi lled intervals of the Zhada basin were delin- 2SZ51.5AD4 384.6 –0.1 –2.1 2SZ51.5AD5 384.6 0.3 –1.6 eated using defi nitions modifi ed from Bohacs et 2SZ51.5AD6 384.6 0.9 –0.8 al. (2000). In contrast to the evaporative facies 2SZ51.5AD7 384.6 0.0 –0.3 association presented by Bohacs et al. (2000) as 2SZ51.5AD8 384.6 0.0 –0.3 2SZ51.5AD9 384.6 –0.1 –0.4 typical of underfi lled lake basins, evaporites are 2SZ55 388.1 –0.7 –2.3 present, though not dominant within the Zhada 3SZ0.15 389.25 0.9 –1.7 sections. It may be argued that no sections were 3SZ24 413.1 –0.3 –2.2 3SZ24.1A 413.2 1.6 –2.8 measured in the basin center and so the possi- 3SZ24.25A 413.35 0.8 –1.4 bility exists that that is the locus of evaporite 3SZ24.25B 413.35 –1.1 –1.8 deposition. However, that is unlikely given the 3SZ24.25C 413.35 0.6 –1.6 3SZ24.3 413.4 0.9 –2.0 lateral facies continuity in the Zhada Formation 3SZ27 416.1 –2.1 0.7 and the number of measured sections close to 3SZ49 438.1 –3.3 –13.7 3SZ50.5 439.6 –1.2 –1.0 the basin center. A more plausible explanation is 3SZ55 444.1 1.2 –2.5 that discharge into the basin by the paleo–Sutlej Note: VPDB—Vienna Peedee belemnite; SZ–South Zhada. River was consistent and large enough that the

Geosphere, April 2010 85

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Sr Legend Mh Mh/Sr 460n = 19 Mud cracks St Convoluted bedding/ Gmm n = 17 Mh soft sediment deformation Sc St Hummocky cross- Mh stratification Mr/Mc St Oscillatory current ripples Mh Sr Sf 440 St Unidirectional ripples

Mh Erosional surface Sh

Mh Gastropods Sh St Hcs Plants/Plant fragments St 420 Gct Bivalves P Mr St Mr Ostracods St St Sr Root traces Mh St Ml Fish skeletons/fragments St Mh Sh Terrestrial mammal fossils Stratigraphic height (m) 400 Mh Mh St Climbing ripples Start leg 9 at Sr N 31˚ 24.158’ Mh Shell fragments E 79˚ 45.442’ Sh 4057 ± 6 m Mh Rip-up clasts End leg 8 at Number of paleocurrent n = 17 N 31˚ 24.449’ St measurements E 79˚ 45.342’380 Mr/Sr Paleocurrent direction from 4057 ± 13 m trough cross beds Paleocurrent direction from Ml imbricated clasts Sr Paleocurrent direction from ground water tubes St Mh Lithofacies Sr 360 Gcmi n = 16 St St/Gct Gm Mh St Gt Sf St St Sh Sr St Sh/Sm Start leg 8 at 340 Sr n = 13 Mr Sf N 31˚ 24.584’ Mh Sc E 79˚ 45.371’ Ml 4001 ± 9 m Mh Mr St End leg 7 at Ml N 31˚ 25.280’ Mh Mh E 79˚ 44.916’ Ml 4001 ± 10 m 320 St Mm Ml/St Mc Sr

Mh Ml n = 14 St Ml Figure 10. South Zhada lithologic section Start leg 7 at St Sr and associated δ18O values of aquatic gas- N 31˚ 25.275’ Ml E 79˚ 44.989’300 Mh tropods. Horizontal black lines represent 3966 ± 6 m Ml End leg 6 at St parasequence boundaries. Thick verti- Mh N 31˚ 25.281’ St Mh cal green boxes indicate the sequences E 79˚ 44.986’ Ml Sr that were used to construct Figure 12. 3966 ± 7 m Mh Sr/Sc Within all fi ve sequences where a trend Mh Sr/Sc δ18 280 is evident, Occ values increase from Mh/Sr the fl ooding surface to the maximum Mh Start leg 6 at regression surface. Vertical orange boxes N 31˚ 25.507’ Ml St indicate sequences that show a possible, E 79˚ 45.118’ Mh/Sh 3933 ± 8 m St though not clear, trend. Vertical red End leg 5 at Mh Ml N 31˚ 26.121’260 Sh/St boxes indicate sequences with suffi cient E 79˚ 45.421’ 18 Mh sampling density that trends in δ O val- 3929 ± 10 m cc St -20-18-16-14--10-8-6-4-20 12 ues may be expected, but where no trends Mh are observed. VPDB—Vienna Peedee C S SS CgmMl 18 Sh δ O (VPDB%) Mh belemnite; C—claystone; S—siltstone; St 240 St/Sh SS— sandstone; Cgm—conglomerate.

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lake rarely desiccated though surface outfl ow not directly underlie the coarse-grained facies. basin rarely became desiccated. If water, and was minimal (e.g., Lake Naivasha; Barton et al., Rather, the profundal lacustrine facies coars- thus sediment, infl ux were relatively stable, 1987; Duhnforth et al., 2006). ens upward gradually and shows evidence of regression would be marked by progradation The overfi lled interval was determined based traction transport, including oscillatory current and, during maximum regression, which marks on the prevalence of fl uvial input, indistinctly ripples, throughout regression. The coarse- the time when the lake has the smallest volume expressed parasequences (Bohacs et al., 2000), grained facies exhibit evidence of subaerial and is the most restricted, the relative infl uence and the dominance of sedimentary structures exposure including preferential weathering of fl uvial input would be greatest. indicating traction transport. The overfi lled and cementation, and root traces. In addition, The evolution of the Zhada basin followed interval extends from the base of the section the coarse-grained facies are often interbed- a typical pattern from a fl uvial system to an to 1TS (which is the same surface as 2TS1; ded with organic-rich siltstone and sandstone underfi lled lacustrine basin (Fig. 11) (Bohacs et δ18 Fig. 7). The Occ values from this interval facies (lithofacies association F3) indicative al., 2000). However, the top of the Zhada For- are extremely negative due to the low water- of marshy wetlands, such as might occur on mation is dominated by coarse-grained, basin- residence times associated with river through- lake margins or between fl uvial channels. We margin equivalents of type B sequences. There fl ow (Saylor et al., 2009). therefore interpret the coarse-grained facies is no change in large-scale sedimentary environ- The balanced fi ll interval is identifi ed by as the maximum progradation of lake-margin ment indicated prior to an abrupt truncation of a dominantly retrogradational parasequence depositional environments (Figs. 4C, 4D, panel the Zhada Formation by a paleodepositional stacking pattern. The balanced fi ll interval is I). One possible explanation for the difference plain. By implication, there was no return to a characterized primarily by the rising water between type B cycles and those in the basins balanced fi ll or overfi lled basin type. The return table inferred from the increased prevalence of studied by Bohacs et al. (2000) and Carroll to fl uvial conditions often observed was discon- marshy intervals. Though the basin was inter- (1998) is that fl uctuations in infl ux were not as tinuous in that it bypassed the balanced fi ll and mittently open, fl uvial infl ux was greater than great in the Zhada basin, and that the Zhada overfi lled intervals (Fig. 11). effl ux via outfl ow and evaporation and so the basin was being slowly drowned. The balanced Accommodation supply fi ll interval extends from 1TS to 2MFS1 (Fig. Accommodation < supply δ18 7). The trend toward more positive Occ values in this interval and the inferred increase in water Overfilled

residence times (Saylor et al., 2009) are consis- =~ tent with this interpretation. The underfi lled interval is well represented in the Zhada Formation and was identifi ed based Balanced fill on the occurrence of well-expressed fl ood- ing surfaces that separate distinct lithologies. Parasequences are well developed and record Fluvial a combination of progradational and aggrada-

tional stacking patterns. Depositional geome- T h tries (fl ooding surfaces) are generally parallel or i c subparallel and well-expressed parasequences k S converge and become indistinct toward the T o h u basin center. Within parasequences, trans- i r n c Accommodation > supply e gressive deposits are thin (<0.5 m) or absent, S I Underfilled o nt whereas progradational deposits are thick, well u er rc va developed, and dominated by traction transport e ls In (oscillatory current ripples, climbing ripples). te The underfi lled interval extends from 2MFS1 to supply Sediment and water rv als the paleodepositional surface. A B Type B parasequences occur primarily in the underfi lled portion of the Zhada basin. How- C ever, they differ from previous descriptions of underfi lled basin lithofacies (e.g., Carroll, Eolian 1998; Bohacs et al., 2000). The primary dif- ference is that coarse-grained facies were pre- Accommodation (height of sill above base level) sented as the result of transgression by Bohacs et al. (2000) and Carroll (1998), whereas in Figure 11. The trajectory of Zhada basin evolution in accommodation and the Zhada basin they typically constitute the sediment-supply and water-supply space. Also shown are fi elds occupied by over- regressive portion of the parasequence. There fi lled, balanced fi ll, and underfi lled basins. The Zhada basin followed a typical are several reasons for interpreting coarse- evolutionary pattern from fl uvial to underfi lled basin due to an increase in accom- grained facies as the regressive part of the cycle modation (solid black arrow A) until a new sill was breached (B). At this point the in the Zhada basin. Unlike the cycles presented basin underwent a discontinuous return to fl uvial conditions, bypassing the usual by Bohacs et al. (2000) and Carroll (1998), progression back through the balanced fi ll and overfi lled fi elds (dashed black arrow fi ne-grained, subaerial exposure surfaces do C). Modifi ed from Bohacs et al. (2000).

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Frequency Analysis insolation-driven climate changes (fourth-order (usually climatically driven) and movement on sequences of Vail et al., 1977) due to changes faults (tectonically driven) are both primary The two independent time-series analyses in the orbital characteristics of the Earth (i.e., controllers of systems tracts. Sediment supply described above indicate that ~100 k.y. cycles Milankovitch cycles). If parasequences are rep- is linked to water infl ux. This creates the para- are present in the Zhada Formation. In addition resentative of Milankovitch cycles, the driving doxical situation where the infl ux of water and to a peak at 91.7 k.y., univariate spectral analy- process behind high-frequency environmental lake volume can be high, and yet lake volume sis reveals a peak at 22 k.y. These are within cyclicity in the Zhada basin was not tecton- can be decreasing (net evaporation > net infl ux; 1/2 bandwidth (6 dB bandwidth = 2.4) of the ics. Rather, lacustrine expansion and contrac- Figs. 4C, 4D). eccentricity and precession frequencies. Har- tion was caused by a change in the long-term Water and sediment infl ux are thus decoupled monic analysis does not reveal the 22 k.y. peak precipitation to evaporation ratio. Long-term from base-level changes but are primary con- indicated by univariate analysis, but does show changes in the precipitation/evaporation ratio trollers of shoreline trajectory, and therefore peaks at 91.7 and 379 k.y., both of which are have been linked to strengthening or weakening parasequence evolution. Lithofacies distribution consistent with the eccentricity cycle (Figs. 9B, of the monsoon due to increases or decreases, and thus lithologic stacking patterns appear to 9C). Coherence analysis shows coherence with respectively, in insolation (Kutzbach, 1981; be controlled primarily by the location of the both eccentricity and insolation records (Laskar Prell and Kutzbach, 1992; Gupta et al., 2001; shoreline and so are also decoupled from lake et al., 2004) only at the eccentricity frequency Shi et al., 2001; Ruddiman, 2006; Thompson et volume. This means that parasequence fl ood- (Fig. 9D). The fact that both frequency analysis al., 2006). Shi et al. (2001) suggested a causal ing surfaces correspond to lake expansion due and an average cycle duration shows 100 k.y. link between monsoon strength and Tibetan to a drop in the evaporation/precipitation ratio. δ18 cyclicity indicates that this signal is robust. lake expansion and, in the absence of a change Thus, the lowest Occ values of aquatic gas- Sequences and parasequences in the Zhada in winter rainfall in Tibet, we link Zhada paleo- tropods and, implicitly, of the lake water, are Formation are either tectonic or climatic in lake size to insolation-driven monsoon inten- found at the fl ooding surface, even though the origin. The correlation between the fi rst-order sity. It is not surprising that climatically driven coarsest material is associated with maximum transgressive surface (Fig. 7, 1TS) and major parasequences are most distinctly expressed in regression (Fig. 12). Particularly when the basin tectonic reorganization in the Zhada region the underfi lled interval of the Zhada Formation, was underfi lled, the highest isotopic values (Saylor, 2008) points to a tectonic origin for the because during this interval the lake would occur at the time of maximum regression (Figs. fi rst-order sequence. Likewise, the correlation be most susceptible to changes in hydrology 4D and 10). This apparent discrepancy can be between the fi rst second-order transgressive (Kelly, 1993; Bohacs et al., 2000). accounted for by understanding that though surface (Fig. 7, 2TS1) and maximum fl ooding water and sediment infl ux were both relatively surface (Fig. 7, 2MFS1) with the major tectonic Isotopes in Zhada Formation Cycles high and stable, climatically driven evapora- reorganization and an increase in the exhuma- tion/precipitation controlled lake level and thus δ18 tion rate on of the Leo Pargil Range, respectively, Lakes respond to changes in hydrology on the Osw value of lake water. When effl ux was δ18 (Thiede et al., 2006; Saylor, 2008) also points much shorter time scales than do oceans because greater than infl ux, the Osw value increased, to a tectonic origin for second-order sequences. they have much smaller water and sediment vol- the lake shrank, and the coarse-grained material The number and consistent and short duration umes (see Kelts, 1988; Sladen, 1994; Bohacs et was carried further into the basin (Fig. 4C, panel of parasequences rule out a tectonic origin. al., 2000). In addition, in lacustrine settings the I). Conversely, when infl ux was greater than δ18 Parasequences are consistent in duration with relative proportions of water infl ux and effl ux effl ux, the Osw value decreased, the lake grew,

1 Parasequence 1 Parasequence 2 0.75 Parasequence 3 Parasequence 4 Parasequence 5 0.5

0.25

Normalized height above flooding Sfc above floodingheight Sfc Normalized 0 Flooding Regression -4 -3 -2 -1 0 1 2 3 -16 -14 -12 -10 -8 -6 -4 -2 0 2 δ13 δ13 Ccc(‰ VPDB) Ccc(‰ VPDB)

Figure 12. δ18O and δ13C values (Vienna Peedee belemnite, VPDB) of aquatic gastropods from fi ve sequences (indicated in Fig. 10) are plot- ted against their normalized height above the fl ooding surface (Sfc). The lowest values occur just above the fl ooding surface and represent lake expansion associated with a decrease in the evaporation/precipitation ratio. However, continued evaporative enrichment and isotopic δ18 δ13 evolution means that Occ and Ccc values increase through most of the regressive sequence.

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and the coarse-grained material was trapped at eventually breached after 1 Ma. At this point, we can reasonably expect that fl oral and faunal the basin margins (Fig. 4C, panel II). the system abruptly returned to fl uvial condi- communities on the Tibetan Plateau would also The foregoing discussion indicates that the tions and began incising through the Zhada have responded to global climate change. The δ13 δ18 primary control of Csw and Osw values Formation. The sudden return to fl uvial condi- shift from C3-dominated forests to mixed C3

was volume-weighted average water residence tions via the integration of the modern Sutlej and C4 or C4-dominated grasslands observed in time. Just prior to fl ooding, when lake vol- River system truncated the typical basin evolu- the Zhada basin (Zhang et al., 1981; Zhu et al., ume was small, the average water residence tion pattern described by Bohacs et al. (2000). 2006, 2007; Yu et al., 2007; Saylor et al., 2009) δ13 δ18 time, and hence Csw and Osw values, was was not the result of basin uplift, because an signifi cantly altered by addition of a small vol- Global Climate Change and Its Impact on identical change is observed in low-elevation ume of water. However, after signifi cant fl ood- the Southern Tibetan Plateau deposits in nearby northern India. Further, anal- ing, the lake was suffi ciently large and the water ysis of oxygen isotopes from aquatic gastro- suffi ciently evolved that the continued input of Numerous authors have reported 100 and pod shells from the Zhada Formation indicates water during fl ooding had only a minor effect on 400 k.y. cycles in the Miocene (Van Wagoner a probable decrease in elevation of the basin average water residence time. Though the dis- et al., 1988a; Kashiwaya et al., 2001; Zachos et since the Late Miocene (Saylor et al., 2009). A cussion here refers primarily to individual para- al., 2001; Di Celma and Cantalamessa, 2007; more likely scenario is that a regional or global sequences, the effect may span several parase- Holbourn et al., 2007), although none from climatic change affected both low- and high- quences and point to climatic control at multiple high elevations such as the Tibetan Plateau. elevation environments and favored a shift from frequencies (Fig. 10). The Zhada basin therefore presents an excel- forest to grassland. δ18 The correlation between low Osw val- lent opportunity to study high-frequency cli- A possible scenario is that the vegetation ues and fl ooding described here is confi rmed matically driven environmental change at high shift began at high elevations due to a global in the modern analog of Kungyu Co. Water elevations in the Miocene–Pleistocene. Expan- or regional climate change. Suggested factors samples collected on 25 July, 2006, from the sion and contraction of lakes and wetlands have include the onset of rapidly decreasing global lake and from the sole river fl owing into the been linked to variability in the strength of the temperatures in the latest Miocene–Pliocene lake had δ18O values of −14.8‰ and −15.6‰ Indian monsoon (Shi et al., 2001). The Quater- (Zachos et al., 2001) or increased monsoon (VSMOW), respectively. Stranded shorelines nary monsoon is thought to be modulated by intensity (Kroon et al., 1991) and associated with aquatic grasses and evaporites on the lake orbital cyclicity (Clemens et al., 1991; Prell and increased aridity and seasonality of precipita- margins showed that the lake was recently at Kutzbach, 1992; Jian et al., 2001; Wang et al., tion (Guo et al., 2002; Garzione et al., 2003; higher levels. The samples were collected at the 2005; Nie et al., 2008; Y. Wang et al., 2008b), Molnar, 2005). Increased warm-season precipi-

start of the monsoon season and the interpreta- though there is disagreement about which fre- tation and increased aridity favor C4 grasses (An tion is that the lake level had fallen to extremely quencies are dominant (Clemens and Prell, et al., 2005). As is thought to be the case in the low levels and was now in the process of refi ll- 2003; Nakagawa et al., 2008). Data from this foreland, the fl oral shift was accompanied by ing (Fig. 4D; black star denotes the interpreted study support previous work indicating that the faunal change at high elevations. location of Kungyu Co within the fi lling and/or monsoon has long varied at eccentricity fre- The possibility remains that these climate emptying cycle at the time of sampling). quencies (Dupont-Nivet et al., 2007). changes were driven by expansion of the region We turn next to another challenge presented of high elevations, particularly on the northern Basin History by the Zhada basin, i.e., the explanation of the and eastern margins of the Tibetan Plateau (e.g., fl oral and faunal changes observed within the An et al., 2001). However, any such models Combining the observations made above Zhada Formation and between the Late Mio- must take into consideration long-lived high with previous studies (Saylor, 2008; Saylor et cene and the present. The Zhada basin con- elevations in the southern and central Tibetan al., 2009) points to the following basin history. tained a host of plants that are typically thought Plateau (Garzione et al., 2000a; Rowley et al., Through arc-parallel extension, a sill was cre- of as native to warm, humid, and, as inferred 2001; Currie et al., 2005; Cyr et al., 2005; Row- ated that caused ponding of the river, leading to by some, low-elevation climates (Li and Zhou, ley and Currie, 2006; DeCelles et al., 2007; deposition of the lowest strata of the Zhada For- 2001a, 2001b; Zhu et al., 2004, 2007). In addi- Dupont-Nivet et al., 2008; Saylor et al., 2009). mation. The accumulating sediment onlapped tion, a broad cross section of mammal mega- the preexisting Tethyan sequence topogra- fauna lived in the Zhada basin area, including CONCLUSIONS phy, forming the observed buttress or angular Hipparion zandaense, Nyctereutes, Palaeotra- unconformities. The ancestral Sutlej River gus microdon, and rhinoceri that have variously 1. Lithologic cycles (types A and B) in the continued to fl ow from its source, increasing been identifi ed as Hyracodon or Dicerorhinus Zhada basin are Waltherian parasequences. the sediment pile. The exhumation rate of the (Liu, 1981b; Zhang et al., 1981; X. Wang, 2006, 2. Sedimentology and sequence stratigraphic Leo Pargil–Qusum Range to the northwest of personal commun.; E. Lindsay, 2006, personal analysis indicate that the Zhada basin evolved the Zhada basin between 10 and 5.6 Ma was commun.; Li and Li, 1990; Meng et al., 2004). from a fl uvial system to an overfi lled basin. The the same as the sediment accumulation rate in This is in striking contrast to the basin today, in overfi lled basin was marked by a broad deposi- the Zhada basin (Thiede et al., 2006; Saylor, which the only large mammalian fauna are the tional plain dominated by wetlands bordering a 2008), indicating that the uplifting range may kiang (Tibetan wild asses) and extremely rare large braided river. From there, the basin evolved have acted as a sill. After 5.6 Ma both the exhu- chiru (small, long-horned antelope). sequentially to a balanced fi ll basin and an under- mation rate and the sediment accumulation The recognition of Milankovitch cycles in fi lled basin. The fi nal stage was marked by open rate increased, the basin became closed, and the Zhada Formation indicates that insolation- lacustrine conditions, which give way to pro- lacustrine sedimentation commenced. These driven global or regional climate change drove grading basin-margin alluvial fans. The typical conditions continued, despite progradation of environmental changes in basins at high eleva- regression through the basin-type sequence was basin-margin alluvial fans, until a new sill was tions on the southern Tibetan Plateau. Thus, bypassed by an abrupt return to fl uvial conditions.

Geosphere, April 2010 89

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