Gondwana Research 32 (2016) 64–75

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The youngest marine deposits preserved in southern Tibet anddisappearance of the Tethyan Ocean

Tian Jiang a,b, Jonathan C. Aitchison b,c,⁎, Xiaoqiao Wan a

a State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Xueyuan Lu 29, Beijing 100083, Chinab School of Geosciences, The University of Sydney, Sydney, NSW 2006, Australiac School of Geography, Planning and Environmental Management, The University of Queensland, Brisbane, QLD 4072, Australia

⁎ Corresponding author at: School of Geography,Management, The University of Queensland, Brisbane, Q3346 7010.

E-mail address: jona@uq.edu.au (J.C. Aitchison).

http://dx.doi.org/10.1016/j.gr.2015.01.0151342-937X/© 2015 International Association for Gondwa

a b s t r a c t

a r t i c l e i n f o

Article history:Received 31 October 2014Received in revised form 15 January 2015Accepted 30 January 2015Available online 11 March 2015

Handling Editor: A.S. Collins

Keywords:Southern TibetMarine sedimentLate EoceneTethyan OceanIndia–Asia collision

Fossil ages as young as Priabonian (38–34 Ma) are reported for the last marine sedimentary rocks in southernTibet. Correlation is based on examination of foraminifers and nannofossil biostratigraphy of youngest preservedsediments in sections at Gamba (Zongpu), Tingri (Qumiba) as well as a previously unreported section at Yadong.Our results demonstrate that a marine seaway remained in existence south of the Yarlung Tsangpo suture zoneuntil at least Priabonian time. Notably this remains a maximum age estimate in this area as all sections are trun-cated by erosion or faulting. We compare our results with sections throughout the Himalaya region to demon-strate that shallow marine conditions existed widely during the Eocene period. In fact, it seems likely that themarine conditions in the Tethyan Himalaya did not entirely disappear by the end of Priabonian, especially inthe eastern Himalaya. The data presented in this study place direct constraints on the elimination of the TethyanOcean and thus have important implications for timing of the India–Eurasia collision.

© 2015 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

The collision of India with Eurasia, which has profoundly influencedthe geological, geochemical, climatic and oceanographic evolution ofthe Earth, is one of themost significant tectonic events during the Ceno-zoic (Butler, 1995). In the last few decades, many researchers haveattempted to reconstruct the evolution of the Himalayan orogen and Ti-betan uplift. However, timing of the initiation of continental collision,key to all reconstruction models, is still strongly disputed, and the esti-mated time ranges from 70Ma to 25Ma (Searle et al., 1987; Beck et al.,1995; Yin and Harrison, 2000; Zhu et al., 2005; Aitchison et al., 2007,2011; Aitchison and Ali, 2012; van Hinsbergen et al., 2012; Wanget al., 2012; Singh, 2013; Xu et al., 2015).

Many approaches are used to date the timing of the collision includ-ing the relative position of the Indian plate, the commencement of theinflux of sedimentary detritus onto the Indian passive plate marginalong the Indus Yarlung Tsangpo Suture Zone (IYTSZ), initiation ofmajor collision related thrust systems in the Himalaya Ranges, and theend of calc-alkaline magmatism along the Trans-Himalayan batholith,etc. (Searle et al., 1988; Aitchison et al., 2007; Najman et al., 2010).The cessation of marine sedimentation on the Indian margin, which is

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closely related to the elimination of the Tethyan Ocean, is widelyregarded as placing a direct constraint on timing of the initiation of con-tinent–continent collision and has long been investigated in differentHimalayan regions (Rowley, 1996; Green et al., 2008).

In the Lesser Himalaya, the final marine deposits are commonly pre-served as part of a nappe or klippe. They commonly underlie regional hi-atus and are themselves thrust over Oligocene to Miocene continentalclastic sediments. The age of the final marine deposits varies fromwest to east. In the frontal region of the Kashmir syntaxis, the marinePatala Formation has been dated at 55–50 Ma where it underlies thecontinental Balakot Formation (Najman et al., 2001, 2002). Southeastof the Kashmir syntaxis, the Subathu Formation outcropswidely in loca-tions from Simla to Tansen (Fig. 1a), and preserves the Eocene finalmarine deposits of 44 Ma (Bhatia and Bhargava, 2006). South of theeastern Himalayan syntaxis, the collision-related final marine sedimentsoutcrop as the lower Eocene Rengging Formation and the upper lowerEocene toMiddle Eocene Yinkiong Formation in the Siangwindow regionof Assam–Arakan Basin (Tripathi et al., 1978, 1981; Tripathi andMamgain, 1986; Tripathi et al., 1988; Acharyya, 2007; Acharyya andSaha, 2008). Even younger final marine deposits occur in the Bengalbasin, where the Sylhet limestone and overlying Kopili Formation areof Middle to upper Eocene age (Samanta, 1965, 1968, 1969; Singh andPratap, 1983; Alam et al., 2003).

In the western Tethyan Himalaya, limestone of the Kong Formation(48.6 Ma) in the Zanskar Basin and nummulitic limestone (50.8–49.4 Ma) that outcrop near the Indus Suture Zone are regarded as the

V. All rights reserved.

Fig. 1. (a) Simplified map of the Himalaya orogen showing the main regional tectonic elements and locations discussed in the text, (b) Simplified map of Gamba–Tingri region. Modifiedfrom Aitchison et al. (2007), Yin (2006).

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youngest record of the Tethyan Sea (Searle et al., 1988; Green et al.,2008; Mathur et al., 2009). However, it must be noted that nummuliticlimestones in the Zanskar valley are overlain by unfossiliferous massflow deposit (turbidites and deep marine conglomerates) that never-theless indicate ongoing marine conditions. In southern Tibet, severalsedimentary successions containing final marine sediments have beenreported, including the Tingri (Qumiba) section (Li et al., 2000; Xu,2000;Wang et al., 2002; Zhu et al., 2005) and the Gamba (Zongpu) sec-tion (Wan, 1987; Willems, 1993; Li and Wan, 2003; Wan et al., 2006,2010). However, documentation of these sections is commonly incom-plete or important questions remain regarding stratigraphic continuity,the reliability of fossil identifications and their depositional setting. Inthis study, we critically assess existing data of the youngestmarine stra-ta in Gamba–Tingri region in order to correlate and compare betweensections in southern Tibet. This also includes the Yadong section, a pre-viously unreported succession 70 km east of Gamba, fromwhichwe re-corded planktonic foraminifers and nannofossils. We endeavor toestablish the precise timing offinalmarine sedimentation in the easternTethyan Himalaya, so that we can correlate it with the results fromother regions throughout the Himalaya to reconstruct the eliminationof the Tethyan Ocean.

2. Geological setting

From south to north, the Himalaya is divided into four zones: theOuter or Sub-Himalaya, the lesser or lower Himalaya, the Greater orHigher Himalaya, and the Tethyan or Tibetan Himalaya. Farther northlies the Indus Yarlung Tsangpo Suture Zone beyond which is theTrans-Himalayan (Gangdese) batholith, or Lhasa Terrane, which repre-sents the original southern margin of Asia prior to Indus collision(Thakur, 1992; Yin, 2006). The Sub-Himalaya, mainly consist of theNeogene Siwalik molasse, bounded by the Main Boundary Thrust(MBT) to the north and the Main Frontal Thrust (MFT) to the south.The Lesser Himalaya consists of low-grade metasedimentary rocks,with discontinuous exposures of Paleogene sediments (Yin, 2006;Acharyya, 2007). The Greater Himalaya, separated from the LesserHimalaya by the Main Central Thrust (MCT), mainly contains meta-sedimentary rocks ranging in age from Proterozoic to Ordovician andaffected by Oligo–Miocene Himalayan metamorphism. The TethyanHimalayan zone is separated from the Greater Himalaya across theSouth Tibetan Detachment Surface (STDS). Paleozoic to Paleogene ma-rine sediments are widely exposed in the Tethyan Himalaya. The Ceno-zoic pre-collision related Tethyan Himalayan sequence varies between

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locations and indicates a northward-deepening environment fromcontinental shelf to slope. In the northern part of Tethyan Himalaya,the sequences are mainly composed by deformed mélange with con-glomerates, cherts and sandstones (e.g. Saga–Gyangze belt, Zandaarea) (Liu and Einsele, 1994; Ding et al., 2005; Wan et al., 2014). Theshelf deposits are mainly preserved in the Gamba–Tingri region of theeastern Tethyan Himalaya, which is the focus of this study.

The Gamba–Tingri region is located in southern Tibet and liesaround 100 km south of the Yarlung Tsangpo Suture Zone (YTSZ).Gamba is located about 90 km SE of Tingri, which is 70 km NW ofMount Everest (Qomolangma). Yadong is about a further 70 km SE ofGamba (Fig. 1b). The Paleogene sequence in these areas broadly consistsof three parts: marine sandstonewith intercalations of sandy limestone,massive limestone beds, gray and reddish siltstone and shale intercalat-ed with sandy limestone (Wan et al., 2014). The Paleogene sedimentsoverlie thick massive Cretaceous limestone uncomformably, and thetops of all sections are truncated by erosion or faulting (Wan et al.,2002a).

3. Stratigraphy

3.1. Yadong section

Yadong County is located south of Gyantse in the border zonebetween the Greater Himalaya and Tethyan Himalaya. A TethyanHimalaya sequence crops out around Tüna village NE of Yadong(Fig. 1; GPS location 28°04′02″N 89°11′32″E). Xiu (2011) reported os-tracod assemblages from the uppermost non-calcareous part of this sec-tion. The Cretaceous to Paleogene lithological succession correlates wellwith those described from Gamba and Tingri byWillems (1993). In thisstudy, we stratigraphically logged the non-carbonate Yadong sectionand correlate it with the Eocene non-calcareous Zongpubei Formation(Wan et al., 2014) at Gamba and Tingri. The Yadong section also con-tains representatives of the youngest marine sediments of the TethyanHimalaya known as the Zongpubei Formation at both Gamba and Tingri.

3.1.1. LithologyThe 46m thickYadong section (Fig. 2), dips gently toward the south-

west and consists of gray and reddish mudstone intercalated with thinsiltstone and sandstone layers (Fig. 3). Graded bedding and cross-lamination indicate that the section is right-way up. Quaternary fluvialcover sediments obscure both the top and bottom of the section.

3.1.2. BiostratigraphyThe Yadong section yields numerous fossils, including foraminifers,

nannofossils, ostracods, radiolarians and ichthyodonts. Planktonic fora-minifer and nannofossil assemblages provide biostratigraphic ageconstraints.

According to our present work, six samples from the Yadong sectionyielded identifiable foraminifers (Fig. 4), most of which are planktonicforaminifer species like Morozovelloides coronatus, Morozovelloidescrassatus, Subbotina angiporoides, Subbotina gortanii, Turborotaliacerroazulensis, Turborotalia cocoaensis, Chiloguembelina cubensis andsmall benthic foraminifers Glomospira charoides, and Fissurinamargirinata. We use youngest species co-occurrences to correlatewith the sub-tropical zonation of planktonic foraminifers (Pearsonet al., 2006). Results show that species like C. cubensis, andChiloguembelina ototara, occur in most samples. These taxa first ap-pear at least in E9 zone. Taxa such as Turborotalia cocoaensis, foundin the uppermost two samples, first appear in E13 zone (Fig. 5).The planktonic foraminifer species range zones constrain theYadong section to E10–E13 covering the Middle Eocene BartonianStage (38–41.3 Ma).

Nannofossil results relate to a B.Sc. Honors research project by aUniversity of Sydney student (Harvey, 2012). Most of the nannofossilspecies present here have long ranges. However, in the uppermost

two samples, Sphenolithus obtusus, which has short range upperLutetian to lower Priabonian (NP16 to NP18) range (Martini, 1971), ispresent. Therefore, deposition of the uppermost portion of the Yadongsection can at least be correlated to the NP16 zone (upper Lutetian),corresponding with the foraminiferal result. Other reports includethose of ostracod assemblages, which constrain the top of the formationto the lower Priabonian (Xiu, 2011).

Among the recovered taxa, reworked fossils such as Cretaceous radi-olarians, the lower Eocene foraminifer Morozovella sp. and Cretaceousnannofossil species like Micula murus, Loxolithus armilla, Manivitellapemmatoidea are also observed. As reported from Gamba and Tingri,reworking of fossils in the Zongpubei Formation is common. This iscited by some researchers to question the biostratigraphic results andthe marine origin of the Zongpubei Formation (Zhu et al., 2005; Huet al., 2012). However, it has been demonstrated that reworking isvery common as a result of marine circulation (Moore et al., 2012).Reworked fossils from Zongpubei Formation present a record of subma-rine erosion and transportation from surrounding seamounts or topo-graphically higher regions within the sedimentary basin into thedepressed Gamba–Tingri area. Alternatively, they may represent theeffects of upwelling movement or circulation of the water mass in thisregion, rather than representing products of any terrestrial fluvial envi-ronment (Takahashi, 1990). Reworking notwithstanding, the youngestfossils identified in the Zongpubei Formation in this and former studiesare marine and as such can unequivocally be used to constrain themaximum age of the final marine sedimentation. Based on thesefossil records, the age of the Zongpubei Formation of Yadong can beconstrained to the Bartonian to early Priabonian.

3.2. Gamba (Zongpu) section

The Zongpu section (GPS location 28°16′57″N88°31′46″E) is locatedabout 1 km east of Gamba County. As it is a typical shelf deposit sectionin the Tethyan Himalaya, many researchers have studied this sectionbefore. However, biostratigraphic constraints for this section, mainlybased on the larger benthic foraminiferal faunal changes, are still thesubject of discussion, especially for the Zhepure Formation (Wan,1987; Willems, 1993; Wan et al., 2002b; Li and Wan, 2003; Wan et al.,2010; Zhang et al., 2013). In this study, we reassess the biostratigraphicresults from earlier investigations of this section.

3.2.1. LithologyGamba (Zongpu) section is around 600 m thick and contains four

formations ranging from Paleocene to Eocene (Fig. 6). The Jidula Forma-tion (~180m thick) is dominated by yellowishwhite, indurated, homo-geneous sandstone, with sandy limestone intercalations in the middleand upper part. The overlying Zongpu Formation (~140 m) consists ofmassive limestone (dolomitized in the lower section), nodular lime-stone and minor amounts of calcareous marlstone and marl. TheZhepure Formation (~80 m) is composed of gray, massive and thicklybedded Alveolina packstone, with limestone beds separated by shaleyintercalations. The Zongpubei Formation (~180m) at the top of the sec-tion is gray and reddish shale intercalated with thin-bedded sandylimestone.

3.2.2. BiostratigraphyThe Zongpu section yields larger and small benthic foraminifers,

planktonic foraminifers aswell as ostracods. The larger benthic foramin-ifers, that dominate the fauna, are mainly preserved in the massivelimestone of Zongpu and Zhepure formations and the limestone inter-calations of the Jidula and Zongpubei formations. The larger foraminiferassemblages in the Zongpu section include Rotalia–Lockhartia andMiscellanea–Daviasina–Operculina assemblages of the Jidula andZongpu formations, and Alveolina–Orbitolites assemblages of ZhepureFormation (Wan et al., 2010). The P/E (Paleocene to Eocene) boundary,whichmarks the base of the Zhepure Formation, has been identified by

Fig. 2. Photographs from study sections in southern Tibet. (a) Yadong section. (b) Gamba (Zongpu) section. (c) Tingri (Qumiba) section.

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the turnover of larger foraminifer assemblages and a CarbonateIsotopic Excursion (CIE) (Wan et al., 2010; Zhang et al., 2013). Zhanget al. (2013) reported an SBZ 7 (Serra-Kiel et al., 1998) foraminiferassemblage from the Zhepure Limestone at Gamba and SBZ 6–10from Tingri. This contrasts with the reported Early to Middle Eoceneage of Wan (1987, 1990) and Willems (1993) and Willems et al.(1996). We reassess the species reported by these authors from theZhepure Limestone, and suggest that some taxa like Nummuliteslaevigatus, andNummulites obesus, identified by bothWan andWillems,are common in Tethyan region and appear after the Ypresian in SBZ 13

(Serra-Kiel et al., 1998) (Fig. 7). Other species, identified in bothGamba and Tingri regions, such as Nummulites globulus, Nummulitespengaroensis, Assilina granulose, Assilina spinosa, Assilina subspinosa,Assilina sublaminosa, Discocyclina sowerbyi, are widespread in upperLower to Middle Eocene strata of the Himalayan region (Singh andPratap, 1983; Tripathi and Mamgain, 1986). Therefore, we suggestthe Zhepure Formation in the Gamba–Tingri region ranges from theYpresian to Lutetian.

Li and Wan (2003) reported a planktonic foraminifer assemblagefrom the Morozovella spinulosa–Acarinina bullbrooki zone for the

Fig. 3. Lithostratigraphic log of Yadong section with sample distribution and foraminiferal age range, Sub-tropical zone is based on Berggren and Pearson (2005).

Fig. 4. Scanning electronmicrographs of Eocene foraminifers in the Yadong section. 1–2.Morozovelloides crassatus (Cushman) 1925; 3. Planorotalites capdevilensis (Cushman and Ponton)1949; 4. Subbotina eocaena (Guembel) 1868; 5. Subbotina gortanii (Borsetti) 1959; 6–7. Subbotina linaperta (Finlay) 1939; 8. Subbotina angiporoides (Hornibrook) 1965; 9. Turborotaliacerroazulensis (Cole) 1928; 10.Malimbus coronatus (Blow) 1979; 11. Globorotaloides eovariabilisHuber and Pearson 2006; 12. Ammodiscus latusGrzybowski 1898; 13. Streptochilus martini(Pijpers) 1933; 14–15. Chiloguembelina cubensis (Palmer) 1934; 16–17. Chiloguembelina ototara (Finlay) 1940; 18. Subbotina crociapertura Blow 1979. Scale bar = 100 μm.

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Fig. 5. Biostratigraphic range chart for fossils occurring in the Yadong section. Nannofossil zonation is based on Martini (1971), sub-tropical planktonic foraminiferal zone is based onBerggren and Pearson (2005). Sample distribution is illustrated on Fig. 3.

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Zongpubei Formation. For this research, we re-analyzed the species oc-curring in the M. spinulosa–A. bullbrooki range zone of Li and Wan(2003) and other planktonic taxa of Xu et al. (1990). The faunal

Fig. 6. Correlation of stratigraphic logs of Paleocene to Eocene strata in Gamba and Tingri areas.Fossil assemblages are as reported byWan (1987, 1990, 1991), Willems et al. (1996) and Wan

assemblage present indicates that deposition of the Zongpubei Forma-tion could not have occurred before the late Lutetian (~44.5 Ma), andcorresponds with our results from the Yadong section.

Stratigraphy is mainly adapted fromWillems et al. (1996) andWan et al. (2002a,b, 2014).g et al. (2002).

Fig. 7. Biostratigraphic range chart for fossils in the Zhepure and Zongpubei formations in Gamba and Tingri areas. Planktonic foraminifer species were reported by Li and Wan (2003).Larger benthic foraminifer species were reported by Wan (1987, 1990) and Willems et al. (1996). Sub-Tropical zonation is based on Berggren and Pearson (2005). Benthic zonation isbased on Serra-Kiel et al. (1998).

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3.3. Tingri (Qumiba) section

The Cretaceous to Paleogene stratigraphy of the Tingri area is wellexposed in the Zhepure Mountain area, west of Tingri (GPS location28°41′23″N 86°43′51″E). Various researchers have measured differentsection traverses using their own stratigraphic subdivisions in thisarea, especially for the uppermost non-calcareous beds (Xu et al.,1990; Willems, 1993; Li et al., 2000; Zhu et al., 2005). According toWan et al. (2014) and observations of the authors for this study, thestratigraphic units of Tingri can be correlated with those of Gamba.Therefore, in this research, we will follow the same classification(Wan et al., 2014) for both the Gamba and Tingri areas.

3.3.1. LithologyThere is little facies variation betweenGamba and Tingri. At Tingri, the

Jidula Formation also consists of a group of sandy beds but with differentthicknesses. The Zongpu Formation is correlatedwithmember I–IV of theZhepure Shan Formation (Willems et al., 1996), and consists of massivelimestone that is dolomitized in the lower part. The main difference isthat nodular limestone does not occur until the upper part of the ZongpuFormation (member IV of the Zhepure Shan Formation), whereas atGamba the nodular limestone occurs at the bottom of Zongpu Formation.The Zhepure Formation is a massive limestone with abundant macro-scopic Nummulites and Alveolina. The Zongpubei Formation at Tingri iscorrelated with the Pengqu Formation of Li et al. (2000), and is also char-acterized by gray and reddish shale, sandstone and siltstone. As there is adistinct color difference between the lower and upper part of theZongpubei Formation,many researchers subdivide the Zongpubei Forma-tion into two members, such as the Enba and Zhaguo members (Wanget al., 2002), or Youxia and Shenkeza formations (Zhu et al., 2005).

3.3.2. BiostratigraphyLike the Gamba (Zongpu) section, larger foraminifers occurring in

the limestone layers dominate the Tingri (Qumiba) section. The JidulaFormation and Lower Zongpu Formation yield aMiscellanea–Rotalia as-semblage similar to Gamba (Wanet al., 2002a).Willems et al. (1996) re-ported Miscellanea–Operculina and Alveolina zones from the upper

Zongpu Formation, Nummulites–Alveolina–Orbitolites and Assilina–Asterocyclina–Nummulites zones from the Zhepure Formation, and theoccurrence of N. laevigatus, Assilina dandicotica and Asterocyclina cf.stellata within assemblages from the Zhepure Formation indicating aLutetian age. The Zongpubei Formation at Tingri yields foraminifersand nannofossils. According to Li and Wan (2003), the M. spinulosa–A. bullbrooki range zone was also discovered at Tingri and has similarconstituent species to those in Gamba.

Wang et al. (2002) reported a NP15 to NP20 nannofossil assemblagefrom the Zongpubei Formation, which constrains the timing of deposi-tion to the late Priabonian (34.5 Ma). However Zhu et al. (2005) chal-lenged this arguing that the upper reddish part of the formationrepresents fluvial channels and floodplain deposits and suggested allthe nannofossils Wang et al. (2002) reported from upper part of theZongpubei Formation are reworked from older strata. During ourinvestigations we noted that sedimentological characteristics of theZongpubei Formation are shallow marine rather than fluvial. Moreover,it contains not only marine microfossils but also in situ macrofossilssuch as gastropods and bivalves. In this study, we sampled the top ofthe Zongpubei Formation at Tingri and recovered both nannofossilsand planktonic foraminifers. Bartonian planktonic foraminifer specieslike C. cubensis also occur at Tingri as well as in Yadong. The presenceof the reworked fossils is noted not only at Tingri but also at Gambaand Yadong. However reworking is common in most microfossil studiesespecially for nannofossils from silty-clay particles (Ferreira et al., 2008)and we certainly do not regard all of our fossils as reworked. Therefore,together with our results from the Yadong section and the age of the un-derlying Zhepure Formation, we suggest that the Zongpubei Formationin the Gamba–Tingri area is of Bartonian to Priabonian age. Importantly,whether reworked or not, if Priabonian nannofossils are the youngestmarine fossils from this area they indicatemarine conditions at that time.

4. Regional correlations

4.1. Tethyan Himalaya

In the Tethyan Himalaya, the final marine stratigraphic succession isalso well studied in the Zanskar region (Searle et al., 1988). Lutetian

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age deltaic deposits of the Chulung La Formation of the south ZanskarBelt mark the youngest preserved marine sediments on the Zanskarshelf with any formerly overlying units removed by faulting or erosion(Fig. 8). Marine shelf deposition ended on the Zanskar shelf by theend of shale deposit in the Kong Formation, which overlies the upperPaleocene to lower Eocene Spanboth Formation (Lingshet Formation).The Kong Formation (48.6 Ma) contains Assilina and Nummulites domi-nated larger foraminifer assemblages from SBZ 7 to SBZ 12 (Gaetaniet al., 1986; Gaetani and Garzanti, 1991; Mathur et al., 2009). North ofthe Zanskar shelf, in the Indus suture zone (Searle et al., 1988), somescholars (Searle et al., 1990; Clift et al., 2002) insist that marine sedi-mentation ended with a change from the Khalsi Flysch to the overlyingChogdo Formation of 51Mawhich they interpreted as Eurasian-derivedcontinental molasses. However, after reassessment of the studies ofChogdo Formation, Henderson et al. (2010, 2011) recently disputedwhether or not it could constrain the timing of the collision on thebasis of both provenance data and stratigraphic position. They sug-gested that the lowest strata that contain an unequivocal mixed influxof Indian–Eurasian detritus belong to the Choksti Formation, suggestingthat the final marine deposits in this area are nummulitic limestone(50.8–49.4 Ma) and possibly the overlying Nurla Formation. This resultcorrelates with final marine deposit on the Zanskar shelf and suggeststhat the marine sedimentation ended in the Zanskar region after theend of the Ypresian.

Our own observations from the Zanskar valley and other transects inthe Ladakh region suggest that significant problems still exist with bothstratigraphic correlation and interpretation of depositional environment.For example, the name Chogdo Formation is applied (Clift et al., 2002) totwodistinctly different sedimentary packages (although bothmapped asChogdo Formation the ophiolite-derived conglomerates at Chilling areclearly different from volcanic arc derived conglomerates at Sumda-do). Moreover, although regarded as non-marine, Bouma sequencesand A-axis clast imbrication in the Choktsi Formation are indicative ofdeep marine mass flows (turbidite sandstones and conglomerates).Although unfossiliferous, such sedimentary facies clearly indicate thatmarine deposition continued after the accumulation of nummulitic

Fig. 8. Stratigraphic correlation of upper Paleocene to Miocene strata through

limestones. We suggest that this particular lithostratigraphic unit(Choktsi Formation) was deposited in a forearc basin setting along thesouthern margin of Eurasia given that it is dominated by continentalmargin arc-derived volcanic detritus and devoid of any ophiolitic orIndian plate derived material.

The larger foraminifer assemblages in the Kong Formation and thenummulitic limestone are dominated by species ofNummulites, Assilina,Alveolina, Daviesina, and Lockhartia, which is similar to the compositionof assemblages from the Zhepure Formation in the Gamba–Tingri area.Previous work indicates that more than 15% of species within theZhepure Formation are the same as those in the Gamba–Tingri region(Wan, 1987, 1990; Willems et al., 1996; Mathur et al., 2009).

4.2. Lesser Himalaya

In the Lesser Himalaya, Oligocene to Miocene continental sedimentsoverlie Middle Eocene marine strata above a major regional unconfor-mity, indicating a significant hiatus, which marks the change frommarine conditions in the Himalayan foreland (Najman et al., 2004).Balakot is a frontier area on the northern Indian margin in the Kashmirsyntaxis. The Balakot Formation consists of terrestrial sediments and isassigned to the late Priabonian (b35 Ma) (Najman et al., 2001, 2002). Itunconformably overlies the marine Patala Formation of Late Paleoceneto Early Eocene age (Bossart and Ottiger, 1989). The Patala Formationis dominated by larger foraminifers, and contain SBZ 4–5 assemblages(Hanif et al., 2013). The dominant species like Miscellanea miscella,Lockhartia conditi, Lockhartia tipper and Lockhartia haimei within thePatala limestone are the same as species that dominate assemblagesof the Zongpu Formation in the Gamba–Tingri region (Wan, 1991;Hanif et al., 2013). The different age of the last marine deposits beneaththe unconformity in the Zanskar and Kashmir syntaxis areas, even thesouthern Tibet region, denotes a different level of erosion in those regions.The same stratigraphic relationship also occurs in most areas west of theKashmir syntaxis.

Southeast of the Kashmir syntaxis, the Simla basin has beenregarded as an excellent location for investigations of the final marine

out the Himalaya. Blue colored units are marine, white are non-marine.

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deposits (Bera et al., 2008). The Subathu Formation dated at 44 Ma(Bhatia and Bhargava, 2006), can be correlated with the Bhainskati For-mation of central Nepal. It represents the youngest preserved marinesediments in the Himalaya foreland basin, and is unconformably over-lain by alluvial sediments of the Dagshai and Kasauli formations(dated at 31 Ma) (Najman et al., 2004; Najman, 2006; Bera et al.,2008). Unlike the massive limestones of the Kohat and Patala forma-tions, the Subathu Formation consists of variegated shales, bioclasticlimestone intercalations and fine grained sandstone beds (Singh,1978). The base of the Subathu Formation is marked by the Assilinaspira abrardi bed (2–3 m thick packstone with large sized Assilines,and Nummulites) of SBZ 13. Along with the Assilina exponens–Assilinapapillata–Nummulites discorbinus assemblage, the upper limit of theSubathu Formation is assigned a 44 Ma age (Mathur, 1978; Bhatia andBhargava, 2006). In addition, Bera et al. (2008) proposed that theupper limit of the Subathu Formation must be significantly youngerthan 44 Ma based on reworked fossils within it. White sandstone(31 Ma) of the basal Dagshai Formation is interpreted to represent thechange from marine to continental conditions.

The foraminiferal faunal composition within Subathu Formation issimilar to the Zhepure Formation. More than 30% of species from theSubathu Formation are also found in the Zhepure Formation (Mathur,1978; Bhatia and Bhargava, 2006). Notably a N10 my erosional hiatus(41–31 Ma) exists between Subathu and Dagshai formations and thebase of the lattermarks the oldest non-marine sediments (Singh, 2013).

In the easternHimalayan syntaxis, collision-relatedfinalmarine sed-iments are also preserved in the Siang window, NE of the Assam Basin(Acharyya, 2007; Acharyya and Saha, 2008). The Eocene marine stratain this region are not a continuous sequence. Tripathi et al. (1978,1981) reported an Early Eocene limestone bed in the Rengging Forma-tion, outcropping along the banks of the Siang River, intercalated be-tween the Miocene clastics and overthrust by the Abor volcanics. TheRengging Formation contains abundant Nummulites assemblages in-cluding N. globulus, and Nummulites atacicus. More than 60% of thefauna is the same as species that are present in theGamba–Tingri region(e.g. M. miscella, Globorotalia sp., L. haimei, L. conditi). The late EarlyEocene to Middle Eocene Yinkiong Formation, sandwiched betweenthe Abor volcanics (Ali et al., 2012) and the Miocene Dafla Formation,outcrops in the Yamne Valley, north of the Rengging Formation (Singhand Pratap, 1983; Tripathi and Mamgain, 1986; Tripathi et al., 1988;Acharyya, 1994). This unit contains thick beds of greenish gray calcare-ous fossiliferous shales interbedded with dark gray limestones. Thelimestone beds and shales yield rich Nummulites and Assilina assem-blages, which can also be found in other Eocene strata that are wide-spread in the Himalaya (Singh and Pratap, 1983; Acharyya, 2007)(Fig. 9). These striking similarities provide convincing evidence for cor-relation of the Gamba–Tingri–Yadong sections with other sections inthe Tethyan and Lesser Himalayas as described above and the existenceof a Tethyan seaway across the Lesser Himalaya.

4.3. Sub-Himalaya and Bengal basin

Other Tethyan marine deposits are also well preserved in the Sub-Himalaya zone and even in the Bengal basin, far from the IYTSZ. In theKohat area of the western part of the Sub-Himalaya, Miocene fluvialshale, sandstone and conglomerate of the Murree Formation overliethe Mid-Eocene Kohat Formation. The Kohat Formation consists ofshale and limestone in the lower part, and Nummulites limestone andAlveolina limestone in the middle and upper part, respectively (Pivnikand Wells, 1996). The Nummulites, Assilina and Alveolina assemblages

Fig. 9. Dominant species occurring in the Zhepure Formation and other correlative stratain the Himalayan regions. Fossils are reported by Afzal et al. (2009), Mathur et al.(2009), Mirza et al. (2005), Samanta (1968), Singh and Pratap (1983), Tripathi et al.(1988), Tripathi and Mamgain (1986), Wan (1987, 1990), Willems (1993), Willemset al. (1996).

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within the Kohat limestone are of SBZ14–16 ages, and they can be cor-related with those of the Zhepure Formation in the Gamba–Tingriregion with more than 50% of species in common for both formations(Mirza et al., 2005; Afzal et al., 2009).

In the Garo Hill region of the Bengal Basin (Jauhri et al., 2006), SE ofthe Siang window, the collision-related final marine sediments includethe Lower Paleocene toMiddle Eocene Sylhet (or Siju) LimestoneGroupand overlying Upper Eocene Kopili (shale) Formation. These units aresucceeded by the Barail Formation (Samanta, 1965, 1968, 1969; Singhand Pratap, 1983; Alamet al., 2003; Lokho and Tewari, 2011). The SylhetLimestone Group yields Nummulites, Assilina and Discocyclina assem-blages, which indicate shallow marine deposition similar to that in thelesser Himalaya region. The Kopili Formation consists of shale andmarl, with abundant foraminifers and molluscs in the lower part andis poorly fossiliferous in the upper part, indicating a change from shal-low marine to deltaic sedimentation (Samanta, 1968; Najman et al.,2008). Some species from the Sylhet Limestone also appear in theupper part of the Zhepure Formation. The facies change from limestoneto shale and marl in the Garo Hill sequence is correlated with the lime-stone to shale sequence in the Gamba–Tingri region. The planktonic andlarger foraminiferal record in the Kopili Formation indicates an upperEocene age (Samanta, 1969), which is consistent with the age fromthe Zongpubei Formation.

5. Discussion

During the Early to Middle Eocene, marine shelf deposition waswidespread in the Himalayan region, indicating the existence of a Te-thyan seaway along the strike of the present day IYTSZ, extendingfrom Kohat, Zanskar and Simla to southern Tibet and as far as the east-ern syntaxis area (Singh and Pratap, 1983). According to correlation ofthe larger foraminifer assemblageswithin thesefinalmarine sediments,we suggest that the faunas were similar and probably shared the samehabitat during the pre-collision period. This seaway may even havebeen connected with the Bengal Basin and extended as far as theIndo–Burman suture zone. Shallow marine conditions did not changeuntil at least the end of the Lutetian in most parts of the Himalaya re-gion. This indicates that those regions were not close enough to thetrench to undergo any subsidence (Rowley, 1998), and collision-related uplift did not occur at that time. As subduction continued, theIndian and Eurasian continental masses finally collided with eachother and the Tethyan seaway disappeared as a result of syn-collisional uplift. According to our results from investigation of the lastmarine deposit in the Gamba–Tingri region, a Tethyan seaway stillexisted into the Priabonian (~38–34 Ma).

Among the stratigraphic data, the age of the youngest preservedma-rine deposit in the Himalaya region varies from west to east. Rowley(1996) proposed a diachronous collision to explain the differentiationbetween western and eastern Himalaya. However, Najman et al. (2001,2002) questioned this in dating the Balakot Formation to no older than36–40 Ma and suggested re-evaluation of the older age for the collisionin western Himalaya was necessary. Our results show a markedlyyounger age for the change from marine to continental deposition inthe Eastern Himalaya compared with the Western Himalaya, especiallythe Zanskar region. However, we note that the ages of the youngestmarine strata underlying any unconformity or at the top of structurallytruncated sections remain a maximum age estimates and the apparentdifferent in maximum ages may in fact be due to a different level oferosion instead of the diachronous collision. We also note recent workthat suggests the northern margin of Greater Indian was not necessarilyrelatively linear nor was it a simple continent-ocean boundary (Ali andAitchison, 2014). The Bartonian to Priabonian Zongpubei Formationindicates that closure of the Tethys seaway in the Eastern Himalayaoccurred after Priabonian, which is compatible with the inferenceof a younger collision age suggested by Aitchison et al. (2007) inthis region. This result is also compatible with the Early Miocene

depositional age of the first influx of Indian margin clastics onto theLhasa Terrane (Aitchison et al., 2002), the late onset of the HigherHimalaya erosive clastics influx to the Himalayan foreland basin(Ravikant et al., 2011), the significant disgorging of detritus transportedfrom the Himalaya to the Bengal basin by Neogene time (Johnson andAlam, 1991), and the earliest terrestrial sedimentation in the LesserHimalaya during the Oligocene (Srivastava et al., 2013).

Although the youngestmarine strata in Tibet arewidely regarding asa key to placing a maximum age limit on the onset of continental colli-sion (Rowley, 1996; Searle et al., 1988, pg. 117) this may, in fact bemis-leading. We note the following observations from modern collisionsettings such as Taiwan/mainland China (Huang et al., 2005, 2006;Byrne et al., 2011) and Timor/northern Australia (Harris et al., 2009;Harris, 2011; Haig, 2012) where collision, defined by continental crusthaving entered the subduction zone (beneath a colliding island arc)has begun. In both these systems a shallow marine seaway (TaiwanStrait and Timor Sea respectively) still exists somewhat inboard of thecollision zone and is associated with flexural loading of the continent.On the basis of available evidence we cannot exclude the possibilitythat an analogous paleogeography may also have existed in southernTibet. Given the rate of plate convergence between India and Asia theexistence of anymarine seaway south of the immediate zone of collisionmay have been short-lived and any flexural depressionmay have rapid-lymigrated southwards. If so this should be reflected in the sedimentaryrecord. Importantly, we also note that at the same time collision hasgiven rise to substantial relief associated with the development of sig-nificant orogenic mountain ranges. A considerable volume of sedimentis being mass-wasted off this growing relief and if a similar scenarioexisted in Tibet we would predict the existence of synorogenicconglomerates.

6. Conclusions

Through biostratigraphic analysis of the Yadong section and reas-sessment of the Gamba and Tingri Paleocene to Eocene sequence, thefinal marine deposit in Gamba–Tingri region is assigned a Bartonian toPriabonian age. Importantly, as all sections are truncated by erosion orfaulting we note that this remains a maximum estimate for the age ofthe last marine sedimentation in this area. By regional correlationthroughout the Tethyan and Lesser Himalaya, the existence of a wide-spread shallow marine seaway can be reconstructed for the MiddleEocene. This Tethyan seaway ranged from western to eastern Himala-yan syntaxes and may have begun to disappear at different times fromwest to east. As the disappearance of Neotethys is closely related tothe India–Eurasia collision, and the uppermost marine deposits areregarded as an important criterion to constrain the timing of thecollision, we infer that collision occurred in the eastern Himalayaduring or shortly after deposition of the Zongpubei Formation in theGamba–Tingri region.

Acknowledgments

The authors thank the two anonymous reviewers and journal Asso-ciate Editor Alan Collins for their constructive comments that helped toimprove themanuscript. Tian Jiangwould like to thank Tom Savage andSarah Kachovich for assistance in processions of all samples in the mi-crofossil lab in the University of Sydney. All the images of microfossilsin this work are taken with the scientific and technical assistance ofDr Patrick Trimby at the Australian Microscopy & MicroanalysisResearch Facility, University of Sydney. Jianzhong Jia and Guobiao Liare thanked for accompanying Jiang on expeditions in southern Tibet.This study was supported by the Chinese Scholarship Council andthe National Basic Research Program of China (973 Program no.2012CB822002), the National Natural Science Foundation of China(Project no. 41172037, 41302023) and Beijing Higher Education YoungElite Teacher Project (no. YETP0665). The study was also supported by

74 T. Jiang et al. / Gondwana Research 32 (2016) 64–75

the China Geological Survey (Project no. 1212011120142). JCA wouldalso like to thank several indomitable individuals including ThomasHarvey, Hui Lan, Alan Baxter, Jason Ali, Aileen Davis and Sergei Ziabrevfor accompanying him on expeditions to locate and examine youngestmarine sections in Ladakh, Tibet and Aranachal Pradesh. This work wasalso supported with resources provided by the University of HongKong and The University of Sydney.

Appendix A. Supplementary data

Supplementary data associated with this article can be found in theonline version, at http://dx.doi.org/10.1016/j.gr.2015.01.015. Thesedata include Google maps of the most important areas described inthis article.

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