Tectono-Climatic Implications of Eocene Paratethys Regression in the Tajik Basin of Central Asia ∗ Barbara Carrapa A, , Peter G

Earth and Planetary Science Letters 424 (2015) 168–178

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Earth and Planetary Science Letters

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Tectono-climatic implications of Eocene Paratethys regression in the Tajik basin of central Asia ∗ Barbara Carrapa a, , Peter G. DeCelles a, Xin Wang b, Mark T. Clementz c, Nicoletta Mancin d, Marius Stoica e, Brian Kraatz f, Jin Meng g, Sherzod Abdulov h, Fahu Chen b a Department of Geosciences, University of Arizona, Tucson, AZ, USA b Key Laboratory of Western China’s Environmental Systems (Ministry of Education), Lanzhou University, Lanzhou, China c Department of Geology and Geophysics, University of Wyoming, Laramie, USA d Department of Earth and Environmental Sciences, University of Pavia, Italy e Department of Geology and Paleontology, Faculty of Geology and Geophysics, Bucharest University, Balcescu,˘ Romania f Western University of Health Sciences, Pomona, USA g American Museum of Natural History, New York, USA h Institute of Geology, Earthquake Engineering and Seismology of the Academy of Sciences, Tajikistan a r t i c l e i n f o a b s t r a c t

Article history: Plate tectonics and eustatic sea-level changes have fundamental effects on paleoenvironmental conditions Received 9 March 2015 and bio-ecological changes. The Paratethys Sea was a large marine seaway that connected the Received in revised form 14 May 2015 Mediterranean Neotethys Ocean with Central Asia during early Cenozoic time. Withdrawal of the Accepted 18 May 2015 Paratethys from central Asia impacted the distribution and composition of terrestrial faunas in the region Available online xxxx and has been largely associated with changes in global sea level and climate such as cooling associated Editor: A. Yin with the Eocene/Oligocene transition (EOT). Keywords: Whereas the regression has been dated in the Tarim basin (China), the pattern and timing of regression Paratethys in the Tajik basin, 400 km to the west, remain unresolved, precluding a test of current paleogeographic Tajik basin models. Here we date the Paratethys regression in Tajikistan at ca. 39 million years ago (Ma), which Tarim basin is several million years older than the EOT (at ca. 34 Ma) marking the greenhouse to icehouse foreland basin climate transition of the Cenozoic. Our data also show a restricted, evaporitic marine environment since Pamir the middle–late Eocene and establishment of desert like environments after ca. 39 Ma. The overall Tibet stratigraphic record from the Tajik basin and southern Tien Shan points to deposition in a foreland basin setting by ca. 40 Ma in response to active tectonic growth of the Pamir–Tibet Mountains at the same time. Combined with the northwestward younging trend of the regression in the region, the Tajik basin record is consistent with northward growth of the Pamir and suggests signiﬁcant tectonic control on Paratethys regression and paleoenvironmental changes in Central Asia. © 2015 Elsevier B.V. All rights reserved.

1. Introduction tem and related sedimentary basins including the Tajik and Tarim basins. Aridiﬁcation and monsoonal climate in Asia have been linked During early Cenozoic time the Paratethys Seaway (Popov et al., to transgressive–regressive cycles of the Paratethys and global 2004), a northern branch of the Neotethys Ocean, connected the climatic changes (Bosboom et al., 2011, 2013, 2014a, 2014b; central Atlantic Ocean with a complex system of basins, covering Dupont-Nivet et al., 2007; Licht et al., 2014). In particular, the a roughly 1200 km wide and >3000 km long east–west swath of study by Bosboom et al. (2014a) suggests a stepwise westward Europe and southern central Asia (Fig. 1). The eastern extremity of retreat of the Paratethys between 41 Ma and 37 Ma. Although this vast region is now occupied by the Tibet–Pamir orogenic sys- this study is important for paleogeographic reconstruction of the Paratethys, no data exist from the Tajik basin, located ∼400 km to the west of the Tarim basin (Fig. 1). Therefore, the relative controls * Corresponding author. of tectonics and global climate on the retreat of the Paratethys E-mail address: [email protected] (B. Carrapa). remain unresolved. Competing models attribute aridity and mon- http://dx.doi.org/10.1016/j.epsl.2015.05.034 0012-821X/© 2015 Elsevier B.V. All rights reserved. B. Carrapa et al. / Earth and Planetary Science Letters 424 (2015) 168–178 169

Fig. 1. (A) Paleogeographic reconstruction of the central Mediterranean and Central Asia at ca. 35 Ma modified after Blakey (2011). (B) Digital elevation model with simplified geological map of the study area modified after Burtman and Molnar (1993) and Robinson et al. (2012). soonal climate in central Asia to the Tibetan Plateau (e.g., Molnar pared with the relative timing of the regression available for the et al., 2010). The timing of Paratethys regression and attainment of Tarim basin, ranging from ca. 41 Ma to ca. 37 Ma (Bosboom et high topography and elevations in the region are thus critical for al., 2014a), our study suggests a northwestward younging of the evaluating current climate and tectonic models. regression consistent with northward growth of the Pamir Moun- A several km thick succession of Cenozoic sedimentary rocks tains and development of a foreland basin system in the area now deposited in the Tajik basin (Nikolaev, 2002)preserves a record occupied by the Tajik–Tarim basins by late Eocene time. Growth of of paleoenvironmental changes, erosion and tectonic activity of the Pamir in the Cenozoic has most likely affected land–sea distri- the surrounding Pamir and Tien Shan Mountains. This study ap- butions and climate in Central Asia. plies sedimentology, paleontology, sandstone petrography and zircon U–Pb geochronology to lower Cenozoic deposits in the Tajik 2. Geological setting depression and southern Tien Shan to constrain the timing of Paratethys regression in Tajikistan and to understand changes in depositional environments in response to tectonics and climate. The Tajik basin was connected with the Tarim basin during the The zircon geochronology (from a volcanic tuff) anchors the tran- early Cenozoic, but subsequent northward growth of the Pamir sition from marine to non-marine deposition at ca. 39 Ma in the orogen has severed this connection. The geologic history of the region. This datum is the first radiometric age ever reported from Tajik and western Tarim basins (Fig. 1) is thus crucial to under- Paratethyan rocks in central Asia, and also constrains the time of standing the tectonic evolution of the Pamir and the Tajik–Tarim the regression in Tajikistan to ∼39 Ma. The 39 Ma age is signifi- connection through the Paratethys Seaway. The Paratethys Seaway cantly older than global sea-level changes and biotic turnovers of was part of a larger epicontinental sea that connected the Mediter- the Eocene/Oligocene transition (EOT) at ca. 34 Ma. When com- ranean Tethys to the Tarim basin until at least the early Cenozoic 170 B. Carrapa et al. / Earth and Planetary Science Letters 424 (2015) 168–178

(Fig. 1A). It covered a vast area and as such probably had a major tained suitable microfossils (ostracods, planktonic foraminifera and effect on paleoenvironments and climate in central Asia. calcareous nannofossils) for age determination (Fig. 2); samples During the EOT, the Paratethys Seaway was surrounded by from the WA sections were barren and only a few samples from land and connected to the Mediterranean Tethys and other oceans the PE section contained limited foraminifera (Supplementary ma- through narrow passages. The Paratethys Seaway was located up- terial; Table S2). wind from and served as a major moisture source for the Tarim Samples were disaggregated by mortar and washed and sieved basin and large parts of continental Asia. through meshes of 180 and 125 μm; the washed residues were ◦ The westward retreat of the Paratethys Seaway has been pri- then dried in an oven at about 40 C. Approximately 200 forami- marily associated with eustatic sea level changes (Bosboom et al., nifera were counted under a stereomicroscope in random aliquots 2013, 2014a, 2014b). Eocene–Oligocene redbeds and late Miocene of both the >180 and 180–125 μm washed fractions to obtain cen- loessite in China are interpreted as indicating arid climate and sus counts of the more common species recorded in the studied winter monsoonal conditions (Bosboom et al., 2014a; Licht et al., samples. Census counts are reported in the Supplementary mate- 2014)related to either orographic effects associated with the Ti- rial (Table S2). Planktonic foraminiferal taxonomy follows the guide betan Plateau (Molnar et al., 2010)or a combination of tectonics of Toumarkine and Luterbacher (1985) partly updated by Pearson and global sea level changes affecting Paratethys regression and et al. (2012). The biostratigraphic attribution is primarily based on global cooling (e.g., Bosboom et al., 2014b; Dupont-Nivet et al., the standard zonations (Berggren and Pearson, 2005; Wade et al., 2007). Although growth of the Pamir has been loosely suggested as 2011). The same washed residues analyzed for foraminifera were a possible control on paleoenvironmental changes in Central Asia, investigated also for ostracods. the role of the Pamir on Paratethys regression and timing of tec- For nannofossil analyses, simple smear-slides were prepared tonic activity remain largely unconstrained. An orographic effect from the same ZD samples used for foraminiferal study and were associated with the Pamir has been suggested to have played a analyzed under a polarizing light microscope at 1250X magnifi- significant role on erosion since the Miocene (Carrapa et al., 2014); cation. The nannofossil abundance was quantified by counting all however, uplift and northward growth of the Pamir and its possi- the nannofossil specimens occurring in 400 random fields of view, ble effect on paleoenvironments during the early Cenozoic is not corresponding approximately to an area of 6mm2, in each sam- known. ple. The biostratigraphic attribution is primarily based on standard Data from the Tibetan Plateau suggest significant shortening zonations (Martini, 1971; Okada and Bukry, 1980). Results are re- and crustal thickening during the Cretaceous and early Cenozoic ported in the Supplementary material (Table S2). (Yin and Harrison, 2000; Kapp et al., 2005) consistent with high Invertebrate fossils, primarily shells of bivalves of the orders elevations of at least parts of Tibet since the Eocene. Paleoaltime- Ostreida and Pectinida, were observed in the PE, WA, and ZD try data indicate high elevations since at least the late Oligocene in sections. When of suitable quality, specimens were identified to central Tibet (DeCelles et al., 2007). The Pamir, which is the west- lowest taxonomic-level following previous studies (Lan, 1997) and ern continuation of the Tibetan Plateau (Yin and Harrison, 2000; were compared with faunas of comparable age from the Tarim Robinson et al., 2012), preserves evidence of thick crust and Oligo- basin (Bosboom et al., 2011, 2013, 2014a). Miocene high-grade metamorphism and exhumation (e.g., Schmidt et al., 2011) but its earlier tectonic history remains largely un- 3.3. U–Pb geochronology of detrital zircons (Nu HR ICPMS) known, leaving open the question of its potential role in control- ling land–sea reorganizations and Asian climate during the early Zircon crystals were extracted from six selected samples by tra- Cenozoic. ditional methods of crushing and grinding, followed by separation In the Tarim basin, Paratethys regression has been dated be- with a Wilfley table, heavy liquids, and a Frantz magnetic separa- tween ca. 47 and 37 Ma (Bosboom et al., 2011, 2013, 2014a; Sun tor. Samples were processed such that all zircons were retained in and Jiang, 2013). In particular, Bosboom et al. (2014a) suggested a the final heavy mineral fraction. A large split of these grains (gen- million-year scale stepwise retreat of the Paratethys in the Eocene, erally several hundreds of grains) was incorporated into a 1 epoxy consistent with a tectonic control, and hydrological connection be- mount together with fragments of our Sri Lanka standard zircon. tween the Tarim and Tajik basins until ca. 34 Ma. The mounts were sanded down to a depth of ∼20 μm, polished, imaged, and cleaned prior to isotopic analysis. U–Pb geochronology 3. Methods of zircons was conducted by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) at the 3.1. Field work and stratigraphic correlations Arizona LaserChron Center (Gehrels et al., 2008). 100 grains were targeted for sandstone samples and 38 grains for the volcanic tuff; We investigated three widely spaced Paleogene stratigraphic results are shown in Fig. 3. Additional analytical information can sections, up to 1500 m thick, within the Tajik basin (Figs. 1 be found in the Supplementary material (Table S3). and 2; WA and PE) and southern Tien Shan (ZD) and measured them at the m scale using a Jacob’s staff. Out of a total of 130 3.4. Sandstone petrography samples collected, 48 were selected, on the basis of paleontological content and mineral yield, for biostratigraphic, geochrono- Modal sandstone petrographic data were collected from six se- logic, petrographic, and sedimentologic analyses. Facies analysis lected standard petrographic thin sections from medium-grained and interpretations of paleodepositional environments are based sandstones from the WA and PE measured sections (Fig. 2). Each on detailed stratigraphic sections presented in Fig. 2. Facies were thin section was stained for calcium and potassium feldspars and identified and recorded using the classification scheme of Miall 450 grains were counted according to the Gazzi–Dickinson method (1978) (Supplementary material; Table S1). Correlations between (Ingersoll et al., 1984). Results are shown in Fig. 4 and additional sections were based on facies, fossil assemblages, and zircon U–Pb material and information can be found in the Supplementary ma- geochronological data. terial (Tables S4, S5).

3.2. Biostratigraphy 3.5. SEM and grain size analyses

Microfossil analyses were performed on sixteen samples from Three hundred and eighty bulk samples were collected from all three sections; however, only samples from the ZD section con- the lower part of the PE section (0–850 m) for grain size analysis. B. Carrapa et al. / Earth and Planetary Science Letters 424 (2015) 168–178 171

Fig. 2. Measured stratigraphic sections from the Tajik basin (PE and WA sections) and southern Tien Shan (ZD section); facies codes and facies associations are described in Table S1 in the Supplementary material. 172 B. Carrapa et al. / Earth and Planetary Science Letters 424 (2015) 168–178

Fig. 3. Detrital zircon U–Pb ages; MSWD: mean square weighted deviation. Refer to the Supplementary material and Table S3 for additional analytical information.

Twenty samples from eolian dune and loessite strata within the 4. Results and interpretations PE section were selected for Scanning Electron Microscope (SEM) analysis. Each sample was treated with 10% H2O2 and 10% HCl to 4.1. Sedimentology and depositional environments remove organic matter and carbonates, and the grain morphology was observed on a Hitachi S4800 SEM. 2–4 g of air-dried mate- The studied sections are dominated by fine-grained siliciclas- rial for each sample was firstly treated with 10 ml of 10% H2O2 tic and carbonate lithofacies in their lower parts and sandstone to remove organic matter and with 10 ml of 10% HCl to remove in their upper parts. Lithofacies exhibit an overall transition up- carbonates; 100 ml of distilled water was then added and the sam- section from shallow marine to continental characteristics (Fig. 2). ple solution was kept for 12 h to rinse acidic ions. The sample The shallow marine to marginal marine deposits are characterized residues were dispersed with 10 ml of 0.5 N (NaPO3)6 in an ultra- by black to predominantly green–gray colors (Supplementary ma- sonic vibrator and were measured on a Malvern Mastersizer 2000 terial, Fig. S1) and oolitic grainstone, fossiliferous packstone, lami- laser grain-size analyzer. All the pretreatments and measurements nated carbonate mudstone, dolomitic marl, calcarenite, and rippled were performed in the Key Laboratory of Western China’s Envi- fine-grained sandstone with occasional gypsum. Gypsum was iden- ronmental systems, Lanzhou University. Results are presented in tified in deposits of the ZD section and tufa mounds were iden- Fig. 5. tified in section WA (Fig. 2) indicating shallow–marginal marine B. Carrapa et al. / Earth and Planetary Science Letters 424 (2015) 168–178 173

Fig. 4. (A) Typical composition and (B) petrography of sandstones from the investigated samples (Supplementary Tables S4, S5); ternary diagrams and tectonic fields after Dickinson (1974). facies and warm, restricted water (Winsor et al., 2012). A single of these bivalves were collected from levels in the ZD section and bed (20 cm thick) of white volcanic ash crops out at the 47 m were identified (Dr. Oleg Mandic, pers. comm.) as belonging to level of section PE, within the transition between marine and non- the species Sokolowia buhsii (Supplementary material; Fig. S2). The marine deposits (Fig. 2). same species has been described in deposits from the Tarim basin Fluvial facies generally consist of red, trough cross-stratified and interpreted to be middle–late Eocene in age (late Lutetian to and rippled sandstone, and laminated to massive siltstone typical early Priabonian) (Bosboom et al., 2011). of high-sinuosity fluvial deposits (Fig. 2; Supplementary material). In section ZD massive red siltstone with mottles and carbonate 4.3. Foraminifera and calcareous nannofossils nodules is interpreted as calcic paleosols. Red siltstones with occasional oscillatory current ripples are interpreted to represent a Planktonic foraminifera from the PE and WA sections are rare transitional environment (possibly tidal flats) (Dalrymple and Choi, or missing and scarcely preserved (Supplementary material; Ta- 2007). ble S2); only in one sample from the PE section (PE42), very Eolian deposits are characterized by large-scale (several meters rare marker species (Acarinina gr. broedermanni, Turborotalia cer- thick) cross-stratified sandstone (foresets) with planar tops and roazulensis cerroazulensis and Pseudohastigerina micra) are obtained, tangential bases, trough cross-stratified and rippled sandstone, and which are indicative of a middle–late Eocene age (Bartonian– siltstone typical of eolian dune and interdune deposits (Kocurek, early Priabonian). Planktonic foraminiferal assemblages are quite 1981)(Figs. 2, 5). Massive (structureless), homogeneously sorted, well preserved but less abundant and diverse in ZD section from but disorganized (massive) and poorly consolidated red sandstone the southern Tien Shan. In samples ZD-062 (1–2) planktonic and siltstone are interpreted as eolian loess deposits (Figs. 2, 5) foraminifera do not exceed 5% of the total foraminiferal assem- similar to sandy loessite described by Zheng et al. (2003) from the blages and their abundance decreases up-section (<1% in samples southern Tarim basin. Scanning electron microscopy (SEM) of sand ZD-063 (1–2)). The marker species are always very rare and quite grain surface textures and grain-size analyses confirm this inter- discontinuously distributed; nevertheless the co-occurrence of the pretation (see below). species Morozovella formosa, M. aragonensis and M. subbottinae suggest an Ypresian age (E4–5 Zones; Berggren and Pearson, 2005) for 4.2. Invertebrate paleontology sample ZD-062 (1–2). The poor planktonic assemblages and the possibility of reworking do not, however, exclude a younger age. Typical macrofossils of marine strata from the investigated sec- Benthic foraminifera from the PE section are less abundant and tions include epifaunal bivalves of the orders Pectinida and Os- mostly characterized by textulariids, Cibicides and other rotaliids treida, supporting a shallow-marine environment. Trace fossils in- such as the genera Ammonia and Rotalia; these two genera usu- clude Skolithos, which is typical of marginal and shallow marine ally inhabit shallow water environments and are typical of the environments. littoral zone (e.g., Van Morkhoven et al., 1986). On the contrary, Ostreida fossils (e.g., oysters) indicate eutrophic conditions (i.e., benthic foraminifera from the lower ZD section (samples ZD-062 high nutrient, high content of organic matter); the best specimens and 063, Fig. 2) are usually very abundant, quite well preserved, 174 B. Carrapa et al. / Earth and Planetary Science Letters 424 (2015) 168–178

Fig. 5. Lithologies, scanning electron microscope (SEM) photographs, and grain-size distribution of representative samples from the PE section. See text for explanation. and mostly characterized by agglutinated taxa. The overall benthic ataxia, Textularia, Spiroplectammina, Dorothia and Reophax) that are foraminiferal fauna (Table S2) is composed of representatives of typical of the neritic domain (<200 m water depth). The mixed the agglutinated genera Tritaxia, Textularia, Psammosphaera, Spiro- foraminiferal assemblage is interpreted to be the result of rework- plectammina, Dorothia and Reophax together with the calcareous ing. The studied washed residues also contain rare ﬁsh teeth, sea- hyaline genera Cibicidoides (both biconvex and planoconvex mor- urchin spines and gypsum crystals, which are particularly abun- photypes), Anomalinoides, Praeglobobulimina and Lenticulina with dant in samples ZD-063 (1–2) and are consistent with an evapor- rare to very rare admixtures of representatives of Pullenia, Trifa- itic environment. rina, Vaginulinopsis, Gyroidinoides, Oridorsalis (mostly O. umbonatus) In general, nannofossil abundance and preservation are quite and fragmented nodosariids (Nodosaria, Vaginulina, Dentalina). We limited in the ZD samples (1–2) (Table S2); the other collected note that the benthic foraminiferal assemblage is characterized by samples are barren of nannofossils. In the studied assemblages, mixed shallow and deep water index taxa (e.g., Jorissen et al., the co-presence of Discoaster dyastipus and D. multiradiatus, and 2007 and references therein). Specimens of Lenticulina spp., No- the absence of Discoaster lodoensis and Tribrachiatus contortus, sug- dosariids and of the species Oridorsalis umbonatus and Anomalina gest an Ypresian age (NP11 Zone) (Martini, 1971); however, this pompilioides, all indicative of upper depth limit distribution of ca. assemblage may be reworked, similar to our interpretation for the 600–1000 m, co-occur with shallower-water indexes (such as Tri- foraminifera assemblage. B. Carrapa et al. / Earth and Planetary Science Letters 424 (2015) 168–178 175

4.4. Ostracods layer is located within the transition from marine to non-marine facies, it provides the only absolute geochronological age constrain- Ostracods were only available for the ZD section and are very ing the timing of the transition from marine to continental envi- rare and poorly preserved. The specimen from the sample ZD-062 ronments during the middle Eocene (Bartonian) in Tajikistan. (1) (Supplementary material; Fig. S3) is recognized as Cytheridea Zircon U–Pb ages from five detrital sandstone samples from the eocaenica, which has been described in Priabonian deposits from ZD, WA, and PE sections (Figs. 2 and 3) show a strong 40 Ma signal, the Bashibulake Mine section in the Tarim basin (Bosboom et al., typical of Pamir rocks (Lukens et al., 2012; Bershaw et al., 2012). 2014a). C. eocaenica is also described in the Oligocene of south- This requires that the depositional ages of the host sedimentary ern Romania (Olteanu, 2006). Sample ZD-062 (1) also contains the strata must be younger than ∼40 Ma and suggests active mag- genus Loxoconcha (Fig. S3, images C, D, E). We note that the species matism (U/Th ratios <1; Supplementary material, Table S3) in the Loxoconcha ex. gr. nystiana occurs from late Eocene to Oligocene sediment source area at this time. (Keen, 1978; Pirkenseer and Berger, 2011) and has been found in Modal petrographic compositions of selected sandstones from deposits from the Tarim Basin that were interpreted to be late the investigated strata document provenance typical of foreland Eocene in age (Bosboom et al., 2011). Fragments from sample basin deposits (e.g., Dickinson, 1974) (Fig. 4). Sandstone frame- ZD-063 (1) (Fig. S3, images A, B) and complete shells from sample work grain modes are dominated by quartzolithic compositions, ZD-063 (2) are identified as the species Cytheridea ex. gr. pernota, plagioclase feldspar, and heterogeneous labile lithic populations. which is one of the most common species described in the Tarim On quartz–feldspar–lithic grain diagrams, the samples plot in the basin in the upper Eocene (Bosboom et al., 2011); although, the recycled orogenic and magmatic arc provenance fields, typical of same species is also mentioned from the lower Oligocene of west- sandstones deposited in retroarc foreland basins. The most impor- ern Europe. tant lithic grain types include quartz–mica tectonite, quartz–mica The specimen from sample ZD-063 (2) (Fig. S3, images A, B) schist, micritic limestone, and various types of intermediate vol- is difficult to identify but it may represent the anterior part of canic lithic grains (Supplementary material; Tables S4 and S5). Vol- the genus Pterigocythereis. The species P. ceratoptera has been iden- canic lithic grains indicate active magmatism in the source regions tified in Priabonian strata from the Tarim basin (Bosboom et al., during the time of deposition, consistent with the abundance of ca. 2014a). The same species has been described from the upper Ru- 40 Ma detrital zircon U–Pb ages and the presence of the ∼39 Ma pelian of the upper Rhine Graben, the Oligocene of the Paris Basin volcanic ash layer in section PE. and the lower Rupelian from the Swiss Jura (Ducasse et al., 1985; Picot et al., 2008; Pirkenseer and Berger, 2011). This marine genus 4.7. Characterization of eolian deposits is used as a fossil marker for water depth, indicating minimum water depth of approximately 60 m in open marine coasts and SEM analyses were performed on eolian sandstone and loessite less than 10 m in restricted marine environments (e.g., Libeau, from the PE section deposited on top of the last marine interval 1980). Overall the ostracod assemblages suggest a late Eocene– (Fig. 2). A majority of the grains, mostly quartz, from eolian dune early Oligocene age for the lower part of the ZD section. and loess strata, have irregular shapes often characterized by sharp edges and conchoidal surfaces (Fig. 5) that are characteristic of eo- 4.5. Integrated biostratigraphy and chronostratigraphy lian deposits (Whalley et al., 1982). Grain-size distributions of eolian dune sandstone from the PE section are bimodal (Fig. 5I) with The paleontological data described above are apparently in dis- a well-sorted coarse component (ca. 150 μm) and a poorly-sorted agreement: oyster and ostracod assemblages indicate a mostly late fine component (ca. 10 μm). Grain-size distributions of eolian loess Eocene to early Oligocene age, whereas the foraminiferal and nan- material have trimodal distributions (Fig. 5J), with a well-sorted nofossil assemblages, coming from the same ZD samples, indicate coarse component (ca. 50 μm), a poorly sorted fine component (ca. an older early Eocene (Ypresian) age. These results can be ex- 10 μm), and a minor ultrafine component (<2μm).Our data are plained by erosion and reworking of older foraminiferal and nannofossil taxa derived from Ypresian strata and later deposition as consistent with the grain-size distribution of modern eolian dune residues in late Eocene–early Oligocene deposits. The scarcity of and loess deposits from western China (Sun et al., 2002). The grain younger (late Eocene–lower Oligocene) foraminiferal and nanno- morphology, grain-size distribution and lithological evidence fur- fossil specimens in our samples could be justified by the particular ther confirm the eolian origin of these strata (Whalley et al., 1982). depositional conditions. It is plausible that in a shallow and restricted marine environment, characterized by deposition of gyp- 5. Discussion sum (indicative of an evaporitic environment with a restricted cir- culation and high salinity), the paleontological assemblages were The sedimentology, paleontology and geochronology of the in- scarce and mostly made of heavy-shelled mollusks and ostracods, vestigated sections suggest that a marginal marine environment while foraminifera (mostly the planktonic specimens) and nanno- and restricted conditions were present during mid–late Eocene fossils were very rare to absent. Also, in the ZD section we find ex- time and a transitional to fluvial environment was established by clusively planktonic foraminifera and nannofossil specimens older ca. 39 Ma in the Tajik depression. Similar facies in the Tajik basin in age and benthic foraminifera indicative of both neritic and deep- and southern Tien Shan indicate that the two regions were part of water conditions (depth below 600 m), which are inconsistent with the same basin filled by detritus derived from the nascent Pamir. the sedimentological and other paleontological data reinforcing the Furthermore, our data imply that the Tien Shan did not exist as interpretation that these assemblages are reworked residues. In- a topographic feature during the late Eocene–early Oligocene and stead, the biostratigraphic data from the lower portion of the PE instead a continuous basin covered the region between the Pamir section (sample PE-042) indicates a late Bartonian–early Priabo- and southern Tien Shan. nian age. This is consistent with the age derived from ostracod A restricted evaporitic marine environment was established in and oyster assemblages for the lower portion of the ZD section. the late Eocene–early Oligocene as indicated by the presence of gypsum and tufa mounds typical of arid coastal conditions. More 4.6. Geochronology and sediment provenance than 1000 m of eolian deposits (sand dunes and loess) on top of the last marine interval (PE section) indicate that a desert-like en- The volcanic ash layer at the 47 m level of section PE produced vironment was established directly following Paratethys regression. a zircon U–Pb final age of 38.93 ±0.54 Ma (Fig. 3). Because this ash Our results are consistent with data from the Chinese Loess Plateau 176 B. Carrapa et al. / Earth and Planetary Science Letters 424 (2015) 168–178

Fig. 6. (A) Digital elevation model of the study area with location of the sections investigated in this study and in Bosboom et al. (2011, 2013, 2014a, 2014b, 2014c) used for comparison; (B) modified sea level curve based on the New Jersey record (Cramer et al., 2011)and PETM global regression–transgression sequences. (C) Paleogeographic cartoon showing timing of Paratethys regression (blue: marine; gray: continental/uplifting area) based on this study and literature data; sample locations as in A. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) and the Tarim basin, indicating that aridity was established by the progradation owing to high ratio of sediment supply to basin ac- Oligocene (Bosboom et al., 2013) and as early as 40 Ma (Bosboom commodation at this location. In particular the transition from et al., 2014a, 2014c). marginal marine gypsiferous facies to transitional and continen- Aridification in central Asia has been linked to a global cooling tal deposits with occasional paleosols (e.g., ZD section) observed trend following the early Eocene, characterized by climatic vari- in upper Eocene strata of the Tajik basin including the southern ability (e.g. Middle Eocene climate optimum) and leading to the Tien Shan, is interpreted to represent deposition in a distal fore- Eocene–Oligocene transition (Dupont-Nivet et al., 2007; Bosboom land basin. The up-section coarsening trend reflects progressive et al., 2014c). The EOT corresponds with an abrupt decline of at- encroachment of the growing Pamir thrust belt. Synorogenic de- mospheric CO2 concentration and associated glaciation in Antarc- position, magmatic activity, and crustal thickening in the Pamir as tica (Pagani et al., 2011; Zachos et al., 2008)around 34 Ma. early as 40 Ma are consistent with early Cenozoic radial thrust- Paratethys regression in the Tarim basin of Central Asia has been ing in the Pamir (Bosboom et al., 2014c) and Paleogene shortening linked to global sea level fall related to Eocene–Oligocene cool- and magmatism in central Tibet (Kapp et al., 2005), suggesting sig- ing (Bosboom et al., 2014a). However, the correlation between the nificant crustal thickening and elevation in the region since the Tajik basin stratigraphic record of Paratethys regression and global Eocene (Fig. 7B). High elevation in the Pamir by Eocene time is also sea level shows that marine regression predated the EOT by as supported by oxygen isotopic compositions of Eocene–Oligocene much as 4.8 million years in Tajikistan (Fig. 6). Combined with data carbonates in the Tarim basin (Bershaw et al., 2012). from the Tarim basin, our study indicates that the marine connection between the Tarim and Tajik basins was disrupted between 6. Conclusions ca. 39 and 37 Ma and followed a northwestward younging trend (starting at ca. 41 in southern Tarim basin; Bosboom et al., 2014a) Regression of the Paratethys in Tajikistan occurred during the supporting a tectonic control by the growing Pamir (Fig. 6). middle–late Eocene (∼39 Ma). Our data combined with data from Sediment provenance is consistent with a recycled orogen and the Tarim basin in China show a northwestward migration of the foreland basin setting as supported by the convex-upward shape shoreline between 41 Ma and 37 Ma, which is consistent with (Van Hinte, 1978)of the post-40 Ma subsidence curve (Leith, 1985) northward indentation and growth of the Pamir. Whereas sea level for the Tajik basin (Fig. 7A). The late Eocene–early Oligocene re- changes associated with global cooling and glaciations may have gression in the Tajik–Tarim basins can thus be explained by coastal controlled Paratethys regression during the early Cenozoic, growth B. Carrapa et al. / Earth and Planetary Science Letters 424 (2015) 168–178 177

Fig. 7. (A) Subsidence analysis of the Tajik depression modified after Leith (1985). (B) Schematic cross section across the Pamir and Tajik–Tarim basin at ca. 40 Ma. of the Pamir has exerted a primary control on sediment supply retreat from the Taim Basin (west China) and concomitant Asia paleoenviron- and accommodation, depositional environments, and subsequent ment change. Palaeogeogr. Palaeoclimatol. Palaeoecol. 299, 385–398. deformation in the Tajik–Tarim basins by ca. 40 Ma. Topographic Bosboom, R., Dupont-Nivet, G., Grothe, A., Brinkhuis, H., Villa, G., Mandic, O., Stoica, M., Huang, W., Yang, W., Guo, Z., Krijgsman, W., 2013. Linking Tarim Basin sea loading by the Pamir was responsible for the formation of a fore- retreat (west China) and Asian aridification in the late Eocene. Basin Res. 26. land basin system including the Tajik depression and Tarim basin http://dx.doi.org/10.1111/bre.12054. during the late Eocene. Combined with evidence of thick crust and Bosboom, R., Dupont-Nivet, G., Grothe, A., Brinkhuis, H., Villa, G., Mandic, O., Stoica, active Cenozoic tectonics in the Pamir, this suggests significant el- M., Kouwenhoven, T., Huang, W., Yang, W., Guo, Z., 2014a. Timing, cause and im- pact of the late Eocene stepwise sea retreat from the Tarim Basin (west China). evation in the region which needs to be considered in discussions Palaeogeogr. Palaeoclimatol. Palaeoecol. 403, 101–118. on aridity and monsoonal climate in central Asia. We suggest that Bosboom, R.E., Abels, H.A., Hoorn, C., van den Berg, B.C.J., Guo, Z.J., Dupont-Nivet, G., the formation of high topography in the Pamir and the associated 2014b. Aridification in continental Asia after the Middle Eocene Climatic Opti- orographic barrier likely severed the seaway connection between mum (MECO). Earth Planet. Sci. Lett. 389, 34–42. Bosboom, R., Dupont-Nivet, G., Huang, W., Yang, W., Guo, Z.-J., 2014c. 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