Earth and Planetary Science Letters 424 (2015) 168–178
Contents lists available at ScienceDirect
Earth and Planetary Science Letters
www.elsevier.com/locate/epsl
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
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, Bălcescu, 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
b
a r t i c l e
i n f o
Article history:
Received 9 March 2015
Received in revised form 14 May 2015
Accepted 18 May 2015
Available online xxxx
Editor: A. Yin
Keywords:
Paratethys
Tajik basin
Tarim basin
foreland basin
Pamir
Tibet
a b s t r a c t
Plate tectonics and eustatic sea-level changes have fundamental effects on paleoenvironmental conditions
and bio-ecological changes. The Paratethys Sea was a large marine seaway that connected the
Mediterranean Neotethys Ocean with Central Asia during early Cenozoic time. Withdrawal of the
Paratethys from central Asia impacted the distribution and composition of terrestrial faunas in the region
and has been largely associated with changes in global sea level and climate such as cooling associated
with the Eocene/Oligocene transition (EOT).
Whereas the regression has been dated in the Tarim basin (China), the pattern and timing of regression
in the Tajik basin, 400 km to the west, remain unresolved, precluding a test of current paleogeographic
models. Here we date the Paratethys regression in Tajikistan at ca. 39 million years ago (Ma), which
is several million years older than the EOT (at ca. 34 Ma) marking the greenhouse to icehouse
climate transition of the Cenozoic. Our data also show a restricted, evaporitic marine environment since
the middle–late Eocene and establishment of desert like environments after ca. 39 Ma. The overall
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
During early Cenozoic time the Paratethys Seaway (Popov et al.,
2004), a northern branch of the Neotethys Ocean, connected the
central Atlantic Ocean with a complex system of basins, covering
a roughly 1200 km wide and >3000 km long east–west swath of
Europe and southern central Asia (Fig. 1). The eastern extremity of
this vast region is now occupied by the Tibet–Pamir orogenic sys-
*
Corresponding author.
E-mail address: bcarrapa@email.arizona.edu (B. Carrapa).
http://dx.doi.org/10.1016/j.epsl.2015.05.034
0012-821X/© 2015 Elsevier B.V. All rights reserved.
tem and related sedimentary basins including the Tajik and Tarim
basins.
Aridification and monsoonal climate in Asia have been linked
to transgressive–regressive cycles of the Paratethys and global
climatic changes (Bosboom et al., 2011, 2013, 2014a, 2014b;
Dupont-Nivet et al., 2007; Licht et al., 2014). In particular, the
study by Bosboom et al. (2014a) suggests a stepwise westward
retreat of the Paratethys between 41 Ma and 37 Ma. Although
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
of tectonics and global climate on the retreat of the Paratethys
remain unresolved. Competing models attribute aridity and mon-
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
et al., 2010). The timing of Paratethys regression and attainment of
high topography and elevations in the region are thus critical for
evaluating current climate and tectonic models.
A several km thick succession of Cenozoic sedimentary rocks
deposited in the Tajik basin (Nikolaev, 2002) preserves a record
of paleoenvironmental changes, erosion and tectonic activity of
the surrounding Pamir and Tien Shan Mountains. This study applies sedimentology, paleontology, sandstone petrography and zircon U–Pb geochronology to lower Cenozoic deposits in the Tajik
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 zircon geochronology (from a volcanic tuff) anchors the transition from marine to non-marine deposition at ca. 39 Ma in the
region. This datum is the first radiometric age ever reported from
Paratethyan rocks in central Asia, and also constrains the time of
the regression in Tajikistan to ∼39 Ma. The 39 Ma age is significantly older than global sea-level changes and biotic turnovers of
the Eocene/Oligocene transition (EOT) at ca. 34 Ma. When com-
pared with the relative timing of the regression available for the
Tarim basin, ranging from ca. 41 Ma to ca. 37 Ma (Bosboom et
al., 2014a), our study suggests a northwestward younging of the
regression consistent with northward growth of the Pamir Mountains and development of a foreland basin system in the area now
occupied by the Tajik–Tarim basins by late Eocene time. Growth of
the Pamir in the Cenozoic has most likely affected land–sea distributions and climate in Central Asia.
2. Geological setting
The Tajik basin was connected with the Tarim basin during the
early Cenozoic, but subsequent northward growth of the Pamir
orogen has severed this connection. The geologic history of the
Tajik and western Tarim basins (Fig. 1) is thus crucial to understanding the tectonic evolution of the Pamir and the Tajik–Tarim
connection through the Paratethys Seaway. The Paratethys Seaway
was part of a larger epicontinental sea that connected the Mediterranean Tethys to the Tarim basin until at least the early Cenozoic
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(Fig. 1A). It covered a vast area and as such probably had a major
effect on paleoenvironments and climate in central Asia.
During the EOT, the Paratethys Seaway was surrounded by
land and connected to the Mediterranean Tethys and other oceans
through narrow passages. The Paratethys Seaway was located upwind from and served as a major moisture source for the Tarim
basin and large parts of continental Asia.
The westward retreat of the Paratethys Seaway has been primarily associated with eustatic sea level changes (Bosboom et al.,
2013, 2014a, 2014b). Eocene–Oligocene redbeds and late Miocene
loessite in China are interpreted as indicating arid climate and
winter monsoonal conditions (Bosboom et al., 2014a; Licht et al.,
2014) related to either orographic effects associated with the Tibetan Plateau (Molnar et al., 2010) or a combination of tectonics
and global sea level changes affecting Paratethys regression and
global cooling (e.g., Bosboom et al., 2014b; Dupont-Nivet et al.,
2007). Although growth of the Pamir has been loosely suggested as
a possible control on paleoenvironmental changes in Central Asia,
the role of the Pamir on Paratethys regression and timing of tectonic activity remain largely unconstrained. An orographic effect
associated with the Pamir has been suggested to have played a
significant role on erosion since the Miocene (Carrapa et al., 2014);
however, uplift and northward growth of the Pamir and its possible effect on paleoenvironments during the early Cenozoic is not
known.
Data from the Tibetan Plateau suggest significant shortening
and crustal thickening during the Cretaceous and early Cenozoic
(Yin and Harrison, 2000; Kapp et al., 2005) consistent with high
elevations of at least parts of Tibet since the Eocene. Paleoaltimetry data indicate high elevations since at least the late Oligocene in
central Tibet (DeCelles et al., 2007). The Pamir, which is the western continuation of the Tibetan Plateau (Yin and Harrison, 2000;
Robinson et al., 2012), preserves evidence of thick crust and OligoMiocene high-grade metamorphism and exhumation (e.g., Schmidt
et al., 2011) but its earlier tectonic history remains largely unknown, leaving open the question of its potential role in controlling land–sea reorganizations and Asian climate during the early
Cenozoic.
In the Tarim basin, Paratethys regression has been dated between ca. 47 and 37 Ma (Bosboom et al., 2011, 2013, 2014a; Sun
and Jiang, 2013). In particular, Bosboom et al. (2014a) suggested a
million-year scale stepwise retreat of the Paratethys in the Eocene,
consistent with a tectonic control, and hydrological connection between the Tarim and Tajik basins until ca. 34 Ma.
3. Methods
3.1. Field work and stratigraphic correlations
tained suitable microfossils (ostracods, planktonic foraminifera and
calcareous nannofossils) for age determination (Fig. 2); samples
from the WA sections were barren and only a few samples from
the PE section contained limited foraminifera (Supplementary material; Table S2).
Samples were disaggregated by mortar and washed and sieved
through meshes of 180 and 125 μm; the washed residues were
then dried in an oven at about 40 ◦ C. Approximately 200 foraminifera were counted under a stereomicroscope in random aliquots
of both the >180 and 180–125 μm washed fractions to obtain census counts of the more common species recorded in the studied
samples. Census counts are reported in the Supplementary material (Table S2). Planktonic foraminiferal taxonomy follows the guide
of Toumarkine and Luterbacher (1985) partly updated by Pearson
et al. (2012). The biostratigraphic attribution is primarily based on
the standard zonations (Berggren and Pearson, 2005; Wade et al.,
2011). The same washed residues analyzed for foraminifera were
investigated also for ostracods.
For nannofossil analyses, simple smear-slides were prepared
from the same ZD samples used for foraminiferal study and were
analyzed under a polarizing light microscope at 1250X magnification. The nannofossil abundance was quantified by counting all
the nannofossil specimens occurring in 400 random fields of view,
corresponding approximately to an area of 6 mm2 , in each sample. The biostratigraphic attribution is primarily based on standard
zonations (Martini, 1971; Okada and Bukry, 1980). Results are reported in the Supplementary material (Table S2).
Invertebrate fossils, primarily shells of bivalves of the orders
Ostreida and Pectinida, were observed in the PE, WA, and ZD
sections. When of suitable quality, specimens were identified to
lowest taxonomic-level following previous studies (Lan, 1997) and
were compared with faunas of comparable age from the Tarim
basin (Bosboom et al., 2011, 2013, 2014a).
3.3. U–Pb geochronology of detrital zircons (Nu HR ICPMS)
Zircon crystals were extracted from six selected samples by traditional methods of crushing and grinding, followed by separation
with a Wilfley table, heavy liquids, and a Frantz magnetic separator. Samples were processed such that all zircons were retained in
the final heavy mineral fraction. A large split of these grains (generally several hundreds of grains) was incorporated into a 1 epoxy
mount together with fragments of our Sri Lanka standard zircon.
The mounts were sanded down to a depth of ∼20 μm, polished,
imaged, and cleaned prior to isotopic analysis. U–Pb geochronology
of zircons was conducted by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) at the
Arizona LaserChron Center (Gehrels et al., 2008). 100 grains were
targeted for sandstone samples and 38 grains for the volcanic tuff;
results are shown in Fig. 3. Additional analytical information can
be found in the Supplementary material (Table S3).
We investigated three widely spaced Paleogene stratigraphic
sections, up to 1500 m thick, within the Tajik basin (Figs. 1
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
samples collected, 48 were selected, on the basis of paleontological content and mineral yield, for biostratigraphic, geochronologic, petrographic, and sedimentologic analyses. Facies analysis
and interpretations of paleodepositional environments are based
on detailed stratigraphic sections presented in Fig. 2. Facies were
identified and recorded using the classification scheme of Miall
(1978) (Supplementary material; Table S1). Correlations between
sections were based on facies, fossil assemblages, and zircon U–Pb
geochronological data.
Modal sandstone petrographic data were collected from six selected standard petrographic thin sections from medium-grained
sandstones from the WA and PE measured sections (Fig. 2). Each
thin section was stained for calcium and potassium feldspars and
450 grains were counted according to the Gazzi–Dickinson method
(Ingersoll et al., 1984). Results are shown in Fig. 4 and additional
material and information can be found in the Supplementary material (Tables S4, S5).
3.2. Biostratigraphy
3.5. SEM and grain size analyses
Microfossil analyses were performed on sixteen samples from
all three sections; however, only samples from the ZD section con-
Three hundred and eighty bulk samples were collected from
the lower part of the PE section (0–850 m) for grain size analysis.
3.4. Sandstone petrography
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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.
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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
PE section were selected for Scanning Electron Microscope (SEM)
analysis. Each sample was treated with 10% H2 O2 and 10% HCl to
remove organic matter and carbonates, and the grain morphology
was observed on a Hitachi S4800 SEM. 2–4 g of air-dried material for each sample was firstly treated with 10 ml of 10% H2 O2
to remove organic matter and with 10 ml of 10% HCl to remove
carbonates; 100 ml of distilled water was then added and the sample solution was kept for 12 h to rinse acidic ions. The sample
residues were dispersed with 10 ml of 0.5 N (NaPO3 )6 in an ultrasonic vibrator and were measured on a Malvern Mastersizer 2000
laser grain-size analyzer. All the pretreatments and measurements
were performed in the Key Laboratory of Western China’s Environmental systems, Lanzhou University. Results are presented in
Fig. 5.
4. Results and interpretations
4.1. Sedimentology and depositional environments
The studied sections are dominated by fine-grained siliciclastic and carbonate lithofacies in their lower parts and sandstone
in their upper parts. Lithofacies exhibit an overall transition upsection from shallow marine to continental characteristics (Fig. 2).
The shallow marine to marginal marine deposits are characterized
by black to predominantly green–gray colors (Supplementary material, Fig. S1) and oolitic grainstone, fossiliferous packstone, laminated carbonate mudstone, dolomitic marl, calcarenite, and rippled
fine-grained sandstone with occasional gypsum. Gypsum was identified in deposits of the ZD section and tufa mounds were identified in section WA (Fig. 2) indicating shallow–marginal marine
B. Carrapa et al. / Earth and Planetary Science Letters 424 (2015) 168–178
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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
bed (20 cm thick) of white volcanic ash crops out at the 47 m
level of section PE, within the transition between marine and nonmarine deposits (Fig. 2).
Fluvial facies generally consist of red, trough cross-stratified
and rippled sandstone, and laminated to massive siltstone typical
of high-sinuosity fluvial deposits (Fig. 2; Supplementary material).
In section ZD massive red siltstone with mottles and carbonate
nodules is interpreted as calcic paleosols. Red siltstones with occasional oscillatory current ripples are interpreted to represent a
transitional environment (possibly tidal flats) (Dalrymple and Choi,
2007).
Eolian deposits are characterized by large-scale (several meters
thick) cross-stratified sandstone (foresets) with planar tops and
tangential bases, trough cross-stratified and rippled sandstone, and
siltstone typical of eolian dune and interdune deposits (Kocurek,
1981) (Figs. 2, 5). Massive (structureless), homogeneously sorted,
but disorganized (massive) and poorly consolidated red sandstone
and siltstone are interpreted as eolian loess deposits (Figs. 2, 5)
similar to sandy loessite described by Zheng et al. (2003) from the
southern Tarim basin. Scanning electron microscopy (SEM) of sand
grain surface textures and grain-size analyses confirm this interpretation (see below).
4.2. Invertebrate paleontology
Typical macrofossils of marine strata from the investigated sections include epifaunal bivalves of the orders Pectinida and Ostreida, supporting a shallow-marine environment. Trace fossils include Skolithos, which is typical of marginal and shallow marine
environments.
Ostreida fossils (e.g., oysters) indicate eutrophic conditions (i.e.,
high nutrient, high content of organic matter); the best specimens
of these bivalves were collected from levels in the ZD section and
were identified (Dr. Oleg Mandic, pers. comm.) as belonging to
the species Sokolowia buhsii (Supplementary material; Fig. S2). The
same species has been described in deposits from the Tarim basin
and interpreted to be middle–late Eocene in age (late Lutetian to
early Priabonian) (Bosboom et al., 2011).
4.3. Foraminifera and calcareous nannofossils
Planktonic foraminifera from the PE and WA sections are rare
or missing and scarcely preserved (Supplementary material; Table S2); only in one sample from the PE section (PE42), very
rare marker species (Acarinina gr. broedermanni, Turborotalia cerroazulensis cerroazulensis and Pseudohastigerina micra) are obtained,
which are indicative of a middle–late Eocene age (Bartonian–
early Priabonian). Planktonic foraminiferal assemblages are quite
well preserved but less abundant and diverse in ZD section from
the southern Tien Shan. In samples ZD-062 (1–2) planktonic
foraminifera do not exceed 5% of the total foraminiferal assemblages and their abundance decreases up-section (<1% in samples
ZD-063 (1–2)). The marker species are always very rare and quite
discontinuously distributed; nevertheless the co-occurrence of the
species Morozovella formosa, M. aragonensis and M. subbottinae suggest an Ypresian age (E4–5 Zones; Berggren and Pearson, 2005) for
sample ZD-062 (1–2). The poor planktonic assemblages and the
possibility of reworking do not, however, exclude a younger age.
Benthic foraminifera from the PE section are less abundant and
mostly characterized by textulariids, Cibicides and other rotaliids
such as the genera Ammonia and Rotalia; these two genera usually inhabit shallow water environments and are typical of the
littoral zone (e.g., Van Morkhoven et al., 1986). On the contrary,
benthic foraminifera from the lower ZD section (samples ZD-062
and 063, Fig. 2) are usually very abundant, quite well preserved,
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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
foraminiferal fauna (Table S2) is composed of representatives of
the agglutinated genera Tritaxia, Textularia, Psammosphaera, Spiroplectammina, Dorothia and Reophax together with the calcareous
hyaline genera Cibicidoides (both biconvex and planoconvex morphotypes), Anomalinoides, Praeglobobulimina and Lenticulina with
rare to very rare admixtures of representatives of Pullenia, Trifarina, Vaginulinopsis, Gyroidinoides, Oridorsalis (mostly O. umbonatus)
and fragmented nodosariids (Nodosaria, Vaginulina, Dentalina). We
note that the benthic foraminiferal assemblage is characterized by
mixed shallow and deep water index taxa (e.g., Jorissen et al.,
2007 and references therein). Specimens of Lenticulina spp., Nodosariids and of the species Oridorsalis umbonatus and Anomalina
pompilioides, all indicative of upper depth limit distribution of ca.
600–1000 m, co-occur with shallower-water indexes (such as Tri-
ataxia, Textularia, Spiroplectammina, Dorothia and Reophax) that are
typical of the neritic domain (<200 m water depth). The mixed
foraminiferal assemblage is interpreted to be the result of reworking. The studied washed residues also contain rare fish teeth, seaurchin spines and gypsum crystals, which are particularly abundant in samples ZD-063 (1–2) and are consistent with an evaporitic environment.
In general, nannofossil abundance and preservation are quite
limited in the ZD samples (1–2) (Table S2); the other collected
samples are barren of nannofossils. In the studied assemblages,
the co-presence of Discoaster dyastipus and D. multiradiatus, and
the absence of Discoaster lodoensis and Tribrachiatus contortus, suggest an Ypresian age (NP11 Zone) (Martini, 1971); however, this
assemblage may be reworked, similar to our interpretation for the
foraminifera assemblage.
B. Carrapa et al. / Earth and Planetary Science Letters 424 (2015) 168–178
4.4. Ostracods
Ostracods were only available for the ZD section and are very
rare and poorly preserved. The specimen from the sample ZD-062
(1) (Supplementary material; Fig. S3) is recognized as Cytheridea
eocaenica, which has been described in Priabonian deposits from
the Bashibulake Mine section in the Tarim basin (Bosboom et al.,
2014a). C . eocaenica is also described in the Oligocene of southern Romania (Olteanu, 2006). Sample ZD-062 (1) also contains the
genus Loxoconcha (Fig. S3, images C, D, E). We note that the species
Loxoconcha ex. gr. nystiana occurs from late Eocene to Oligocene
(Keen, 1978; Pirkenseer and Berger, 2011) and has been found in
deposits from the Tarim Basin that were interpreted to be late
Eocene in age (Bosboom et al., 2011). Fragments from sample
ZD-063 (1) (Fig. S3, images A, B) and complete shells from sample
ZD-063 (2) are identified as the species Cytheridea ex. gr. pernota,
which is one of the most common species described in the Tarim
basin in the upper Eocene (Bosboom et al., 2011); although, the
same species is also mentioned from the lower Oligocene of western Europe.
The specimen from sample ZD-063 (2) (Fig. S3, images A, B)
is difficult to identify but it may represent the anterior part of
the genus Pterigocythereis. The species P. ceratoptera has been identified in Priabonian strata from the Tarim basin (Bosboom et al.,
2014a). The same species has been described from the upper Rupelian of the upper Rhine Graben, the Oligocene of the Paris Basin
and the lower Rupelian from the Swiss Jura (Ducasse et al., 1985;
Picot et al., 2008; Pirkenseer and Berger, 2011). This marine genus
is used as a fossil marker for water depth, indicating minimum
water depth of approximately 60 m in open marine coasts and
less than 10 m in restricted marine environments (e.g., Libeau,
1980). Overall the ostracod assemblages suggest a late Eocene–
early Oligocene age for the lower part of the ZD section.
4.5. Integrated biostratigraphy and chronostratigraphy
The paleontological data described above are apparently in disagreement: oyster and ostracod assemblages indicate a mostly late
Eocene to early Oligocene age, whereas the foraminiferal and nannofossil assemblages, coming from the same ZD samples, indicate
an older early Eocene (Ypresian) age. These results can be explained by erosion and reworking of older foraminiferal and nannofossil taxa derived from Ypresian strata and later deposition as
residues in late Eocene–early Oligocene deposits. The scarcity of
younger (late Eocene–lower Oligocene) foraminiferal and nannofossil specimens in our samples could be justified by the particular
depositional conditions. It is plausible that in a shallow and restricted marine environment, characterized by deposition of gypsum (indicative of an evaporitic environment with a restricted circulation and high salinity), the paleontological assemblages were
scarce and mostly made of heavy-shelled mollusks and ostracods,
while foraminifera (mostly the planktonic specimens) and nannofossils were very rare to absent. Also, in the ZD section we find exclusively planktonic foraminifera and nannofossil specimens older
in age and benthic foraminifera indicative of both neritic and deepwater conditions (depth below 600 m), which are inconsistent with
the sedimentological and other paleontological data reinforcing the
interpretation that these assemblages are reworked residues. Instead, the biostratigraphic data from the lower portion of the PE
section (sample PE-042) indicates a late Bartonian–early Priabonian age. This is consistent with the age derived from ostracod
and oyster assemblages for the lower portion of the ZD section.
4.6. Geochronology and sediment provenance
The volcanic ash layer at the 47 m level of section PE produced
a zircon U–Pb final age of 38.93 ± 0.54 Ma (Fig. 3). Because this ash
175
layer is located within the transition from marine to non-marine
facies, it provides the only absolute geochronological age constraining the timing of the transition from marine to continental environments during the middle Eocene (Bartonian) in Tajikistan.
Zircon U–Pb ages from five detrital sandstone samples from the
ZD, WA, and PE sections (Figs. 2 and 3) show a strong 40 Ma signal,
typical of Pamir rocks (Lukens et al., 2012; Bershaw et al., 2012).
This requires that the depositional ages of the host sedimentary
strata must be younger than ∼40 Ma and suggests active magmatism (U/Th ratios <1; Supplementary material, Table S3) in the
sediment source area at this time.
Modal petrographic compositions of selected sandstones from
the investigated strata document provenance typical of foreland
basin deposits (e.g., Dickinson, 1974) (Fig. 4). Sandstone framework grain modes are dominated by quartzolithic compositions,
plagioclase feldspar, and heterogeneous labile lithic populations.
On quartz–feldspar–lithic grain diagrams, the samples plot in the
recycled orogenic and magmatic arc provenance fields, typical of
sandstones deposited in retroarc foreland basins. The most important lithic grain types include quartz–mica tectonite, quartz–mica
schist, micritic limestone, and various types of intermediate volcanic lithic grains (Supplementary material; Tables S4 and S5). Volcanic lithic grains indicate active magmatism in the source regions
during the time of deposition, consistent with the abundance of ca.
40 Ma detrital zircon U–Pb ages and the presence of the ∼39 Ma
volcanic ash layer in section PE.
4.7. Characterization of eolian deposits
SEM analyses were performed on eolian sandstone and loessite
from the PE section deposited on top of the last marine interval
(Fig. 2). A majority of the grains, mostly quartz, from eolian dune
and loess strata, have irregular shapes often characterized by sharp
edges and conchoidal surfaces (Fig. 5) that are characteristic of eolian deposits (Whalley et al., 1982). Grain-size distributions of eolian dune sandstone from the PE section are bimodal (Fig. 5I) with
a well-sorted coarse component (ca. 150 μm) and a poorly-sorted
fine component (ca. 10 μm). Grain-size distributions of eolian loess
material have trimodal distributions (Fig. 5J), with a well-sorted
coarse component (ca. 50 μm), a poorly sorted fine component (ca.
10 μm), and a minor ultrafine component (<2 μm). Our data are
consistent with the grain-size distribution of modern eolian dune
and loess deposits from western China (Sun et al., 2002). The grain
morphology, grain-size distribution and lithological evidence further confirm the eolian origin of these strata (Whalley et al., 1982).
5. Discussion
The sedimentology, paleontology and geochronology of the investigated sections suggest that a marginal marine environment
and restricted conditions were present during mid–late Eocene
time and a transitional to fluvial environment was established by
ca. 39 Ma in the Tajik depression. Similar facies in the Tajik basin
and southern Tien Shan indicate that the two regions were part of
the same basin filled by detritus derived from the nascent Pamir.
Furthermore, our data imply that the Tien Shan did not exist as
a topographic feature during the late Eocene–early Oligocene and
instead a continuous basin covered the region between the Pamir
and southern Tien Shan.
A restricted evaporitic marine environment was established in
the late Eocene–early Oligocene as indicated by the presence of
gypsum and tufa mounds typical of arid coastal conditions. More
than 1000 m of eolian deposits (sand dunes and loess) on top of
the last marine interval (PE section) indicate that a desert-like environment was established directly following Paratethys regression.
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
Oligocene (Bosboom et al., 2013) and as early as 40 Ma (Bosboom
et al., 2014a, 2014c).
Aridification in central Asia has been linked to a global cooling
trend following the early Eocene, characterized by climatic variability (e.g. Middle Eocene climate optimum) and leading to the
Eocene–Oligocene transition (Dupont-Nivet et al., 2007; Bosboom
et al., 2014c). The EOT corresponds with an abrupt decline of atmospheric CO2 concentration and associated glaciation in Antarctica (Pagani et al., 2011; Zachos et al., 2008) around 34 Ma.
Paratethys regression in the Tarim basin of Central Asia has been
linked to global sea level fall related to Eocene–Oligocene cooling (Bosboom et al., 2014a). However, the correlation between the
Tajik basin stratigraphic record of Paratethys regression and global
sea level shows that marine regression predated the EOT by as
much as 4.8 million years in Tajikistan (Fig. 6). Combined with data
from the Tarim basin, our study indicates that the marine connection between the Tarim and Tajik basins was disrupted between
ca. 39 and 37 Ma and followed a northwestward younging trend
(starting at ca. 41 in southern Tarim basin; Bosboom et al., 2014a)
supporting a tectonic control by the growing Pamir (Fig. 6).
Sediment provenance is consistent with a recycled orogen and
foreland basin setting as supported by the convex-upward shape
(Van Hinte, 1978) of the post-40 Ma subsidence curve (Leith, 1985)
for the Tajik basin (Fig. 7A). The late Eocene–early Oligocene regression in the Tajik–Tarim basins can thus be explained by coastal
progradation owing to high ratio of sediment supply to basin accommodation at this location. In particular the transition from
marginal marine gypsiferous facies to transitional and continental deposits with occasional paleosols (e.g., ZD section) observed
in upper Eocene strata of the Tajik basin including the southern
Tien Shan, is interpreted to represent deposition in a distal foreland basin. The up-section coarsening trend reflects progressive
encroachment of the growing Pamir thrust belt. Synorogenic deposition, magmatic activity, and crustal thickening in the Pamir as
early as 40 Ma are consistent with early Cenozoic radial thrusting in the Pamir (Bosboom et al., 2014c) and Paleogene shortening
and magmatism in central Tibet (Kapp et al., 2005), suggesting significant crustal thickening and elevation in the region since the
Eocene (Fig. 7B). High elevation in the Pamir by Eocene time is also
supported by oxygen isotopic compositions of Eocene–Oligocene
carbonates in the Tarim basin (Bershaw et al., 2012).
6. Conclusions
Regression of the Paratethys in Tajikistan occurred during the
middle–late Eocene (∼39 Ma). Our data combined with data from
the Tarim basin in China show a northwestward migration of the
shoreline between 41 Ma and 37 Ma, which is consistent with
northward indentation and growth of the Pamir. Whereas sea level
changes associated with global cooling and glaciations may have
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
and accommodation, depositional environments, and subsequent
deformation in the Tajik–Tarim basins by ca. 40 Ma. Topographic
loading by the Pamir was responsible for the formation of a foreland basin system including the Tajik depression and Tarim basin
during the late Eocene. Combined with evidence of thick crust and
active Cenozoic tectonics in the Pamir, this suggests significant elevation in the region which needs to be considered in discussions
on aridity and monsoonal climate in central Asia. We suggest that
the formation of high topography in the Pamir and the associated
orographic barrier likely severed the seaway connection between
the Tarim and Tajik basins and contributed to aridification of central Asian since ca. 40 Ma.
Acknowledgements
We acknowledge Miriam Cobianchi for her help in the analysis of nannofossils and Oleg Mandic for his aid in identification
of marine bivalves. We thank Nikolai Ishuck for help in the field
and Anatoly Ishuck, Oimahmadov Ilhomjon, Yunus Mamadjanov
and Ansor Nyozov for help with permits and logistics. We thank
Jay Chapman for help with drafting and discussions. Editor An Yin,
John Bershaw and two anonymous reviewers are acknowledged
for constructive criticisms. Funding was provided by National Geographic Society/Waitt Grants ##W226-12, NGS grant #8442-08 to
Kraatz, by Exxon Mobil Exploration to Carrapa, and by a National
Natural Science Foundation of China Grant (41302144). We thank
Mark Pecha and George Gehrels, at the Arizona Laserchron laboratory, for technical assistance.
Appendix A. Supplementary material
Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.epsl.2015.05.034.
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