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Article

The History of Transgressions during the Late Paleocene-Early Eocene in the Kuqa Depression, Tarim Basin: Constraints from C-O-S-Sr Isotopic Geochemistry

by
Yang Xu
1,*,
Yangtong Cao
2,
Chenglin Liu
2,*,
Hua Zhang
2 and
Xiao Nie
2
1
State Key Laboratory of Nuclear Resource and Environment, East China University of Technology, Nanchang 330013, China
2
MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China
*
Authors to whom correspondence should be addressed.
Minerals 2020, 10(9), 834; https://doi.org/10.3390/min10090834
Submission received: 25 August 2020 / Revised: 20 September 2020 / Accepted: 21 September 2020 / Published: 22 September 2020
(This article belongs to the Special Issue Mineralogy, Petrology and Geochemistry of Evaporites)
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Abstract

:
The Tethys Sea extended into the Kuqa Depression from the Paleocene to the late Eocene and provided an abundant provenance for the deposition of evaporite sequences. Until now, detailed research on the history of transgressions during the late Paleocene-early Eocene in the Kuqa Depression has been limited. Therefore, in this study, we took the upper Paleocene Talak section and the lower Eocene Xiaokuzibai section in the western part of the Depression as the research objects and analyzed the petrology, the carbon and oxygen isotopes of carbonate rocks, and the sulfur and strontium isotopes of gypsum rocks to systematically study the above issues. The δ13C, δ18O and δ34S values of the upper Paleocene evaporite sequences were determined to be between 4.2‰ and 5.7‰, between −5.2‰ and 2.4‰, and between 16.5‰ and 17.9‰, respectively. The δ13C, δ18O, δ34S, and 87Sr/86Sr values of the lower Eocene evaporite sequences were determined to be between −6.9‰ and −2.0‰, between −9.0‰ and −4.5‰, between 10.5‰ and 17.0‰, and between 0.708642 and 0.709883, respectively. The analysis results show that the evaporite sequence of the upper Paleocene was formed by transgression. The deposition of the evaporite sequence changed from continental to marine deposition, and then gradually transitioned to continental during the Early Eocene. This paper is of great significance for reconstructing the history of transgressions in the Tethys tectonic realm during this period.

Graphical Abstract

1. Introduction

Multiple transgression-regression cycles of Tethyan seawater occurred during the Cretaceous-Paleogene, and these cycles represent major events during the geological evolution of the Tarim Basin. In the early Cretaceous, seawater invaded from west to east in the southwest of the Tarim Basin, and the transgression scope increased during the Paleocene. Consequently, seawater began to extend into the northern Tarim Basin (the Kuqa Depression) (Figure 1). Therefore, how the transgressions of the late Paleocene-early Eocene affect the Kuqa Depression is an urgent problem that remains to be solved, with obvious significance for revealing the evolution of the Tethyan Sea.
Marine fossils (such as foraminifera, gastropods, and calcareous fossils) have been found in the Paleocene and Eocene strata in this area [3,4]. Fu (1996) and Jia (2002) [5,6] discussed the sedimentary environment by analyzing the spatial distribution and carbon and oxygen isotopic compositions of Paleogene carbonate rocks in the Kuqa Depression. Liu et al. (1987) and Zhang et al. (2013) [7,8] analyzed the provenance based on the sulfur isotopic composition of gypsum from multiple Paleogene stratigraphic sections. Although previous studies on the provenance and sedimentary environments of the evaporite sequences have been conducted, no systematic studies or analyses of typical upper Paleocene-lower Eocene sections have been conducted, especially in terms of the integration of multiple isotope geochemical methods. To some extent, this has affected the ability of researchers to deeply understand and recognize the history of transgressions during the late Paleocene-early Eocene. Therefore, it is of great significance to systematically study the sedimentary environment of the upper Paleocene and lower Eocene strata by combining multiple isotope geochemical methods.
Isotope geochemical proxies can provide valid evidence for explaining the material sources and sedimentary environments of evaporite sequences. The carbon and oxygen isotopic compositions of carbonate rocks are significant paleoenvironmental indicators and can be used to estimate the material sources, paleosalinity, paleotemperature, and other parameters [9,10,11]. Because sulfur isotope fractionation during gypsum deposition is very weak or even negligible, the sulfur isotopic composition of gypsum is often used to determine the material source during the evaporite deposition period [12,13]. The element strontium does not exhibit isotope fractionation during chemical and biochemical processes, although geological processes, such as evaporation and concentration, can change the concentration of strontium; however, the 87Sr/86Sr ratio remains almost constant within a given geological time and sea area [14]. Thus, the strontium isotopic composition can be used as an effective tracer for studying the migration and transition of materials and is often combined with sulfur isotopic compositions to study the material sources [15,16,17,18].
Typical Talak and Xiaokuzibai sections of the upper Paleocene and lower Eocene in the Kuqa Depression are the objects of this research, and the C, O, S and Sr isotopic compositions of these evaporite sequences were systematically studied. Then, the provenance and sedimentary environment of the upper Paleocene and lower Eocene carbonate–gypsum sequences in the Kuqa Depression were discussed to reveal the transgression history of the late Paleocene-early Eocene.

2. Geological Setting

The Kuqa Depression is located between the southern Tianshan tectonic belt and the Tarim Basin [19] and features a nearly east-west orientation (Figure 2). It is approximately 410 km long from east to west and approximately 20 to 60 km wide from north to south, with an area of approximately 16,000 km2. The Hercynian Movement at the end of the early Permian formed the basic appearance of the basement of the Kuqa Depression [20], and the region then entered the evolution stage of a foreland basin. In the Late Triassic-Early Jurassic, the tectonic setting began to evolve from compressional to extensional. In the Early Cretaceous, the Tarim Basin again featured a compressional tectonic stress field due to the intense collision between the Indian plate and the Eurasian plate [21,22].
Five major transgressions affected the development of marine sediments in the Tarim Basin during the Late Cretaceous-Paleogene period [23]. The western part of the Kuqa Depression began to receive seawater at the beginning of the Paleocene, and the key factor controlling the retreat of seawater from the Tarim Basin was mainly the northward growth of the Pamirs rather than the India–Eurasia collision [24,25]. At present, the final phase of seawater retreat from the Tarim Basin occurred mainly between 47 and 33 Ma [26,27,28]. In this period, thick salt-bearing strata were deposited in the western part of the basin, and the lithology of the Paleocene Talak Formation consists of a succession of thick maroon conglomerate, sandy conglomerate, thin dolomite, mud-bearing gypsum, and silty mudstone from bottom to top. The lower part of the Eocene Xiaokuzibai Formation consists of dark gray silty mudstone, grayish-green silty mudstone, limestone, gypsum and rock salt with dark gray boulder-clay, and the upper part consists of thick rock salt with maroon boulder-clay.

3. Materials and Methods

3.1. Materials

A total of 33 samples were collected for isotope analysis from the upper parts of the Talak Formation and the lower parts of the Xiaokuzibai Formation in the northwestern Kuqa Depression, and each sample was collected at equal intervals (Figure 2). There is no direct and precise dating of the carbonate-gypsum deposition in the Kuqa Depression. However, Zheng and Meng (2006) [29] precisely identified the Paleogene strata age through paleomagnetic data and suggested that the Talak Formation and the Xiaokuzibai Formation formed from 60.5 to 38 Ma. Teng et al. (1997) [30] indicated that the age of the top boundary of the Xiaokuzibai Formation is 38 Ma based on magnetostratigraphic research of the Cenozoic section of the Kuqa River area. The Paleocene and Eocene samples used in this study are from the Talak Formation and the bottom of the Xiaokuzibai Formation, respectively. Hence, they are inferred to have formed in the late Paleocene and early Eocene, respectively.
The Talak and Xiaokuzibai sections located in the western Kuqa Depression are typical upper Paleocene and lower Eocene sections in the basin, respectively. The Talak section is located in Talak village in the western Kuqa Depression. This section contains evaporite strata deposited in the late Paleocene, which consist of thick maroon conglomerate, thin dolomite, and thick gypsum deposits (Figure 3a–d). Nine dolomite samples and six gypsum samples were collected from the upper Paleocene Talak section (the thickness of the dolomite layer is approximately 1.6 m, and the thickness of the gypsum layer is approximately 13 m). The Xiaokuzibai section is located in the Xiaokuzibai ravine in the western Kuqa Depression. This section contains evaporite strata deposited in the early Eocene, which consist of thick limestone and thick gypsum deposits (Figure 3e,f). Seven carbonate samples and eleven gypsum samples were collected from the lower Eocene Xiaokuzibai section (the thickness of the limestone layer is approximately 9 m, and the thickness of the gypsum layer is approximately 18 m).
The dolomite in the Talak section is muddy dolomite, the hand specimen is dark gray in color, and the appearance is dense and uniform (Figure 4a). Observation under a polarizing microscope showed that the carbonate minerals are dolomite, with a micritic texture and a particle size of generally less than 5 μm (Figure 4d). These petrographic characteristics show a sedimentary environment with weak hydrodynamic conditions and no terrigenous material supply.
The lower part of the Xiaokuzibai section is sand-bearing micrite, and the upper part is micrite. The hand specimen of the sand-bearing micrite is light brown, and quartz particles are visible to the naked eye (Figure 4b). Observation under a polarizing microscope showed that the minerals mainly include calcite and quartz clasts. The calcite crystals are fine-grained with a micritic texture and a grain size of generally less than 5 μm (Figure 4e). The terrigenous quartz particles are distributed irregularly and chaotically and exhibit angular shapes, low textural maturity, and grain sizes of 50~1200 μm. The hand specimen of the upper micrite is gray, and calcite veins are present in it (Figure 4c). Observation under a polarizing microscope showed that the carbonate minerals are fine-grained calcite with grain sizes of generally less than 7 μm and a micritic texture. Therefore, the lower limestone was deposited in a sedimentary environment characterized by weak hydrodynamic conditions and a terrigenous debris supply; over time, the terrigenous debris supply of the sedimentary environment disappeared.
Fluid inclusions in gypsum are considered to be a source of direct information about primary parent liquor [31]. The upper Paleocene gypsum crystals are mainly characterized by single-phase liquid inclusions with diameters in the range of ~3 to 40 µm (Figure 5a,b). The lower Eocene gypsum crystals are mainly characterized by single-phase liquid inclusions with diameters in the range of ~5 to 50 µm (Figure 5c,d). These inclusions are arranged in planes parallel to the growth zones of gypsum crystals, showing a well-preserved primary texture.

3.2. Experimental Methods

No calcite veins were observed within the upper Paleocene dolomite samples. Some calcite veins were observed in the lower Eocene limestone, so we used an electric drill with a diamond bit to acquire limestone samples from between the veins and observed whether or not there was recrystallization under the microscope. First, the samples were determined to be unaltered before analysis; then, all selected samples were ground to 200 mesh and baked at 110 °C for 3 h. The C and O isotopic compositions and the Mn and Sr contents of the carbonate rocks were measured at the Analytical Laboratory of the Beijing Research Institute of Uranium Geology. The C and O isotopic measurement instrument was a MAT252 mass spectrometer (Finnigan, San Francisco, CA, USA) with an error of less than ±0.2‰. Then, 25 mg were placed at the bottom of the main reaction tube and 5 mL of 100% orthophosphoric acid were poured into the branch tube of the reaction bulb. The two were mixed after vacuumization to react for 16 h with a reaction temperature of 75 °C. Then, a liquid-nitrogen cooled trap (−80 to −70 °C) was used to collect carbon dioxide gas, which was placed into a mass spectrometer for analysis once purified. The standard sample number was GBW04405 and the unit standard was V-PDB. The contents of Mn and Sr were determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS, Agilent, Palo Alto, CA, USA) with an error of less than ±5%. The samples were first reacted with HCl 5 M to remove carbonate and subsequently dissolved in 18 mΩ cm−1 Milli-Q water. To avoid gypsum saturation in the final solution and to ensure a similar range of concentrations, a constant dilution ratio of 1:500 was maintained for all samples. The samples were then filtered through a 0.45 μm cellulose acetate membrane filter to remove any quartz or other insoluble mineral particles.
To prevent other minerals in the original gypsum samples from affecting the effectiveness and accuracy of the test results, we first performed X-ray diffraction analysis on the selected sample before analyzing the sulfur isotopic composition and then selected pure unweathered gypsum to place in an ultrasonic cleaning machine for 2 times with pure water, then the cleaned samples were dried and crushed to 200 mesh. The S and Sr isotopic compositions were measured at the Institute of Mineral Resources, Chinese Academy of Geological Sciences. The S isotopic measurement instrument was an MAT-251EM mass spectrometer. The sample was purified by Eschka reagent fusion sample into pure BaSO4, and then V2O5 oxidant was used to prepare SO2, and then SO2 was used for sulfur isotope test and analysis. Based on repeated measurements of the standard sample, the error was less than ±0.1‰, and the results are expressed in δ values relative to the Vienna Canyon Diablo Troilite (V-CDT) standard. The Sr isotopic measurement instrument was a Thermal Ionization Isotope Mass Spectrometer (TIMS), and the instrument model was Triton Ti. The process involved weighing 150 mg of the sample, dissolving it with deionized water, and centrifuging to remove the insoluble matter to eliminate the influence of other minerals. The Rb and Sr content of the samples were measured with the atomic absorption spectrometer (AAS); we weighed 1g of the sample, dissolved in deionized water and filtered to remove insolubles. The 87Sr/86Sr measured by the international standard NBS-987 is 0.710266with a 2σ uncertainty of ± 0.000007 (external precision).

4. Results

The analysis results of 16 carbon and oxygen isotopes and the corresponding Mn and Sr contents are shown in Table 1. The δ13C values of the upper Paleocene micritic dolomite are positive, and the variation range of the values is small, with values ranging from 4.2‰ to 5.7‰ (mean 5.3‰, S.D. = 0.4, n = 9). The δ18O values vary widely, ranging from −5.2‰ to 2.4‰ (mean −1.7‰, S.D. = 2.5, n = 9). The variation in the δ13C values of the lower Eocene micritic limestone is large, ranging from -6.9‰ to −2.0‰ (mean −5.3‰, S.D. = 1.9, n = 7); the δ18O values vary widely, ranging from −9.0‰ to −6.4‰ (mean = −7.3‰, S.D. = 1.0, n = 7).
The results from the analysis of 17 sulfur isotopes and 11 strontium isotopes are shown in Table 2. The variation in the δ34S values of the upper Paleocene gypsum is very small, with values ranging from 16.5‰ to 17.9‰ and an average of 17.3‰. The δ34S values of lower Eocene gypsum vary widely, ranging from 10.5‰ to 17.0‰, with an average of 13.2‰; the 87Sr/86Sr ratios of the gypsum range from 0.708642 to 0.709883, with an average of 0.709184.

5. Discussion

5.1. Significance of the Carbon and Oxygen Isotopic Geochemistry

5.1.1. Validity of the Data

When using carbon and oxygen isotopes of carbonate rocks from marine strata to study the paleoenvironment, it is first necessary to determine if the samples are primary [32,33]. Carbonate rocks may undergo a series of alterations in the process of diagenesis and postdiagenesis that alter the original isotopic compositions, such as the effect of diagenetic fluids (especially atmospheric precipitation), recrystallization, and metamorphism. At present, to determine that the samples have not undergone diagenetic alteration, the following three conditions should be met: (1) In general, an Mn/Sr ratio of less than 3 is a criterion for determining the original carbon isotopic composition of a sample [34,35,36]; (2) It is generally believed that samples with δ18O values less than −10‰ are altered and that the original information has been lost [35]; (3) A good correlation between the δ13C values and the δ18O values of carbonate rocks reflects the occurrence of strong diagenetic alteration, whereas a poor correlation reflects the occurrence of weak diagenetic alteration [37,38].
Three of the 16 samples have Mn/Sr ratios greater than 3 and are considered to exhibit nonprimary carbon isotopic compositions (Table 1). The δ18O values of all samples were greater than −10‰ and could therefore be considered to be the original oxygen isotopic compositions. To eliminate the disturbance of diagenetic alteration, samples TLK-05, TLK-06, and TLK-07 were excluded, and the δ13C and δ18O values of the remaining samples were not significantly correlated (Figure 6a,b), with R2 values of 0.1821 and 0.2252 for the Paleocene and Eocene rocks, respectively. Therefore, except for the above three samples, the carbon and oxygen isotope data of the remaining samples reflect primary features and represent valid information.

5.1.2. Upper Paleocene Carbonate Rocks

Having demonstrated that the samples retain primary information, we can determine the salinity of the diagenetic medium of the carbonate rocks based on the δ13C values [39]. Keith and Weber (1964) [39] studied the carbon and oxygen isotopic compositions of limestone and calcium-bearing fossils. It is believed that δ13C and δ18O values can be used to determine the diagenetic medium salinity of carbonate rocks that formed before the Jurassic. When the salinity Z value (Z = 2.048 (δ13CPDB + 50) + 0.498 (δ18OPDB + 50)) is greater than 120, the carbonate formed under marine conditions; when the Z value is less than 120, the carbonate formed under freshwater conditions. The Z values of the Paleocene carbonate rocks range from 135.81 to 139.07, with an average of 137.67, all of which are much higher than 120, indicating a high salinity and stable marine sedimentary characteristics during formation of the carbonates.
The δ13C values of the Talak Formation have a small variation range, whereas the δ18O values vary widely. This may be because the carbon isotopic composition is more stable than the oxygen isotopic composition during diagenesis, so the carbon isotopic composition better reflects the original composition of the Tethyan seawater [40]. Figure 7 shows that the range of δ13C values from the upper Paleocene carbonate rocks is very small and similar to the range of δ13C values of marine carbonates. Furthermore, the range of values overlaps the range of δ13C values of evaporation-produced carbonate. Therefore, the upper Paleocene carbonate rocks have seawater sedimentary features. The higher δ13C values indicate that the ancient seawater did not receive freshwater replenishment with low carbon isotopic compositions, but it did experience a strong evaporation and concentration effect.
Cao et al. (2016) [42] noted that the Talak Formation of the Kuqa Depression formed in the same period as the upper Paleocene Aertashi Formation in the southwestern Tarim Basin. Bosboom et al. (2011; 2014) [27,43] found that Paleocene and Eocene Tethyan seawater simultaneously invaded the southwestern Tarim Basin and the Kuqa Depression from the west to the east through the Alay Valley. The above evidence indicates that the upper Paleocene salt-bearing strata in the southwestern Tarim Basin and the Kuqa Depression are products of the same era and are comparable. Previous studies on the carbon and oxygen isotopes of Paleocene marine carbonate rocks in the southwestern Tarim Basin show that the δ13C values are positive (1.45~3.5‰) and the δ18O values are negative (−5.9~−0.46‰) [44,45]. The distribution of oxygen isotopic values in the upper Paleocene carbonates is very similar to the previously published data, but the carbon isotopic values exhibit weak positive anomalies, which may be related to the burial of organic carbon by marine organisms within the strata [3].

5.1.3. Lower Eocene Carbonate Rocks

The main differences between the Xiaokuzibai Formation and the Talak Formation in terms of the carbon and oxygen isotopic compositions are that the former has significantly lower isotopic values than the latter (Figure 8) and that both δ13C values and δ18O values show negative bias characteristics. The range of measured δ13C values overlaps with the range of δ13C values for marine and freshwater carbonates (Figure 7), and the values tend to increase from the bottom of the section to the top, indicating that the input of terrestrial material enriched in the light isotope 12C to the basin gradually decreased. The petrographic characteristics and salinity Z values of the carbonate samples are also consistent with this conclusion. The Z values of the lower Eocene carbonate rock range from 108.94 to 119.52 and increase significantly from the bottom of the section to the top. The above research results and petrographic characteristics of the carbonate rocks indicate that the bottom limestone deposits formed in a sedimentary environment with weak hydrodynamic conditions and the input of terrestrial debris upwards in the stratigraphy and then gradually transformed into an environment with less terrestrial debris input, possibly due to a transient increase in the supply of marine materials.
The carbon and oxygen isotopic trends are not related (Figure 6b), indicating an open sedimentary environment. A large amount of terrestrial material or freshwater enriched in the light isotope 12C was frequently input into the basin during this period, resulting in low carbon isotopic values. The Paleocene and Eocene lithofacies paleogeographic features of the Kuqa Depression have been described [46]. The Paleocene Tethyan seawater first invaded the Kuqa Depression from the west, and the salt lake that formed was shallow. The Talak section was located on the western edge of the salt lake, which may be the reason why the carbonate rock layer in the Talak section is thin and the carbon and oxygen isotopic values are somewhat high. The salt lake was affected by intermittent transgressions, and the southern Tianshan mountain front and other sources provided terrigenous materials to the lake at the beginning of the Eocene, which led to further increases in the depth of the salt lake and the area of the lake surface. This may explain the thick carbonate rocks and the low carbon and oxygen isotopic values in the Xiaokuzibai section.

5.2. Significance of the Sulfur Isotopic Geochemistry

The sulfur isotopic characteristics of sulfate minerals in evaporite basins are mainly affected by bacterial sulfate reduction, reservoir effects, and the composition of the source. Bacterial sulfate reduction causes the resulting sulfide to be significantly enriched in the light isotope 32S, while the residual sulfate minerals are enriched in the heavy isotope 34S [47]. For example, the δ34S values of Early Triassic marine gypsum in the Sichuan Basin range from 27.8‰ to 35.9‰ [48], and the δ34S values of Paleogene continental gypsum in the Dongpu Depression range from 28‰ to 33‰ [49]. Reservoir effects can cause the δ34S values of postdeposition gypsum in a stratigraphic sequence to be slightly less than those of gypsum prior to deposition [47,50]. Sources of the sulfur present in evaporites include marine, terrestrial, and volcanic sources. The δ34S value of marine sulfur is similar to the δ34S value of contemporary seawater, and its range is small. The δ34S values of terrestrial and volcanic sulfur are obviously lower than that of contemporary seawater, and their ranges are larger [50,51].
The δ34S values of the upper Paleocene and lower Eocene gypsum in the Kuqa Depression are generally low, which is in sharp contrast with the high δ34S values formed under strongly reducing conditions, indicating that sulfur isotope fractionation was induced by bacteria in the process of sulfate formation in the Kuqa Depression. Moreover, the petrologic characteristics do not show the presence of sulfides or other minerals indicative of a reducing environment, and the widely developed red layer indicates that the depositional environment had a high redox potential, which would not have been conducive to the survival of sulfate-reducing bacteria. Therefore, it is inferred that the sulfate samples in this paper are not associated with bacterial reduction. Further supporting the lack of the effect of bacterial reduction on sulfur isotope fractionation, the δ34S values of gypsum from the Talak section are rather high and feature a narrow range (16.5~17.9‰). These δ34S values all fall within the same range as those of contemporary marine evaporites. Additionally, the δ34S values of the strata decrease gradually from bottom to top, indicating that the reservoir effects or a small input of terrigenous materials may have played an important role in this variation. The above evidence indicates that these gypsum deposits formed in a marine sedimentary environment. However, the δ34S values of the gypsum from the Xiaokuzibai Formation decrease to a greater degree, and most δ34S values are lower than the δ34S values of contemporary marine sulfate, indicating that the seawater was frequently affected by continental freshwater during the process of evaporation, which shows that the sedimentary environment gradually changed from marine to continental.
Zhang et al. (2013) and Sun (2014) [8,52] performed sulfur isotope analysis on gypsum samples from the Paleocene Artashi Formation collected from surface sections and drill wells and showed that the δ34S values ranged from 16.6‰ to 20.6‰ and 16.4~19.1‰, respectively, exhibiting typical marine source characteristics. The sulfur isotope values of the gypsum from the upper Paleocene Talak section in the Kuqa Depression are similar and within these ranges. Therefore, the upper Paleocene gypsum deposits formed in a marine sedimentary environment, while the lower Eocene gypsum deposits formed in a sedimentary environment that gradually changed from marine to continental.

5.3. Significance of the Strontium Isotopic Geochemistry

Primary gypsum carries and usually retains the strontium isotopic composition of the water body from which the gypsum precipitated. The strontium isotopic ratio of marine gypsum is similar to the strontium isotopic ratio of the contemporary seawater, and the variation range is very small. The radioactive strontium isotopic ratio of continental gypsum is larger than the strontium isotopic ratio of contemporaneous seawater and plots far from the strontium isotope curve of contemporaneous seawater. For example, the 87Sr/86Sr ratio of Paleogene continental gypsum in the Dongpu Depression ranges from 0.7116 to 0.7137 [53]. The strontium isotopic ratio of gypsum from the sea-land transition phase is commonly affected by the mixing of seawater and terrestrial water and has the characteristics of both. For example, the strontium isotope ratios of gypsum from a Jurassic marginal sea environment range from 0.70836 to 0.71174 [54], and the strontium isotope ratios of middle Miocene marine gypsum affected by terrestrial radioactive strontium range from 0.708873 to 0.709106 [9].
The strontium isotope ratio of gypsum samples from the Xiaokuzibai section has an increasing trend; the lowest ratio (0.708642) is higher than the strontium isotopic ratio of contemporaneous seawater (Figure 9b) and higher than the maximum strontium isotopic ratio of marine foraminifera from the Gamba area of Tibet in the Tethyan realm during the same period [55]. The strontium isotopic ratio of minerals in evaporites associated with seawater is higher than that of contemporaneous seawater, and such a difference is usually interpreted as the effect of secondary precipitation or the recrystallization of minerals under the action of radioactive 87Sr-bearing fluids [55,56]. However, under the conditions of well-preserved primary fluid inclusions and X-ray diffraction microanalysis, the gypsum samples in this study have not experienced recrystallization. Therefore, the strontium isotopic signatures reflect a gradual increase in radiogenic Sr due to the constant input of nonmarine material during the gypsum formation period.
Our results are similar to the strontium isotopic ratios (0.708718~0.709326) of Eocene marine-continental rock salt in the Kuqa Depression, as measured by Wang et al. (2015) [59], but they are clearly different from the strontium isotopic ratios (0.709909~0.710868) of Miocene continental rock salt in the Kuqa Depression. In addition, the strontium isotopic ratios have a good correlation with the sulfur isotopic values from the same samples (R2 = 0.7, Figure 10), indicating that the source of the sulfur was related to the source of the strontium (calcium) in the gypsum and that the sulfur and strontium isotopic characteristics are a result of mixing between marine and continental materials. Therefore, the continuous mixing of terrestrial materials was the main factor causing sulfur isotopic values and strontium isotopic ratios to gradually move away from the isotopic values of contemporaneous seawater.

6. Conclusions

The petrologic characteristics of the studied carbonate rocks indicate that the Kuqa Depression featured a sedimentary environment characterized by weak hydrodynamic conditions and no terrigenous material input in the late Paleocene. Later, during the early Eocene, the sedimentary environment continued to feature weak hydrodynamic conditions, but the supply of terrigenous debris first increased and then decreased and disappeared. The carbon and oxygen isotopic compositions of carbonate rocks and the strontium isotopic compositions of gypsum rocks show that the Kuqa Depression experienced a Tethys transgression in the Late Paleocene. In contrast, the Tethys retreated during the early Eocene, began to accept continental deposits, and then experienced a transient transgression. Finally, it gradually evolved into continental deposits (Figure 11). The isotope geochemistry characteristics are consistent with the petrographic characteristics of carbonate rocks.

Author Contributions

Y.X. performed the data analyses and wrote the manuscript; Y.C. helped perform the analysis with constructive discussions; C.L. contributed to the conception of the study and editing; H.Z. and X.N. performed the experiment; All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the Basic Research Project for the Central Public Welfare Scientific Institutions (YYWF201716) granted by the Institute of Mineral Resources Chinese Academy of Geological Sciences and National Natural Science Foundation of China (No. 41902064 and No. 41602100).

Acknowledgments

We especially thank the Editor-in-Chief and anonymous reviewers for constructive comments and suggestions that have greatly improved the quality of this manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Paleogeographic sketch map of the Paleocene and Eocene in the Tethys Realm (modified from Abels et al. 2011 [1]); (b) Eocene paleogeographic map of the Tarim Basin (modified from Wang 1985 [2]).
Figure 1. (a) Paleogeographic sketch map of the Paleocene and Eocene in the Tethys Realm (modified from Abels et al. 2011 [1]); (b) Eocene paleogeographic map of the Tarim Basin (modified from Wang 1985 [2]).
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Figure 2. Simplified geological map of the Kuqa Depression and the locations of the Paleocene and Eocene stratigraphic outcrops (modified after the 1:200,000 Geological Map of the Kuqa Depression, 1967).
Figure 2. Simplified geological map of the Kuqa Depression and the locations of the Paleocene and Eocene stratigraphic outcrops (modified after the 1:200,000 Geological Map of the Kuqa Depression, 1967).
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Figure 3. Photographs of the outcrops of upper Paleocene and lower Eocene carbonate-gypsum rock sedimentary sequences. (a) upper Paleocene maroon conglomerate, dolomite, and gypsum; (b) upper Paleocene conglomerate; (c) upper Paleocene dolomite; (d) upper Paleocene gypsum; (e) lower Eocene limestone; (f) lower Eocene laminar gypsum.
Figure 3. Photographs of the outcrops of upper Paleocene and lower Eocene carbonate-gypsum rock sedimentary sequences. (a) upper Paleocene maroon conglomerate, dolomite, and gypsum; (b) upper Paleocene conglomerate; (c) upper Paleocene dolomite; (d) upper Paleocene gypsum; (e) lower Eocene limestone; (f) lower Eocene laminar gypsum.
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Figure 4. Petrographic characteristics of the upper Paleocene and lower Eocene carbonate rocks. (a,d): upper Paleocene micritic dolomite; (b,e): lower Eocene sandy micritic limestone (sample XKZB-02); (c,f): lower Eocene micritic limestone (sample XKZB-06).
Figure 4. Petrographic characteristics of the upper Paleocene and lower Eocene carbonate rocks. (a,d): upper Paleocene micritic dolomite; (b,e): lower Eocene sandy micritic limestone (sample XKZB-02); (c,f): lower Eocene micritic limestone (sample XKZB-06).
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Figure 5. Primary fluid inclusions characteristics of the upper Paleocene (a,b) and lower Eocene (c,d) gypsum.
Figure 5. Primary fluid inclusions characteristics of the upper Paleocene (a,b) and lower Eocene (c,d) gypsum.
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Figure 6. Correlations between δ13C and δ18O values of the upper Paleocene (a) and lower Eocene (b) carbonate rocks in the Kuqa Depression.
Figure 6. Correlations between δ13C and δ18O values of the upper Paleocene (a) and lower Eocene (b) carbonate rocks in the Kuqa Depression.
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Figure 7. Distribution of carbon isotopic values in various carbonate rocks and related materials (modified from Schopf 1980 [41]). The gray bars E1 and E2 represent the distribution ranges of δ13C values in the upper Paleocene and lower Eocene strata, respectively.
Figure 7. Distribution of carbon isotopic values in various carbonate rocks and related materials (modified from Schopf 1980 [41]). The gray bars E1 and E2 represent the distribution ranges of δ13C values in the upper Paleocene and lower Eocene strata, respectively.
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Figure 8. Lithology histogram of evaporite sequences in the upper Paleocene and lower Eocene strata, Kuqa Depression.
Figure 8. Lithology histogram of evaporite sequences in the upper Paleocene and lower Eocene strata, Kuqa Depression.
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Figure 9. Plots of the δ34S values (a) and 87Sr/86Sr ratios (b) of the Paleocene and Eocene gypsum in the Kuqa Depression and contemporaneous seawater. Squares represent the values of the Paleocene gypsum samples, and triangles represent the values of the Eocene gypsum samples. The δ34S seawater curve is from Paytan et al. (1998) [57]. The 87Sr/86Sr seawater curve is from McArthur et al. (2001) [58].
Figure 9. Plots of the δ34S values (a) and 87Sr/86Sr ratios (b) of the Paleocene and Eocene gypsum in the Kuqa Depression and contemporaneous seawater. Squares represent the values of the Paleocene gypsum samples, and triangles represent the values of the Eocene gypsum samples. The δ34S seawater curve is from Paytan et al. (1998) [57]. The 87Sr/86Sr seawater curve is from McArthur et al. (2001) [58].
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Figure 10. Correlation between sulfur and strontium isotopic compositions within the gypsum in the lower Eocene strata of the Kuqa Depression.
Figure 10. Correlation between sulfur and strontium isotopic compositions within the gypsum in the lower Eocene strata of the Kuqa Depression.
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Figure 11. Evolution of late Paleocene-early Eocene transgressions in the Kuqa Depression.
Figure 11. Evolution of late Paleocene-early Eocene transgressions in the Kuqa Depression.
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Table
Table 1. Carbon and oxygen isotope compositions and related data for carbonate rocks from the upper Paleocene and lower Eocene strata in the Kuqa Depression.
Table 1. Carbon and oxygen isotope compositions and related data for carbonate rocks from the upper Paleocene and lower Eocene strata in the Kuqa Depression.
SeriesLithologySample Numberδ13C
(PDB, ‰)		δ18O
(PDB, ‰)		Z ValuesMn/SrRemarks
Lower Eocenemicritic limestoneXKZB-01−6.6−8.5109.550.93
XKZB-02−6.9−8.5108.940.48
XKZB-03−6.0−6.4111.822.44
XKZB-04−5.4−6.9112.800.45
XKZB-05−6.4−9.0109.710.36
XKZB-06−2.0−7.4119.521.24
XKZB-07−2.1−7.0119.510.87
Upper Paleocenemicritic dolomiteTLK-015.70.2139.071.85
TLK-025.50.4138.760.17
TLK-035.2−4.3135.810.36
TLK-045.5−5.2135.970.32
TLK-055.4−3.9136.4213.36exclude
TLK-065.3−4.0136.1613.49exclude
TLK-074.22.4137.106.28exclude
TLK-085.5−0.1138.512.10
TLK-095.3−0.5137.911.37
Table
Table 2. Sulfur and strontium isotopic compositions of gypsum from the upper Paleocene and lower Eocene strata in the Kuqa Depression.
Table 2. Sulfur and strontium isotopic compositions of gypsum from the upper Paleocene and lower Eocene strata in the Kuqa Depression.
SeriesLithologySample Numberδ34S (‰)87Sr/86Sr
Lower EocenegypsumXKZB-S0117.00.708642 ± 11
XKZB-S0215.70.708929 ± 9
XKZB-S0314.40.709017 ± 6
XKZB-S0412.20.709062 ± 7
XKZB-S0514.10.709060 ± 7
XKZB-S0615.10.709193 ± 10
XKZB-S0711.10.709495 ± 12
XKZB-S0810.60.709494 ± 8
XKZB-S0910.80.709252 ± 6
XKZB-S1010.50.709883 ± 8
XKZB-S1113.30.709002 ± 7
Upper PaleocenegypsumTLK-S0117.9
TLK-S0217.8
TLK-S0317.3
TLK-S0417.2
TLK-S0517.2
TLK-S0616.5

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Xu, Y.; Cao, Y.; Liu, C.; Zhang, H.; Nie, X. The History of Transgressions during the Late Paleocene-Early Eocene in the Kuqa Depression, Tarim Basin: Constraints from C-O-S-Sr Isotopic Geochemistry. Minerals 2020, 10, 834. https://doi.org/10.3390/min10090834

AMA Style

Xu Y, Cao Y, Liu C, Zhang H, Nie X. The History of Transgressions during the Late Paleocene-Early Eocene in the Kuqa Depression, Tarim Basin: Constraints from C-O-S-Sr Isotopic Geochemistry. Minerals. 2020; 10(9):834. https://doi.org/10.3390/min10090834

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Xu, Yang, Yangtong Cao, Chenglin Liu, Hua Zhang, and Xiao Nie. 2020. "The History of Transgressions during the Late Paleocene-Early Eocene in the Kuqa Depression, Tarim Basin: Constraints from C-O-S-Sr Isotopic Geochemistry" Minerals 10, no. 9: 834. https://doi.org/10.3390/min10090834

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