Introduction

The evaporites is typically closely associated with tectonic activities, transgression-regression cycles, closed basins, and arid climatic conditions1,2,3,4. During the Late Cretaceous to Paleogene period, the western Tarim Basin (also referred to as the Yarkand Basin) underwent multiple transgression-regression cycles5,6,7,8,9leading to the development of two distinct evaporites sequences: the Late Cretaceous Tuyiluoke Formation, primarily composed of sandstone, mudstone, silty mudstone, gypseous mudstone, thin gypsum rock, and lenticular rock salt, formed under a marine regressive settings during the Late Cretaceous; and the Aertashen Formation, mainly consisting of thick-massive gypsum and anhydrock layers interbedded with mudstone (sandstone) and limestone (argillaceous limestone), which developed in an intermittent marine transgressive sedimentary environment during the Paleocene (Fig. 1a). Over the past few decades, paleontological and mineralogical indicators have significantly advanced the reconstruction of paleotemperatures. However, there are limited reports on Late Cretaceous paleotemperature records from the Yarkand Basin in the eastern Tethys Region. The extremely thick evaporites deposited under arid conditions in this basin largely restrict the applicability of traditional climatic and environmental indicators, thereby impeding a comprehensive understanding of the Late Cretaceous paleotemperatures in the Yarkand Basin.

Due to the high sensitivity of rock salt to temperature fluctuations in evaporative basins, it plays a crucial role in paleoclimate reconstruction. This significance is demonstrated by the fact that during the formation process, rock salt crystals capture salt lake brines and form fluid inclusions. The homogenization temperatures of these fluid inclusions record detailed information about water and atmospheric temperatures at the time of their formation10,11. Previous studies have indicated that since halite forms in shallow brine environments during evaporation (typically no deeper than 2 m), the homogenization temperatures of fluid inclusions in primary pure liquid-phase halite not only reflect the surface brine temperature of ancient salt lakes during the crystallization process12 but also approximate the temperature conditions under ancient evaporative environments11,13. Furthermore, given the strong correlation between water and air temperatures, which are representative paleoclimate indicators, the homogenization temperatures can serve as proxies for water or air temperatures at the time of formation14,15,16.

However, research on evaporites in the Yarkand Basin has predominantly focused on salt-bearing strata, mineral assemblages, and geochemical characteristics17,18as well as the extent of Mesocenozoic transgressions19,20,21,22,23 for many years, but no studies have been conducted on salt inclusions from the ancient salt lake in the basin. Consequently, the surface of brines or paleoclimate temperatures during the sedimentation process of the ancient salt lake remain unknown. Over the past decade, investigations into the temperature measurement of salt inclusions in ancient evaporative basins across China (e.g., the Lanping-Simao Basin in Yunnan Province, Sichuan Basin, Jiangling Depression in Hubei Province, Kuqa Depression in the northern Tarim Basin, etc.) have been carried out, with numerous relevant publications emerging. However, such studies have not yet been extended to the Yarkand Basin in the western Tarim Basin. In this study, halite cuttings from the Wb1 borehole, which are well-developed in the central part of the basin, were selected for salt inclusion temperature measurements. This aims to determine the surface temperature of brines during the halite sedimentation process and further reflect the temperature characteristics of the paleoenvironment during this period.

Geological background

The Yarkand Basin is located in the western Tarim Basin, as a foreland basin which developed from the Pre-Sinian basement has been reformed multiple times by tectonic superimpositions24,25. From the West Kunlun piedmont to the interior of the basin, it is characterized by a piedmont thrust belt, central sag, which divided the Kashgar and Yecheng-Hotan sags, the Markit slope belt, and the Bachu frontal uplift zone sequentially26,27 (Fig. 1b). The Yarkand Basin was an inherited graben basin in the early Cretaceous, but during the late Cretaceous-Paleogene it had begun to develop into a foreland basin, with its depositional center distributed along the West Kunlun to South Tianshan piedmonts20. Since the Cenozoic era, the basin has subsided and deformed, developing several subsiding centers and very thick depositional cap rocks due to the collision of the Indian and Eurasian plates28. In the Neogene, the maximum depositional thickness of this basin was located in the Yecheng-Hotan sag, but its depositional center has already migrated along the northwest line to the Kashgar sag since the Quaternary29.

In the early Cretaceous, the Yarkand Basin was a strip-shaped graben basin with NW-SE orientation located at the West Kunlun piedmont with a largest subsiding center30. Sedimentation into the basin was controlled by the provenances of the South Tianshan and West Kunlun orogens. Sediments were distributed along the long strip-shaped West Kunlun piedmont, with thickness gradually decreasing from SW to NE orientation, developing into an alluvial fan, fan delta, and shore-neritic sub-facies31. In the late Cretaceous, a braided river delta, supratidal evaporated sand-mud flat, and a carbonate platform developed sequentially23 and a salt-gypsum flat was deposited in the basin. In the Paleocene-early Eocene, this basin developed into a semi-closed estuarine and lagoon environment. Affected by terrigenous debris and subtropical dry climate21,22clastic rocks, in addition to evaporites and carbonates, were developed during this period. The main evaporative sequences are the Tuyiluoke and Aertashen Formations, but the former is smaller than the latter on the extent or scale.

Fig. 1
figure 1

Rock salt outcrop and isopach map of gypsum rock in Yarkand Basin (a) and its tectonic division (b).

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Materials and methods

​A total of 35 halite cuttings samples from well Wb1 (the Tuyiluoke Formation of Upper Cretaceous) were selected for thin section grinding (Fig. 2a), and temperature measurement of salt fluid inclusion was carried out. First select the crystal form of regular and pure halite, and thoroughly clean with anhydrous ethanol to ensure that its surface is clean and flawless. Then, the treated halite sample was cut along its cleavage plane with a knife to a 1–2 mm slice, and then microscopic petrographic observation was carried out to record the size, shape and distribution characteristics of the fluid inclusions in halite. The primary salt fluid inclusions were analyzed and selected, and the selected primary halite crystals were sealed with plastic self-sealing bags and stored in desiccant. The temperature measurement and analysis of fluid inclusions were carried out in the Ministry of Natural Resources Key Laboratory of Metallogeny and Mineral Assessment.

The fluid inclusions were observed under microscope and the pure liquid phase primary inclusions were selected. The single liquid phase inclusion sheet is placed under the microscope for overall observation and photography, focusing on recording the combination and morphology of the primary inclusion formed in the early diagenesis and the secondary inclusion formed by recrystallization, and the measured area is marked out. Then the inclusion sheet is cut into small pieces suitable for temperature measurement (generally about 1 cm × 1 cm), and the small inclusion sheet to be measured is placed on the hot and cold platform. Constant temperature freezing is carried out, and the general freezing temperature is − 18 ℃32. After bubbles appeared in freezing nucleation of a single liquid phase inclusion, the homogenization temperature test was carried out. The Linkam THMSG600 cold and hot platform was used to complete the homogenization temperature test, and the heating rate was 0.5 ℃/min. When the bubbles gradually became smaller, the temperature was temporarily reduced to 0.1 ℃/min. The homogenization temperature (Th) test of fluid inclusion in rock salt is divided into two parts: single liquid inclusion and gas–liquid two-phase inclusion. The homogenization temperature of the primary single liquid phase inclusions can reflect the surface temperature of the brine during crystallization. The time of thermal event can be predicted by the homogenization temperature obtained from the test of gas-liquid two-phase inclusions. Because the repeated temperature-cooling test process may lead to certain errors in the results13 in order to ensure the accuracy of the test results, only one temperature measurement experiment was conducted for all samples.

Fig. 2
figure 2

Rock salt of well Wb1 from Tuyiluoke Formation (a), Primary fluid inclusions (b), secondary fluid inclusions (c), and histogram of the homogenization temperature with 6 samples and 31 data statistics (d).

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Results

Fluid inclusions are well developed in the halite of the Cretaceous Tuyiluoke Formation in well Wb1, the Yarkand Basin, and primary and secondary fluid inclusions are found under microscope. The fluid inclusions are single liquid and gas–liquid two phases. The gas-liquid ratio of the two-phase fluid inclusions is usually less than 5%, and the individual fluid inclusions can reach 8%. The primary fluid inclusions are mainly distributed in strips of light and dark, and individual fluid inclusions appear as negative crystals, square or rectangular, with sizes ranging from 2 to 70 μm (Fig. 2b,c). The secondary fluid inclusions are mostly irregular, without edges, and have large size changes, usually cutting through crystals, and most of them are gas-liquid two-phase, and the gas-liquid ratio changes greatly.

In this temperature measurement experiment, after screening 35 experimental samples, the pure liquid phase homogenization temperature of 31 data points of 6 samples was measured, among which the homogenization temperature of halite fluid inclusions of Wb1-6191.20 was between 9.1 ℃ and 20.3 ℃, with an average of 15.5 ℃. The homogenization temperature of halite fluid inclusions in Wb1-6193.25 was between 13.1 ℃ and 32.8 ℃, with an average of 25.0 ℃. The homogenization temperature of fluid inclusions in halite sample Wb1-6193.50 is 25.7 ℃. Similarly, in Wb1-6193.70 samples is 23.9 ℃, in Wb1-6194.35 sample was between 15.7 ℃ and 17.7 ℃, with an average of 16.7 ℃. The homogenization temperature of halite fluid inclusions of Wb1-6194.95 samples ranged from 13.5 ℃ to 35.0 ℃, with an average of 23.3 ℃ (Table 1). The minimum value of the overall homogenization temperature is 9.1 ℃, the maximum is 35 ℃, and the average is 22.7 ℃ (Table 1). The distribution trend of temperature data is shown in Fig. 2d.

Table 1 Homogenization temperatures of cumulate fluid inclusions of the Tuyiluoke formation in Yarkand Basin.
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Discussion

Rationality of data

Because the halite is easy to deliquesce and dissolve, it may easily change in the process of burial and preservation, which will affect the reliability and stability of Homogenization temperature (Th) data, especially for the halite samples in ancient salt lakes11. However, when the halite crystal precipitates in a shallow water environment, the temperature captured by the fluid inclusion is approximately Th, without pressure correction, and can directly reflect the temperature condition when the rock salt is deposited11. In this study, we rigorously verify whether the primary fluid inclusions are altered or destroyed by thermal rebalancing through two approaches. On the one hand, the consistency of Th data in a given fluid inclusion assemblage is analyzed. Goldstein and Reynolds33 pointed out that if the fluctuation range of about 90% Th data within a single fluid inclusion assemblage is less than 15 ℃, it can indicate that the primary fluid inclusions are not affected by obvious thermal rebalancing. The results of this paper conform to this standard (Table 1). The temperature range of all Th data of fluid inclusion assemblage samples measured is less than 15 ℃, with the minimum value of 2 ℃ and the maximum value of 13.2 ℃, which strongly proves that the rock salt crystal remains relatively stable in terms of thermal rebalancing and does not cause significant interference to Th data.

In addition, large halite inclusions are theoretically more malleable than small halite inclusions, and their Th value will be higher than the actual trapping temperature if stretched34,35,36. But the results of this study show that: There is no obvious correlation between the size of fluid inclusions and the corresponding Th (Fig. 3), which further supports the conclusion that thermal rebalancing has not caused substantial changes or damage to the halite crystals and Th data, and fully indicates that the Th data obtained by us can more accurately reflect the surface temperature and atmospheric temperature of the ancient salt lake brine during the Late Cretaceous Tuyiluoke Formation development.

Fig. 3
figure 3

Cross-plot of the Th data plotted against size of fluid inclusions.

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Temperature characteristics

Based on the mean and maximum homogenization temperature data of halite at different locations in the drilling profile (Fig. 4), the variation characteristics of brine temperature are preliminarily inferred. The results show that the brine temperature or atmospheric temperature has experienced a great fluctuation from the bottom of the drilling profile to the top, showing a cooling–warming–cooling trend on the whole (average homogenization temperature ranging from 23.3 ℃−16.7 ℃−23.9 ℃/25.7 ℃/26.7℃–15.5 ℃). There are two cycles of temperature change from high to low. The average temperature of the sample 6194.95 m at the bottom of the profile is 23.3 ℃, which represents the average temperature of the paleo-lake surface during this period (halite deposited period), but the maximum value of 35.0 ℃ may record the highest atmospheric temperature and paleo-brine temperature during this period. The average and maximum temperature of samples 6194.35 m were 16.7 ℃ and 17.7 ℃ respectively, showing a rapid downward trend. In contrast, temperature warming is revealed in the upper part (sample 6193.70 m, 6193.50 m and 6193.25 m), the average homogenization temperature rose to 26.7 ℃, and finally dropped to 15.5 ℃ in sample 6191.20 m, which reflects the periodic change of paleoenvironmental temperature during the salt lake developmental period. Due to the small thickness of the salt section (3.75 m in total), the temperature change will not be too long on the time scale, and can only reflect the change of the surface temperature of the brine in the ancient salt lake during the halite depositional period.

Fig. 4
figure 4

Temperature change trend indicated by Th from core Wb1.

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Comparison of ancient seawater temperature in Tethys domain

The comparison of the paleotemperature reflected by the fluid inclusions of halite with other regions of the world at the same time can provide a key clue for understanding the regional geological evolution and paleoclimate pattern. The average temperature of halite fluid inclusion in Mengyejing potash deposit in Yunnan Province, which is also a Late Cretaceous Tethys seawater deposit, is 23.8 ℃37which is similar to the average temperature value in this study. In addition, according to the study of Marine oyster fossils, the average temperature of paleo-seawater surface in the Eocene of Yarkand Basin is 27–28 ℃38. From the salt-forming belt, the evaporative series in Yarkand Basin mainly developed in the late Cretace-Paleocene, and the Kuqa Basin in the east of the basin, which has the same salt-forming material source, and later salt-forming age, Paleocene-Eocene. Xu Yang et al.15,16 recovered an average temperature of 31.8 ℃ from the early Eocene halite fluid inclusion in Kuqa Basin, and the homogenization temperature of the Middle Eocene halite fluid inclusion showed that the average temperature was 25.6 ℃. Early Eocene Marine mollusc assemblage in the Paris Basin of the Tethys Domain, was studied with high precision, with an average temperature of 31 ℃39. The Late Cretaceous-Early Eocene-Middle Eocene as a whole showed a process of first increasing and then decreasing, which is consistent with the global trend of paleoclimate and sea water temperature changes in the same period40.

Conclusions

In this study, the homogenization temperature data of primary fluid inclusions in the Upper Cretaceous Tuyiluoke Formation in Yarkand Basin can provide quantitative evidence of paleo-seawater temperature. The homogenization temperature ranges from 9.1 to 35.0 ℃, with an average of 22.7 ℃, which represents the brine temperature or the paleo-seawater temperature of the halite depositional period. From the bottom up, the brine surface or atmospheric temperature of the ancient salt lake experienced two periodic fluctuation cycles from high to low, and the maximum temperature difference of the average temperature was 10.2 ℃. Considering the small thickness of halite profile, the paleoenvironment will not be too long on the time scale. This temperature difference reflects the temperature change of ancient brine at that time, and is basically consistent with the temperature change range of the Cretaceous-Paleogene geological history interface period. During the Late Cretaceous-Early Eocene-Middle Eocene, along with the gradual intrusion of seawater from the northwest margin of the Tarim Basin, the data of halite fluid inclusions show that the paleo-seawater temperature as a whole increased first and then decreased, which is consistent with the global paleoclimate and seawater temperature change trend in the same period. These values provide new evidence for the ancient seawater temperature or paleoenvironment temperature recorded of the northwestern Tarim Basin during the late Cretaceous period.