The major composition of a middle–late Eocene salt lake in the Yunying depression of Jianghan Basin of Middle China based on analyses of fluid inclusions in halite

Journal of Asian Earth Sciences 85 (2014) 97–105

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The major composition of a middle–late Eocene salt lake in the Yunying depression of Jianghan Basin of Middle China based on analyses of fluid inclusions in halite Fan-Wei Meng a, A.R. Galamay b, Pei Ni c,⇑, Chun-He Yang d, Yin-Ping Li d, Qin-Gong Zhuo e a

State Key Laboratory of Paleobiology and Stratigraphy, Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing, China Institute of Geology and Geochemistry of Combustible Minerals, NAS of Ukraine, Lviv, Ukraine State Key Laboratory for Mineral Deposits Research, Institute of Geo-Fluid Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing, China d State Key Laboratory of Geo-mechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan, China e Research Institute of Petroleum Exploration and Development, CNPC, Beijing, China b c

a r t i c l e

i n f o

Article history: Received 30 May 2013 Received in revised form 22 January 2014 Accepted 26 January 2014 Available online 6 February 2014 Keywords: Salt lake Composition Fluid inclusion Halite Eocene Yunying depression

a b s t r a c t During the Cretaceous–Tertiary transition in eastern China, abundant halites formed in non-marine areas. Many continental salt deposits from inland salt lakes were formed in eastern China in faulted basins as a result of the northward movement and collision of the Indo-China Plate with the Eurasian Plate, including the Bohai Gulf Basin. However, a marine transgression versus a non-marine origin of these evaporites remains to be determined. Primary fluid inclusions trapped in halite deposits can directly record the composition of evaporated seawater or salt lake water, such as those in the Cretaceous halite in the Khorat Plateau (Laos and Thailand) area can resolve the origins of the evaporate deposits; recent fluid inclusions data in the Khorat Plateau coincide with the predicted secular variation of seawater and are comparable to other fluid inclusions in Cretaceous marine halite, indicating these fluid inclusions are directly related to a marine transgression. Our analyses in this study shows that the average K+, Mg2+, contents are 8.8, 5.0, and 6.8 g/l, respectively, in the primary fluid inclusions in halite of and SO2 4 middle–late Eocene from the Yunying depression of China. These numbers are much less than those in the contemporary Spanish primary fluid inclusions in halite precipitated from seawater (16.4, 36.3, and 12.5 g/l for K+, Mg2+, and SO2 4 , respectively). Furthermore, Br contents of all fluid inclusion samples in halite from the Yunying depression are lower than 2 ppm (vs. 55–58 ppm at the base of Spanish contemporary marine halite), and their d37Cl values range from 0.11‰ to +2.94‰ (vs. 0.09‰ to 0.24‰ in sylvite of Spanish deposit), indicating that the compositions of the middle–late Eocene brines trapped in halite in the Yunying depression of China are very different from those derived from the contemporary seawater, and are considered to be resulted from evaporation of an inland saline lake water with little influence of seawater. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Fluid inclusions in halite can record the composition of evaporated seawater or salt lake water directly (Kovalevich et al., 1998; Timofeeff et al., 2001; Lowenstein et al., 2001; Meng et al., 2011a). Published data on fluid inclusions in halite have been largely focused on marine halite (Ayora et al., 1994; Kovalevich et al., 1998; Khmelevska et al., 2000; Lowenstein et al., 2001; Brennan and Lowenstein, 2002; Galamay et al., 2003; Kovalevych et al., 2006), although there are some published examples from ⇑ Corresponding author. Tel.: +86 25 83597124; fax: +86 25 83592393. E-mail address: [email protected] (P. Ni). http://dx.doi.org/10.1016/j.jseaes.2014.01.024 1367-9120/Ó 2014 Elsevier Ltd. All rights reserved.

ancient non-marine salt lakes (Lowenstein et al., 1994; Benison et al., 1998; Timofeeff et al., 2001; Benison, 1996, 2013). In most of mainland China, the seawater withdrew after the Triassic Period, and the restricted Cretaceous–Tertiary continental basins became the local areas for salt deposits (Yuan, 1963, 1980, 1982; Wei, 1999; Wang, 2007) as well as oil and gas deposits in China (Fu et al., 1986; Philp and Fan, 1987; Grice et al., 1998). The Pacific plate and Paleo-Tethyan oceanic crust initially subducted and extruded to the Eurasian Plate, followed by the Indo-China Plate moving northward to collide with and fuse with the Eurasian Plate. These tectonic activities (largely the Yanshan and Himalayan movements) resulted in many folds, uplifts, and faults that produced a number of faulted basins in mainland China

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during Cretaceous–Tertiary (Yuan, 1963, 1980, 1982; Wei, 1999; Wang, 2007). The waters in many faulted basins became saline over time, leading to the formation of deposits of salt, crude oils, and gases (Fu et al., 1986; Philp and Fan, 1987; Grice et al., 1998) (Fig. 1). The known oil-fields on land in China are mostly derived from ancient non-marine sediments, including those in some ancient salt lakes (Fu et al., 1986; Fu and Sheng, 1989; Meng et al., 2012). It is likely that some non-marine salt lakes in China were affected by marine transgressions, which are referred to as ‘inland salt lakes with seawater input’. Evaporites are closely related to the occurrences of crude oil and gases (Fig. 1), and for this reason Cretaceous–Tertiary halites were widely studied in China during oil and gas exploration. However, it is still unknown whether marine transgression either happened during Cretaceous–Tertiary in the entire eastern China or only affected restricted inland salt lakes, including Bohai Gulf Basin, eastern North China Basin, and Jianghan Basin. Paleontological and geochemical evidence is ambiguous and paradoxical (Zhu, 1979; Chen and Wu, 1979; Sun et al., 2002). Carbonate reefs in Eocene Shahejie Formation of the Shengli Oilfield of Bohai Gulf Basin mainly consist of Cladosiphonia fossils (calcareous algae, classified into Derbesia, Siphonales, Chlorophyts), which Zhu (1979) suggested as evidence of marine transgression in eastern China, because the modern Siphonales are mostly marine (Bessey, 1907). Serpulidae fossils were also found in similar strata in the Shengli Oilfield of Bohai Gulf Basin as an evidence of marine transgression (Zhu, 1979; Chen and Wu, 1979). Serpulidae is a family of sessile, tube-building annelid worms in the class Polychaeta. There are about 300 known extant species in the Serpulidae family, and all but one of them live in saline waters (occasionally found at the freshwater end of estuaries) (Williams, 1980). Calcareous nannoplankton, unicellular photosynthetic protists with coccolith calcium carbonate platy shells, were also found in the Shengli Oilfield of Bohai Gulf Basin as evidence of marine

transgression (Hao and Li, 1984). Calcareous nannoplankton are predominantly found in tropical marine environments, but a few do live in brackish water and even in fresh water (Brasier, 1980). Calcareous nannoplankton can also be found in Cenozoic-Recent sediments of terrestrial salinized lakes in the northwest and northeast China, far removed from the ocean (Sun et al., 2002). The strontium contained in carbonate shells of marine organisms records Sr isotope data (the 87Sr/86Sr ratio) of the oceans at the time the shells were formed (DePaolo and Ingram, 1985). The 87Sr/86Sr of calcareous nannoplankton in Oligocene Shahejie Formation No. 1 Section of the Shengli Oilfield is 0.71146 on average, higher than those from Oligocene seawater (Sr87/Sr86: 0.7076–0.7084). The 87Sr/86Sr of calcareous nannoplankton in Eocene Shahejie Formation No. 4 Section in this area is 0.7115 on average, also different from Eocene seawater 87Sr/86Sr (Liu et al., 2002). Cretaceous evaporites of the Maha Sarakham Formation which occur on the Khorat Plateau of Thailand and Laos have three depositional cycles (Lower, Middle, and Upper Members), and each cycle contains an evaporite sequence capped by non-marine siliciclastic redbeds. Although the marine versus non-marine origin of the Maha Sarakham Formation is debatable, sulfur isotopes of anhydrites of the Maha Sarakham evaporites have Cretaceous seawater values ðd34 S ¼ 14:3  17:7‰Þ, whereas sulfur isotopes in the anhydrites of the redbeds do not have Cretaceous seawater values ðd34 S ¼ 6:4  10:9‰ÞÞ. Br content and the gradual increasing trend in halites from the base to the surface (from 40–60 to 200– 300 ppm) also indicate a marine origin. Fluid inclusions of halite can provide the direct evidence of marine or non-marine origins of halite (Timofeeff et al., 2006). Fluid inclusions in Cretaceous halite of the Maha Sarakham also coincide with the calculated chemical composition of Cretaceous evaporated seawater and similar to those in other Cretaceous evaporites basins, indicating the marine origin of the Maha Sarakham evaporite Formation. Were there marine transgressions in eastern China? If yes, how extensive were the transgressions that affect the salt lakes? Until

Fig. 1. Oil and gas basins in the mainland of China and the position of Jianghan Basin (after Li and Lv, 2002).

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now there were no published reports on the chemistry of these ancient salt lakes. In this report, we used fluid inclusions in halite in the Yunying depression of Jianghan Basin to reveal the salt lake water composition of southeast China in order to understand brine evolution. The studied halite deposits were derived from the Yunying depression of Jianghan Basin, southeast China, where the Jianghan Oilfield is located because Jianghan Basin was concluded to be affected by a transgression but without the geochemical data, somebody thought Jianghan Basin is a seaway of marine transgressions in eastern China (Fig 1). Cl isotope of reported marine halites changed between narrow range of near to 0‰ (0.0 ± 0.9‰) (Eastoe et al., 2007), so we try to use Cl isotope of halite to understand the origin of halites. Br values in halite are taken to judge if halite is formed in marine or nonmarine for long time. Br content in halite below 30 ppm represents either non-marine deposition of the halite or the leaching or dilution of bromine in halite after marine deposition (Holser, 1966). In this study, we use primary fluid inclusions in halite with the data of Cl isotope, Br content and B content together to judge if there are a transgression in halite of the Yunying depression in Jianghan Basin. 2. Geological setting The greater inland Jianghan Basin consists of eleven depressions and five structurally positive regions that were formed during the late Cretaceous–early Tertiary (Zhang et al., 2003; Meng et al., 2013). The eleven depressions are the Yunying, Xiaoban, Mianyang, Qianjiang, Jiangling, Zhijiang, Chentuokou, Yuanan, Herong, Jingmen, and Jiangshui depressions. The five structurally positive regions are the Longsaihu Low Uplift, the Yuekou Low Uplift, the Chenhu Low Uplift, the Tonghaikou Uplift and the Yajiao-xingou Low Uplift. Following the accumulation of siliciclastics, some of the individual basins (the Yunying, Jiangling and Qianjiang depressions) accumulated evaporite sediments. The depressions

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are separated by structurally positive regions that began to subside during the late Cretaceous and accumulated a substantially thick layer of sediments, including evaporites (Fig. 2). The Yunying depression is located in the northeastern Jianghan Basin of mid-China, and is an inland salt lake of faulted basin formed during the late Cretaceous–Paleogene (Zhang et al., 2003; Yang et al., 2006). The Yunying Salt Mine, a famous salt and anhydrite mine, is located at Yingcheng City in the middle of the depression, and the main salt and anhydrite deposits come from the Gaoyan Formation. Pollen fossil correlations indicate that the Gaoyan Formation dates to the middle–late Eocene (Tong, 1989). The stratigraphy of Yunying Salt Mine is correlated with the Gaoyan Formation (up to 1598 m in thickness), which conformably contacts the underlying Baishakou Formation (Yang et al., 2006) (Table 1). The sequences of Gaoyan Formation are interbedded with grayish green mudstone, fuchsia sandy mudstone and siltstone with abundant anhydrite, gypsum, glauberite, and halite. According to sedimentary and lithological characteristics, the Gaoyan Formation is divided into 5 sections from the base to the top: lower anhydrite section; lower glauberite section; halite section; upper glauberite section; and upper anhydrite section. These cycle sets contain 7 sub-cycles, with each sub-cycle bounded with regionally-widespread gray and red mudstones/siltstones. These sub-cycles can be further subdivided into 83 elementary cycles, usually containing calcium sulfate at the base (representing the desalination layer), and coated with sodium chloride at the top (desiccation layer) (Meng et al., 2013). Our samples come from the halite section. We collected eight samples from drill cores (Fig. 3), and every halite sample is adjacent to bedded anhydrite/gypsum (i.e., the base of the halite during precipitation). 3. Petrography Three types of fluid inclusions in halite can be distinguished according to crystal origins and fluid inclusion distributions

Fig. 2. The map of the Yunying depression in Jianghan Basin (after Li and Lv, 2002).

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Table 1 The stratigraphy of the Yunying depression of Jianghan Basin in China (after Meng et al., 2013). Geological time

Formation

Thickness (m)

Lithology

Quaternary

Pingyuan Formation

11–153

Neogene

Duodaoshi Formation Wenfengta Formation

0–50

Pleistocene series include yellow, red and gray-white clay soil layer, sand, sandy gravel and clay gravel layer. Holocene Series is light yellow, shaded brown and gray sandy clay. These quaternary sediments unconformable contact the underly Formation Gray and yellow mudstone and clay rocks with basal yellow brown Conglomerate. Basal contact is unconformable

Paleogene

Cretaceous

3–480

Gaoyan Formation

394–1598

Baishakou Formation Yuntaishan Formation Gonganzhai Formation

694–920 727 >350

Lower part is grayish green and ocher calcareous mudstone and calcareous siltstone containing striate gypsum and ostracods; Upper part is light green, ash gray and ochre marl interbedded with siltstone. The Formation conformably contact underlying Formation According to sedimentary and lithological characteristics, the Formation can be divided as five sections including Lower anhydrite section, Lower glauberite section, Halite section, Upper glauberite section, Upper anhydrite section. The Formation conformably contact underlying Formation Lower part is red sandstone, pebbled sandstone; Upper part is red siltstone, muddy siltstone interbedded with thin striate gypsum. The Formation conformably contact underlying Formation The Formation includes rhythmic red sandstone, muddy siltstone, sandy shale and conglomerate. The Formation conformably contact underlying Formation Red middle-thick siltstone, muddy siltstone, sandy mudstone interbedded with pebbled sandstone. Muddy siltstone contains chalk

Fig. 3. The drilling cores of halite from the Yunying depression (halite with anhydrite, from YC-1 to YC-8).

(Kovalevich et al., 1998). The first type is primary fluid inclusions with alternating fluid-inclusion-rich and fluid-inclusion-poor bands in chevron or cumulate primary halite. Primary halite crystals include cumulate crystals (formed at the air-brine interface) and chevron crystals in sedimentary surrounding (Roberts and Spencer, 1995; Benison and Goldstein, 1999; Lowenstein et al., 1998; Meng et al., 2011b, 2013). These primary fluid inclusions can directly record the composition of seawater or salt lake water (Fig. 4). The second type is represented by large fluid inclusions in transparent and recrystallized parts at the edge of the primarybedded halite. These are relics of primary fluid inclusions in chevron or cumulate halite (Kovalevich et al., 1998). The composition of fluid inclusions in transparent parts in the primary-bedded halite crystals is very close to the primary fluid inclusions preserved in the same crystals (Fig. 5). They should be formed in syndepositional or early diagenetic process (Fig. 5). However, sometimes the composition of fluid inclusions in transparent parts is a little different from the primary fluid inclusions in the same crystal. The third type of halite is transparent and recrystallized halite crystals with large (often 250 lm) inclusions (Fig. 6, Samples from Pakistan), which formed during different postsedimentary stages (Roedder,

1984; Kovalevich et al., 1998). In halite samples of the Yunying depression, there are only fluid inclusions of the first and second type. All halite samples in the Yunying depression in this study have abundant and well-preserved primary fluid inclusions in chevron and cumulate crystals, and the outer parts of these crystals also have some large fluid inclusions, so these bedded halites are primary rather than recrystallized during different postsedimentary stages. Chevron and growth bands are composed of cubic, negative crystal-shaped primary fluid inclusions. Each inclusion-rich chevron or cumulate growth band is considered an individual fluid inclusion assemblage. There is a wide range of inclusion sizes, but most are between 3 and 50 lm in diameter, with some as large as or greater than 200 lm. Whether these large cubic fluid inclusions are primary fluid inclusions formed during evaporation process or secondary fluid inclusions formed during diagenesis is difficult to judge via petrography, but can be determined from compositions of fluid inclusions (Fig. 5). The majority of primary fluid inclusions are one-phase-liquid inclusions at room temperature. Some contain crystals trapped during halite crystal growth, and most of these ‘‘accidental’’ crystals are anhydrite needles. They

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Fig. 4. Photomicrographs showing primary fluid inclusion banding in chevron halite (Halite from the Yunying depression sample YC-3).

Fig. 5. Photomicrographs showing large fluid inclusion at the edge of the fluid inclusion bands of the transparent halite part (Halite from the Yunying depression sample YC3).

Fig. 6. Photomicrographs showing large secondary fluid inclusions in Precambrian recrystallized halite of Salt Range Formation of Pakistan.

are interpreted to be ‘‘accidental’’ because they occur in different volume ratios and do not decrease in size upon heating, as true daughter crystals would. Very few contain gas–liquid inclusions. The presence of the gas phase probably was resulted from stretching due to salt overheating at some post-sedimentary stage, because the gas–liquid inclusions tend to be large in size. However, it is also possible that the gas–liquid fluid inclusions in halite capture ancient air at the surface of brine during the salt precipitation. 4. Methods The analyses of individual brine inclusions were carried out by the ultramicrochemical method (or the method of glass capillaries) as described in Petrichenko (1973). The method can determine the content of major ions (K+, Mg2+, Ca2+, and SO2 4 ) in brine inclusions (except for Na+ and Cl) with an error of ca. 20%. The halite is dissolved in a thin jet of water within a few tens of micrometers of the inclusion walls. After the halite crystal is dried, the inclusion is opened. This method can examine fluid inclusions

>20 lm (Kovalevich et al., 1998). The inclusion fluid is extracted with a conical capillary tube (3–5 lm in diameter at the sharp end of the cone), and reagents are then added to determine the solutes in the inclusion fluid, based on the recommendations of Korenman (1955): 30% solution of BaCl2 for the determination of SO4 ion, 5% solution of (NH4)2C2O4 for Ca ion, 3% solution of Na3Co(NO2)6/(1/2)H2O for K ion, and one part of a 30% solution of urotropine and one part of a 15% solution of K4[Fe(CN)6]3H2O for Mg ion (assuming that its content is <6 g/L) or 15% solution of (NH4)2C2O4 (for higher Mg content). The reagent is added until the process of precipitation is completed according to the following reactions (Korenman, 1955):

SO24  þBaCl2 ! BaSO4 # Ca2þ þ 5% solution of ðNH4 ÞC2 O4 ! CaC2 O4  3H2 O # Kþ þ 3% solution of Na3 CoðNO2 Þ6  1=2H2 O ! K2 Na½CoðNO2 Þ6 # Mg2þ þ 30% solution of urotropine þ 15% solution of K4  ½FeðCNÞ6 3H2 O ! 2MgCrO4  3C6 H12 N4  15H2 O #

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Table 2 The major composition of fluid inclusions in middle–late Eocene halite of the Yunying depression. Evaporite basin

Content, g/l solution K

Marine Eocene Navarra in Spain (Eoc  38–52 Ma) Jianghan, China (Eoc2–3  38–52 Ma) (primary fluid inclusions formed during sedimentary process) Jianghan, China (Eoc2–3  38 Ma) (secondary fluid inclusions formed during early diagenesis) Modern seawater saturated to the beginning of precipitation of halite Modern seawater saturated to the beginning of precipitation of epsomite

+

Janecke units, % Mg

2+

SO2 4

Literature

2K

Mg

SO4

16.4 8.9; 7.9; 9.7 (8.8)

36.3 5.3; 4.7 (5.0)

12.5 6.4; 7.2 (6.8)

11.4 30.0

81.5 53.0

7.1 18.0

Ayora et al. (1994) Our data from YC-3 sample

3.3; 3.3; 3.1 (3.2)

1.8; 2.1 (2.0)

3.1; 3.5 (3.3)

25.5

52.5

22.0

Our data from YC-3 sample

3.9

12.6

17.6

6.6

69.0

24.3

McCaffrey et al. (1987)

26.1

85.9

115.0

6.6

69.8

23.6

McCaffrey et al. (1987)

Fig. 7. The results of fluid inclusion in the Eocene non-marine halite (our data) with modern seawater (McCaffrey et al., 1987), marine Eocene halite (after Ayora et al., 1994) on a Jänecke diagram (after Valyashko, 1962).

The capillary is then sealed and centrifuged. To determine each element, separate aliquots of solution are used for each titration followed with centrifugation, and the amount of the reagent added is visually controlled under the microscope by the moment of termination of chemical reaction. The amount of reagent added depends on the concentration of the element studied, usually double to triple the volume of the original inclusion fluid; it always exceeds the amount of the element to be precipitated. The volumes of precipitate and solution formed during this process are measured and then compared with those formed from a standard solution, according to formulas published by Petrichenko (1973, Table 4). To decrease the error of determination, a number of analyses of each component in inclusion brines of each sample were carried out; two to three parallel analyses decrease the error rate to 16–17% (see Petrichenko, 1973, Table 4). The minimum quantity of the studied ions needed for such an error rate is (in g/l) 0.8 for K+, 1.0 for Mg2+, 0.9 for Ca2+, and 0.5 for the sulfate ion, and the smaller values are not very precise. This method has been used to analyze ancient seawater chemistry from fluid inclusions in halite throughout geologic time (Kovalevich et al., 1998; Khmelevska et al., 2000; Galamay et al., 2003; Petrychenko and Peryt, 2004; Kovalevych et al., 2002, 2006, 2009). The inclusions close to 50–100 lm were used for analyses. The ultra-microchemical analysis of individual brine inclusions was performed in the Institute of Geology and Geochemistry of Combustible Minerals, NAS of Ukraine. We chose one sample of YC-3 halite with abundant primary fluid inclusions to study the chemistry of salt lakes (Fig. 3). The Br, Cl, and B contents, as well as the Cl isotope of eight halite samples, were analyzed in the Chemical Analysis Department of Institute of Salt Lakes, Testing Center of Lanzhou Branch, Chinese Academy of Sciences. The analytical methods utilized in this study

follow those of Tan et al. (2006). Br and B contents were measured by absorption photometers. The Cl content of the halite was determined by the AgNO3 titration method. The precision of the Br and Cl analysis was about 1%. Pure halite crystals selected under a microscope were dissolved in high-purity water produced by sub-boiling distillation. This solution passed through a Ba-resin column to remove interfering SO2 4 , followed by passing through an H-resin column to remove cations and convert the Cl into HCl. At last, the pure HCl solution passed through a Cs-resin to produce a CsCl solution, which was then used for the AVG 354 thermal mass spectrometry analysis (Xiao and Zhang, 1992; Tan et al., 2006).

5. Results Different parts of the drill core YC-3 were investigated to confirm the presence of marine or non-marine evaporites. Fluid inclusion bands were formed by many very small inclusions, from 1–2 lm to 40–50 lm, and inclusions of up to 100 lm were also found. Cubic inclusions of large sizes, ranging from 200 to 1300 lm, were scattered randomly in the zonal fluid inclusion bands and beyond, and most likely are not primary, but rather post-sedimentary. When we analyze these fluid inclusions of large size beyond fluid inclusion bands, we find that their geochemistry is similar to fluid inclusions in bands, so these fluid inclusions can reflect the compositions of brines during deposition. However some fluid inclusions outside the bands of fluid inclusions show a different composition, indicating these fluid inclusions are secondary fluid inclusions formed during post-sedimentary processes. The results of our analyses are shown in Table 2 and Fig. 7. To present the composition of fluid inclusions in Jänecke units, the g/l values in Table 2 were converted into molar (mol/l)

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concentrations, and then the fraction (expressed in terms of a percentage) of every component was established based on the assumption that the sum of K2, Mg and SO4 is 100% (Kovalevych et al., 2002). The Br contents of all samples in this study are lower than 2 ppm, and their d37Cl values range from 0.11‰ to +2.94‰. 6. Discussion The Bohai Gulf Basin is located in North China, in an area of salt deposits that is proximal to input from the Pacific Ocean, and thus influence of marine transgression to the salt lakes in this region is possible (Ren et al., 2000). However, a so-called ‘‘marine transgression seaway’’ was not found. Previous studies have concluded that probable marine transgressions are dual; in one case with seawater coming from the Jianghan Basin and Nanxiang Basin into the huge Bohai Gulf Basin (Wu and Ren, 2004), and in the other with seawater coming from north to south China into the Dongpu depression near the Yunying depression of Jianghan Basin (Li, 1986). 6.1. Major composition of brine As shown in Table 2, brine in Jianghan Basin is sulfate-rich and is different from the Eocene seawater with a high content of potassium and sulfate ions. Average K+, Mg2+, and SO2 4 contents in the primary fluid inclusions of middle–late Eocene non-marine halite from the Yunying depression of China are 8.8, 5.0, and 6.8 g/l, respectively; however the average K+, Mg2+, and SO2 4 contents in the primary fluid inclusions of Spanish marine halite precipitated at the same period of time are 16.4, 36.3, and 12.5 g/l, respectively. On a Jänecke diagram (Fig. 7), one can compare the compositions of brines (the ratio between the ions) of different seawater/salt lakes water, regardless of the degree of evaporation of the brine (Kovalevych et al., 2002). Normally, the process of continental salinization is the main source of salt-bearing rock. The process of salt accumulation can be represented as continuous leaching and removal of salts from the continent, leading to their accumulation in the drainage basin. The seawater chemistry over geologic time changes significantly between the Na–K–Mg–Ca–Cl (Ca-rich) type and the Na–K–Mg–Cl–SO4 (SO4-rich) type according to primary fluid inclusions. The seawater during Eocene is also the Na–K– Mg–Cl–SO4 (SO4-rich) type. The composition of Eocene salt lake in Yunying depression fits with the Na–K–Mg–Cl–SO4 (SO4-rich) type but with much lower K+, Mg2+, and SO2 contents. Conse4 quently, the composition of fluid inclusions in halite indicates the Yunying depression is mainly affected by inland drainage, rather than marine transgression. 6.2. Br contents The Br content of normal marine-derived halite ranges from 50 to 100 ppm, and will increase to a maximum value of 270 ppm at the onset of bitter salt precipitation (Valyashko, 1956). The Br contents of all samples in this study are lower than 2 ppm (Table 3), Table 3 Chlorine isotope and chlorine, bromine, and boron contents of middle–late Eocene halite from the Yunying depression. Samples

d37Cl

Error SD (n = 3)

Cl (%)

Br (ppm)

B2O3 (%)

YC-1 YC-2 YC-3 YC-4 YC-5 YC-6 YC-7 YC-8

1.70 2.49 1.82 0.89 2.04 0.11 0.14 2.09

0.03 0.01 0.10 0.14 0.11 0.14 0.22 0.16

60.31 60.23 60.26 60.41 58.87 60.36 60.20 60.25

<2 <2 <2 <2 <2 <2 <2 <2

<0.0006 <0.0006 0.0023 <0.0006 <0.0006 <0.0006 0.0009 <0.0006

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thus indicating that they do not have a marine origin (McCaffrey et al., 1987; Holser, 1979). 6.3. Cl isotopes Halite crystallizing and separating from 0‰ of a NaCl solution has an initial d37Cl value of 0.2–0.3‰ at 22 ± 2 °C (Eggenkamp et al., 1995), and halite formed from seawater by fractional crystallization has a range of 0.2‰ (at the start of halite precipitation) to 0.5‰ (at the appearance of hexahydrite). Halite precipitated from seawater should have a range of 0.4‰ to 0.6‰ (considering error) (Eastoe et al., 1999; Eastoe and Peryt, 1999). The d37Cl values of halite from the Yunying depression are between 0.11‰ and +2.49‰ (Table 2) (vs. 0.09‰ to 0.24‰ in sylvite of Spanish deposit, Sun et al., 1998); these high d37Cl values of the halites imply they are not formed from seawater. The samples of both YC-1 and YC-3 have fluid inclusions with alternating fluid-inclusion-rich and fluid-inclusion-poor bands, which is interpreted to suggest that fluid inclusion bands formed during day and night cycles indicating that the samples of YC-1 and YC-3 are primary halites formed in shallow water without recycling and recrystallization. However, the d37Cl of YC-1 and YC-3 are 1.7‰ and 1.82‰ which are much higher than those in halite formed from seawater (between 0.4‰ and 0.5‰). High positive d37Cl values require a large input of chloride with positive d37Cl into the basin (Eastoe and Peryt, 1999). Chlorine isotope geochemical analyses were performed by Liu et al. (1997) with samples from different origins: salt lake brine coexisting with halite, oil-field water and river water from Qaidam Basin (Qinghai, China). The d37Cl values range from 2.05‰ to +2.94‰, with average values of +2.94‰ for hot spring water, +1.35‰ for river water, 0.38‰ for oil-field brine, 0.40‰ for saline water, and 0.65‰ for salt lake brine. The fractionation of chlorine during salt precipitation can account for the negative d37Cl values of salt lake brine (Liu et al., 1997). Hot spring water has the highest d37Cl value, and river water also has a high d37Cl values, ranging from +0.77‰ to +2.22‰ (Liu et al., 1997). The high positive d37Cl value of halite in the Yunying depression (above +0.4‰) should come from the input of ancient rivers during this time or/and hydrothermal fluids. Although dissolution and reprecipitation can also explain the high d37Cl value of halite; the halite that has fluid inclusions with obvious alternating fluid-inclusion-rich and fluidinclusion-poor bands (e.g., YC-1 and YC-3) also have high d37Cl values (above +0.4‰) (Fig. 4), indicating that the primary salt lake brine should have a high d37Cl value. The main evaporite section of the Gaoyan Formation is interbedded with mudstone with abundant evaporites, including anhydrite, gypsum, glauberite, and halite; thus, when these evaporites are formed there should be frequent river or flood inputs with siliciclastic sediments. Some halite formed in the salt lake probably dissolved and re-precipitated, leading to halite with an even higher d37Cl value (e.g., YC-2 without obvious fluid-inclusion-rich and fluid-inclusion-poor bands). The Cl isotope composition of Yunying Salt Mine is different from the limited data of the Shengli Oilfield and Jintan Salt Mine, whose Cl isotope composition are near 0‰ (unpublished data). These differences indicate that the salt lakes are isolated, and that the composition of ancient salt lake water from the Yunying depression is mainly derived from inland drainage. 6.4. B content Natural B was derived from geothermal discharges, leaching from a large variety of rocks, or the mixing of groundwater with oil field water or connate or fossil brines (Gemici et al., 2008). The B (boron) content of global river system is generally very low, about 0.01 ppm (Livingstone, 1963), the seawater has higher

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boron content with an average of around 4.5 ppm. The boron concentrations of the minerals and residual brines in the partial evaporation experiments indicate distribution coefficient is lower (0.0066–0.031) than those of absorbed clay (0.75–2.85) (Palmer et al., 1987), therefore boron is difficult to be adsorbed into halite, and instead mainly exists in fluid inclusions in halite (Liu et al., 2000). Our halite samples with abundant fluid inclusions have B2O3 concentrations below 6 ppm, and B (boron) content in our halite samples are below 1.8 ppm (Table 3). Boron contents in marine halite range from 0 to 50 ppm (Stewart, 1963), so it is difficult to judge if the halites were affected by seawater input only based on the information on boron contents. However, all 8 samples have low boron contents implying that the halite samples had no obvious seawater impacts. 6.5. Palaeontological evidence Dinoflagellate cyst fossils have 34 genera and more than 80 species in the Dongpu depression, 17 genera and 27 species in the Qianjiang depression of Jianghan Basin. While, in normal marine sediments, there are more than 50 genera and over 120 species, indicating a lack of influence from marine transgression in these areas (Sun et al., 1997). However, Ji et al. (2011) only found only a single species of dinoflagellate cyst from Eocene saline lacustrine sediments of Qaidam basin in Chinese hinderland. The Qaidam basin saline lake is related to the aridity of northwest China, and not related to marine transgression. Qinghai Lake located in northwest China is the largest salt water lake in China today, and represents a good analog as Qaidam Basin’s ancient salt lake. In modern Qinghai Lake, there is only a single species of dinoflagellate; thus, according to the numbers of genera and species of dinoflagellate fossils, we can conclude that seawater must have influenced Cretaceous– Tertiary salt lakes in east China to certain extent, even though their water compositions were largely controlled by inland drainage. 7. Conclusions During Eocene, many oilfields were formed with bedded halite in ancient saline lakes (e.g., Bohai Gulf Basin of China, the biggest oil and gas basin in China; The Green River Formation of USA, the largest oil shale deposits in the world) (Fang, 2006; Tissot and Deroo, 1978; Higley, 1983; Mason and Surdam, 1992; Tuttle and Goldhaber, 1993). The water chemistry however of these ancient saline lakes is not known. Previous speculations take transgression has two possible seaways, in one case with seawater coming from the Jianghan Basin into the huge Bohai Gulf Basin (Wu and Ren, 2004), and in the second case with seawater coming from North to South China into the Dongpu depression (Li, 1986). Our data indicates that the salt lake water composition in the Yunying depression of Jianghan Basin, China, is very different from Eocene seawater. Our data indicates that the composition of these salt lakes water are mainly affected by inland drainage, while seawater probably affected the composition of these ancient salt lakes to a certain extent according to dinoflagellate cyst fossils. Acknowledgments This investigation was supported by Major State Basic Research Development Program of China (973 Program) (No. 2011CB403007), China Postdoctoral Science Foundation (2007), National Natural Science Foundation of China (40703018, 41173051, and 41172131) and Bureau of International Co-operation, Chinese Academy of Sciences. We are very grateful to Dr. James D. Schiffbauer of Department of Geological Sciences, the University of Missouri for his helpful review and comments.

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