Sedimentary Environment Analysis of Eocene in Pingtai Area of Qaidam Basin, China

The Pingtai area is a relatively new oil and gas exploration area in Qaidam Basin, China. As one of the most important sandstone reservoir in the Qaidam Basin, Eocene Lulehe Formation and Xiaganchaigou Formation. Based on the petrological characteristics and element geochemistry of core samples, the paleosalinity and paleoclimate changes are discussed. The results show that the clay mineral assemblage of the Lulehe Formation consists of smectite, chlorite, and illite; while the clay minerals in the Xiaganchaigou Formation consist of illite/smectite mixed layer, chlorite, and illite. The paleosalinity calculated using the B content and Sr/Ba and Rb/Sr ratios indicates that the Lulehe Formation was formed in a freshwater environment, and the Xiaganchaigou Formation was formed in alternating brackish water and fresh water environments. The chemical weathering index (CIA), La/Th ratio, and Eu anomaly index indicate that the overall chemical weathering in the Pingtai area was weak, the provenance was relatively stable, and the inuence of diagenesis on the clay minerals and the trace element contents was negligible. From the Lulehe Formation to the Xiaganchaigou Formation, the paleoclimate gradually changed from warm and humid to cold with humid-dry seasonal changes, which is consistent with the global decrease in temperature in the Eocene. Moreover, because of the uplift of the Altun Mountain and the global cooling, the rainfall decreased, resulting in the Xiaganchaigou Formation being formed in a higher salinity environment. studies when clay minerals are used to analyze paleoclimates, the results are consistent with those obtained using other Furthermore, the wide distribution of clay minerals and the ease of sampling make clay minerals a particularly effective method for paleoclimate research(Jain In this study, we will study the paleoclimate change during the sedimentary through the paleosalinity value calculated by clay mineral assemblage and boron element.


Introduction
In recent years, with the development of geochemistry and sedimentology, the variations of paleosalinity and paleoclimate in different geological periods can be recognized through multiple geochemical parameters of sediments and clay minerals (Wei and Algeo 2019; Ye et al. 2016). Paleosalinity, which refers to the salinity of ancient water recorded in Paleosediments, can be used as an important indicator to analyze the characteristics of sedimentary environment in geological history (Degens et al. 1957;Harder 1970). The recovery of paleosalinity is an important way to reconstruction of paleoenvironments and further investigate the process and mechanism of environmental changes. The conventional paleosalinity reconstruction methods mainly include (Jiang et al. 2011; Kai et al. 1982;Walker and Price 1963;Walker 1968;Zheng and Liu 1999): qualitative analysis using fossil, mineralogy, and some geochemical parameters of sediments; quantitative calculation based on geochemical experiments; analysis and test on pore uid and liquid inclusion. At present, geochemical analysis is the most commonly used method in the study of paleosalinity. The element decomposition and migration processes are affected with respect to the environmental conditions and element types, which results in the fact that element concentration varies in different environmental conditions. Therefore, the variation in content of certain elements in sediments can re ect the change of paleosalinity in the depositional period, e.g., the content of B and Na, the ratio of Ba/Ca and B/Ga, as well as the carbon and oxygen isotopes (Couch 1971; Rohling and Eelco 2007;Seward 1978;Veizer et al. 1977).
Boron content is a very common parameter in paleosalinity and sedimentary environment study. Boron exists in seawater mainly as B(OH) 3  With the emphasis on global climate change research, the method of using clay minerals as a means of paleoclimate reconstruction has attracted the attention of scholars from worldwide. This method has been applied in numerous studies, such as studies of glacial polar regions (Zhao et al. 2005), plateau weathering products (Ding et al. 1998), lakes (Dera et al. 2008), and marine sediments (Cai et al. 2020). Clay minerals are widely distributed in various types of sediments, and their genesis is affected by many factors such as the source area lithology, mode of transport, paleoclimate, sedimentary environment, and diagenetic processes (Abdullayev and Leroy 2016;Fagel 2007;Fagel et al. 2003;Gylesjö and Arnold 2006;Hong et al. 2017; N. and D. 2020; Xu et al. 2016).Current studies have found when clay minerals are used to analyze paleoclimates, the results are consistent with those obtained using other methods (Gao et al. 2013;Liu et al. 2003). Furthermore, the wide distribution of clay minerals and the ease of sampling make clay minerals a particularly effective method for paleoclimate research (Jain and Tandon 2003). In this study, we will study the paleoclimate change during the sedimentary period through the paleosalinity value calculated by clay mineral assemblage and boron element.

Samples And Methods
The experimental samples mainly come from the drilling cores from well Ping 2 in the northern margin of the Qaidam Basin. The focus of the sample collection was to collect continuous whole-rock samples from the pro le of the Xiaganchaigou Formation and from the entire Lulehe Formation. The sample collection depths were 644.77-653.26 m and 928.23-1002.78 m. Before the trace element analyses, the samples were baked in an oven at 105°C for about 3 hours to remove moisture and to enable more accurate weighing, and then, the samples were dissolved in HF and HNO 3 in sealed containers. Finally, they were analyzed using a laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS).
The data for the clay minerals were obtained by x-ray diffraction (XRD) analysis of the clay minerals (< 2 µm). An ~ 1 cm 3 sample was crushed (no grinding), soaked in deionized water for 24 hours, and 0.5% dilute hydrochloric acid was added to remove the CaCO 3 until the pH test paper remained acidic after constant testing. The sample was allowed to settle naturally, and then, it was washed repeatedly with deionized water until the sample exhibited an antiocculation effect. The sedimentation time was determined using the Stoke's law, and then, the < 2-µm particles were removed using a syringe. They were precipitated using a centrifuge. Then, the sample was made into oriented thin sections using the scraping method, after which the slides were air-dried. The natural akes remaining after the tests were soaked in saturated glycol steam for 24 hours to create glycol akes. Finally, the tested akes were heated in an oven at 490°C for 2h to create heated akes. The identi cation and interpretation of the clay minerals were mainly based on comprehensive comparisons of the XRD superimposed spectra obtained under three different testing conditions. The semi-quantitative calculation of each peak parameter was performed on

Bulk mineralogy and Clay mineralogy
By testing of core samples from the Pingtai area, 18 whole-rock components and 19 clay mineral components were obtained (Table 1 and

Couch's paleosalinity
The main disadvantage of the equivalent B method is that the accuracy of its calculation largely depends on the abundance of illite and other clay minerals in the sample. Because of the different combinations of clay minerals in geological samples, each clay mineral has a different B absorption capacity. Couch used the B in kaolinite to further correct the boron content( (Couch 1971)). The equation used for this correction is kaolinite boron = B k / (4X i + 2X m + X k ), where X i is the illite content, X m is the smectite content, and X k is the kaolinite content.
Couch's paleosalinity calculation equation is lg(kaolinite boron) = 1.28×lgSp + 0.11, where Sp is paleosalinity. In the past, scholars have divided paleosalinity into three types: paleosalinities of < 10‰ is slightly freshwater-slightly saline water type; paleosalinities of 10‰-18‰ are brackish water type; and paleosalinities of > 18‰ are salt water type. The average paleosalinity of the Xiaganchaigou Formation in the Pingtai area is 10.13‰, and that of the Lulehe Formation is 6.26‰ ( Table 2). The changes in Couch's paleosalinity value in the Pingtai area are very similar to the changes in the equivalent B content. These two ancient salinity calculation methods have a relatively high positive correlation (Fig. 5), indicating that these methods are highly reliable.  Thus, the paleosalinity of the depositional environment of the Xiaganchaigou Formation was higher than that of the Lulehe Formation, which is consistent with the conclusions drawn from their Sr/Ba ratios (Fig. 6).
The correlation between Rb/Sr ratio and the intensity of the water ow into the lake is relatively signi cant , so the Rb/Sr ratio corresponds to changes between dry and wet paleoclimate conditions. in continental basins, under wet conditions, chemical weathering is strong. Rb has a large ion radius, so it is very soluble and lters out of the parent minerals. Then, it is adsorbed by clays. However, these clays do not remain in place, and most of them are transported into lake sediments due to denudation(Ingram and Sloan 1992). At the same time, the dissolved Sr element that enters the lake basin is generally deposited during the dry, so the Rb/Sr ratio is high in the humid environment. Therefore, in terrestrial basins, the meaning of the Rb/Sr ratio is actually the opposite of that under marine conditions, that is, a high Rb/Sr ratio represents a humid climate and a low Rb/Sr ratio represents an arid climate. The Rb/Sr values of the Lulehe Formation are 0.31-0.44, with an average of 0.38; and those of the Xiaganchaigou Formation are 0.13-0.55, with an average of 0.32.

Diagenesis
The trace element contents of clastic sedimentary rocks are comprehensively affected by the mineral composition of their parent rocks, weathering and transportation processes, sedimentary environment, and post-sedimentary diagenesis. The older the rock, the greater the in uence of uid diagenesis in the later period. However, based on the analysis of the whole rock compositions and burial depths in the Pingtai area, the burial depth is relatively shallow, and the core samples contain almost no diagenetic minerals, such as dolomite and iron dolomite. Therefore, the in uence of diagenesis on these rocks was weak, which is not conducive to the conversion of smectite to illite during rock formation (Chamley 1994).
The intensity of the chemical weathering of the rocks in the source area can be determined using the chemical weathering index (CIA) of the ne-grained debris (Nesbitt and Young 1982).  The La/Th ratio and Eu anomalies (Eu/Eu*) are usually regarded as provenance indicators (Cullers et al. 1988). In the Pingtai area, the La/Th and Eu/Eu* ratios exhibit stable changes with limited variability. These ratios are also close to their respective upper continental crust (UCC) values, indicating that the impact of the provenance was negligible. Based on these results and the CIA, the clay mineral contents of the Lulehe Formation and the Xiaganchaigou Formation in the Pingtai area were less affected by the provenance area and the clay minerals produced during chemical weathering. Therefore, trace elements and clay minerals can be effectively used in paleoenvironment reconstruction.

Paleosalinity and clay minerals
The types and their combinations of the clay minerals in sediments are closely related to the climatic conditions under which they were deposited. When the climatic conditions change, a suitable combination of clay minerals will be formed (Liu et al. 2020;Rodrigues et al. 2018). Studies have shown that illite and chlorite are generally formed in dry and cold climates ; smectite is usually formed in warm and humid climates as a weathering product of alkali-neutral volcanic rocks, metamorphic rock minerals, and the low-temperature self-generating rock minerals of the weathering products (Taheri et al. 2019).
The formation of smectite is related to an enhanced degree of hydrolysis. Relatively warm and humid climatic conditions are conducive to the formation of smectite, and its content decreases as the climate becomes warmer. Illite/smectite mixed layers clay minerals are formed under seasonal dry and humid climatic conditions (Jain and Tandon 2003). In this study, the clay minerals in most of the samples from the Lulehe Formation were dominated by smectite, re ecting relative warm and humid climate conditions. Among them, three samples were dominated by illite/smectite mixed layers; however, the smectite content is as high as 91,92,88 in the illite/smectite mixed layers, indicating that the clay mineral content is actually dominated by smectite, and it is believed that the climate during the deposition of the Lulehe Formation was mainly warm and humid. The clay minerals in the Xiaganchaigou Formation mainly consist of illite/smectite mixed layers, indicating that the climate had entered an alternating wet and dry phase during the deposition of this part of the formation, but it is mainly cold and dry with small uctuations. The ubiquitous illite/smectite mixed layers in the Oligocene strata in the Linxia Basin re ect the relatively small uctuations in climate (Hong et al. 2007). From the Lulehe Formation to the Xiaganchaigou Formation, the paleosalinity changed from a freshwater environment to a brackish water environment, which was caused by this climate change. Chlorite can generally only be preserved in areas where chemical weathering is inhibited. The previous nding that chlorite is widespread in these two formations is with the results obtained using the CIA index.

Paleoclimate analysis
According to the comprehensive judgment of the above major trace elements and clay minerals, the paleoclimate of Lulehe formation is mainly warm and humid, while that of Xiaganchaigou formation is mainly cold and dry, supplemented by seasonal humidity. The paleosalinity of Lulehe formation to Xiaganchaigou formation has changed from fresh water environment to brackish water environment. The causes of the above paleosalinity and paleoclimate changes will be analyzed from two aspects of climate and structure In the early Eocene, due to the collision of Eurasian plate, the Qaidam Basin moved northward slowly, resulting in the increase of latitude and altitude (Zhang et  Xiaganchaigou formation may also be attributed to the balance between evaporation and precipitation, which may be related to the suspicious climate driven by orbital cycle (Abels et al. 2011;Xiao et al. 2010). In continental basins, the deposition and dissolution of calcite mainly depend on the saturation of calcium carbonate. Higher rainfall will lead to lower concentrations of calcium and cobalt ions in water, which is conducive to the dissolution of calcite. On the contrary, lower rainfall resulted in increased concentrations of calcium and carbon dioxide in water due to lower ow, and promoted the deposition of calcite (Hong et al. 2007). The high calcite content and salinity of Xiaganchaigou formation support this view. The main reason for this climate is global cooling, followed by the uplift of Altun mountain.

Conclusions
Through the study of the clay mineralogy and geochemistry of the strata the Pingtai area of the Qaidam Basin, it was found that during the Lulehe Formation-Xiaganchaigou Formation period, the clay mineral composition transitioned from smectite dominated to illite/smectite mixed layer dominated, but the chlorite and illite contents did not change signi cantly. This re ects the fact that at this time, the climate changed from warm and humid to cold and dry with seasonal humidity. This cooling event is consistent with the global climate. Owing to the uplift of the Altun Mountain and the continuous cooling of the climate during the Eocene, the water supply in the inland areas decreased and the rainfall decreased, resulting in a transformation in paleosalinity from a freshwater sedimentary environment to a brackish water sedimentary environment. The average paleosalinity reached 10.13‰ during the Xiaganchaigou Formation period. The paleosalinity is not only conducive to evaluating the sedimentary environment at this time, but it also provided support for the climate change that occurred during this period. Correlation between paleosalinity and B, B/Ga