Element Geochemical Characteristics and Geological Significance of Mudstones from the Middle Jurassic Shaximiao Formation in Sichuan Basin, Southwest China

Thick sequences of terrestrial multicolored mudstones of the Middle Jurassic Shaximiao Formation in the Sichuan Basin, Southwest China, effectively recorded paleoclimate and paleoenvironment changes. The paleoenvironment of the Shaximiao Formation is reconstructed by using detailed sedimentological and elemental geochemical analysis of the multicolored mudstones. The provenance, paleoclimate, paleosalinity, and paleoredox conditions are distinguished by using the discriminant indicators of CIA, C-value, Sr/Cu, Rb/Sr, Th/U, V/Cr, and V/(V + Ni). The results show that all samples derive primarily from felsic igneous rocks and intermediate rocks rather than recycled sediments. The mudstone sequences were deposited under semiarid and semihumid regions with paleoclimate evolved to drier and cooler conditions from lower to upper Shaximiao Formation. Such a paleoclimate coincided with the records of several basins in the lower paleolatitudes of the Northern Hemisphere and were possibly affected by the Middle Jurassic global geological events such as wildfire, paleogeographic reorganizations, and seaway dynamics change. The paleowater body belongs to a typical terrestrial freshwater environment, although the paleosalinity increased significantly during arid periods. The multicolored mudstones were deposited in oxidation and weak-oxidation to weak-anoxic conditions. We also propose a detailed conceptual paleoenvironment model for Shaximiao Formation, with a large perennial lake surrounded by limited alluvial plain during a period of high lake level and small ephemeral lakes scattering extensive alluvial plain during a phase of low lake level.


INTRODUCTION
The element content of mudstone and the ratio of related elements in stratigraphic records are used to qualitatively and quantitatively reconstruct the paleoclimate and paleoenvironment. 1−3 Chemical weathering caused by temperature and humidity changes, salinity changes in the environmental water body, and different redox conditions will result in the selective loss or enrichment of sensitive elements in sedimentary materials, which is essential for revealing the veil of paleoclimate and paleoenvironment conditions. 4−6 Generally, the intensity of chemical weathering depends mainly on climate, and intensified chemical weathering occurs in warmer and wetter conditions. Climate-sensitive elements, such as Fe, Mn, Cr, V, Ni, and Co, are more readily enriched under humid climates, while in arid conditions, Ca, Mg, K, Na, Sr, and Ba are relatively enriched. Paleoclimatic conditions can be indirectly inferred from the ratio of various elements, such as Mg/Ca, Sr/Cu, and Th/U. CIA and C-values are also widely used as paleoclimate proxies. 7−9 Salinity is an essential chemical feature of the water body, and elemental paleosalinity proxies, such as Sr/Ba, Rb/K, V/Ni, and Th/U are effective indices to judge the salinity of paleo-water body. 3,10 In addition, trace elements can well reflect the redox conditions of the paleoenvironment. Previous studies have revealed that the Ni/Co, U/Th, V/(V + Ni), and V/Cr ratios could be used to reconstruct the paleoredox conditions. 1,11,12 The Sichuan Basin, an intracontinental sedimentary basin in southwest China, rich in oil and gas resources, is widely famous for dinosaur research in the Middle Jurassic. 13 The multicolored mudstone, Estherian fossil, and glauconite in sandstone preserved in the stratigraphic record are the result of the complex climate and environmental conditions that evolved during the Middle Jurassic. The element characteristics show that the western edge of the basin was warm and dry in the Middle Jurassic, and turned cool in local periods. 14 The characteristics of clay minerals in the southwestern margin of the basin indicate a semi-arid climate dominated by intermittent cold and drought when Shaximiao Formation was deposited. 15 The analysis of the paleosol and carbonate nodule in the northeast of the basin shows that this period is in the semiarid to semihumid climate and the dry-humid and cool/warm temperate climates alternately. 16 These studies reveal the paleoclimate of each region of the basin but lack of comprehensive analysis combined with mudstone type, paleowater body, and oxidation conditions.
In terms of the sedimentary environment, sedimentary indicators of the delta front, such as channel and mouth bars, have been found in cores in central Sichuan, while meandering river and floodplain deposits are dominant in outcrops in western and eastern Sichuan. 17 Dark-gray and gray-green mudstones, rich in bivalves, and other paleontological fossil  assemblages, indicate the existence of local large lakes during the flood/wet period in central and southern Sichuan, whereas purple-red soil layers and cross-bedded sandstone represent alluvial deposits during the dry period in the northeastern and western Sichuan Basin. 17,18 Obviously, climate fluctuations have profoundly affected the distribution and evolution of the environment in the basin from the proximal to the distal domain, especially the size of the lake and the spatial arrange of the alluvial plain and lake, and changed the weathering intensity and paleoredox conditions of the basin, resulting in changes in the lithofacies of Shaximiao Formation. The existing research still fails to put forward an applicable environmental evolution model in combination with climate and sedimentary facies, which hinders the prediction and evaluation of tight sand reservoirs of Shaximiao Formation.
As the reconstruction of the paleoenvironment typically has been of particular benefit to promote the understanding of the distribution pattern of the sand body and predict the favorable lithologic reservoir, this paper describes the sedimentological characteristics of mudstone, emphatically systematically analyzes the geochemical characteristics of major elements, trace elements, and rare earth elements(REE) in mudstone, to reconstruct the paleoenvironment of the Shaximiao Formation by using applicability methods such as the ratio of each element to determine the paleoclimate, paleosalinity, paleoredox, and other environmental conditions. Furthermore, based on the characteristics of the paleoenvironment of the Sichuan Basin in this period, we propose an alluvial plain-lake model for the Shaximiao Formation.

GEOLOGICAL SETTING
The Sichuan Basin, geographically located in the eastern part of Sichuan Province and the western part of Chongqing in southwest China, has an area of approximately 18 × 10 4 km 2 ( Figure 1). The Sichuan Basin, which comprises the northwestern segment of the Yangtze Block in southern China, is located on the southern side of the Qinling orogenic belt, and at the intersection of the North China plate, the Yangtze plate, the Qinghai-Tibet Plateau, and other terranes. 19,20 It is structurally located in the superimposed area of the Micangshan-Dabashan front thrust nappe belt and the eastern Sichuan arc-shaped fold belt. The basin has evolved from a passive continental margin to a foreland basin since the Middle Triassic, in response to multiphase collisions of the surrounding tectonic blocks. 21 Considerable collisions between these blocks distinctly shaped the development from a shallowmarine carbonate platform to an intercontinental sedimentary basin. 22 The Qinling orogen was thrust southwestward over the northeastern Sichuan basin along the Dabashan fold-andthrust belts from the Middle Triassic to the Late Jurassic, forming the northeastern Sichuan foreland basin. 23, 24 The northeastern Sichuan foreland basin experienced rapid flexural subsidence in the Middle and Late Jurassic, resulting in the deposition of thick-to medium-bedded sandstone interbedded with conglomerates. During the Middle Jurassic, the sedimentary stratigraphy distributed throughout the basin with thickness characterized by being thick in the northeast and thin toward the southwest. The Middle Jurassic strata in the basin consists of the Shaximiao Formation and the Lianggaoshan Formation (Figure 2A). The Lianggaoshangou Formation is mainly composed of purplish-red mudstone and siltstone with gray sandstone. The upper part conforms with the Suining Formation of the Upper Jurassic, and the lower part is in unconformity with the Shaximiao Formation. The Shaximiao Formation consists of purplish-red, gray-green mudstone and siltstone as well as interbedded gray fine-to medium-grained sandstone. According to well-developed paleosols, most of the mudstones and siltstones are interpreted to be deposits of alluvial plain. 23 Indicated by trough crossbedding, planar cross-bedding, parallel bedding, and massive bedding in sandbody, interbedded sandstones are generally interpreted to be channel deposits of a river or delta system. 17 enrichment or loss of the relative content of elements in the sediment, defined as follows: where X and Al represent respectively the content of trace element and aluminum in the sediments. The samples are usually normalized using the composition of the post-Archean Australian shale (PAAS) from Taylor and McLennan (1985). 25 X EF > 1 indicates that the sample X element is enriched relative to the average shale, X EF > 10 indicates that the sample X element is strongly enriched, and X EF < 1 indicates that the sample X element is relatively deficient. This method is used to evaluate the enrichment degree of major and trace elements in mudstone samples from the Shaximiao Formation.

Paleoweathering Indices.
Several researchers have utilized the weathering index of ancient detrital sediments to trace changes in chemical weathering. Different degrees of chemical weathering occurs in moist or arid environments due to differences in temperature or humidity acting on patent rocks. 2,6 Hence, the Chemical Index of Alteration (CIA )values represent degrees of chemical weathering and have been widely used to reconstruct the paleoclimate and weathering intensity. The CIA value was calculated using molecular proportions in mudrocks 2 as follows: where elemental concentrations are presented as ppm. The Cvalue may be used to evaluate the changes in paleoclimate from warm and humid to hot and arid. 29 0 < C-value < 0.2 indicates an arid environment, 0.2 < C-value < 0.6 indicates a semi-arid to semi-humid environment, and C-value > 0.6 indicates a humid environment. 30,31 4. RESULTS

Sedimentary Characteristics of Mudstone.
The origin and cause of the colors in the mudstone are generally emphasized, including the implications of environmental, climatic, and diagenetic factors. 32,33 Field observation of outcrops and cores from the Shaximiao Formation shows that the color of mudstone is mainly purplish-red, grayishgreen, and locally dark gray. The strong vertical heterogeneity of color indicates that the sedimentary environment changes frequently on the surface. The mudstone of the Shaximiao Formation is divided into purplish-red mudstone lithofacies (PRM), gray-green mudstone lithofacies(GGM), and darkgray mudstone lithofacies(DGM) according to fresh surface color. We have calculated the fraction of each color facies in the Sha-1 and Sha-2 Member, respectively. DGM consist mainly of shale and laminated mudstone. Minute plant debris can be observed in all gray mudstones and is the chief coloring agent ( Figure 3A). Plant fragments are common and increase in abundance in darker mudstone, resulting in a higher fraction of organic matter in DGM than PGM and GGM. This color, which results in an anoxic deposition condition, typically indicates a water body with a certain depth that has been stable for some time. 34 DGM is only visible at the top of Sha-1 Member, accounting for about 18% ( Figure 3D).
PRM is the main mudstone color, characterized by rapid physical weathering and slow chemical pedogenetic processes ( Figure 3B). Purple color is considered to be a function of the amount of hematite present related to oxidation. 33,35 PRM comprises 55.2% of Sha-1 members and 80.1% of Sha-2 members ( Figure 3D).
GGM is predominantly composed of claystone and mudstone. Most green rocks are uniformly colored throughout, even when in abruptly contact with PRF ( Figure 3C). Green colors are deriving from the ferrous iron in chlorite and illite and indirectly to the lack of other coloring agents, representing the product of a hypoxic environment. 36    , which are consistent with PAAS. In addition, major elements of Sha-1 and Sha-2 Member show obvious similarity. The commonly used indicators of paleoclimate based on major elements include C-value, CIA, and T also, which are provided in Table 1. Table 2 shows the trace element data of mudstone samples from the Shaximiao Formation. Generally, the degree of enrichment or consumption of trace elements in the sample is evaluated relative to the concentration in the reference of the post-Archean Australian shale (PAAS). 25 The average enrichment factors of Zn, Ga, Sr, and Pb are greater than 1, indicating relatively enriched. A few trace elements are characterized by enrichment factor less than 1 including V, Co, Ni, Cu, Cs, W, Nb, and Zr, all of which exhibit depleted concentrations. Other elements fall in the variable range of enrichment and depletion ( Figure  5).

Trace Element Geochemistry.
Other indirect geochemical parameters are shown in Table 2 including Ni/Co, U/Th, V/Cr, Sr/Cu, Sr/Ba, V/(V + Ni), etc., most of which shows a co-evolution of trace element concentration and sedimentary environment. The Sr/Cu ratio is higher in the Sha-2 Member than in the Sha-1 Member, with average values of 7.66 and 6.82, respectively. Ni/Co, V/Cr, and Rb/Sr are slightly larger in Sha-1 Member, whereas Sr/Ba, U/ Th, and V/(V + Ni) ratios are similar in the Sha-1 and Sha-2 Member.

Rare Earth Element Geochemistry.
The results of the REE concentration of the mudstone from the Shaximiao Formation are shown in Table 3. The chondrite-normalized REE patterns of the studied sample are given in Figure 6. The content of total rare earth elements (ΣREE) in mudstone from the Shaximiao Formation ranges from 125. 35 Figure 6). The characteristics of REE in Sha-1 and Sha-2 Member show no significant difference.

Provenance.
The geochemistry of fine-grained clastic sediments has been widely used to identify the provenance of the sedimentary system in the sink area. The ratios of immobile elements such as Al 3+ , Zr + , and Ti 4+ oxides in sedimentary rocks are usually similar to those of parental rocks. 31,38−40 The high ratios of Al 2 O 3 /TiO 2 for the studied samples range from 19.99 to 37.71 (average = 23.23), indicating sediments mainly derive from felsic parent rocks in the source area ( Figure 7A). 41 In addition, the TiO 2 /Zr ratio of the samples varies from 2.14 to 7.72 (average = 4.89), suggesting a dominant felsic and intermediate source composition (Figure 7B). 41 A scatter plot of La/Th versus Hf suggests derivation from a source area composed of mixed felsic/basic and felsic sources ( Figure 7C). 42 McLennan et al. (1993) insisted that Zr/Sc and Th/Sc ratios can be used to understand the variations of sediments from the source region in mineral composition, degree of sorting, and heavy mineral content. 43 In the mudstone samples, these values range from 6.42 to 11.26 (average = 8.09) and from 0.75 to 1.39 (average = 0.96), respectively. A bivariate plot of Zr/Sc versus Th/Sc suggests that the studied mudstone samples derive principally from felsic igneous rocks, and the data fall far from the sedimentary recycling line indicating the clastic materials have

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http://pubs.acs.org/journal/acsodf Article not experienced the process of sediment recycling ( Figure 7D). Felsic source rocks generally respond to higher LREE/HREE ratios and negative Eu anomalies, whereas lower LREE/HREE ratios and the absence of Eu anomalies are characteristic of mafic rocks. 44 The studied mudstone samples are extremely enriched in LREE (LREE/HREE = 9.40 on average) and deficient in HREE, with obviously slight negative anomalies (Eu anomaly = 1.51 on average), exhibiting a felsic provenance. These results uniformly indicate the studied mudstones of the Shaximiao Formation mainly derive from felsic and intermediate igneous rocks other than sediment recycling. Trace elements whose concentration depends on their relative insensitivity to weathering and transportation processes provide important evidence to explain the tectonic environment. 25 On the discrimination diagrams of La-Th-Sc, 45 most of the data fall in or adjacent to the passive margin field ( Figure 7E). The Sc/Cr versus La/Y binary plot shows that the majority of samples were plotted in the passive marginal setting with a few data been grouped into continental island arc, an active continental margin or mixed continental island arc-active continental margin ( Figure 7F). 46 In summary, the overall geochemical characteristics of the discriminate diagrams from the Shaximiao Formation suggest that the tectonic setting of the source area of the main provenance was most likely a passive margin. Such interpretations are supported by the widely accepted tectonic evolutionary model, which insists that the Sichuan Basin has evolved from a passive continental margin to a foreland basin since the Middle Triassic, affected by multiphase collisions of tectonic blocks. 21 During this period, due to the influence of the Yanshan movement, the Qinling orogenic belt was napped and uplifted, and the felsic igneous material of the Micangshan-Dabashan, characterized by low quartz, high feldspar and high content of metamorphic rock debris, was denuded and transported, feeding sediments to the Sichuan Basin as a main provenance. At the same time, western provenance with high quartz, low feldspar, and relatively low metamorphic rock debris from Longmenshan also provided clastic materials to the basin.

Paleoclimate and Paleo-weathering Conditions.
The paleoclimate determines the mineralogy and chemical composition of siliciclastic sediments and is the key factor controlling the weathering degree of sediments. 47,48 The concentrations of Fe, V, Ni, Ba, Co, and other elements are very sensitive to paleoclimatic conditions. The changes in their contents and corresponding ratios can be used to analyze the changes in paleoclimate and judge the sedimentary environment of a specific period. Geochemical indicators such as CIA, major oxide ratio (Al 2 O 3 /SiO 2 ), and climate-sensitive trace elements (e.g., Ga, Rb, Sr, and Cu) have been widely used to reconstruct paleoclimatic conditions. 2,49−51 In this study, the CIA value of Sha-1 Member mudstone ranges from 63.23 to 77.88 (average = 74.89), reflecting moderate chemical weathering, belonging to a semi-humid climate. Differently, the CIA value of Sha-2 Member mudstone varies from 56.01 to 73.42 (average = 68.76), indicating a lower degree of chemical weathering than Sha-1 Member, which belongs to a semiarid and semihumid climate ( Figure  8A). The ratio of wet elements (e.g., Rb and Cu) to dry elements(e.g., Sr)is often used to analyze the paleoclimatic characteristics. The high Sr content of sedimentary rocks may be caused by the concentration and precipitation of lake water under dry and hot climate conditions. 11 (Figure 8d), which indicates that Shaximiao Formation mudstone was deposited under a semihumid and semiarid climate ( Figure 8D).
For CIA values are particularly sensitive to land surface temperature (LST), latitude of the lake basin, and soil depth in the drainage area, an experiential relationship between the CIA and LST as a paleothermometer was quantified as T (°C) = 0.56 × CIA − 25.7(R 2 = 0.50), which is robust over a temperature range of 3−25°C and has an uncertainty of approximately ±5°C, corresponding to a CIA range of approximately 50−90. 53 This method has been used to quantify the paleotemperature of land evolution in deep times. 54,55 We use the relationship between CIA and LST to evaluate chemical weathering and temperature changes of Shaximiao Formation. The paleotemperature of Sha-1 Member ranges from 9.71 to 16.24°C, with an average of 16.9°C, and the value of Sha-2 Member is from 5.67 to 15.53°C, with an average of 12.81°C. These results are consistent with the paleotemperatures (7.9−17.4°C) calculated by the PWI index and the SAL index based on paleosol outcrops. 16 In summary, the paleoweathering indices and climatic indicators demonstrate that the studied samples of Shaximiao Formation experienced moderate chemical weathering and formed in semiarid to semihumid conditions. Moreover, paleotemperature T, C-value, and CIA index all suggest that the Sha-1 Member is wetter and warmer than the Sha-2 Member and experiences stronger weathering. These interpretations agree fairly well with association of mudstone lithofacies which is characterized by higher proportion of GGM, lower proportion of PRM, and wide occurrence of Estheria fossils in the shale interval at the top of the Sha-1 Member.

Paleosalinity in a Paleo-water Environment.
To ensure accurate determination of the paleosalinity of the paleowater body, we use multiple effective indexes to support paleosalinity interpretation. For Sr has higher solubility and migration ability in water than Ba, the Sr/Ba ratios of sediments usually indicate the salinity of the water body. 56, 57 The value of Sr/Ba in the Shaximiao Formation ranges from 0.16 to 0.55, with an average value of 0.29, which is far less than the critical value of freshwater (<0.6) ( Figure 9A). K is mainly related to the content of clay minerals in mudstone, while Rb is more easily adsorbed by clay and organic matter, and its concentration increase with higher salinity. The ratio 1000 × Rb/K 2 O of less than 6 is used to represent freshwater. 30,58 In this study, the value of 1000 × Rb/K 2 O in Sha-1 Member ranges from 1.77 to 4.39, with an average value of 3.70, and the value in Sha-2 Member ranges from 3.35 to 5.63, with an average value of 4.09, which indicates a freshwater environment with higher salinity than Sha-1 Member ( Table 2). The Th/U ratio can also be used as a  parameter to identify paleosalinity. In general, the Th/U ratio in freshwater environments is more than 2, and in saline water environments, it is less than 2 ( Figure 9B). 3,11 The Th/U ratio of the Shaximiao Formation is 1.54 to 8.68, with an average value of 5.08 and 4.95 for Sha-2 and Sha-1 Member, respectively (Table 2), indicating freshwater environments. To sum up, it is inferred that the Shaximiao Formation mudstone is mainly formed under a typical continental freshwater lake basin. 5.4. Paleoredox Conditions. V, Cr, U, Th, Co, Ni, and other oxidation-sensitive elements are easier to dissolve under oxidation conditions than under anoxic conditions. 1,59 Therefore, they are used as good indicators to judge paleoredox conditions. According to previous studies, the redox state of the sedimentary environment can be characterized according to the ratios of V/Cr, V/(V + Ni), etc. 60,61 The higher the V/ (V + Ni) ratio, the lower the degree of oxidation of the water during deposition. It is widely accepted that the V/(V + Ni) ratio > 0.84, 0.54−0.82, and 0.46−0.6 represent anoxic, hypoxic, and oxygen-enriched sedimentary conditions, respectively ( Figure 10A). 62 The V/(V + Ni) range of Sha-1 Member is 0.58 to 0.82 (average = 0.71), and the value in Sha-2 Member varies from 0.68 to 0.87 (average = 0.75) ( Table 2), indicating a hypoxic environment. The V/Cr value can also be used as a parameter to identify the redox environment. Generally, the V/Cr ratios >4.25, 2−4.25, and <2 indicate anoxic, dysoxic, and oxic conditions, respectively ( Figure  10B). 63 The V/Cr values of mudstone samples in Sha-1 Member range from 0.68 to 1.77, with an average of 1.33, and in Sha-2 Member, they vary from 0.48 to 2.21(average = 1.10), suggesting a weak to strong oxidation environment. The generally oxygen-enriched conditions are also supported by the widespread occurrence of purplish-red mudstone and the carbonate nodules of paleosols in the Shaximiao Formation.
In addition, the correlation between the Rb/Sr value and the Sr/Ba ratio ( Figure 9A) shows a significant exponential correlation (r = 0.82), indicating that the local climate drought would lead to intense evaporation, reduce the lake level, and increase the salinity of the water body, resulting in a significant fluctuation of the Sr/Ba and Th/U values. However these processes do not significantly change the paleoenvironment of the Sichuan Basin during deposition of Shaximiao Formation, resulting in a relative stable lake water body dominated with freshwater and oxidation-weak oxidation conditions. 5.5. Reconstruction of the Paleoenvironmental Model. The sediment sources in the Shaximiao Formation in the Sichuan Basin are mainly affected by Dabashan and Micangshan followed by Longmenshan in the west and Xuefengshan in the southeast, resulting in the distribution of the sedimentary system in the direction of NE−SW. 21,64 The sedimentary environment elements, which include meandering rivers, lakes, and paleosols, are identified by outcrop and core evidences. 18 The stratigraphic sequence shows a high ratio of mudstone to sandstone. The fine-grained sediments are dominated with meandering river mudstones and lake mudstones intercalated with paleosol intervals. The seismic profiles in central Sichuan show imbricated foreset reflections, representing the progradation of river-delta systems into the shallow water environment, feeding the lake basin with clastic materials derived from Dabashan, Micangshan, Longmenshan, Figure 6. Chondrite-normalized REE patterns of mudstone from the Shaximiao Formation. 25 and Xuefengshan. 65,66 Therefore, the subenvironment of Shaximiao Formation can be divided into alluvial fan, river, delta, and lake from the source to sink.
The long-term similarity and short-term local differences in the evolution history of geochemical indicators in the Shaximiao Formation are evident. A subhumid to semi-arid climate with strong seasonal aridity is interpreted by the absence of lignite and carbonaceous plant debris and the abundance of purple-red mudstones that developed under a lowered water table condition. As interpreted to be deposited in a weak-oxidation environment (indicated by paleoredox indices, Figure 10) during a relative humid climate (indicated by C-value and paleotemperature T, figure 8) with relative intense chemical weathering (indicated by paleo-weathering indices, Figure 8), the dark-gray mudstone/shale interval on upper part of Sha-1 Member is speculated to be deposited in a shallow water lake during a short-term humidity phase. It is concluded that although semiarid and semihumid climates dominate the Shaximiao Formation stage, the sedimentary basin would experience a short-term humidity phase in the late stage of Sha Member characterized by shallow water depth ( Figure 11).
Based on the characteristics of mudstone sedimentology and element geochemistry, a sedimentary environmental model is proposed for the Shaximiao Formation, including two stages: (1) the alluvial plain-perennial lake stage at flood/wet periods and (2) the alluvial plain-ephemeral lake stage at dry periods. During the flood/wet period, due to the abundant seasonal precipitation, the riverine discharge and sediment budget increase significantly, resulting in the rapid expansion of the lake area, the formation of a unified large lake in center of the basin, and widely distribution of delta and lacustrine deposits, with limited sourceward retreated alluvial plain ( Figure 12A). This period is characterized by stronger chemical weathering and a higher sedimentation rate. Given the "short term" attribute of wet period and shallow water depth, extensive reduction conditions would not develop, and most of the detrital material was preserved in weak oxidation to weakanoxic environment. DGM and GGM are the dominated mudstone lithofacies in this stage, with PRM distributed at the margin of the basin. This stage exists mainly for Sha-1 Member. Under the long-term semiarid and semihumid climate, Shaximiao Formation is dominated with the alluvial plain-ephemeral lake stage, especially in the Sha-2 Member  Figure 12B). During this dry stage, for rare precipitation and intense evaporation, the lake experiences low levels with the extensive basin floor having been exposed as alluvial/lake plain. The alluvial/lake plain deposits, which include channel sandstones and floodplain mudstones, dominate the stratigraphic record. During this stage, the sedimentary basin which is characterized by weak chemical weathering and strong oxidation, is dominated by sporadic small lakes/playas, ephemeral rivers, and discontinuous paleosols. PRM, distinctive soil horizons, carbonate nodules, and root traces are the main indicators of this stage. The sediments in Sha-2 Member mainly deposited in this stage, although it also contains interval developed from flood/wet stage. 5.6. Correlation of Regional Climate in Middle Jurassic. Global geodynamics during the Middle Jurassic may have exerted an influence on climatic fluctuations, thus shaping the alluvial plain-lake environment model of the Shaximiao Formation. The Middle Jurassic global climate was characterized by zonal climate characteristics of arid, tropical, northern tropical, boreotropical, warm temperate, and cold temperate environmental conditions from the equator to the poles. 67 The Jurassic is a typical greenhouse climate period, with long-term climate changes and short-term fluctuations recorded in continental basins. 68,69 Plate tectonics is the key driving force for shaping paleoclimate and paleogeography. 70 The climate of the depositional basin is generally considered to be closely related to paleogeography. As the supercontinent Pangea split apart, intense plate tectonic movement led to significant volcanic activity, mountain-building events, and the attachment of islands onto continents during the Jurassic, 71,72 resulting in reorganization of paleogeographic and paleoclimatic zones of the Sichuan Basin. 73 During the Middle Jurassic, been interpreted to be located between 20°N and 30°N in the northern tropical climate zone, 74 the paleotemperatures of the Sichuan Basin are supposed to be ∼20 for geochemical evidence suggests that surface waters in the low latitudes were ∼20°C (68°F). The assumption is confirmed by temperature calculated from the relationship between weathering index and LST during the Shaximiao Formation (ranging from 11.6 to 17.7°C) and calculated from the  salinization (SAL) index (ranging from 10 ± 4.4°C to 13 ± 4.4°C). 16 Middle Jurassic sedimentary sequences, which are dominated with red sediments deposited in similar paleoclimates and paleoenvironments, have been discovered in many sedimentary basins in the Northern Hemisphere including the Junggar Basin, Qaidam Basin, and Santanghu Basin. In the Junggar Basin, the sedimentary environment changes from deltaic to fluvial depositional systems with enhanced aridity. 75 In the Qaidam Basin, an upward weakening and even arrested coal accumulation caused by dry and/or arid episodes have been confirmed by the high diversity of spores and pollen

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http://pubs.acs.org/journal/acsodf Article grains. A sudden decrease of the chemical weathering index, which reveals a rapidly transformed climatic state from humid to seasonally arid also been revealed in the Aalenian-Bajocian of the Santanghu Basin. 76 These strata records from different sedimentary basins of the Northern Hemisphere may be related to a transient global warming phenomenon or a strong greenhouse effect 77 as well as Shaximiao Formation in the Sichuan Basin. Global wildfires have probably consumed enormous amounts of O 2 from the atmosphere, and huge amounts of greenhouse gases were released, inducing the Jurassic global warming event, with palaeoatmospheric CO 2 four times higher than today in the Early Mesozoic period. 78 Estimated pCO 2 fluctuations in the Sichuan basin coincide with paleotemperature variations in the low paleolatitudes of the Northern Hemisphere. In addition, according to the seaway dynamics model, which emphasizes the potential significance of the seaway for heat transport, major modifications in Mesozoic oceanic current patterns may have a significant influence on a large-scale, abrupt climate change and potentially govern transformations between warm/cool or dry/humid climate modes. 79 As Korte (2015) proposed, the uplift of the North Sea Dome that impeded northward oceanic heat transport was considered to cause an especially abrupt mid-latitude cooling of seawater by as much as 10°C in the north−south Laurasian Seaway during the Jurassic. 80 Interestingly, Shaximiao Formation shows an earlier climate change from humid to seasonally arid during Middle Jurassic indicated by small cumulative thickness of dark-gray shale and deficiency of a coal bed comparing with equivalent stratigraphic records in the Junggar Basin, Qaidam Basin, and Santanghu Basin. The lithological difference of Shaximiao Formation probably due to reorganization of paleoclimate zones of Sichuan Basin beyond orogen belts along plate sutures (e.g., Qinling orogen). As the lee side (or rain shadow) of the Qinling Orogen and Longmenshan Orogen, the Sichuan Basin may receive considerably less precipitation as Tethys trade winds zone.
In summary, possibly having been controlled by global geological events and local topography, the paleoclimate of Middle Jurassic Shaximiao Formation in the Sichuan Basin is primarily semiarid and semihumid, with Sha-2 Member precipitated in a drier, cooler, and more oxygen-rich environment.

CONCLUSIONS
Combined with the analysis of element geochemistry and sedimentology of mudstone, different types of mudstone lithofacies are identified, the characteristics of the paleoclimate, paleowater body, and paleooxidation environments are recovered, and a sedimentary environmental model including wet and dry stages is proposed for Middle Jurassic Shaximiao Formation in the Sichuan Basin.
(1) Three mudstone lithofacies can be identified based on color including purplish-red mudstone (PRM), graygreen mudstone (GGM), and dark-gray mudstone (DGM), with DGM exists only in the Sha-1 Member, and the proportion of PRM gradually increases from Sha-1 to Sha-2 Member. (2) The geochemical characteristics of mudstone indicate that the main source of sediments are felsic and intermediate igneous rocks derived from passive continental margin. (3) The Shaximiao Formation mudstones, which suffered moderate chemical weathering, are mainly deposited in a shallow oxygen-rich freshwater lake basin under semiarid−semihumid, cool-cold climatic conditions, with Sha-2 Member precipitated in a drier, cooler, and more oxygen-rich environment. The regional paleoclimate is possibly controlled by global geological events and local topography. (4) A two-stage sedimentary environmental model is proposed for the Shaximiao Formation. The wet stage environment is dominated by a large perennial lake surrounded by limited alluvial plain, and the dry stage