Characteristics and formation mechanism of siltstone-mudstone rhythmic sedimentary sections in the Lower Silurian Longmaxi Formation in the Changning area, South Sichuan Basin, southwest China

Abstract

The Lower Member of the Longmaxi Formation is generally dominated by siliceous shale, but recently we found some siltstone-mudstone rhythm sections developed in the Lower Member of the Longmaxi Formation. The study of formation mechanism of siltstone-mudstone rhythmic sedimentary sections may provide new insights into the shale sedimentary environment. Therefore, we studied the characteristics and formation mechanism of siltstone-mudstone rhythmic sedimentary sections in the Lower Member of the Longmaxi Formation in the Changning area based on core observation, thin section identification, major elements and trace elements analysis. The results show the following: 1) Two siltstone-mudstone rhythmic sedimentary sections are characterized by frequent interbed between black or gray-black shale and light gray siltstone, abundant argillaceous laminas and silty laminas, with obvious lithological boundaries having developed. Horizontal laminas and rhythmic laminas are well-developed in the shale layer, while the wavy laminas are well-developed in the siltstone layer. 2) The major compositional elements are SiO₂, Al₂O₃ and CaO, followed by Fe₂O₃, MgO, K₂O and Na₂O. 3) Compared with the world average shale, these siltstone-mudstone rhythmic sedimentary sections are rich in Mo, U and Ba, but less in V, Co, Ni, Cu. Compared with the shale layer, the siltstone layer has lower contents of V, Co and Ni. 4). The geochemical redox indices, Mo-U and CIA values suggest the formation of the siltstone-mudstone rhythmic sedimentary sections are related to influences from bottom currents in an oxic condition with a warm and humid paleoclimate.

Graphical abstract

Keywords

shale / Longmaxi Formation / bottom current deposit / Sichuan Basin

1 Introduction

The black and organic-rich marine shale of the Upper Ordovician Wufeng-Lower Silurian Longmaxi Formation in Sichuan Basin, characterized by great thickness, wide distribution, high total organic carbon (TOC) content, high total gas content, brittle minerals, richness and moderate burial depth, and regarded as the most favorable strata for the exploration and development of marine shale gas in China (Zou et al., 2010; Chen et al., 2015a; Zou et al., 2015; Wang et al., 2016; Zou et al., 2016; Shan et al., 2017; Ma and Xie, 2018).Since shale gas was first obtained on an industrial scale from the Wufeng-Longmaxi Formation in the south Sichuan Basin in 2010, other shale gas fields such as Weiyuan, Changning, Fuling, Zhaotong, Luzhou and Wuxi have been discovered across China (Dai et al., 2014; Guo and Zhang, 2014; Jiang et al., 2017; Chen et al., 2020; Qiu and Zou, 2020; Shu et al., 2020). It is generally believed that the Wufeng-Longmaxi shale is deposited in a low-energy, under-compensated and anoxic deep water restricted environment in Sichuan Basin (Algeo et al., 2016; Zhao et al., 2017; Yan, et al., 2019). As related studies and explorations achieved progress, it became known that anoxic deep water environment is not the only sedimentary environment where marine shale may be found. The organic-rich shale could also be deposited in shallow water areas (Macquaker, et al., 2010; Smith et al., 2019; He et al., 2020). Some scholars have found that centimeter-subcentimeter-scale bioturbation occurred in the organic-rich shale, suggesting that long-term anoxic water column is not a prerequisite for the enrichment of organic matter (Macquaker et al., 2010; Aplin and Macquaker, 2011). Other researchers have confirmed that biological deposition (Wang et al., 2014a), storm deposition (Drummond and Sheets, 2001) and bottom current deposition (Lu et al., 2017) could take place during the deposition and formation process of organic-rich shales. There are preliminary studies mainly focused on the formation mechanism of the lower part organic-rich shale in the Wufeng-Longmaxi Formation, and a general viewpoint is that biological deposition and suspended deposition at the bottom of the Longmaxi Formation is related to the the formation of high-quality organic-rich siliceous shale (Han et al., 2019; Yan et al., 2018), while storm deposition is associated with the grading sedimentary structure of the shell limestone in Guanyinqiao Member (Zhao et al., 2016). Calcareous laminas shale was developed in the middle and upper part of Longmaxi Formation with wavy laminas and cross-laminas, indicating that the sedimentary process of the shale involved bottom currents. Shale with wavy laminas at the upper part of the Longmaxi Formation is characterized by siltstone interbedded with shale, suggesting that periodic high-energy events occurred during the shale depositional process (Wang et al., 2014b). Some proposals put forth attempting to explain this phenomenon, include gravity flow, storm deposition or bottom flow (Li et al., 2016; Liang et al., 2016; Ma et al., 2016; Zhao et al., 2017). However, there is still no report related to the siltstone sections in the organic-rich shale sections at the bottom of the Longmaxi Formation.

Recently, from three drilling wells (N1, N2 and N3) in the South Sichuan Basin we found some siltstone-mudstone rhythmic sedimentary sections that had developed in the Low Member of the Longmaxi Formation, which are characterized by frequent interbed between black or gray-black shale and light gray siltstone. For this paper, two siltstone-mudstone rhythmic sedimentary sections (Section-A: 2453.93‒2454.25 m; Section-B: 2460.96‒2461.54 m) from the Lower Member of the Longmaxi Formation in the N1 well in South Sichuan Basin were studied. The characteristics, palaeoenvironment and the formation mechanism of these two siltstone-mudstone rhythmic sedimentary sections were analyzed through the drilling core, thin section, and analysis of major element and trace elements.

2 Geological setting

Due to the convergence between the Cathaysia Terrane and the Yangtze Plate during the Middle Ordovician, the Yangtze Plate ushered in the foreland basin tectonic evolution stage, while the Sichuan Basin became a part of backbulge of the Yangtze Plate foreland basin (Liu et al., 2011). The Chuanzhong uplift, Qianzhong uplift and Xuefengshan uplift were formed because of the intensification of compression during the Early Silurian. Such developments subsequently led to a restricted deep-water environment formed in the Sichuan Basin and its adjacent areas surrounded by these uplifts, in which the black shale of the Wufeng-Longmaxi Formation was deposited (Chen et al., 2004; Su et al., 2009; Yan et al., 2009; Wang et al., 2015).

At the transition phase between the Ordovician and Silurian, the areas from the Upper Ordovician Wufeng Formation to the Lower Silurian Longmaxi Formation are characterized by organic-rich shale developed in the Yangtze Platform. The Wufeng Formation is divided into two parts. The lower part is composed of black organic-rich siliceous shale, with a thickness ranging from several meters to more than ten meters. And an abundance of graptolite, and radiolarian can be found. The upper part of the Wufeng Formation is the Guanyinqiao Member, which is dominated by argillaceous limestone, with a thickness of no more than one meter (Chen et al., 2004; Guo et al., 2004). The Longmaxi Formation can be divided into its Upper Member and Lower Member. The Lower Member primarily consists of black carbonaceous shale and black calcareous shale, with copious graptolite, high TOC content, plentiful pyrite and bentonite laminas (Chen et al., 2019; Su et al., 2009). The Upper Member is predominantly made up of dark-gray shale, argillaceous siltstone and gray siltstone, with bountiful laminas but lower graptolite content (Guo et al., 2004).

The drilling wells N1, N2 and N3 are located in the Changning Tectonic Belt of the Sichuan Basin (Fig. 1(a)). The cores of the Longmaxi Formation were obtained from this trio of drilling wells. Cores from the N1 well show that the Lower Member of the Longmaxi Formation is dominated by black organic-rich shale with dark-gray silty shale interbedded, while the Upper Member of the Longmaxi Formation consists of dark-gray silty shale and gray siltstone (Fig. 1(b)).

3 Data and methods

This study is focused on the cores from two sections (Section-A: 2453.93‒2454.25 m; Section-B: 2460.96‒2461.54 m) of the Lower Member of the Longmaxi Formation in the Changning area of the South Sichuan Basin obtained from the N1 well. This study performed the description, thin section analysis, X-ray diffraction (XRD), major elements and trace elements test of these cores.

3.1 X-ray diffraction (XRD)

Seven samples from Section-A and ten samples from Section-B were selected for XRD test. The samples were crushed to a size of less than 1mm using a crushing machine, and grounded until the particle size was less than 40 µm using a grinder miller or agate mortar. Thereafter, the total mass of protolith powder of sedimentary rock was measured, and clay minerals with particle size less than 10 µm were extracted by suspension. The total amount of clay minerals was obtained by “Weighing Method”, the content of each non-clay mineral was measured using the “K-value Method”. For the test, the X-ray diffractometer was configured with a scanning angle of 5°‒45°, step length of 0.02°, step speed of 2°/min, working voltage of 40 kV, and working current of 150 mA.

3.2 Major and trace elements test

For the major and trace elements test, dilithium tetraborate was used to melt the sample, ammonium nitrate was used as the oxidant, and lithium oxide and a small amount of lithium bromide were used as flux and mold release agent. An automatic fusion machine was utilized for fusion at a temperature range of 1150°C‒1250°C, a glass sample wafer was made and measured with X-ray fluorescence spectrophotometer, and the contents of major and trace elements were calculated respectively according to fluorescence intensity.

3.3 Data calculations

To study the sedimentary environment of the siltstone-mudstone rhythmic sedimentary sections, the relevant parameters were calculated by using trace elements, and the specific calculation is as follows: influence of terrigenous detritus was eliminated through aluminum-standardization (Tribovillard et al., 2006).

Degree of enrichment of Mo and U are expressed using Mo-EF and U-EF, and the relevant formulas are: Mo-EF= (Mo-sample/Al-sample) / (Mo-average shale/Al-average shale); U-EF= (U-sample/Al-sample) / (U-average shale/Al-average shale) (Tribovillard et al., 2006).

Paleoclimate and weathering conditions were assessed by calculating the chemical index of alteration (CIA) using the following formula: CIA= Al₂O₃/ (Al₂O₃ + CaO* + Na₂O+ K₂O) × 100 (Nesbitt and Young, 1982; Price and Velbel, 2003), but they were omitted in the present study because of difficulty in measuring the CaO* content of calcareous sedimentary rocks (Liu et al., 2019).

4 Characteristics of siltstone-mudstone rhythmic sedimentary sections

4.1 Sedimentary structures

The lithology of siltstone-mudstone rhythmic sedimentary sections are characterized by interbedded layers of shale, silty shale, argillaceous siltstone and siltstone (Figs. 2 and 3). The laminas are well-developed in the siltstone-mudstone rhythmic sedimentary sections. A large number of rhythmic laminations composing of silt laminas and argillaceous laminas were developed with lamina thickness between 0.2 mm and 1.5 mm (Figs. 2 and 3). The shale layer mostly developed continuous horizontal rhythmic laminas (Figs. 3(a), 3(c) and 4(a)). Wavy laminas (Figs. 3(b) and 4(f)), boulder-clays (Fig. 4(b)) and deformation lamination (Fig. 4(c)) can be found in siltstone layer. The boulder-clays are oval in shape, with a size between 0.5 and 2.5 cm, and demonstrate a certain directional arrangement (Fig. 4(b)). The lithological interfaces of the siltstone-mudstone rhythmic sedimentary sections are obvious and exhibit various forms, including direct sharp contact (Figs. 3(c) and 4(b)), oblique crossing contact (Fig. 4(g)), wavy contact (Figs. 3(d) and 4(c)) and gradual contact (Fig. 4(d)). In addition, small scouring surface (Fig. 4(c)), flamelike structure (Fig. 4(e)), lenticle (Fig. 4(d)) and bioturbated structures (Fig. 5(f)) can be found in the siltstone-mudstone rhythmic sedimentary sections.

Thin section analysis show that the siltstone-mudstone rhythmic sections are characterized by the development of various types of laminas, including horizontal laminas, rhythmic laminas, and wavy laminas. Horizontal laminas are mainly developed in the black shale, which are argillaceous laminas with a thickness of about 0.3 mm (Fig. 5(e)). Rhythmic laminas feature silty laminas interbedded with argillaceous laminas. The thickness of silty laminas differs greatly, ranging from 0.1 to 2 mm, while that of argillaceous laminas varies little, ranging from 0.2 to 0.5 mm. In addition to silty laminas interbedded with argillaceous laminas of equal thickness, the rhythmic laminas also show a pattern of thick silty laminas interbedded with thin argillaceous laminas, and silty laminas with varying thicknesses in the vertical direction (Figs. 5(a) and 5(h)). In addition, boulder-clays (Fig. 5(c)), flamelike structures (Fig. 5(d)) and bioturbated structures (Fig. 5(f)) could also be found.

4.2 Sedimentary sequence of the silty-mudstone rhythmic sedimentary sections

The total length of Section-A is 0.32 m, and distributed within are, in sequence from bottom to top, 0.2 m dark-gray shale (A-1), 0.04 m light-gray siltstone (A-2) and 0.08 m dark-gray shale (A-3) (Fig. 6(a)). Looking from the bottom, up, Section A-1 shows lithology changing from shale to silty shale, color gradually becoming lighter, an increase in silty content and a rise in coarseness, while the laminas gradually changed from argillaceous laminas to silty laminas and from horizontal laminas to wavy cross laminas. The flamelike structure caused by differential compaction can be found in the middle part of Section A-1 and the boulder-clays in the silty mudstone is observed in the upper part of Section A-1. Meanwhile, Section A-2 is dominated by light-gray massive siltstone with a sharp contact that forms the Section A-1. The lithology of Section A-3 is mostly dark-gray shale with an abundance of horizontal laminas.

The total length of Section-B is 0.58 m, and distributed within are, in sequence from bottom to top, 0.15 m dark-gray shale (B-1), 0.35 m gray silty mudstone-siltstone (B-2) and 0.08 m gray black shale (B-3) (Fig. 6(b)). Dark-gray shale dominates Section B-1 with plentiful laminas having developed. Argillaceous laminas constitute the main staple in this section while silty laminas are occasionally observed. Looking at Section B-1 from the bottom up, the color becomes darker and clay content increases in. The lithological interfaces between Section B-1 and Section B-2 are obvious. Section B-2 is mostly comprised of silty mudstone and siltstone. While its clay content exhibits an increasing trend and then a decreasing when viewed from the bottom up, and argillaceous wavy laminas are developed in middle part of Section B-2. Lithology changes frequently from 2461.17 to 2461.04 m, with black-gray shale, gray silty shale and gray siltstone developing successively from the bottom up and the oblique or wavy lithological interfaces between these three kinds of lithology are obvious. Boulder-clays and silty lenticle could be found in the silty shale, and wavy bedding can be seen in siltstone. Gray-black shale are developed in Section B-3 with horizontal laminas having formed.

4.3 Geochemical characteristics of siltstone-mudstone characteristics of siltstone-mudstone rhythm sections

The siltstone-mudstone rhythmic sedimentary sections are mainly composed of SiO₂, Al₂O₃ and CaO and the sum of these three components add up to 80.32% (Table 1). The content of SiO₂ in Section-A was higher than that in Section-B, respectively in the ranges of 53.08%‒58.41% and 35.94%‒63.58%, with the respective average values of 56.04% and 43.42%. Al₂O₃ content in these two sections are similar, respectively in the ranges of 6.25%‒14.22% and 4.80%‒12.64% with respective average values of 10.76% and 7.26%. CaO content in Section-A was lower than that in Section-B, respectively in the ranges of 6.20%‒13.73% and 4.10%‒17.88%, with the respective average values of 9.55% and 14.41%. Other than the three main components, Fe₂O_3, MgO, K₂O and Na₂O were also identified. The Fe₂O₃ contents of Section-A and Section-B are respectively in the ranges of 4.19%‒5.13% and 4.10%‒6.41%, with the respective mean values of 4.71% and 5.78%. The MgO contents of Section-A and Section-B are respectively in the ranges of 2.82%‒4.13% and 3.05%‒8.33%, with the respective mean values of 3.28% and 6.96%. The K₂O contents of Section-A and Section-B are respectively in the ranges of 1.06%‒3.78% and 1.08%‒3.24%, with the respective mean values of 2.54% and 1.77%. The Na₂O contents of Section-A and Section-B are respectively in the ranges of 1.33%‒1.70% and 0.74%‒1.56%, with the respective mean values of 1.47% and 1.03%. MnO₂ contents and TiO₂ contents are less than 1.0%, while the content of P₂O₅ is about 0.1%. Compared with the Average Shale (Tribovillard et al., 2006), the Al₂O₃ content in siltstone-mudstone rhythm sections is higher, indicating that there may be an influence of terrigenous clastic material input.

Only the authigenic trace elements in the sediments can accurately determine the palaeoenvironment. Therefore, the effect of terrestrial components in the trace elements in sediments should be removed, and a common method adopted in this regard is Al-standardization (Tribovillard et al., 2006). After the standardization of trace elements, the comparison with the world average shale (AS) is shown in Figs. 7 and 8. Mo, U and Ba in the siltstone-mudstone rhythmic sedimentary sections of the Lower Member of the Longmaxi Formation are all higher than the world average shale, showing an obvious abundance. The content variation of Mo ranges from 1.04‒5.04 µg/g with an average of 2.88 µg/g. The content of U spans from 1.33 to 4.22 µg/g and the average value figure is 2.61 µg/g. Ba content varies from 325.0 µg/g to 861 µg/g with an average value of 599.27 µg/g. The content of V, Co, Ni and Cu in the siltstone-mudstone rhythmic sedimentary sections of the Lower Member of the Longmaxi Formation are lower than the world average shale. The content variation of V is 13.5‒88.1 µg/g with an average of 47.82 µg/g, while the content variation of Co is 3.02‒12.9 µg/g with an average of 22.33 µg/g. The content variation of Cu is 6.79‒25.9 µg/g with an average of 47.82 µg/g, while the content variation of Co is 3.02‒12.9 µg/g with an average of 16.10 µg/g. Compared with the shale layer, the siltstone layer has lower V, Co and Ni contents (Table 2).

5 Discussion

5.1 Hydrodynamic condition

For marine shale, effects of hydrodynamics will gradually weaken with the increase of seawater depth because of lower seawater temperature, decreasing oxygen content and fewer biological disturbance (Wang et al., 2014b). In general, strong hydrodynamic conditions correspond to a relatively shallower, oxygen-rich, warmer and suitable environment, while weak hydrodynamic conditions are typically found at relatively deeper, anoxic and colder environment (Wang et al., 2014b). Relatively weak hydrodynamic condition is seen in the shale layer, which is mainly comprised of suspended deposition (Wang et al., 2014b). Suspended clay and silt particles sank to the bottom of the sea and formed horizontal laminas, which have been preserved because of the absence of influence form bottom current and bioturbation.

Sand-body reshaped by bottom current could be identified by primary sedimentary structures, and the tractive structure is the only reliable source to identify sand-bodies reshaped by bottom current (Shanmugam, 2000, 2011; Zhao et al., 2017). Unlike turbidite deposit, siltstone deposited by the bottom current usually features a top interface that makes abrupt contact with the overlying strata, and also develops an internal erosion interface (Zhao et al., 2017). The rhythmic laminas (Fig. 4(a)), wavy laminas (Fig. 4(e)), small scouring surface (Fig. 4(c)) and directional arrangement of boulder-clays can be seen in the siltstone-mudstone rhythmic sedimentary sections (Fig. 4(b)), which are all the products of tractive current (Shanmugam, 2011; Zhao et al., 2016). In addition, the abrupt interface between shale and siltstone (Figs. 4(b), 4(f) and 4(g)) shows that there were sudden changes in hydrodynamics. Therefore, the siltstone layer in siltstone-mudstone rhythmic sedimentary sections were deposited under a relatively strong hydrodynamic conditions involving bottom current, and bottom current contributed to the formation of the siltstone.

5.2 Palaeoredox conditions

U, V, Ni, Mo, Th and Cr are reliable indices to interpret palaeoredox conditions (Jones and Manning, 1994), with Ni and Co often employed as redox index. High Ni/Co ratio (>7.0) corresponds to a reducing atmosphere, and a Ni/Co ratio of 5.0‒7.0 indicates an anoxic atmosphere, while low Ni/Co ratio (<5.0) correlates to anoxic atmosphere (Jones and Manning, 1994). Ni/Co values vary from 2.57 to 4.08 with an average of 3.18 in Section-A, and range from 2.78 to 3.59 with an average of 3.07 in Section-B, with all values pointing to an oxic atmosphere. The U/Th ratio is also a reliable redox index. High values of U/Th (>1.25) indicate an anoxic atmosphere, and low values of U/Th (<0.75) represent an oxic atmosphere, while a U/Th ratio between 0.75 and 1.25 is linked to a dysoxic atmosphere (Wilkin et al., 1997). There is little change between the U/Th ratios of Section-A and Section-B, which respectively range from 0.21 to 0.25 and 0.18 to 0.27, with respective averages of 0.23 and 0.22, meaning that the environment had an oxic atmosphere. The values of V/(V+ Ni)<0.46 are thought to represent oxic depositional conditions. Values ranging from 0.46 to 0.57 are indicative of the weak-oxidizing depositional conditions, while values between 0.57 and 0.83 suggest dysoxic-anoxic depositional conditions, and values between 0.83 and 1 are understood as indicator of euxinic environment (Wingenall, 1994). The values of V/(V+ Ni) range from 0.64 to 0.72 with an average of 0.69 in Section-A, while Section-B data range from 0.55 to 0.70 with an average of 0.64. All data reflect that the depositional environment had a dysoxic-oxic condition. Furthermore, V/Cr has also been used as a redox index. Low values (V/Cr<2) imply oxic conditions, values ranging from 2 to 4.25 designate dysoxic conditions, and a high value (V/Cr>4.25) represents anoxic conditions (Jones and Manning, 1994). Values of V/Cr were between 0.23 and 1.41 with an average of 1.10 in Section-A, while values range from 0.06 to 0.24 with an average of 0.12 in Section-B. All values of V/Cr indicate an oxic environment. According to the four redox indexes, oxic conditions prevailed in the palaeoenvironment of the siltstone-mudstone rhythm sections in the Lower Member of the Longmaxi Formation (Figs. 9 and 10).

The cross plot of Mo-U shows that the values of Mo/U in siltstone-mudstone rhythmic sedimentary sections are mainly between 0.3 × SM and SM, except two data from section-A slightly higher than SM. Which indicates that the siltstone-mudstone rhythmic sedimentary sections are formed in the oxic environment with the anoxic environment occasionally happened during the depositional process (Fig. 11).

The CIA value can be used as a proxy for interpreting the intensity of chemical weathering in the provenance (Young and Nesbitt, 1998). Sediments deposited in cold and arid climates have CIA values from 50 to 70, those in a warm and humid climate have CIA values of 70‒80, and CIA values of 80‒100 correspond to hot and humid climate (Nesbitt and Young, 1982; Liu et al., 2019; Yan et al., 2010). The CIA values are between 69.96 and 74.46 with an average of 72.44 in section-A, while between 70.74 and 73.67 with an average of 71.97 in section-B. All data from CIA values indicate the warm and humid paleoclimate during the depositional period of the siltstone-mudstone rhythm (Figs. 9 and 10).

5.3 Sedimentary model

Sea level fell due to the Gondwana glacier event during the late depositional stage of the Wufeng Formation (Couto et al., 2013). The sedimentary environment transformed from deep water and anoxic environment during the early depositional stage of the Wufeng Formation to the shallow and oxic environment during the late depositional stage of the Wufeng Formation, resulting in sediments that changed from organic-rich shale to bioclastic limestone or bioclastic mudstone (Zhang et al., 2019). In the early depositional stage of the Longmaxi Formation, sea level rose as a result of the melting of the Gondwana glacier, dysoxic-anoxic deep water shelf dominated the majority of the Sichuan Basin area, leading to a broad distribution of organic-rich shales (Fig. 12(a)) (Wang et al., 2014b; Chen et al., 2004; Chen et al., 2015b). As the sea level dropped, the oxygen content of water column increased as a result of the influence of deep water bottom current and terrigenous supply, which led to the development of siltstone-mudstone rhythmic sedimentary sections (Fig. 12(b)). Subsequently, the sedimentary environment gradually changed to an oxic environment due to the continuous decline of sea level, with gray argillaceous siltstone and siltstone depositing in the Upper Member of the Longmaxi Formation (Rong et al., 2019).

6 Conclusions

The two siltstone-mudstone rhythmic sedimentary sections of the Longmaxi Formation in the Changning area analyzed from samples of the N1 well are characterized by frequent interbed between gray-black shale and light-gray siltstone. Argillaceous laminas and silty laminas are well developed in siltstone-mudstone rhythmic sedimentary sections. The shale layer mainly features well-developed horizontal rhythmic laminas, while the siltstone layer has well-developed wavy laminas. The major compositional elements are SiO₂, Al₂O₃ and CaO, followed by Fe₂O₃, MgO, K₂O and Na₂O. Compared with the world average shale, the siltstone-mudstone rhythmic sedimentary sections are rich in Mo, U and Ba but loss in V, Co, Ni and Cu. Compared with shale, siltstone has lower V, Co and Ni values. The geochemical redox indices, Mo-U and CIA values suggest the formation of the siltstone-mudstone rhythmic sedimentary sections are related to the influence of bottom current in oxic conditions with a warm and humid paleoclimate.

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