The Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation are China’s major targets for shale gas exploration^[1]. These strata have organic-rich shale (TOC>2%), large thickness and broad distribution, abundant graptolitic^{[1,2,3,4,5,6,7]} and bentonite^{[2, 5, 8-15]}. Currently, geologists commonly use biological fossils such as graptolite and brachiopoda as the basis for dividing strata, and then carry out the geological study and shale gas exploration assessment^{[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]}. In recent years, the authors found in field study that it is difficult to find the first appearance position (i.e., the boundary of stratification) of the target graptolitic belt, since the rock layers are hard and firm and severely weathered in surface, and rare graptolites are exposed to the surface. A continuously distributed bentonite bed with 5 to 40 cm thickness is present in the lower-bottom part of the Demirastrite triangulatus belt across the Sichuan Basin and its periphery region, which is named by the authors the Aeronian Demirastrite belt thick-bedded bentonite ^{[2, 5]} and can be used as an important reference interface for defining the top of the Rhuddanian^{[2, 5]}. This suggests that, the isochronic and broadly distributed bentonite bed has important reference value for fine division and correlation of black shale.

At present, problems related to the study of the high-frequency bentonite beds include: (1) Little effective data available and weak study foundation, there is no complete outcrop section or well data revealing the development characteristics of bentonite in the Wufeng Formation and Longmaxi Formation, and bentonite in public reports is present mainly in the Wufeng Formation, the basal part of the Rhuddanian and the basal part of the Aeronian, as evidence by limited data acquired from Shuanghe in Changning and Jiaoshiba in Fuling^{[2, 5, 9-14]}. (2) The methods of study and approach to application are limited, and key understandings remain controversial; most researches and applications of bentonite before were limited zircon dating, regional correlation of bentonite beds and the relationship between the volcanic ash and the organic matter richness, and the study results had issues like big error in dating, complex stratigraphic correlation scheme, and highly controversial fertilizing effect of the volcanic ash, making the scientific value of the bentonite undervalued.

In order to solve these problems, based on the Wufeng Formation-Longxi Formation on Qiliao section in Shizhu county, the Eastern Sichuan Basin, the standard profile of bentonite was built by surveying the field section and testing key rock mineral, organic geochemical and elemental geochemical data, the vertical and lateral variation patterns of key bentonite beds and their relationship with the regional tectonic activity, paleo-environmental evolution and formation of high-quality shale were examined carefully to find out the special geological significance of the bentonite bed.

The Qiliao section is located in the central part of the Yangtze Sea Basin (Fig. 1), where the whole Wufeng and Longmaxi formations are continuous and complete. The outcropped black shale is over 130 m thick and contains abundant graptolites and complete fossils. The strata deposited include, from the bottom to top, Katian, Hirnantian, Rhuddanian and Aeronian. The GR value ranges from 150 to 480 cps over the strata and 180 to 300 cps in most of the layers (Fig. 2).

The Wufeng Formation is 10.6 m thick and graptolitic-rich. Its lower part consists of grey black to black, thin-bedded siliceous shale and is in conformable contact with the underlying Linxiang Formation; the middle part comprises interlayers of carbonaceous shale and thin-bedded siliceous shale, with 5 bentonite beds intercalated; the upper section is composed of black, thin-medium thick siliceous shale with some bentonite beds intercalated; and the top part is 0.89 m thick Guanyinqiao member black dolomitic siliceous shale that contains Hirnantian shell fossils. GR is low in the middle and lower parts of the Wufeng Formation (150 to 200 cps), rises significantly in the upper section, and has a peak of the natural gamma in the proximity of the interface between the top part of the Guanyinqiao member and the basal part of the Longmaxi Formation^{[2, 4-5]} (the GR peak of No.9 is 274 to 481 cps) (Fig. 2).

The Longmaxi Formation is over 190 m thick. In its lower part the Rhuddanian, 37.05 m thick, is made up of black, thin-moderate thick siliceous shale layers in the bottom, grey black, moderate- thick siliceous shale and thick-bedded argillaceous and siliceous mixed shale in the middle and upper parts, with multiple bentonite beds in the middle part, and graptolite fossils like the Cystograptus vesiculosus and Coronograptus cyphus (Fig. 2). The GR ranges from 177 to 300 cps, drops slowly from the lower to the upper parts, and spikes at the No.10 to No.15 and No.18 beds (Fig. 2).

The Aeronian in the upper part of the Longmaxi Formation is over 150 m thick and can be divided, from bottom to top, into black shale (over 90 m thick), light siltstone (over 10 m thick) and grey green argillaceous shale (over 50 m thick) members. The black shale member was surveyed in detail. It consists mainly of thick-bedded argillaceous and siliceous mixed shale, argillaceous shale, carbonaceous shale and multiple nodule beds. Its GR ranges from 180 to 250 cps and shows spikes at the No.24, 32, 34, 36 and 38 beds (Fig. 2). It contains graptolitic belts such as the Demirastrites triangularis and Lituigraptus convolutes belts. In particular, a thick bentonite bed (No.24 bed) is present in the lower part of the Demirastrites triangularis belt.

In the Shizhu area, the Wufeng Formation-Longmaxi Formation black shale is comprised mainly of the semi-deep water to deep-water shelf sediments, with Type I to II₁ kerogen, and δ¹³C of -30.9‰ to -29.1‰. The shale has higher organic matter abundance, evidenced by TOC of 1.2% to 11.2% (decreasing slowly upward), and 2.7% on average (Fig. 2). The lower part consists mainly of organic-rich shale with TOC of over 2%, a thickness of 35 m and TOC range from 1.9% to 11.2%, averaging 3.7%. The TOC peaks occur at the Guanyinqiao member (5.4% to 11.2%) and the Normalograptus persculptus belt (5.0% to 7.3%), and low TOC occur at the middle and lower parts of the Wufeng Formation (averaging 2.6%) and the middle and upper parts of the Rhuddanian (averaging 2.4%) (Fig. 2). In the middle and upper parts, the formation with lower organic matter abundance in general have a thickness of 100 m, and TOC from 0.8% to 2.4% (1.7% on average). Shale sections with a TOC of over 2% include the No.21 to 23, 26, 32, 33, 36 and 37 beds (Fig. 2).

Thus, the organic-rich shale in the Shizu area is 85 m thick in total. The layer from the Wufeng Formation to the middle part of the Coronograptus cyphus belt is the best quality shale section, with a mineral brittleness index (the percentage of quartz, dolomite and pyrite in total rock minerals) of over 50% and TOC of over 3%. The upper part of the Coronograptus cyphus belt to the Demirastrites triangulates belt is the second best quality shale section with a mineral brittleness index range from 40% to 50% and TOC range from 1.5% to 3.0%^[5] (Fig. 2).

The Qiliao section in the Shizhu area provides an ideal data point for observing the development characteristics of bentonite, since the section is fresh and contains complete graptolitic belts and black shale bed fully outcropped. The authors saw a total of 31 bentonite beds with over 0.5 cm thickness each on this section from the Katian to the middle part of the Aeronian (about 105 m thick), which are distributed unevenly in 6 graptolitic belts and 18 sub-layers (Fig. 2 and Table 1).

From the development frequency and scale of bentonite (Tables 1 and 2), volcanic ash occurs mainly in the middle and upper parts of the Dicellograptus complexus belt, the top part of the Paraorthograptus pacificus belt, the basal and middle - upper parts of the Coronograptus cyphus belt, the lower part of the Demirastrites triangulatus belt, and the upper part of the Lituigrapatus convolutus belt, and rarely or doesn’t occur in other graptolite belts or sections. From the development scale per Ma (Table 2), the development rate of bentonite is higher in the Dicellograptus complexus (16.7 cm/Ma), Coronograptus cyphus (24.9 cm/Ma), Demirastrites triangulatus (10.8 cm/Ma) and Lituigrapatus convolutus (24.0 cm/Ma) belts. That is to say that the bentonite developed mainly during the initial stage of the Katian, the later stage of the Rhuddanian, and the Aeronian, similar in development characteristics with that in the Changning area^{[2, 5]}. What the bentonite in this area is different from the bentonite in southern Sichuan is that the bentonite in the Coronograptus cyphus belt here has much higher development rate than that in other graptolitic belts.

It is found through study that only the bentonite beds over 5 cm thick individually or the bentonite-rich members with a cumulative bentonite thickness of over 5 cm have great geological significance^{[2, 5]}. Thus, this type of bentonite beds (or bentonite-rich members) was examined carefully in this study. The bentonite-rich member is defined as the black shale section with bentonite over 5 cm thick in individual layer or in total in 1 m thickness. Accordingly, 6 bentonite-rich members were recognized on the Qiliao section in Shizhu, namely the bentonite-rich member ① to ⑥ (Figs. 2 to 4), with a cumulative bentonite thickness of 48 cm (as per average individual thickness), accounting for 76% of the total bentonite thickness on the section.The bentonite-rich member ① is in the middle-upper part of the Dicellograptus complexus belt (extending from the top of the No.5 bed to the middle and upper parts of the No.6-1 bed), with a thickness of about 1.3 m (Fig. 3a). This member is composed of thin interlayers of carbonaceous shale and siliceous shale, with no trace of lamina but abundant radiolarian grains in star- or dot-like distribution under microscopy (Fig. 4a and 4b). It has a TOC range from 1.92% to 2.31%, GR range from 154 to 172 cps, and mineral composition of 73.3% to 78.1% quartz, 0.9% to 3.7% feldspar and 21.0% to 23.0% clay (Fig. 2 and Table 1). It contains 5 bentonite beds, with a cumulative thickness of 10 cm. Two bentonite beds at the base are 0.5 to 1.0 cm thick each and weathered to grey white claystone. Three bentonite beds in the middle and upper parts vary in thickness: the upper one is 4 to 5 cm thick, the middle one is 2 to 3 cm thick and the lower one is 1 to 2 cm thick, they are 10 to 30 cm apart from each other. The bentonite has a GR range from 209 to 218 cps, and is composed of 6.9% quartz, 1.8% quartz, 1.4% calcite, 18.9% pyrite and 71% clay (Fig. 2 and Table 1). Major elements, SiO₂, Al₂O₃, Fe₂O₃+FeO, MgO, and K₂O in the bentonite account for 41.57%, 19.94%, 16.61%, 1.56% and 6.08% respectively.

The bentonite-rich member ② is at the top part of the Paraorthograptus pacificus belt (extending from the top part of the No.6 bed to the upper part of the No.7 bed), with a thickness of about 1 m. It consists of thin-bedded siliceous shale (Fig. 3b), with no lamina but abundant radiolarian grains in star- or dot-like distribution under microscopy (Fig. 4c and 4d). It has a TOC range from 2.68% to 5.55%, GR range from 224 to 252 cps (with low-amplitude peak), and mineral composition of 63.9% to 83.8% quartz, 2.0% to 4.4% feldspar, 1.0% to 1.6% pyrite and 13.2% to 30.1% clay. Two lead grey bentonite beds occur at the base and the upper part and vary in thickness: the lower one is 3 to 4 cm thick and the upper one is 2 to 3 cm, (Fig. 2 and Table 1), with a cumulative thickness of 6 cm (Fig. 2 and Table 1).

The bentonite-rich member ③ is present at the base of the Coronograptus cyphus belt (the top of the No.12 bed), with a thickness of 0.3 m. It consists of moderate-bedded siliceous shale (Fig. 3c) with higher clay content and radiolarian laminae under microscopy (Fig. 4e and 4f). It has a TOC of 3.2%, a GR range from 217 to 231 cps (with low-amplitude peak), and mineral composition of 56.7% quartz, 7.5% feldspar, 1.7% pyrite and 34.1% clay. It contains two bentonite beds, 25 cm apart, with a cumulative thickness of 4 cm. The lower one is 1 to 2 cm thick and the upper one is 2 to 3 cm thick (Fig. 2 and Table 1).

The bentonite-rich member ④ lies in the middle and upper parts of the Coronograptus cyphus belt (extending from the top part of the No.17 bed to the lower part of the No.18 bed), with a thickness of 1.5 m. It consists of thick-bedded argillaceous and siliceous mixed shale with higher clay content (Figs. 2 and 3d), abundant Monograptus and horizontal lamina under microscopy (Fig. 4g and 4h). It has a GR range from 208 to 226 cps (with low-amplitude peak), a TOC range from 1.63% to 1.96%, and mineral composition of 47.7% to 53.7% quartz, 15.3% to 22.1% feldspar, 0 to 1.4% pyrite and 28.5% to 35.2% clay. It contains seven bentonite beds 15 to 50 cm apart, 0.5 to 4.0 cm thick each and 9.9 cm thick combined (Fig. 2 and Table 1).

The bentonite-rich member ⑤ is in the lower part of the Demirastrites triangulatus belt, with a thickness of 8 to 10 cm. It is the Aeronian Demirastrites belt thick-bedded bentonite (Figs. 2 and 3e), monolayered, and lead grey. Stable across the region, it is an important marker bed for regional correlation^[5]. It has a GR range from 216 to 220 cps (with low-amplitude peak) (Fig. 2 and Table 1). Having altered, this bentonite bed is composed of 1.8% quartz, 1.0% feldspar, 67.5% pyrite, 1.9% barite and 27.8% clay (Table 1). It is overlain by argillaceous shale with horizontal laminae, in which bright-colored grains are mostly quartz and radiolarian (Fig. 4i and 4j).

The bentonite-rich member ⑥ is located in the upper part of the Lituigrapatus convolutus belt, with a thickness of 0.1 m. Monolayered, and lead grey (Figs. 2 and 3f), it is the first discovered thick-bedded bentonite in that belt. It has a GR of 237 cps (showing moderate-amplitude peak) (Fig. 2 and Table 1). This bentonite bed is overlain by argillaceous shale with horizontal lamina, in which the bright-colored grains are dominated by quartz and radiolarian (Fig. 4k and 4l).

From the development characteristics of these 6 bentonite-rich sections, it is found that most of these sections are characterized by increase of clay content, occurrence of GR peak response and poor correlation between volcanic ash and total organic carbon content. For example, members ③ to ⑥ all occur in shale sections with significant increase of clay content and abundant laminae; members ② to ⑥ show clearly higher GR than their surrounding rocks and moderate- to low-amplitude peaks of GR (Fig. 2), which is basically consistent with the feature that the clay content increases greatly; and members ① to ⑥ are volcanic-rich but the associated black shale layers (or overlying black shale) show no abnormally high total organic carbon contents, rather are less than 3% in most of the layers. These characteristics have been further proved by LY1, a well drilled in adjacent area (Fig. 5). In this well, members ② to ④ show high GR and high ϕ_CNL, section ④ in particular has high GR, similar with the Shizhu section, indicating the significant increase in clay minerals and radioactive materials; member ① occurs in the middle-lower part of the Wufeng Formation and, similarly, exhibits high GR, high ϕ_CNL and higher clay content; member ⑤ appears in the basal part of the carbonaceous shale in the lower part of the Demirastrites triangulatus belt and shows high TOC, high clay content and high GR, it is represented by a trough response in high GR interval, similar to the Wuxi and Baokang areas^{[2, 5]}; The total organic carbon content doesn’t increase in the members ① to ④, instead, it is below 3% in most of them. Thus, the GR peak of the most of the bentonite-rich members is a direct reflection of the substantial increase of clay minerals and radioactive materials and is less related to the total organic carbon content. This suggests that the 6 bentonite-rich members in the Eastern Sichuan-Western Hubei region share similar properties with the bentonite-rich members in the basal part of the Wufeng Formation in Changning (i.e., structural interfaces between the platform and the shelf^{[2, 5]}) and, unlike the GR peak of the Hirnantian (i.e., the transitional interface between the glacial period and the interglacial period), shall be structural interfaces, which provide a direct evidence of the intra-plate flexural deflection under the continual collision and convergence of the Yangtze Block and surrounding blocks at the turn of the Ordovician-Silurian.

In this study, focusing on the bentonite-rich members ③ to ⑤, the distribution characteristics of the Longmaxi Formation bentonite in the Eastern Sichuan Basin and its adjacent areas were revealed by comparing the development characteristics and GR logs of the bentonite beds on outcrop sections and in many wells, including the Datianba section in Xiushan, Hongyanxi section in Longshan, Sanhu section in Laifeng, Guanwu section in Hefeng, Well HY1 in Enshi, Maoba section in Lichuan, Well LY1, Qiliao section in Shizhu, Bailu section in Wuxi, Shuanghe section in Changning, Bayu section in Daozhen and Huangcao section in Wulong (Table 3, Figs. 6 and 7).

Bentonite-rich member ③ is broadly distributed in the Eastern Sichuan Basin, Southern Sichuan Basin and Northern Guizhou region and occurs in the basal and lower parts of the Coronograptus cyphus belt, with moderate- to low-amplitude GR peak. However, it is commonly absent in the Hunan- Western Hubei region (Table 3, Figs. 6 and 7). This member is thickest in Changning, the Southern Sichuan Basin, and thins to the east and north. Its total thickness is 1.5 to 2.0 m^{[2, 5]} in Changning, but decreases to 0.9 to 1.1 m in Wulong-Daozhen and 0.15 to 0.30 m in Wuxi-Shizhu-Lichuan. It is absent in Longshan-Hefeng-Enshi. The thickness of individual bentonite beds decreases from 8 cm in Changning^{[2, 5]} to 1 to 2 cm in Wulong-Daozhen and 2 to 3 m in Wuxi-Shizhu-Lichuan. It is inferred from the thinning of total thickness and individual maximum thickness of the bentonite beds from southwest to north and east that the volcanic ash of at the initial stage of Coronograptus cyphus belt deposition was originated from the southwestern or southern margin of the Yangtze Sea Basin.

Bentonite-rich member ④ is distributed mainly in the Eastern Sichuan Basin and Hunan-Western Hubei region and in the middle and upper parts of the Coronograptus cyphus belt, with moderate- to high-amplitude GR peak (Table 3, Figs. 6 and 7). It is over 1 m thick in many areas, such as Shizhu, Wuxi, Lichuan, Longshan and Xiushan, but either doesn’t expose or isn’t observed in Changning, Daozhen and Qijiang. There are 8 bentonite beds in Hefeng, 7 in Shizhu and 3 or less in Longshan-Xiushan in this member. It is therefore speculated that, the volcanic ash during the middle and late depositional stages of Coronograptus cyphus belt might be originated from the eastern or northeastern margin of the Yangtze Block, unlike that in the early depositional stage of the graptolite belt.

Bentonite-rich member ⑤ consists of the Aeronian Demirastrite belt thick-bedded bentonite and is distributed broadly over the Middle and Upper Yangtze areas. It may cover larger area than the Guanyinqiao member. Its volcanic ash was originated from the southwestern margin of the Yangtze Sea Basin^{[2, 5]}.

Bentonite-rich members in the Eastern Sichuan Basin and adjacent areas are distributed broadly and commonly show GR peak on well-log, thus, they can serve as important interfaces for stratigraphic correlation and have great significance in revealing the tectonic activities and organic matter enrichment law in this region. Accordingly, in this study, based on development characteristics and major understandings of the bentonite, the regional variation pattern of the major members of the Longmaxi Formation in the Eastern Sichuan Basin and Western Hubei region were looked into by defining major interfaces, and the absence status of the Longmaxi Formation in the Yichang Uplift zone was examined, and the development characteristics of the Longmaxi Formation organic-rich shale were sorted out.

During preparation of the geological map of the Wufeng Formation and Longmaxi Formation and evaluation of the shale gas zone, it is found that the Wufeng Formation shows clear base and top interfaces^{[1-2, 4-5]}, and is geologically clearly understood^{[1, 3, 8, 13, 17]}; while for the 8 to 9 graptolitic belts in Longmaxi Formation, it is difficult to determine the first appearance position (i.e., the interface) of most of them, resulting in different sequence divisions and maps of the Longmaxi Formation^{[1, 3, 8, 13, 17]}.

It is found through above discussion that the bentonite-rich members ③, ④ and ⑤ are important reference interfaces for dividing and determining the base of the Coronograptus cyphus belt and the top of the Rhuddanian in the Eastern Sichuan Basin-Western Hubei region (Figs. 5 and 7). The bentonite-rich member ③ is generally present in the basal part of the Coronograptus cyphu belt and can serve as the reference interface for dividing the base of that belt. The practice is to use the first trough occurring below the GR peak as the boundary that separates the Cystograptus vesiculosus belt from the Coronograptus cyphus belt (Figs. 5 and 7). The bentonite-rich member ④ appears in the middle-upper part of the Coronograptus cyphus belt. The existence of its GR peak illustrates the Rhuddanian has eroded to the upper part of the Coronograptus cyphus belt in the hinterland of the Yichang Uplift zone. The base of this GR peak is defined as the base of the Rhuddanian (Figs. 6c, 6d and 7). The bentonite-rich member ⑤ is an important reference interface for defining the top of the Rhuddanian, which is usually placed on the first trough of the GR below this member^{[2, 5]}.

Taking these bentonite-rich members as the basis for subdividing the Rhuddanian and Aeronian, the distribution of the Rhuddanian and Aeronian black shale in the Sichuan Basin and adjacent areas was systematically mapped by referring to the graptolites (Figs. 1 and 8). The results show that the Rhuddanian is complete in the Southern Sichuan-Eastern Sichuan regions, with a sedimentation time of 3.06 Ma and thickness of 10 to 40 m. Only the 3 to 7 m thick upper part of the Coronograptus cyphus belt exists in the hinterland of the Hunan-Western Hubei Uplift. This belt is the major sedimentary body of the Rhuddanian, and is over 31.25 m thick (84.4% of total thickness) in Sizhu, although the sedimentation time is only 0.8 Ma (26% of total time). The Aeronian black shale is much thicker than the Rhuddanian black shale. It is 50 to 150 m thick in the Southern Sichuan-Eastern Sichuan regions (up to 200 m thick in local parts), 20 to 50 m thick in the Wuxi-northern depression zone of the Central Yangtze region, and 10 to 20 m thick in the eastern slope of the Central Sichuan Uplift, northern slope of the Central Guizhou Uplift and Hunan-Western Hubei region (Fig. 8).

The Yichang Uplift was a large-scale underwater uplift formed at the turn of the Ordovician-Silurian close to the eastern side of the Eastern Sichuan Depression (i.e., the Hunan-Western Hubei Uplift, as shown in Fig. 1) and is deemed to have a strong control on development of the graptolitic shale in the Eastern Sichuan Basin and its adjacent areas^{[1, 4, 6-7, 18-19]}. Predecessors, based on the presence of graptolites, believed that the sedimentary hiatus occurred between the Paraorthograptus pacificu belt and the Coronograptus cyphus belt in the hinterland of the Yichang Uplift, leading to the absence of 5 graptolitic belts^{[6-7, 18-19]}.

Based on the Qiliao section in Shizhu, Well LY1 data and Maoba section in Lichuan (Figs. 2 and 5 and Table 3), 2 bentonite-rich members (③ and ④) are present in the basal and middle-upper part of the Coronograptus cyphus belt in the area adjacent to the west of the Yichang Uplift, which show two peaks on GR log (Fig. 5) 6 m (in Well LY1) to 13.9 m apart (on the Qiliao section in Shizhu). In the hinterland of the Yichang Uplift, such as Enshi, Hefeng and Longshan, only the bentonite-rich member ④ occurs in the Coronograptus cyphus belt, which shows single peak on GR log (Figs. 6c, 6d and 7). As per the data of Well LY1, it is inferred that a 6 m thick black shale is absent in the lower part of this belt, accounting for over 50% of its total thickness, and the sedimentary hiatus time exceeds 0.4 Ma. Thus, it is speculated that, the Rhuddanian in the hinterland of the Yichang Uplift lost at least 3.5 graptolitic belts: i.e., the Akidograptus ascensus to the middle and upper parts of the Coronograptus cyphus belt, and is less than 0.4 Ma in sedimentary time.

It is therefore reckoned that, the strata absent in the hinterland of the Yichang Uplift during the entire uplifting stage included the Hirnantian and the middle and upper parts of the Coronograptus cyphus belt, corresponding to the bentonite-rich member ② to ④ in Shizhu, which means that at least 5.5 graptolitic belts are absent. This suggests that the bentonite-rich member ② and ④ deposited at two critical time nodes: i.e. the start and end of the uplifting of the Yichang Uplift. Affected by the deep-part geotectonic movements occurring in the southern and eastern parts of South China, the two groups of intensive volcanic eruptions represented two times of sharp shifting in the tectonic stress field in the Hunan-Western Hubei region and its periphery (S-N compression at early stage and E-W extension at later stage). As a consequence, this region witnessed uplifting first and then rapid subsidence. More geological evidences are needed to prove the shifting mechanism of the tectonic stress field during the sedimentation of the bentonite-rich member ② and ④.

High-frequency bentonite is an important sedimentary record that represents the intensity of the deflection activity within the Yangtze Sea Basin^{[2, 5]}. Based on the distribution characteristics of the bentonite-rich members on the Qiliao section in Shizhu and the tectonic sedimentary responses in the Southern Sichuan Basin and the Wuxi area^{[2, 5, 16, 20]}, it is concluded that, the deflection depression in the Eastern Sichuan Basin experienced 4 tectonic active stages: i.e., the initial stage of depression, middle-later stage of depression, initial stage of foreland deflection, and development stage of foreland deflection (Fig. 2). These stages varied greatly in sedimentary elements, giving rise to fine-grained sedimentary lithofacies assemblages of different types with significant differences in organic matter abundance (Figs. 1, 2, 8 and 9).

(1) Initial stage of depression: That is the transition from the platform to the shelf (the Dicellograptus complexus belt^{[2, 5]}). This period lasted 0.6 Ma and the development rate of bentonite was 16.7cm/Ma (Table 2), when the Eastern Sichuan Basin experienced fast transformation from platform to shallow-water shelf and then to deep-water depression. In response to the rapid sea level rise, sediments within this area turned quickly from marlstone and grey green argillaceous shale to black graptolitic shale, with thickness of nearly 5 m and TOC increasing from 0.1% to 1.7% and clay content decreasing from 51.2% to 21.0% (Fig. 2).

(2) Middle-late stage of depression: That is the middle period of the Katian stage (the Paraorthograptus pacificus belt) to the late period of the Rhuddanian stage (base of bentonite-rich member ④), when large-scale uplifts and depressions came about^[5] (Fig. 2). This stage lasted over 5.85 Ma, when the regional tectonic activity was gentle and the development rate of bentonite was 0.8 to 3.2 cm/Ma. The Eastern Sichuan Basin- the north of Central Yangtze region was a depression behind the uplift delimited by the Hunan- Western Hubei and Central Sichuan underwater uplifts opening to the north. It was located far from the provenance to the southeast (Fig. 1) and the clay content ranges from 9.3% to 42.6%, averaging at 28.4% (Fig. 2). Sea level remained high, δ¹³C ranged from -30.9‰ to -29.6‰ and mainly showed negative drift, and V/(V+Ni) ranged from 0.67 to 0.91 (averaging 0.81, higher than the oxygen deficit criterion of 0.77^{[16, 21]}), which suggests that the sea bed was oxygen-deficient environment. The S/C ranges from 0.01% to 0.35% (averaging 0.11), and Mo content ranges from 4.2 to 100.0 μg/g (averaging 31.0 μg/g), indicating that the sea area was low in salinity and weakly closed. P₂O₅/TiO₂ ranges from 0.10 to 2.89 (averaging 0.28, peaks at the Guanyinqiao member), and Ba content ranges from 481 to 1885 μg/g (averaging 1347 μg/g), indicating that the sea water was rich in nutrient substances, such as P and Ba, and had relatively high primary productivity (Fig. 2). The sedimentation rate was quite slow, about 1.27 to 3.32 m/Ma (Table 2). The black shale has a TOC of 1.9% to 11.2% (averaging 4.0%) and silica content of 48.6% to 88.8% (averaging 60.8%) (Fig. 2). In particular, the shale of Rhuddanian has a TOC of 2.9% to 5.0% (Fig. 9a).

(3) Initial stage of foreland deflection: That is the period of about 0.4 Ma from the base of the bentonite-rich member ④ to prior to the presence of bentonite-rich member ⑤, which witnessed the transition from the depression to the foreland (Fig. 2) with a development rate of bentonite ranging from 10.8 to 24.9 cm/Ma. In this period, the downward deflection degree of the southeastern part of the Yangtze platform increased gradually, the center of subsidence and sedimentation began to shift from the southeast to northwest^{[1-2, 5]}, the Hunan-Western Hubei uplift subsided rapidly into the southeast slope of the Eastern Sichuan Depression, and substantial clays sourced from the southeast provenance entered into the Eastern Sichuan Depression, leading to the rise of average clay content to 37.4%. The sea level in the Eastern Sichuan Depression dropped from high to moderately high, the δ¹³C generally shows positive drift (-30.0‰ to -29.3‰), suggesting the sea bed was generally under the oxygen-lean to oxygen- deficit environment (Fig. 2). Sedimentation rate increased to 20.57 to 33.75 m/Ma (Table 2), average TOC of black shale decreased to 1.8%, and average silica content decreased to 46.5% (Fig. 2).

(4) Development stage of foreland deflection: That is period after the occurrence of bentonite-rich member ⑤ and is the primary sedimentation period of the Aeronian. The development rate of bentonite rose to 24.0 cm/Ma (Table 2), reflecting that the dramatic increase of the deflection degree of the Yangtze platform, consequently, the center of subsidence and sedimentation shifted from southeast to north and west, the upwelling became active in the northern basin, the relatively deep-water zone shifted to the center of the Southern Sichuan and Eastern Sichuan Depression, Weiyuan, Wuxi and northern part of the Middle Yangtze region (Figs. 8 and 9b), substantial clay entered into the platform-basin area, and the average clay content in Shizhu rose to 43.5% quickly (Fig. 2). At the early and middle stages, the Eastern Sichuan sea area was generally low to normal in salinity and weakly to semi-closed. The Ba content was relatively high (averaging 2477 μg/g) as a result of the upwelling in the northern part (Figs. 2 and 8). The sedimentation rate increased considerably to 20.47-72.82 m/Ma (Table 2) and average TOC decreased to 1.5% to 2.5%. Average TOC in Shizhu decreased to 1.7% (Figs. 2 and 9b).

In conclusion, the Wufeng Formation-Longmaxi Formation high-quality shale in the Eastern Sichuan Depression was formed during the initial and middle-late stages of depression, prior to the presence of the bentonite-rich member ④, and the slow sedimentation in the Eastern Sichuan Basin was longer than in the Southern Sichuan Basin, indicating that the long-term uplifting of the Hunan-Western Hubei Uplift and its hindering effect on the southeast provenance had a strong control on sedimentation of the organic-rich shale in the depression behind the uplift. During the foreland stage after the presence of the bentonite-rich member ④, as the Hunan- Western Hubei Uplift vanished, substantial clay minerals originated from the southeast provenance entered directly into the hinterland of the Eastern Sichuan Depression, leading to the dramatic increase in sedimentation rate, as a result, the organic matter and silica richness conditions became worse.

Totally 6 bentonite-rich members were found on the Qiliao section in Shuzhu, which occur mainly in 6 graptolitic belts of the Katian, Rhuddanian and Aeronian. Most of these members are characterized by dramatic increase in clay mineral content, GR peak response and poor correlation between the volcanic ash and the organic carbon content. The bentonite-rich members on this section have universal development characteristics and logging response across the Eastern Sichuan-Western Hubei region and adjacent areas, and hence can act as important interfaces for stratigraphic correlation of the Longmaxi Formation.

Using the bentonite-rich members as important reference interfaces for defining the Rhuddanian and Aeronian can make up the deficiency in dividing the strata relying only on graptolitic belts. It is concluded that the Rhuddanian deposited continuously in deep-water environment in the Eastern Sichuan-northern part of the Middle Yangtze region and the Southern Sichuan Depression, with the thickness of 10 to 40 m. But in the hinterland of the Hunan-Western Hubei Uplift, only the upper part of the Coronograptus cyphus belt 3 to 7 m thick deposited. The strata absent in the hinterland of the Yichang Uplift during the entire uplifting stage include the Normalograptus extraordinarius belt to the upper part of the Coronograptus cyphus belt, which means that, at least 5.5 graptolitic belts are absent and the sedimentation time of the Rhuddanian is less than 0.4 Ma.

The Eastern Sichuan Depression experienced 4 tectonic active stages at the turn of the Ordovician-Silurian: i.e., the initial stage of depression, middle-later stage of depression, initial stage of foreland deflection, and development stage of foreland deflection (Fig. 2). High-quality shale layers developed largely during the initial stage and middle-later stage of depression, prior to the presence of the bentonite-rich member ④. The long-term uplifting of the Hunan-Western Hubei Uplift had a strong control on sedimentation of the high-quality shale in the depression behind the uplift.