The shales of the Upper Ordovician Wufeng Formation and the Member 1 of Lower Silurian Longmaxi Formation (Long 1 Member) in the Upper Yangtze Platform of the South China Craton are abundant in silica minerals and organic matter. Influenced by environmental changes in geological history, several thin bentonite layers occur in these organic-rich shales, indicating that multiple stages of volcanic activity took place from the Late Ordovician to Early Silurian in the Upper Yangtze area. For these bentonite layers, a number of previous studies focused on their petrological characteristics, tectonic setting and formation age have been carried out in detail^[1,2]. In recent years, some researchers have noticed that the bentonite rich interval has a good correlation with the organic- rich shale section, hereafter, the study of the influence of volcanic activity on the accumulation of organic matter in shale has become an important study subject. For example, Li et al. (2014) analyzed the relationship between the volcanic ash and organic-rich shales of the main shale gas production basins in China^[3]. The widely distributed bentonite layers in the Wufeng Formation and Long 1 Member shales of Upper Yangtze area has become a typical example to further explore the effect of multi-stage volcanic activity on the organic enrichment in shale.

Based on establishment of a sequence stratigraphic framework of the Wufeng Formation and Long 1 Member in the Upper Yangtze area, the vertical differentiation of bentonite development in chronostratigraphic framework was examined, and the biological and geochemical characteristics of organic-rich shale corresponding to the bentonite rich interval were analyzed, then the difference in the intensity of Late Ordovician and Early Silurian volcanisms and its influence on the organic matter enrichment in shale were discussed. This study provides reference for the exploration of shale oil and gas in Upper Yangtze area.

Identification of lithofacies and establishment of lithofacies sequence within chronostratigraphic framework is an important method to research on the types, temporal and spatial distribution and evolution of marine organic-rich shale in southern China. Graptolite biostratigraphy is one of the most effective methods for the division and correlation of Ordovician and Silurian black shale strata in the Upper Yangtze area^[4]. Combined with 13 graptolite zones by Chen et al.^[5], this study integrates lithofacies, geochemical parameters and well logs to establish a sequence stratigraphic framework (using systems tract as unit) of Wufeng Formation and Long 1 Member in Upper Yangtze area^[5,6] (Fig. 1). Based on the identification of the sequence boundaries (SB1, SB2 and SB3) and maximum flooding surfaces (MFS), we interpret the Wufeng Formation and Long 1 Member as two third-order sequences. The Ordovician Wufeng and Guanyinqiao Formations are interpreted as Sequence 1 (Sq1), which consists of a transgressive systems tract 1 (TST1, Wufeng Formation) and a highstand systems tract (HST) Guanyinqiao Formation). TST1 corresponds to Dicellograptus complexus (WF2) -Normalogr. Extraordinarius (WF4) graptolite zones and HST has abundant shelly fossils but few graptolites. The Member 1 of the Longmaxi Formation is identified as Sequence 2 (Sq2), which consists of a transgressive systems tract 2 (TST 2), an early highstand systems tract (EHST) and a late highstand systems tract (LHST) from the bottom to the top. The TST2 corresponds to Persculptogr. Persculptus (LM1)-Cystograptus vesiculosus (LM4) graptolite zones, and EHST and LHST correspond to Coronograptus cyphus (LM5) and graptolite zones above (Fig. 1).

A set of stable distributed organic-rich siliceous shale with bentonite layers deposited during the TST1 (Late Kadian) in the Upper Yangtze area (Figs. 1 and 2a). Fuling-Wulong- Pengshui area was one of the depositional centers of the study area (Fig. 2b). The depositional period of HST corresponds to the major glaciation during the Hirnantian Stage, and the Upper Yangtze area deposited the carbonaceous/siliceous shale with abundant Hirnantia Fauna (Fig. 2c), which is generally less than 0.5 m thick^[7] (Fig. 2d). The depositional period of TST2 corresponds to the Late Hirnantian-Early Rhuddanian, and lithofacies of TST2 in the study area is dominated by organic- rich siliceous shale with bentonite layers (Figs. 1 and 3a). The depositional center was still in the Fuling-Wulong-Pengshui area^[7] (Fig. 3b). The depositional periods of EHST and LHST correspond to Late Rhuddanian and Aeronian, respectively, when a sedimentary sequence of shale interbedded with siltstone and shale/silty mudstone/limy mudstone deposited in the Upper Yangtze area (Figs. 1 and 3c), and the depositional center is same with that in the TST1 and TST2 stages (Fig. 3d).

In this study, high density sampling with spacing of 20-50 cm was conducted in the Wufeng-Long 1 Member shales. A total of 141 shale samples were collected for elemental geochemical analysis (including concentrations of trace elements and rare earth elements). The trace elements and rare earth elements were analyzed using an Optimass 9500 inductively coupled plasma time-of-flight mass spectrometer (ICP-TOF- MS) at the State Key Laboratory of Geological Processes and Mineral Resources at the China University of Geosciences in Wuhan, with an analytical precision of over ±5%.

Based on the observation of bentonite specimen, the bentonite layers in drilling cores are all light to dark gray and brown, distributed in the siliceous shale, and usually associated with pyrite (Figs. 4a and 4b). Due to relatively severe weathering effects, the bentonite layers in outcrops are light-yellow to dark yellow. Bentonites are composed almost entirely of clay minerals, the samples have smooth and silky texture. All these features mentioned above are consistent with the field identification standard of bentonite proposed by Huff et al.^[8] Through thin-section observation, the bentonite is composed of volcanic ash, angular quartz and feldspar fragments, darkened biotite phenocrysts and rare vitric fragments (Fig. 4c), which shows that the volcanic ash experienced strong silicification during the late period of diagenesis. We also discovered a set of thin white sedimentary tuff bed (Fig. 4d) at the top of the HST (Guanyinqiao Formation) that differs from bentonite. This sedimentary tuff bed is grayish white and extremely smooth, and no foam comes out when acid is dripped on fresh surface of the sedimentary tuff. The thin-section photograph shows there are abundant chicken bone-shaped and sickle-shaped vitric fragments in the sedimentary tuff (Fig. 4e). Because the sedimentary tuff is a type of rock in the transition from pyroclastic rock to sedimentary rock, it is often mixed with shelly biogenic fossil fragments and normal sedimentary materials (Fig. 4f).

On the basis of precise description of bentonite development in the Wangjiawan section made by Su et al.^[9], through the observation of five drill cores in the Upper Yangtze area, we found that there only develops a set of sedimentary tuff bed at the top of the Guanyinqiao Formation in the Wangjiawan section, and all bentonite layers in the Upper Yangtze area are mainly in the TST1 and TST2 (using systems tract as the basic unit). The number and thickness of bentonite layers show obvious segmental features (Fig. 5).

The number of bentonite layers in the TST1 is more than that in the TST2, increases upward in the TST1 and then decreases largely in the TST2. In order to quantitatively describe the stage and intensity of volcanic activity, two parameters, the frequency of bentonite development and the cumulative thickness ratio of bentonite, were introduced into the study. The frequency of bentonite development is the number of bentonite layers per unit of time within each systems tract (bentonite layers/time). The bentonite frequency is higher than 1.5 Layer/Ma in the TST1 and lower than 1.5 Layer/Ma in the TST2 (Fig. 5). The bentonite layers in the TST1 (0.2-2.2 cm) are generally thicker than those in TST2 (0.1-1.2 cm). The cumulative thickness ratio of bentonite is cumulative thickness of bentonite layers to total shale thickness in each systems tract. Based on the statistics of five drill cores, the cumulative bentonite thickness ratios are greater than 1% in the TST1 and less than 1% in the TST2 (Fig. 5). According to the vertical heterogeneity of bentonite rich interval, the authors classified TST1 as an interval with dense bentonite layers (dense bentonite interval for short), with a bentonite frequency of more than 1.5 Layer/Ma and cumulative bentonite thickness ratio of more than 1%; and TST2 as an interval with sparse bentonite layers (sparse bentonite interval for short; with bentonite frequency of less than 1.5 Layer/Ma and cumulative bentonite thickness ratio of less than 1 %). Statistics show the TST1 (dense bentonite interval) in the 5 wells have an average TOC of 4.38%, 3.35%, 3.62%, 3.40% and 4.68% respectively, while the TST2 (sparse bentonite interval) in the 5 wells have an average TOC of 3.84%, 3.14%, 3.42%, 3.13% and 4.63% respectively, indicating the dense bentonite interval has slightly higher TOC than sparse bentonite interval (Fig. 5).

Abundant bentonite layers in the study area suggest that intense volcanism occurred from the Late Ordovician to Early Silurian in the Upper Yangtze Platform. Previous studies have proved that the bentonite source material is intermediate- acidic igneous rocks originating in intraplate collision or volcanic arc tectonic settings. Numerous bentonite beds in this set of black shale were related to the collision of the Cathaysia Block with the Yangtze Block during the Middle Caledonian tectonic period^[9,10]. Based on the U-Pb zircon dating, the ages of the bentonite layers ere formed around 443.2 ± 1.6 Ma ago, according to Hu et al.^[1], and 442.2 ± 8.1 Ma according to Xie et al.^[11]. The dated ages correspond most closely to the age (443.8 Ma) of the base of the graptolite sequence LM2 (bottom of TST2) (Fig. 1). Luo et al.^[12] tested that the age of the bentonite at the O-S boundary was 450.0 ± 3.6 Ma, which is slightly earlier than the ages of 443.2 ± 1.6 Ma and 442.2 ± 8.1 Ma but also close to 447.62 Ma, the age of the base of the graptolite sequence WF2 (lowest part of TST 1; Fig. 1). Therefore, we infer that the age of the bentonite marks the beginning of the volcanic event. According to the zircon dating data, these age results all coincide with the age of 443.8 ± 1.5 Ma determined for the O-S boundary published by the International Commission on Stratigraphy^[6] . According to the segmental features of bentonite rich intervals, we infer that the volcanic events occurred in the study area had two episodes: the first stage started at the beginning of the TST1 (around 447.62 Ma ago); the second stage started at the beginning of TST2 (around 443.83 Ma ago).

The TST1 (dense bentonite interval) has slightly higher TOC content than TST2 (sparse bentonite interval), and both TST1 and TST2 contain higher silicon and TOC contents (mean >3.0%) than those in EHST and LHST (Fig. 6). Large-scale volcanic activities occurred during the deposition of the TST1 and TST2, indicating that volcanic activity has a significant effect on the enrichment of organic matter in shale.

4.1.1. The marine biological productivity in bentonite rich interval

Different types of rock assemblages have different fossil records. TST1 (dense bentonite interval) is of sandwich structure with “thick bentonite layer-siliceous shale-thick bentonite layer” assemblage. Except the remaining shelly biodebris in the sedimentary tuff bed at the top of the Wangjiawan section, no biological debris has been found in bentonite layers in drill cores. However, the corresponding siliceous shales are rich in acritarch (Deunffia sp. and Cornutosphaera sp.) (Fig. 7a and 7b) and radiolarians (Figs. 7c and 7d). Shen et al.^[13] concluded - that the main organisms for hydrocarbon formation in the Upper Yangtze area included acritarchs, alginite and zooclasts^[13]. Among them, all the acritarchs are the species of phytoplankton, and the acritarch abundance peak exactly coincides with the top of TST1 (dense bentonite interval; Fig. 6). The TST2 features thick siliceous shale interbedded with thin bentonite layer assemblage (sparse bentonite interval). No biological debris has been found in the bentonite layers either, while corresponding section of siliceous shale has abundant sponge spicules (Fig. 7e), and acritarchs are also found at the bottom of the sparse bentonite interval (Fig. 6). Graptolite fossils in both TST1 and TST2 are complete in shape, random in arrangement without directionality (Fig. 7f), but differ widely in abundance in different intervals. The abundance of graptolite in dense bentonite interval is obviously lower than that in sparse bentonite interval, and it has opposite trend with the bentonite development. Fig. 6 illustrates that bentonite development interval has low graptolite abundance (volcanic eruption period), while the abundance of graptolite increases slightly in non-bentonite interval (volcanism intermission period).

The enrichment of copper (Cu) and Zinc (Zn) in sediments have been widely used to indicate the marine biological productivity^[15,16]. Because most sediments contain terrigenous sources, terrigenous sourced Cu and Zn must be deducted in order to precisely analyze the characteristics of vertical variation of marine biological productivity in the study area. We used non-detrital Cu and Zn as paleoproductivity proxies (i.e., C_Cu and C_Zn). The content of the non-detrital component of Cu and Zn were calculated by^[17]:

Based on data analysis and statistics, the overall marine biological productivity was higher in the TST1 (dense bentonite interval), and the ranges and mean Cu and Zn mass percentage values are 0.003 8%-0.019 1% (mean: 0.010 0%) and 0.006 0%-0.015 4% (mean: 0.009 7%), respectively (Fig. 6). The overall marine biological productivity in the TST2 was relatively lower (sparse interval) than that in the TST1, with the ranges and mean Cu and Zn mass percentage values of 0.000 7%-0.004 1% (mean: 0.002 6%) and 0.004 3%-0.009 3% (mean: 0.006 6%), respectively (Fig. 6). The non-bentonite interval has the lowest marine biological productivity, with the ranges and mean Cu and Zn mass percentage values of 0.000 2%-0.006 4% (0.001 6%) and 0.000 4%-0.008 5% (0.003 4%), respectively (Fig. 6). In a word, the stronger the volcanism, the higher the mass fraction of Cu and Zn, and the higher the marine biological productivity would be.

There is certain positive correlation between the TOC and marine biological productivity of Wufeng-Long 1 Member shales in the Upper Yangtze area. Comparing the correlation between the TOC content and paleoproductivity proxies (C_Cu and C_Zn) in each bentonite interval shows the dense bentonite interval in TST1 has the strongest positive correlation, with multiple correlation coefficients between TOC and C_Cu and C_Zn of 0.54 and 0.76, respectively; followed by sparse bentonite interval (TST2), with multiple correlation coefficients between TOC and C_Cu and C_Zn of 0.50 and 0.55, respectively; the non-bentonite interval has relatively low correlation, with multiple correlation coefficients of 0.47 and 0.32, respectively (Fig. 8).

4.1.2. Relationship between volcanic activity and productivity

During the early stage of TST1 deposition, the tectonic movement caused the strongest collision and conjunction of the Cathaysia and Yangtze Block, which was associated with two episodes of volcanic activities, and the dense bentonite interval (TST1) reflects the strong and frequent volcanic activities. Olgun et al. studied the possible impacts of volcanic ash emissions of Mount Etna on the primary productivity and concluded that volcanic ash was an important nutrient source for the surface ocean^[18]. When volcanic ash particles fell into the seawater, their coatings containing nutrient-bearing soluble salts began to dissolve, releasing substantial amounts of micro-nutrients such as Fe, giving rise to a nutrient-rich basin conducive to massive phytoplankton bloom^[19]. The depositional thickness of volcanic ash is in proportional to the concentration of released nutrients. If the thickness of ash layer is 1 cm, the concentration of released nutrients would increase by about 10 times^[20]. TST1 (dense bentonite interval) has relatively thicker layers of volcanic ash (Fig. 4b), so volcanic ash produced by the volcanic activity provided abundant nutrients for the surface water of the Upper Yangtze Sea and triggered the bloom of ocean surface producers (phytoplankton). Subsequently, the radiolarians feeding on the phytoplankton also began to flourish. The abundance of biological communities contributed the overall high marine biological productivity, and subsequently large amounts of organic matter accumulated in a short time. In addition, siliceous organisms and their symbiotic algae could form larger silicon-rich and organic- rich pellets, increasing their deposition rate in seawater and effectively reducing the oxidized rate of organic matter in the water. One of the most important reasons for marine biological productivity variation in the Upper Yangtze sea is the difference in volcanic activity intensity. The dense bentonite interval (represents high-frequency volcanism) has the highest marine biological productivity, followed by sparse bentonite interval (low-frequency volcanism), and non-bentonite interval has the lowest marine biological productivity.

4.2.1. Characteristics of redox conditions of bentonite rich interval

The preservation of primary sedimentary structures of organic-rich shale is usually the result of anoxic conditions. Enrichment degree and ratio of redox-sensitive trace elements are often used to evaluate paleomarine redox conditions. For example, Th/U, V/Cr and V/(V+Ni)^[21,22,23], Th/U ratio ranges 0-2 in anoxic conditions and reaches >8 in oxidizing conditions^[21]; V/Cr and V/(V+Ni) ratios are >4.25 and >0.60 in anoxic conditions, 2.00-4.25 and 0.45-0.60 in dysoxic conditions, and <2.00 and <0.45 in oxic conditions^[22].

The dense bentonite interval (TST1) represents intense and high-frequency volcanism. Data from Well JY1 and JY11-4 shows that TST1 in the two wells have overall low average Th/U ratios of 0.78 and 0.77, high average V/Cr ratios of 7.23 and 6.68 (peaking at 9.74 and 8.77 in the upper part of TST1), and high average V/(V+Ni) ratios of 0.77 and 0.82, respectively. The low Th/U ratio and high V/Cr and V/(V+Ni) ratios suggest that depositional environment in the TST1 was strongly anoxic-euxinic (Fig. 9). To the depositional period of TST2 (sparse bentonite interval), the volcanic activities weakened. Data from Well JY1 and JY11-4 shows that average Th/U ratios of TST2 are higher (1.63 and 1.25) than those of the TST1, and average V/Cr ratios drop to 3.70 and 3.46 and slightly lower average V/(V+Ni) ratios of 0.71 and 0.60, respectively. The general increase of Th/U ratio and decrease of V/Cr and V/(V+Ni) ratios suggest anoxic-dysoxic conditions during the TST2 depositional period (Fig. 9).

4.2.2. Relationship between volcanic activity and organic matter preservation

Not all marine organisms could adapt to such frequent environmental changes mentioned above. Marine algal bloom could consume too much dissolved oxygen in the water, resulting in hypoxic conditions. Without sufficient dissolved oxygen in the water, a lot of coral might die off in large numbers^[24]. Intense volcanic activity releases a lot of carbon dioxide into the atmosphere. About 30% of atmospheric CO₂ can be absorbed by the ocean to form carbonic acid. Ocean acidification can account for the extinction of calcified marine animals (e.g., tetracoral) and imbalance of the ecosystem^[25,26]. In addition to providing micro-nutrients for the ocean, a large amount of volcanic ash falling into water can also release some other toxic elements (e.g., Pb and Hg) and thus have toxic effects on marine organisms^[27]. Moreover, insoluble matters in volcanic ash might increase the turbidity of sea water^[28] and subsequently destroy the clear and quiet marine environment and then limit the growth and development of some marine plankton such as graptolite. Although intense volcanism is not conducive to the survival of graptolite and coral, it could create excellent conditions for the preservation of organic matter. Intense and high-frequency volcanic activity with short intermission period in the TST1 resulted in imbalance of the marine environment and caused instantaneous and mass death of some marine plankton and benthos. The extremely euxinic-sulfurated conditions trigged by volcanic activity reduced the rate of decomposition of organic matter, thus a large number of dead organisms sunk to seafloor and was eventually preserved due to favorable conditions. Therefore, another reason for the accumulation of organic matter in shale in the dense bentonite interval (TST1) is the rapid burial and high preservation rate of organic matter under extremely euxinic environment. During the depositional period of the sparse bentonite interval (TST2), moderate volcanic activity would not result in mass death of organisms, but the anoxic environment (which is also related to the postglacial transgression during the Late Hirnantian) played a significant role in organic matter preservation.

Gong et al. and Jones et al. demonstrated that major volcanic eruptions might have triggered the O-S mass extinction events^[29,30]. Based on the previous research results, we infer that a large amount of volcanic ash produced by massive volcanic events during the Late Ordovician (dense bentonite interval) could block the sunlight and thus speed up the cooling rate of weather, which is one of the main causes leading to the mass extinction during the Hirnantian.

It can be seen from the above analysis that volcanism is closely related to the formation of organic-rich shale. The differences of volcanic activity intensity and frequency indicated by segmental feature of bentonite rich interval resulted in the heterogeneity of enrichment of organic matter and organic silicon. Intense and frequent volcanic activities have dual promotion effects on the accumulation of organic matter. A large amount of volcanic ash produced by intense volcanic activity provide rich nutrients to surface water and promote blooms of siliceous organism (e.g., radiolarians) and autoeciousness algae, which causes high productivity in the surface water. The organic matter enrichment in shale is also a result of enhanced preservation under extremely anoxic environment caused by volcanism. Water quality deterioration due to toxic substances released by massive volcanic ash could lead to an instantaneous death of some plankton and benthos. After death, biological debris rapidly deposited before oxidation decomposition, and organic matter can then be preserved in anoxic conditions.

The Ordovician Wufeng Formation and Member 1 of the Silurian Longmaxi Formation are interpreted as five systems tracts within two 3^rd order sequences: Sq1 (during Wufeng depositional period) is composed of a transgressive systems tract 1 (TST1) and a highstand systems tract (HST) from the bottom to the top; Sq2 (during depositional period of Member 1 of Longmaxi Formation) consists of a transgressive systems tract (TST2), an early highstand systems tract (EHST) and a late highstand systems tract (LHST) from the bottom to the top. TST1 and TST2 dominated by favorable shale superior to that of the EHST and LHST are characterized by organic-rich siliceous shale interbedded with bentonite layers, and this lithofacies assemblage indicates volcanic activities occurred during this period.

Volcanic activity intensity is different within different systems tracts. Based on frequency of the bentonite development and cumulative bentonite thickness ratio, TST1 is classified as dense interval for bentonite development (bentonite frequency >1.5 Layer/Ma and cumulative bentonite thickness ratio >1%), which indicates intense and high-frequency volcanism; TST2 is classified as the sparse bentonite interval (bentonite frequency <1.5 Layer/Ma and cumulative bentonite thickness ratio <1%), which suggests moderate and low-frequency volcanism.

The difference of volcanic activity intensity has effects on the organic matter enrichment in shale. The organic carbon content of the dense bentonite interval (TST1) is slightly higher than that of the sparse bentonite interval (TST2).

Intense and frequent volcanic activities have dual promotion effects on the accumulation of organic matter. Volcanic ash can provide a significant supply of nutrients and increase marine productivity, which provides material basis for the enrichment of organic silicon and organic matter. The extremely anoxic environment caused by volcanic activity lead to rapid burial and high levels of preservation of deposited organic matter.

Nomenclature

a_Cu—the content ratio of the element Cu to Ti in Post-Archean Australia Shales (PAAS), dimensionless;

a_Zn—the content ratio of the element Zn to Ti in Post-Archean Australia Shales (PAAS), dimensionless;

C_Cut—the total content of Cu element in shale, %;

C_Znt—the total content of Zn element in shale, %;

C_Ti—the total content of Ti in shale, %;

C_Cu, C_Zn—the content of non-detrital origin of Cu and Zn, %;

GR—natural gamma ray, API;

R—multiple correlation coefficient, dimensionless.