Introduction
The Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation is a set of graptolite shale widespread in the Yangtze region of China, which was deposited under the control of the Guangxi movement [1,2,3,4,5,6]. It records a series of major geological events at the turn of Ordovician and Silurian, such as tectonic and volcanic activities, sea level fluctuation, climate change, biological extinction and recovery, and rising ocean current activi-ties [6,7,8,9,10,11,12]. In recent 10 years, with the progress of shale gas exploration and development, the geological and exploration circles have gradually changed their attention to this set of shale from biostratigraphy and Hirnantian glacial paleo-marine environment to the sedimentary environment of black shale (i.e. shale section with TOC>1%), the development mode and main sedimentary controlling factors of organic-rich shale (i.e. black shale section with TOC>2%) [13,14,15,16,17,18,19,20,21,22]. Some scientific problems, such as what form these event records appear in the shale of Wufeng Formation-Longmaxi Formation, what remarkable characteristics they have, and whether they have a control effect on organic-rich sediments, deserve attention and research.
There are two key boundaries at the bottom and top of Wufeng Formation. The bottom boundary is the structural boundary from platform to shelf, which is mainly composed of clayey shale or carbonaceous shale intercalated with bentonite, and shows a peak response on the gamma ray (GR) log. The top boundary is the Hirnantian glacial boundary, that is, the Guanyinqiao Member containing a large number of brachiopod fossils, which is represented by Hirnantian peak on the GR log [18]. These two boundaries are the sedimentary responses of two major events: the structural transformation at the early deposition stage of Wufeng Formation and the sharp decline of sea level in the Hirnantian glacial period, and control the deposition of the organic-rich shale in Katian and of the shell layer in the Guanyinqiao Member respectively. As the top and bottom boundaries of the Wufeng Formation are clear with thickness of 2-11 m, it is usually studied as one third-order sequence. The formation mechanism and distribution law of organic-rich shale in this interval are generally clear [5]. Compared with the Wufeng Formation, the sedimentary thickness of the Longmaxi Formation varies greatly (25-600 m), generally with 6-9 graptolite belts [1,3,5,13-15]. There are significant differences in lithofacies associations vertically and horizontally, and there is a lack of boundaries with distinctive characteristics, unified cognition and wide acceptance. Therefore, the identification of key boundaries and the distribution of high-quality shale in Longmaxi Formation are still the focus of continuous attention in the exploration and geological circles.
Based on the detailed survey of 15 typical sections and the anatomy of 4 key wells in the Sichuan Basin and its periphery, as well as the analysis of typical characteristics of the Guanyinqiao Member shell layer, and focusing on the specific bentonite layers, this paper discusses the basic characteristics of key boundaries in the Wufeng Formation-Longmaxi Formation and their relationship with the deposition of organic-rich shale, so as to provide technical support for geological research and exploration evaluation in the Wufeng Formation-Longmaxi Formation.
1. Typical boundary characteristics of the graptolite belts and concretions
The shale in the Wufeng Formation-Longmaxi Formation has four markers: characteristic graptolite belt, Hirnantian glacial shell layer, specific bentonite (i.e. volcanic ash) and concretion. They are stably distributed in the whole or part of the Yangtze region, similar or partially similar to the top and bottom boundaries of the Wufeng Formation (Table 1).
Table 1 Boundary types and characteristics of Wufeng Formation-Longmaxi Formation
| Boundary types | Recorded geologic events | Major characteristics | Research methods | Application effect | References |
|---|---|---|---|---|---|
| Characteristic graptolite belt | Climate change and ecological differentiation of graptolite fauna | The boundary recognition is determined by the specific graptolite occurrence. It is greatly affected by sedimentary environment, graptolite preservation conditions, cognitive level of researchers on characteristic graptolites and other factors, and the variation of boundary lithofacies is not obvious. It is isochronous and stably distributed in a large area. | Determine the first occurring layer of charac- teristic graptolites through detailed outcrop survey or core observation. | High degree of research and application. In the infralittoral and deeper water areas, graptolites are rich and highly differentiated, and the application effect is better. Graptolites are rare in nearshore, shallow water and near provenance areas, and the application effect is worse. | [1-6, 9, 13-15] |
| Hirnantian glacial shell layer | Climate change and mass extinction in Hirnantian glacial period | The characteristics of lithofacies, biological assemblage and well logging response are distinct and easy to identify. It is isochronous and stably distributed in vertical and horizontal directions. | Determine by detailed outcrop survey or core observation and well logging response characteristics. | High degree of research. In the infralittoral and deeper water area, Hirnantian is rich in fossils and has better application effect. In nearshore, very shallow water or paleo- uplift areas, Hirnantian has rare or no fossils, and the application effect is worse. | [2-3, 7, 12, 14-15, 18] |
| Specific bentonite | Plate movement (Guangxi movement) and volcanic activity | It is isochronous, extensively distributed, and easy to identify. It is greatly affected by the scale of volcanic activity, the source of volcanic ash and the cognitive level of researchers. | Through the detailed survey of typical outcrop section, determine the bentonite dense section; coupling with the calibration of key wells, provide reference to key areas. | Standard sections need to be established in different exploration areas for calibration or reference. It preliminarily meets the requirements of exploration evaluation and geological research. Combined with characteristic graptolite, the effect is better. | [16-17, 19-21] |
| Concretion | Periodic strong activity of sea basins | It is isochronous and easy to identify. The formation and distribution environments are generally adjacent to ancient land (i.e. provenance area), in deep-water area, and in foreland period. It only appears in local areas of a sea basin, with poor distribution stability, large differences in mineral composition and well logging response characteristics, and is only available in local exploration areas. | Through detailed outcrop survey, determine lithofacies association and GR responses, and then provide reference to bottom hole. | It can act as the boundary of fourth-fifth order sequences in some areas. It is in the stage of exploratory research, and has not yet been applied substantively. | [22-24] |
1.1. Graptolite belts
Graptolite is a type of important marine organism in the Ordovician-Early Devonian. It has the characteristics of diverse species, wide distribution, rapid evolution and obvious features, etc. It is an index fossil for the stratigraphic correlation of the Lower Paleozoic [2-3,6,9,11,15]. Graptolites are found in all data points (Fig. 1) in the Lower Silurian shale areas in the Sichuan Basin and its periphery. The bottom boundary of the typical graptolite belt of the Wufeng Formation-Longmaxi Formation is the onset of the generation and evolution of various characteristic graptolites at the turn of Ordovician-Silurian, reflecting the characteristics of paleomarine ecological differentiation and biological evolution in this period [3,4]. The identification completely depends on the occurrence of characteristic graptolites in field outcrops or drilling cores, and a set of mature and effective division scheme (i.e. stratification scheme) for graptolite belt has been formed [1-3, 9]. According to characteristic graptolites, 13 graptolite belts can be identified in the Wufeng Formation-Longmaxi Formation (including 4 in Wufeng Formation and 9 in Longmaxi Formation), i.e. Dicellograptus complanatus (WF1), Dicellograptus complexus (WF2), Paraorthograptus pacificus (WF3), Normalograptus extraordinarius (WF4), Normalograptus persculptus (LM1), Akidograptus ascensus (LM2), Parakidograptus acuminatus (LM3), Cystograptus vesiculosus (LM4), Coronograptus cyphus (LM5), Demirastrites triangulates (LM6), Lituigrapatus convolutes (LM7), Stimulograptus sedgwickii (LM8) and Spirograptus guerichi (LM9) [3,5,15]. The first occurring layer of each graptolite belt corresponds to an isochronous boundary. Therefore, there should be 13 isochronous boundaries in the Wufeng Formation-Longmaxi Formation in the middle-upper Yangtze platform basin, which is an important reference for the calibration of other key boundaries.
Fig. 1.
Fig. 1.
Sedimentary facies and distribution of important data points of Lower Silurian Telychian in Sichuan Basin and its periphery.
1.2. Concretions
Concretion is a common sedimentary structure in the Lower Silurian and Lower Cambrian shales in southern China [22,23,24], which is of great significance in revealing tectonic activities, paleogeography, paleoenvironment, paleoprovenance and diagenesis [25,26,27,28,29]. The concretions of the Longmaxi Formation are distributed in three graptolite belts, Coronograptus cyphus, Demirastrites triangulates and Lituigrapatus convolutus, in South Sichuan and East Sichuan-West Hubei depressions, with the latter two in dominance [22]. They are mainly calcareous (or dolomitic) concretions and siliceous concretions, rich in calcium and silica, with the clay content generally not more than 20%, greatly different from the mineral compositions of surrounding rocks (mostly carbonaceous shale and clayey shale, with clay content higher than 35%). They generally occur in the form of isolated dispersion and intermittent bedding (Fig. 2). They usually present trough on the GR curve, and are found to have formed and distributed remarkably adjacent to ancient land (i.e. provenance area), in deep water area and in foreland period (Table 1). As concretions of the Longmaxi Formation only occur in some areas of South Sichuan-East Sichuan depression, the distribution area is far smaller than the three key boundaries (characteristic graptolite belt, Hirnantian glacial shell layer and bentonite dense section), and the stability of vertical and horizontal distribution is generally poor, so they can only be used as the reference boundary between the fourth-order and fifth-order sequences in some areas (Table 1). So far, no effective correlation scheme has been formed for concretions, due to the prominent problems such as few effective data points, limited and unstable distribution, and large differences in rock mineral compositions and well logging responses. In the field researches, they are mainly utilized as the specific lithofacies of the Upper Rhuddanian and Aeronian [22].
Fig. 2.
Fig. 2.
Concretions of Longmaxi Formation in Sichuan Basin (the geological hammer is 33 cm long, and the arrows point to concretions). (a) Shuanghe section in Changning, discontinuous bedding concretions in upper Rhuddanian; (b) Thin-section photograph of concretions in upper Rhuddanian on Shuanghe section in Changning, calciferous siliceous shale, with TOC of 0.76%; the bright particles are quartz, radiolarian, sponge spicule and dolomite, single polarization; (c) Heizhugou section in Ebian, dispersed concretions in Aeronian; (d) Thin-section photograph of concretions on Heizhugou sectin in Ebian, crystal powder dolomite, with TOC of 1.85%; there are lamina under the microscope, with a large number of dolomites and radiolarians, single polarization.
2. Typical boundary characteristics of Guanyinqiao Member shell layer
The Guanyinqiao Member of Wufeng Formation have the following characteristics [5-9, 18]: (1) the marker is the sedimentary response to the global Hirnantian glacial event; (2) lithofacies changes greatly from area to area: organic-lean calcareous shale and marl in shallow-water areas (e.g. platform-basin slope and nearshore), and organic-rich calcareous siliceous mixed shale and siliceous shale in deep-water areas (e.g. distal); (3) the thickness is generally 9-100 cm; (4) there are Hirnantian cold-water fauna, such as Hirnantia Fauna and Dalmanitina[7]; and (5) near the boundary between the shell layer and the bottom of Longmaxi Formation, abnormal peak response of GR often occurs, i.e. Hirnantian GR peak [18], which is globally comparable, isochronous, stably distributed in a large area and easy to identify (Figs. 3-5). For the Hirnantian in the Sichuan Basin and its periphery, the lithofacies association and regional facies variation are generally clear, but the basic characteristics and regional variation of the GR peak are not understood accurately, which affects the accurate regional correlation of the Guanyinqiao Member.
Fig. 3.
Fig. 3.
Stratigraphic column of Wufeng Formation-Longmaxi Formation in Sanbaiti section, Huaying.
Fig. 4.
Fig. 4.
Boundary characteristics of Wufeng Formation and Longmaxi Formation in key outcrop sections of Sichuan Basin and its periphery (the geological hammer is 33 cm long). (a) Heishui section in Youyang; (b) GR response characteristics of Heishui section in Youyang; (c) GR response characteristics of Xintan section in Zigui; (d) GR response characteristics of Shuanghe section in Changning; (e) Sanhu section in Laifeng; (f) GR response characteristics of Sanhu section in Laifeng.
Fig. 5.
Fig. 5.
Distribution characteristics of Guanyinqiao Member shell layer in North Guizhou-Southeast Chongqing-South Sichuan-Central Sichuan.
The existence and response characteristics of Hirnantian GR peak have become an important basis for judging the depositional continuity (that is, the presence of the Guanyinqiao Member shell layer) and exploration potential of the Wufeng Formation-Longmaxi Formation. Through the survey of sections in and around the Sichuan Basin, it is found that the lithofacies and GR peak response characteristics of Hirnantian are quite different at the edge of Yangtze sea basin, slope, central area and Yichang Rise (Figs. 3-5)
In the shallow-water area at the southeast edge of the sea basin (e.g. Chaole in Meitan), the Guanyinqiao Member is mostly marl or micrite (GR value of 60-70 cps), with the overlying and underlying rocks of organic-lean marl, calcareous shale or clayey shale (GR value of 54-71 cps), so the GR peak response characteristics are not obvious (Fig. 5).
In the southeast slope of the sea basin (e.g. Xiushan, Youyang, Zigui, Changning), with the ancient water depth, the facies of the Guanyinqiao Member is transformed into organic-bearing calcareous shale and calciferous siliceous shale (Fig. 4a). The GR value of the Guanyinqiao Member shows a three-segment style (relatively high→ low→high) (Fig. 4b) or two-segment style (or slope style, low→high) (Fig. 4c, 4d), from bottom to top. The Hirnantian GR peak is generally formed from the upper part of Guanyinqiao Member and continues to the top of the Normalograptus persculptus graptolite belt (or to the Akidograptus ascensus graptolite belt in some areas). In the Heishui section, Youyang, for example, the thickness of Guanyinqiao Member is 0.45 m, and the GR value is 267-276 cps in the lower 15 cm part, decreases to 247-255 cps in the middle 20 cm part, and rapidly increases to 311-363 cps in the upper 10 cm part. The Hirnantian GR peak is formed 10 cm above the Guanyinqiao Member and continues to the top of the Normalograptus persculptus graptolite belt, and the highest GR value (432 cps) appears in the middle of the Normalograptus persculptus graptolite belt (Fig. 4a, 4b).
In the east slope of the central uplift of the South Sichuan depression (e.g. JYT1, L210 and Sanbaiti section in Huaying, etc.), the Hirnantian is a set of continuous deep-water sedimentary siliceous shale with small lithofacies difference and rich organic matter. The GR peak is formed at the top of the Normalograptus extraordinarius graptolite belt, peaked at the Guanyinqiao Member (or maintains peak response) and continues to the top of the Normalograptus persculptus graptolite belt. That is, the Guanyinqiao Member is just located in the middle of the GR peak (Fig. 5). For example, in the Sanbaiti section, Huaying, the thickness of the Hirnantian is about 100 cm, and the Guanyinqiao Member is located in the middle (9 cm thick), which is a deep-water sedimentary siliceous shale. The TOC value of the Hirnantian is 9%-11%, and peaks (11%) in the Guanyinqiao Member. The kerogen δ13C values of the Hirnantian are between -30.2‰ and -29.0‰ (-29.3‰ in the Guanyinqiao Member) and shows a negative drift from bottom to top (Fig. 3). This reflects the characteristics of relative sea level rise, abnormal nutrient enrichment and extremely enriched organic matter in the Huaying sea region during the Hirnantian period, in sharp contrast to the significant decline of sea level and apparent decrease of organic matter abundance in the southeast slope of the sea basin during the Hirnantian period (Fig. 5).
In the Western Hunan-Hubei uplift (i.e. the Yichang Rise), the middle-lower strata of Hirnantian and Rhuddanian are generally absent between Wufeng Formation and Longmaxi Formation, and nearly 10 cm clayey weathering crust even appears in some areas such as Taiyanghe in Enshi [12], resulting in the complete disappearance of Hirnantian GR peak. For example, in the Sanhu section, Laifeng, the Normalograptus extraordinarius-Akidograptus ascensus graptolite belts are absent, so the response of Hirnantian GR peak is missing (Fig. 4e, 4f).
Obviously, the Guanyinqiao Member shell layer is identifiable in lithofacies, characteristic fossils and well logging response, and directly reflects the paleogeographic environment of the Yangtze sea basin during depression at the turn of Ordovician-Silurian (Fig. 5). Therefore, it is the most standard key boundary inside the Wufeng Formation-Longmaxi Formation, and can provide reference for the establishment of other key boundaries. The lithofacies and GR response of this boundary vary greatly from region to region. Such differences of boundary features (especially the Hirnantian GR peak) should be carefully considered in researches.
3. Typical boundary characteristics of bentonite
Wang et al. [19,20] defined the black shale section with the average thickness of shale less than 1 m and the cumulative thickness of bentonite more than 5 cm as the first type of bentonite dense section, and the bentonite layer with a single layer thickness of more than 5 cm as the second type of bentonite dense section. These two types of bentonite are the main research units to reveal the high-frequency volcanic ash system of Wufeng Formation-Longmaxi Formation.
3.1. Main characteristics and regional distribution
According to the research results of bentonite in key areas such as east Sichuan-west Hubei and north Sichuan[17, 19-20], together with the survey of sections (incl. Bailu in Wuxi, Heizhugou in Ebian, Heishui in Youyang, Lujiao in Pengshui, Hongyanxi in Longshan, Bayu in Daozhen and Guanyinqiao in Qijiang), eight bentonite dense sections of two types are found in Wufeng Formation-Longmaxi Formation in the Sichuan Basin and its periphery. They mainly occur in seven graptolite belts (Fig. 6 and Table 2). Four bentonite dense sections (①-④) of the first type are developed in Katian-Rhuddanian. Each section is an interbed of bentonite and shale with a thickness of 0.15-1.50 m, where the bentonite occurs in multiple (usually 2-8) thin volcanic ash layers with individual thickness of 0.5-3.0 cm and cumulative thickness greater than 5 cm (Fig. 7a, 7b). The remaining four bentonite dense sections (⑤-⑧) of the second type are developed in Aeronian and its overlying strata. Each section is mainly a thick (generally 5-15 cm or up to 40 cm) layer of bentonite with a thickness of more than 5 cm, which mainly contains SiO2 (41.57%), Al2O3 (19.94%), Fe2O3 and FeO (16.61%), MgO (1.56%) and K2O (6.08%). The GR value is between 215 and 235 cps, interbedded in carbonaceous shale (Fig. 7c, 7d, Table 2). The bentonite dense section is an important sedimentary response to the episodic activities of Guangxi movement. In addition to stable position in the graptolite belt, obvious increase of clay minerals and peak response of GR, it also has the characteristics of unclear relationship between volcanic ash and TOC and sudden change of lithofacies above and below the boundary at key points [19,20]. This indicates that the bentonite dense section and the Guanyinqiao Member shell layer has similar boundary characteristics in thickness and GR response. They are all event deposits, and can be effectively identified and correlated with the aid of macro geological information such as lithofacies association and GR peak. As the geological properties of the bentonite dense section are greatly affected by later volcanic ash alteration degree, shale lithofacies, structural transformation and other factors, further efforts are needed to determine whether they can be identified and correlated by geochemical, elemental and other indicators.
Fig. 6.
Fig. 6.
Stratigraphic column of Wufeng Formation-Longmaxi Formation in Chengkou area, Northeast Sichuan.
Fig. 7.
Fig. 7.
Sections and GR response of two types of bentonite dense sections of Wufeng Formation-Longmaxi Formation in Sichuan Basin and its periphery (the geological hammer is 33 cm long, and the arrows point to bentonite layer). (a) The bentonite dense section in middle LM5 of Bailu section in Wuxi. Arrows point to thin-layer bentonite, with a single layer thickness of 0.5-3.0 cm, sandwiched in 1.2 m thick black shale; (b) GR response characteristics of the bentonite dense section in middle LM5 of Bailu section in Wuxi; (c) Thick bentonite (8 cm) in LM6 of Heizhugou section in Ebian, intercalated in carbonaceous shale; (d) GR response characteristics of LM6 thick bentonite in Heizhugou section in Ebian. LM5-Coronograptus cyphus graptolite belt; LM6-Demirastrites triangulatus graptolite belt.
Table 2 Basic parameters of bentonite dense sections of Wufeng Formation-Longmaxi Formation in Sichuan Basin and its periphery (modified from References [16-17, 19-20])
| No. of ben- tonite dense section | Horizon | Major features | Region | Calibrating sections |
|---|---|---|---|---|
| ① | Upper part of Dicellograptus complexus graptolite belt | Carbonaceous shale and siliceous shale interbedded with multiple thin volcanic ash layers, with thickness of 0.15-1.50 m. It is characterized by obvious increase of clay minerals, peak response of GR, and no obvious relationship between volcanic ash and TOC. | South Sichuan, East Sichuan-West Hubei | Qiliao in Shizhu, Mingzhong in Chengkou, Xiema in Baokang |
| ② | Top of Paraorthograptus pacificus graptolite belt | East Sichuan- West Hubei | Qiliao in Shizhu, Mingzhong in Chengkou, Xiema in Baokang | |
| ③ | Bottom of Coronograptus cyphus graptolite belt | South Sichuan, East Sichuan | Huangcao in Wulong, Shuanghe in Changning | |
| ④ | Middle part of Coronograptus cyphus graptolite belt | East Sichuan, Western Hunan-Hubei | Qiliao in Shizhu, Bailu in Wuxi and Guanwu in Hefeng | |
| ⑤ | Middle-lower part of Demirastrites triangulatus graptolite belt | A thick layer of bentonite with a thickness of more than 5 cm (5-15 cm, or up to 40 cm). The GR value is 215-235 cps. There are often mutations in lithofacies above and below the boundary in key areas. | All areas of the middle- upper Yangtze region | Shuanghe in Changning, Guanyinqiao in Qijiang, Qiliao in Shizhu, Bailu in Wuxi, Mingzhong in Chengkou, Xiema in Baokang, etc. |
| ⑥ | Upper-top of Lituigrapatus convolutus graptolite belt | East Sichuan, Northwest Hubei | Qiliao in Shizhu, Mingzhong in Chengkou, Xiema in Baokang, Dengjia’ao in Changyang | |
| ⑦ | Bottom of Stimulograptus sedgwickii graptolite belt | Northeast Hubei | Mingzhong in Chengkou | |
| ⑧ | Bottom of Spirograptus guerichi graptolite belt | Northeast Hubei, North Sichuan | Mingzhong in Chengkou, Yangba in Nanjiang |
Depending upon the intensity and scale of episodic volcanic (or tectonic) activities at the turn of Ordovician-Silurian, the eight bentonite dense sections are divided into three distribution scales: whole domain, large area, and local (Table 2). Section ⑤ is a significant sign that the Yangtze sea basin entered the foreland development period as a whole. It is distributed in the whole domain, basically covering all areas of the middle-upper Yangtze region, and with an area not less than that in the Guanyinqiao Member (Fig. 8). Therefore, Section ⑤ can act as the most important third-order sequence boundary in the Longmaxi Formation. Sections ①-④ and ⑥ are distributed in large area, mainly in the South Sichuan depression (①, ③), East Sichuan Depression (①-④, ⑥), Western Hunan-Hubei uplift (④), and Northwest Hubei depression (①, ②, ⑥), which can act as the third-order or fourth-order sequence boundary in Longmaxi Formation in the above areas. Sections ⑦ and ⑧ are locally distributed, mainly in north Sichuan-Northeast Hubei, such as Nanjiang (⑧), and Chengkou (⑦ and ⑧) [20], which can act as the fourth-order sequence boundary in Longmaxi Formation in this area.
Fig. 8.
Fig. 8.
Section of key boundaries in Wufeng Formation-Longmaxi Formation of the Northeast Hubei-Southeast Chongqing-West Hunan region.
Fig. 9.
Fig. 9.
Section of key boundaries in Wufeng Formation-Longmaxi Formation in the South Sichuan-Southeast Sichuan (The data of Shuanghe in Changning and Guanyinqiao in Qijiang are cited from reference [17]).
3.2. Stratification and application
Here, the stratigraphic correlation and application prospect are only described in light of the bentonite dense section in Longmaxi Formation, but not the two bentonite dense sections ① and ② in the Wufeng Formation which is relatively thin with clear top and bottom boundaries. The bentonite dense section in Longmaxi Formation has a stable positional relationship with the top and bottom boundaries of the graptolite belt, and it is widely distributed in the upper Yangtze region, showing universal GR peak behaviors. Hence, it can act as an important marker for stratigraphic correlation in Longmaxi Formation in key exploration areas (Table 2 and Figs. 6-9), and it is also a critical reference for the division of key boundaries such as LM4 top, Rhuddanian top, LM7 top and Telychian bottom (Fig. 6), so as to realize the fine research on black shale.
The number of bentonite layers in Longmaxi Formation is variable in different areas of the Sichuan Basin and its periphery. It is very important to determine the key boundary correlation scheme for different areas according to the bentonite dense section. In East Sichuan depression, North Sichuan, Northwest Hubei and other areas, the bentonite dense sections of Longmaxi Formation are complete and stable, with obvious GR peak behavior, and correspond to many effective data points exposed. Accordingly, the basic scheme for correlation and division of key boundaries in Longmaxi Formation in the middle-north of Yangtze sea basin is formulated (Figs. 6 and 8).
The bentonite dense section ③ is relatively developed at the bottom of Coronograptus cyphus graptolite belt in East Sichuan and South Sichuan depressions (Figs. 8 and 9), and the calibration data points are located in Huangcao in Wulong and Shuanghe in Changning. In Wulong and Changning areas, the strata below the boundary are siliceous shale with TOC>2%, but suddenly become carbonaceous shale or silty shale with TOC of 1%-2% above the boundary. The GR peak can act as the reference for the division of the bottom boundary of the Coronograptus cyphus graptolite belt in the Sichuan Basin, that is, the first trough below the GR peak serves as the division boundary between the Cystograptus vesiculosus graptolite belt and the Coronograptus cyphus graptolite belt.
The bentonite dense section ④ occurs in the middle part of the Coronograptus cyphus graptolite belt from East Sichuan to Western Hunan-Hubei and the surrounding areas (Fig. 8). The calibration data points are located in Qiliao in Shizhu, Bailu in Wuxi and Guanwu in Hefeng. The bottom boundary of the GR peak is the bottom of Rhuddanian in the hinterland of Western Hunan-Hubei uplift.
The bentonite dense section ⑤ is composed of thick bentonite stably distributed in the middle-upper Yangtze region, which can be found in all section points in the Sichuan Basin and its periphery. The GR value is 188-271 cps (mostly 215-235 cps), which is easy to identify on the logging curve. Generally, the top of Rhuddanian is defined at the first GR trough below it, that is, according to the principle that the top of Rhuddanian does not exceed the thick bentonite of Aeronian Demirastrites graptolite belt [17, 19-20].
The bentonite dense section ⑥ is thick bentonite widely distributed in East Sichuan and Northwest Hubei, which generally occurs in the upper-top of the Lituigrapatus convolutus graptolite belt. The calibration data points are located in Qiliao in Shizhu, Dengjia'ao in Changyang, Xiema in Baokang and Mingzhong in Chengkou. The first trough above the GR peak can act as the top boundary of the Lituigrapatus convolutes graptolite belt (i.e. the bottom boundary of the Stimulograptus sedgwickii graptolite belt).
The bentonite dense section ⑦ is distributed in a limited range, only in the Mingzhong section in Chengkou. It is a thick bentonite at the bottom of Stimulograptus sedgwickii graptolite belt. More data points are needed to confirm whether it can act as the basis for dividing the bottom boundary of Stimulograptus sedgwickii graptolite belt.
The bentonite dense section ⑧ is a thick bentonite developed at the bottom of Spirograptus guerichi graptolite belt (0.2-0.5 m from the bottom) in Northeast Hubei and North Sichuan. The calibration data points are located in Yangba in Nanjiang and Mingzhong in Chengkou. The first trough below the GR peak can act as the bottom of Telychian.
Graptolite succession is often found incomplete in the exploration areas of Longmaxi Formation in the Sichuan Basin and its periphery, making it impossible to identify the bottom boundaries of key graptolite belts. Zou et al., Cooper RA et al., and Zhang et al. [6, 10-11] believe that the diverse evolution of graptolites is significantly different in various environments. The graptolite succession is incomplete or missing in varying degree in nearshore and shelf environments, but very continuous in shelf edge-slope environment. Thus, the incomplete graptolite succession is considered as a common phenomenon in the Yangtze platform-basin region, and the bentonite dense section can make up for the lack of graptolite stratification. For example, in East Sichuan-Western Hunan-Hubei (e.g. Longshan, Xiushan, Youyang, Pengshui, Fuling and Shizhu), where the volcanic ash at the turn of Ordovician-Silurian is concentrated, the bentonite dense sections ③-⑥ are found in the Longmaxi Formation. The bentonite dense section ⑤ is about 10 cm thick in this area and well exposed, with obvious GR peak, and it has become an important reference for identifying the top of Rhuddanian [19]. This area is close to Xuefeng-Central Guizhou provenance. Affected by a large amount of terrigenous debris input, the contents of feldspar and clay in Rhuddanian and overlying strata are high. Especially, the average content of feldspar is 9.5%-17.9%, much higher than that in South Sichuan, and North Sichuan-Northwest Hubei. This reflects that the sea water in this sea is relatively turbid, which is not conducive to the growth of Demirastrites that like clean water. Therefore, Demirastrites are generally less, for example, in Youyang, or later, for example, in Xiushan and Fuling, resulting in the first occurrence of the Demirastrites belt at most data points above the bentonite dense section ⑤ (Fig. 8). In other words, the Rhuddanian thickness determined according to the first occurring layer of Rhuddanian is greater than the actual thickness. Through the detailed survey of sections (Hongyanxi in Longshan, Datianba in Xiushan, Heishui in Youyang, Lujiao in Pengshui, Qiliao in Shizhu and Bailu in Wuxi, as shown in Fig. 8) and the research on the distribution of Rhuddanian [19], it is found that the Rhuddanian thickness determined according to the first occurring layer of the Demirastrites in Fuling, Xiushan and Longshan areas is 3 m, 4 m and 12 m greater than the actual thickness respectively (Fig. 8). The determination of top of Rhuddanian in the above areas often refers to the bentonite dense section ⑤.
In the southern Yangtze sea basin (mainly South Sichuan), there are few sections that can completely reveal the bentonite dense sections in the Wufeng Formation-Longmaxi Formation. Only ③ and ⑤ are found in several places, with stable distribution and obvious GR peak (Figs. 7c, 7d and 9). They can act as key boundaries for regional correlation in Longmaxi Formation in South Sichuan and have better application prospect.
4. Relationship between key boundaries and organic-rich shale deposition
Through the study on the relationship between the key boundaries in Wufeng Formation-Longmaxi Formation and the organic-rich shale deposition in Northeast Hubei, Southeast Chongqing, West Hunan and South Sichuan, it is found that the bentonite dense section has a significant control over the deposition of organic-rich shale, and the deposition and lithofacies types of Guanyinqiao Member also have a certain correlation with the development degree of organic-rich shale in key areas (Figs. 8 and 9).
The bentonite-dense section is a structural boundary, and lithofacies mutations often occur near bentonite dense sections ③-⑧ [17, 19-20]. It controls the development of four structural layers from bottom to top in Wufeng Formation-Longmaxi Formation, which correspond to four sea basin active periods, i.e. the initial stage of depression, the middle-late stage of depression, the initial stage of foreland flexure, and the development stage of foreland flexure (Fig. 6). In the development stage of foreland flexure (after the occurrence of bentonite dense section ⑤), there were at least three large-scale northwestward migrations in the Yangtze sea basin, respectively in the formation stage of bentonite dense section ⑤, the formation stage of bentonite dense sections ⑥ and ⑦, and the formation stage of bentonite dense section ⑧. As a result, the platform-basin was uplifted as a whole in the late Aeronian and later, and the subsidence and deposition centers were only distributed in local areas such as Wuxi, Nanjiang and Weiyuan (Fig. 1), far away (nearly 200-300 km) from the deposition center in South Sichuan-East Sichuan [19] formed in the Rhuddanian. The upwelling current flew into the north margin of the Yangtze sea basin and formed organic-rich shale deposits only in smaller areas (Figs. 1 and 6). This suggests that the deep-water sedimentary area of the platform-basin decreased in turn, the stable deposition time was greatly shortened, and the development scale of organic-rich shale was significantly reduced from Rhuddanian to Telychian (Fig. 8). The deep-water area was 19.0×104 km2 in Rhuddanian [19], 10.0×104 km2 in Aeronian[19], and 1.5×104 km2 in Telychian (Fig. 1).
Under the background of the tectonic activity of the sea basin, the deposition of organic-rich shale in different areas of the Yangtze platform-basin was controlled by different bentonite dense sections (Figs. 8 and 9). In the hinterland of the Western Hunan-Hubei uplift (e.g. Hongyanxi in Longshan), the bentonite dense section ④ directly overlies the Paraorthograptus pacificus graptolite belt of Wufeng Formation, reflecting that the Western Hunan-Hubei uplift disappeared with the rapid settlement of the bentonite dense section and entered the foreland deposition period, and the deposition of the organic-rich shale ended basically. In Southeast Chongqing-South Sichuan, the organic-rich shale is mainly endowed below the bentonite dense section ③. The appearance of this bentonite dense section indicates that the area has entered the foreland stage of rapid deposition and ended the deposition of organic-rich shale. In Shizhu area of East Sichuan, the main organic-rich shale is located below the bentonite dense section ④, marking that the East Sichuan depression entered the foreland stage at the period of 0.5 Coronograptus cyphus graptolite belt later than Southeast Chongqing-South Sichuan. In Southeast Sichuan (e.g. Renhuai, Qijiang, and Daozhen), the main organic-rich shale is located below the bentonite dense section ⑤. The appearance of this bentonite dense section marks that the Southeast Sichuan depression after uplift had completely entered the foreland stage, and 0.5-1.0 more graptolite belts were deposited than Southeast Chongqing (Fig. 9). In Wuxi area of Northeast Hubei, the foreland stage appeared actually after the emergence of bentonite dense sections ④ and ⑤. Before the appearance of bentonite dense section ⑤, deposition occurred slowly in hydrostatic shelf. Later, with the large-scale northward movement of the subsidence and deposition centers and the influx of upwelling currents, the area turned into upwelling current deposition, resulting in the development of organic-rich shale below and above the bentonite dense section ⑤ (Figs. 6 and 8).
The relationship between the Guanyinqiao Member shell layer and the organic-rich shale deposition is mainly reflected by the paleogeographic environment at the turn of Ordovician-Silurian (Figs. 8 and 9). The Guanyinqiao Member is generally absent in the uplift areas (e.g. Hongyanxi in Longshan) of Western Hunan-Hubei, which is not conducive to the deposition of organic-rich shale (mostly less than 10 m thick). In the Wuxi, Shizhu, Fuling, Daozhen, Renhuai and Changning depression areas, the Guanyinqiao Member is organic-rich siliceous shale and calciferous siliceous shale, reflecting that the Wufeng Formation-Longmaxi Formation is continuous deep- water deposits, and the water depth is not largely affected by the ice age, which is conducive to the deposition of organic-rich shale (generally 20-40 m thick) [17-19, 30-33]. In the slope areas (e.g. Youyang, Xiushan and Qijiang) of Southeast Chongqing, the Guanyinqiao Member is not missing, but it is mainly organic-lean calcareous shale or marl, reflecting that the ancient water depth was between the Western Hunan-Hubei sea area and the East Sichuan sea area, and the sedimentary thickness of organic-rich shale is 10-20 m. Obviously, the lithofacies type of Guanyinqiao Member can also act as a mark for judging the sedimentary scale of organic-rich shale in key areas.
The relationship between concretion and organic-rich shale deposition is not obvious, and the relationship between the first occurring layer of specific graptolites and organic-rich shale deposition is relatively complex. Concretions mainly occurred in the foreland period above the bentonite dense sections ③ and ⑤ of the platform- basin region, and their distribution is relatively limited. They are mostly not symbiotic with organic-rich shale, with unclear relationship (Figs. 8 and 9). The first occurring layer of a specific graptolite mainly reflects the onset of the generation and evolution of the corresponding graptolite fauna, which is not directly related to the deposition of organic-rich shale. Since the bentonite dense section, the Guanyinqiao Member shell layer and concretion are the event sedimentary responses in the key graptolite belts, it is scientific to explain the relationship between the first occurring layer of a specific graptolite and the deposition of organic-rich shale through the above bentonite dense section and the Guanyinqiao Member shell layer. That is, the major tectonic (volcanic) activities and abrupt events of sedimentary elements in the key graptolite belts control the deposition of organic-rich shale.
5. Conclusions
Four kinds of markers with stable distribution in whole domain or local area and general boundary properties, i.e. characteristic graptolite belt, Guanyinqiao Member shell layer, bentonite dense section and concretion, are found in Wufeng-Longmaxi Formation in the middle-upper Yangtze region. They can serve as the key boundaries for stratification and correlation in graptolite shale.
The Guanyinqiao Member shell layer exhibits the lithofacies and logging responses varying greatly from area to area, and directly reflects the paleogeographic environment of the Yangtze sea basin during depression at the turn of Ordovician-Silurian. It is the most standard boundary of Wufeng Formation-Longmaxi Formation, and can also act as an important marker to judge the sedimentary scale of organic-rich shale.
The bentonite-dense section is the main research unit to understand the high-frequency volcanic ash system of Wufeng Formation-Longmaxi Formation in the Sichuan Basin and its periphery. Eight bentonite dense sections of two types are found, mainly in seven graptolite belts. The bentonite dense section is an important sedimentary response to the flexure, uplift and large-scale migration of subsidence and deposition centers in the Yangtze sea basin at the turn of Ordovician-Silurian. It has similar boundary characteristics in thickness and GR response with the Guanyinqiao Member shell layer. It belongs to structural boundary (i.e. event deposition), and has three distribution scales: whole domain, large area, and local, which can correspondingly become the third-, fourth- and fifth-order sequence boundaries and has differential control on organic-rich shale deposition.
The first occurring layer of the characteristic graptolite belt is an isochronous boundary, which is not directly related to the deposition of organic-rich shale. Its identification fully depends on the occurrence of the characteristic graptolites in the field outcrops or drilling cores.
Concretions occur only in local areas, with the distribution area much smaller than the above three types of key boundaries. Moreover, it has poor stability in terms of vertical and horizontal distribution. It has no obvious relationship with organic-rich shale deposition.
Reference
Toward a stepwise Kwangsian Orogeny
DOI:10.1007/s11430-013-4815-y URL [Cited within: 4]
Circumjacent distribution pattern of the Lungmachian graptolitic black shale (early Silurian) on the Yichang Uplift and its peripheral region
DOI:10.1007/s11430-017-9222-x URL [Cited within: 3]
Stage- progressive distribution pattern of the Lungmachi black graptolitic shales from Guizhou to Chongqing, Central China
DOI:10.1007/s11430-016-9031-9 URL [Cited within: 7]
Wufengian (ashgillian) graptolite faunal differentiation and anoxic environment in South China
Shale gas in China: Characteristics, challenges and prospects (I)
Importance of graptolite evolution and biostratigraphic calibration on shale gas exploration
A deep water shelly fauna from the uppermost Ordovician in northwestern Hunan, South China and its paleoecological implications
DOI:10.1007/s11430-017-9165-y URL [Cited within: 3]
Northward expansion of Central Guizhou Oldland through the Ordovician and Silurian transition: Evidence and implications
Biostratigraphy and geography of the Ordovician-Silurian Lungmachi black shales in South China
Diversity and paleobiogeographic distribution patterns of Early and Middle Ordovician graptolites in distinct depositional environments of South China
DOI:10.1007/s11430-010-4088-7 URL [Cited within: 3]
On the Ordovician-Silurian depositional hiatus at Taiyanghe, Enshi, Hubei Province
Graptolites zone and sedimentary characteristics of Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in Sichuan Basin and its adjacent areas
Biostratigraphy of Ordovician-Silurian black shale at Well Zi 201, South Sichuan
Black shale biostratigraphic characteristics and stratigraphic correlation in the Wufeng and Longmaxi Fms of the Xuanhan-Wuxi areas
Main factors controlling the sedimentation of high-quality shale in Wufeng-Longmaxi Fm, Upper Yangtze region
Distribution characteristics and geological significance of the thickest Aeronian bentonite bed in Middle-Upper Yangtze region
Guanyinqiao member lithofacies of the Upper Ordovician Wufeng Formation around the Sichuan Basin and the significance to shale gas plays, SW China
Developmental characteristics and geological significance of the bentonite in the Upper Ordovician Wufeng-Lower Silurian Longmaxi Formation in eastern Sichuan Basin, SW China
A new discovery and geological significance of thick-layered bentonites in the Upper Member of Lower Silurian Longmaxi Formation in the northern Sichuan-western Hubei area
Sedimentary characteristics of upwelling facies shale in Lower Silurian Longmaxi Formation, northeast Sichuan area
Development characteristics of concretions in the Longmaxi Formation of Lower Silurian in the Sichuan Basin and the indicating significance of their depositional environment
“Three Gorges landscape stones”: The sedimentary concretion in eastern Three Gorges area
Characteristics and genetic mechanism of calcareous concretions in the Early Cambrian Qiongzhusi Formation of northern Sichuan Basin
Septarian crack formation in carbonate concretions from shales and mudstones
DOI:10.1180/claymin.1986.021.4.12 URL [Cited within: 1]
Septarian carbonate concretions in the Permian Rio do Rasto Formation: Birth, growth and implications for the early diagenetic history of southwestern Gondwana succession
DOI:10.1016/j.sedgeo.2015.06.007 URL [Cited within: 1]
Growth mechanisms and geochemistry of carbonate concretions from the Cambrian Wheeler Formation (Utah, USA)
DOI:10.1111/sed.2016.63.issue-3 URL [Cited within: 1]
Oxygen and carbon isotopic composition of marine carbonate concretions: An overview
Deep-burial alteration of early-diagenetic carbonate concretions formed in Palaeozoic deep-marine greywackes and mudstones (Bardo Unit, Sudetes Mountains, Poland)
DOI:10.1111/sed.2014.61.issue-5 URL [Cited within: 1]
LA-ICP-MS zircon U-Pb dating of the bentonites from the uppermost part of the Ordovician Wufeng Formation in the Haoping section, Taoyuan, Hunan
SHRIMP zircon U-Pb dating of the bentonites from the uppermost part of the Ordovician in the Wangjiawan section, Yichang, Hubei, China
DOI:10.1007/s11430-008-0028-1 URL
Hydrocarbon generation and storage mechanisms of deep- water shelf shales of Ordovician Wufeng Formation-Silurian Longmaxi Formation in Sichuan Basin, China
Geological characteristics and high production control factors of shale gas reservoirs in Silurian Longmaxi Formation, southern Sichuan Basin, SW China
