Graptolitic Shales Research Articles

Hydrodynamic performance of Ordovician archaeostracan carapaces.

The diversification of macroscopic pelagic arthropods such as caryocaridid archaeostracans was a crucial aspect of the Great Ordovician Biodiversification Event, and the plankton revolution. A pelagic mode of life has been inferred for caryocaridids from their common presence in black graptolitic shales alongside carapace morphologies that appear streamlined. However, the hydrodynamic performance within the group and comparisons with other archaeostracans were lacking. Here we use a computational fluid dynamics approach to quantify the hydrodynamic performance of caryocaridids, and other early Palaeozoic archaeostracans including Arenosicaris inflata and Ordovician ceratiocaridids. We show that streamlining of the carapace was an important factor facilitating a pelagic mode of life in caryocaridids, in reducing the drag coefficient and facilitating a broader range of lift coefficients at different angles of attack. However, comparable hydrodynamic performance is also recovered for some ceratiocaridids. This suggests that alongside carapace streamlining, adaptations to appendages and thinning of the carapace were also important for a pelagic mode of life in Ordovician caryocaridids.

The Holy Cross Mountains (Poland) terranes palaeoposition and depositional environment in Silurian—new insights from rock magnetic studies

SUMMARY The Holy Cross Mountains (HCM) in Poland, is an isolated natural outcrop of Palaeozoic rocks located within the Trans-European Suture Zone, a tectonic collage of continental terranes adjacent to the Tornquist margin of the Baltica. This uniqueness made the HCM a target for palaeogeographic research. Based on the facies differences, the HCM had been divided into two major units, the southern (the Kielce Unit) and northern (the Łysogóry Unit) part (SHCM and NHCM, respectively). Their position in relation to each other and the Baltica continent during Silurian times is still a matter of discussion, whether both parts of the HCM were separated terranes located along the Baltica margin or they shared in common palaeogeographic history. Here, we present the results of comprehensive rock magnetic measurements applied as a tool to interpret palaeoenvironmental conditions during deposition and burial and therefore allow discussion about the terranes’ relative position. To recognize the magnetic mineral composition and texture of studied Silurian graptolitic shales several rock magnetic measurements were conducted including low-temperature Saturated Isothermal Remanent Magnetization, thermal demagnetization of three-component IRM and hysteresis measurements, as well as anisotropy of magnetic susceptibility (AMS). The sampled rocks come from both units of the HCM. In all analysed samples we found single domain (SD) stoichiometric magnetite of mostly diagenetic (i.e. post-depositional) origin and goethite resulting likely from weathering. In turn, detrital magnetite, even if observed in previously investigated Silurian rocks from the Baltica margin, was not identified in this study, what we attribute to dissolution during diagenesis in the deep-water environment. Solely in the NHCM, SD hematite and maghemite grains were observed, which we interpret as detrital in origin. These grains have been preserved in the suboxic environment of the NHCM sub-basin bottom waters due to their resistance to dissolution in marine waters. Considering the deposition conditions (oxygenation of the near-bottom zone) rather similar for both HCM parts, we associate the presence of aeolian hematite grains solely in the NHCM rocks with a more proximal position of the NHCM than the SHCM in relation to the Baltica continent during late Llandovery (Silurian). This conclusion agrees with some existing palaeogeographic models. In addition to petromagnetic studies focused on the analysis of ferromagnets, AMS measurements were also carried out. The results indicate that the magnetic susceptibility is mainly governed by paramagnetic minerals, mostly phyllosilicates with small ferromagnetic contributions. Oblate AMS ellipsoid and distinct bedding parallel foliation indicate prevailing sedimentary-compactional alignment. Observed magnetic lineation of tectonic origin resulting from weak strain is related presumably to Variscian deformations.

Ordovician of the conterminous United States

AbstractThe Ordovician rocks of the conterminous United States (US) have a complex history, spanning multiple ancient basins, shifting palaeoclimate and evolving tectonic regimes. The US portion of the palaeocontinent of Laurentia occupied a relatively stable and isolated position around the southern tropics during the Ordovician. In general, Lower Ordovician rocks form a vast autochthonous blanket of fine-grained (tropical) carbonates that covered much of Laurentia, named the ‘Great American Carbonate Bank’. Outboard, ribbon carbonates and graptolitic shales are found in allochthonous fragments of the ancient continental margin. Middle Ordovician strata are more lithologically diverse, including the addition of several regionally distributed sandstones of the inner detrital belt, mostly overlying the Sauk–Tippecanoe unconformity. Upper Ordovician strata show the greatest lithologic and faunal diversity, reflecting steepening topography resulting from regional compression along the south Laurentian (Appalachian) margin. Recent advances in the interpretation of the US Ordovician come primarily from studies of carbon and oxygen stable isotopes, sequence stratigraphy, palaeoecology, tephrochronology, redox geochemistry, strontium isotopes and geochronology.

Geochemical Characteristics of Graptolite Shale in the Pingliang Formation of the Ordos Basin, China: Implications for Organic Matter, Thermal Evolution, and Hydrocarbon Reservoir

Graptolite-rich shale is the main layer of shale gas resources in the southern marine sedimentary basin. Recently, shale gas resources were discovered in the Ordovician marine graptolite-rich strata in the Ordos Basin. The graptolite shale in the study area is different from the marine graptolite shale in the Yangtze plate in southern China, and further exploration is needed. This paper presents core samples of the graptolite-rich shale of the Pingliang Formation in the southwest Ordos Basin as research objects. The graptolite genus and graptolite shale characteristics were studied using core observation, electron microscopy, scanning electron microscopy, and geochemical analysis. We determined the role of the sedimentary environment and thermal maturation of graptolite shale in hydrocarbon formation and explored the possibility of hydrocarbon generation. Many graptolite epidermises provide buried organic matter. The quiet sea and low-energy marine environment create favorable conditions for preserving organic matter. The tectonic process resulted in the evolution stage in the oil generation window. Different types of pores formed the spaces of hydrocarbons. Therefore, the shale of the Pingliang Formation has shale oil exploration potential, which complements the shale gas in the northwestern margin of the basin, and provides new venues for shale oil and gas exploration in northern China.

Basic characteristics of key interfaces in Upper Ordovician Wufeng Formation – Lower Silurian Longmaxi Formation in Sichuan Basin and its periphery, SW China

Based on anatomy of key areas and data points and analysis of typical features of shell layer in Guanyinqiao Member, basic characteristics of key interfaces, mainly bentonite layers, in the Upper Ordovician Wufeng Formation—Lower Silurian Longmaxi Formation in the Sichuan Basin and its surrounding areas and the relationship between these key interfaces with the deposition of organic-rich shale have been examined systematically. The Wufeng Formation—Longmaxi Formation has four types of marker beds with interface attributes, namely, the characteristic graptolite belt, Guanyinqiao Member shell layer, section with dense bentonite layers, and concretion section, which can be taken as key interfaces for stratigraphic division and correlation of the graptolite shale. The shell layer in Guanyinqiao Member is the most standard key interface in Wufeng Formation—Longmaxi Formation, and can also be regarded as an important indicator for judging the depositional scale of organic-rich shale in key areas. There are 8 dense bentonite sections of two types mainly occurring in 7 graptolite belts in these formations. They have similar interface characteristics with the shell layer in Guanyinqiao Member in thickness and natural gamma response, and belong to tectonic interfaces (i.e., event deposits). They have three kinds of distribution scales: whole region, large part of the region, and local part, and can be the third, fourth and fifth order sequence interfaces, and have a differential control effect on organic-rich shale deposits. The horizon the characteristic graptolite belt occurs first is the isochronous interface, which is not directly related to the deposition of organic-rich shale. Concretions only appear in local areas, and show poor stability in vertical and horizontal directions, and have no obvious relationship with the deposition of the organic-rich shale.

Lower Silurian graptolitic shales of the Tunguska Syneclise and its oil and gas source properties (East Siberia)

Lower Silurian graptolitic shales of the Tunguska Syneclise and its oil and gas source properties (East Siberia)

A Lithological and Geochemical Description of Lower Silurian Graptolite Shales (Yangtze Platform, People’s Republic of China)

A Lithological and Geochemical Description of Lower Silurian Graptolite Shales (Yangtze Platform, People’s Republic of China)

Upper Silurian (Gorstian, Ludfordian, PŘÍDOLÍ) Graptolite Biozonation in the Muhurr area (Albania)

Three stratigraphical sections and several outcrops through the upper Silurian graptolite shales of the Muhurr area, Albania have been revised. Based on a new detailed re-examination and re-evaluation of graptolite collections and previously unpublished and published studies, a biozonal scheme with much new biostratigraphical data is established. The graptolite assemblages are dominated by Monograptidae: Neodiversograptus, Saetograptus, Bohemograptus, Colonograptus, Pseudomonoclimacis and Lobograptus which are associated with several Retiolitidae. A total of 37 graptolite species has been identified; selected important species are illustrated. The present study recognized and defined the following seven upper Silurian graptolite biozones: nilssoni and scanicus-chimaera in the Gorstian Stage, leintwardinensis, tenuis and formosus in the Ludfordian Stage and parultimus-ultimus and perneri in the Přídolí Series.

A new method for predicting the shale distribution of the Wufeng Formation in the Upper Yangtze Region, China

Taking the Late Ordovician Wufeng Formation (WFF) shale in the Upper Yangtze region as an example, we conducted a lithofacies distribution, thickness quantification, and paleo-topographic reconstruction of the Late Ordovician graptolite zones. Specifically, we focused on the Late Katian Dicellograptus complexus and the Early Hirnantian Metabolograptus extraordinarius within a chronostratigraphic framework, using the Geographic Information System (GIS) and 310 stratigraphic sections (incl. drilling) obtained from the Geobiodiversity Database (GBDB). Reconstruction of the geographic distribution indicates that the WFF and the synchronous sediments in the Upper Yangtze region contain 8 litho-stratigraphic units, which are geographically distributed across 7 provinces/municipalities and do not exhibit significant variations in lithofacies. The black graptolite shale extends in a broad swath from east to west within the basin, while the other lithofacies deposited during the same period are present on the periphery of the basin. All these strata were deposited in a normal neritic epicontinental sea environment, except for the flysch sediments in the southern Hunan area. The thickness reconstruction involves a comparison of three spatial interpolation methods, including Inverse Distance Weighting (IDW), Kriging, and the Radial Basis Function (RBF). Based on a general verification, IDW is considered to be the optimal method since it has the minimum standard deviation and variance. Based on the contours obtained from the IDW model, the WFF black shale is estimated to have an overall area of 0.67 × 106 km2, an average thickness of 6.2 m, and a total volume of 3902 km3. This shale was deposited over a 2.83 Ma period. Therefore, the volume of shale deposited per million years is estimated to be 1379 km3/my and the average thickness of shale deposited per million years is 2.37 m/my. The Hirnantian paleo-water-depth values obtained using 275 sections were used to infer the Late Katian paleo-topography. These results suggest that the Yangtze platform was surrounded by ancient highlands to the west, south, and north, exhibiting a paleo-geographic framework characterized by one uplift and four depressions. This setting blocked water circulation, causing the water to be contained and forming a closed and restricted marine environment, which was one of the major factors controlling the deposition of the organic-rich WFF shale. With the advent of the big data era of geology, the methodology of GIS-based technology is readily exportable to any resource play having spatial distribution pattern. Results can be provided rapidly and efficiently generated from geological data.

Features of the Structure of Carbon-Rich Rocks (Based on the Example of the Lower Silurian Sediments of the Sichuan Depression of the Yangtze Platform)

The petrophysical, geochemical, and lithological features of the structure of graptolite shales from the lower part of the Longmaxi Formation of the Lower Silurian age of the Sichuan Province of the People’s Republic of China are considered. The basic patterns of the formation of carbon-rich sediments in the intrashelf depressions are revealed.

Differential Characteristics of the Upper Ordovician‐Lower Silurian Wufeng‐Longmaxi Shale Reservoir and its Implications for Exploration and Development of Shale Gas in/around the Sichuan Basin

The Upper Ordovician Wufeng‐Lower Silurian Longmaxi shale is widely distributed in the Sichuan Basin and its periphery, which is the key stratum for marine shale gas exploration and development (E&D) in China. Based on sedimentary environment, material basis, storage space, fracability and reservoir evolution data, the reservoir characteristics of the Wufeng‐Longmaxi shale and their significance for shale gas EE&D are systematically compared and analyzed in this paper. The results show that (1) the depocenter of the Wufeng (WF)‐Longmaxi (LM) shale gradually migrates from east to west. The high‐quality shale reservoirs in the eastern Sichuan Basin are mainly siliceous shales, which are primarily distributed in the graptolite shale interval of WF2‐LM5. The high‐quality reservoirs in the southern Sichuan Basin are mainly calcareous‐siliceous and organic‐rich argillaceous shales, which are distributed in the graptolite shale interval of WF2‐LM7. (2) Deep shale gas (the burial depth >3500 m) in the Sichuan Basin has high‐ultrahigh pressure and superior physical properties. The organic‐rich siliceous, calcareous‐siliceous and organic‐rich argillaceous shales have suitable reservoir properties. The marginal area of the Sichuan Basin has a higher degree of pressure relief, which leads to the argillaceous and silty shales evolving into direct cap rocks with poor reservoir/good sealing capacity. (3) Combining shale gas exploration practices and impacts of lithofacies, depth, pressure coefficient and brittle‐ductile transition on the reservoir properties, it is concluded that the favorable depth interval of the Wufeng‐Longmaxi shale gas is 2200∼4000 m under current technical conditions. (4) Aiming at the differential reservoir properties of the Wufeng‐Longmaxi shale in the Sichuan Basin and its periphery, several suggestions for future research directions and EE&D of shale gas are formulated.

Integrated graptolite-conodont biostratigraphy and organic carbon chemostratigraphy of the Llandovery of Kallholn quarry, Dalarna, Sweden

A revised graptolite and conodont biostratigraphy together with new organic carbon isotope data for the middle Aeronian to lower Telychian (Llandovery, lower Silurian) of Kallholn quarry, central Sweden, are presented. The base of the section is marked by an unconformity between the Ordovician Boda Limestone and Aeronian shales with limestone nodules belonging to the Pribylograptus leptotheca Biozone. These are overlain by strata assigned to the lower Lituigraptus convolutus Biozone. The Stimulograptus sedgwickii Biozone, previously recorded from the section, is absent and marked by an unconformity which is overlain by radiolarian-rich graptolitic shales belonging to the Stimulograptus halli Biozone. The overlying Telychian strata are assigned to the Spirograptus guerichi, Spirograptus turriculatus and Streptograptus crispus biozones. Conodonts indicate that the boundary between the Distomodus staurognathoides Biozone and Pterospathodus eopennatus Superzone lies within the Sp. turriculatus Biozone, as it does also in the Ohesaare core, Estonia. The δ13Corg record shows mostly minor fluctuations but with a distinct twin-peaked positive excursion in the upper Sp. turriculatus Biozone, close to the boundary with the overlying Streptograptus crispus Biozone. The excursion in the Kallholn section may correlate with the earliest positive shift in δ13C values at the onset of the excursion associated with the Valgu Event or it may represent a new excursion (provisionally named the Kallholn excursion) preceding this. Many more bentonites (36) occur in the section than previously recorded – geochemical studies are needed to confirm whether the Osmundsberg bentonite (which may or may not be equivalent to the “O” bed in Estonia and Latvia) is present.

Unraveling a distal segment of the West African Craton Paleozoic margin: Stratigraphy of the Mougueur inlier of the eastern High Atlas, Morocco

Because of its location in the boundary zone between the Anti-Atlas and the Atlas–Meseta crustal domains of Morocco, the Mougueur Paleozoic Massif is a key area to decipher the evolution of the Gondwana NW margin during the Paleozoic. In this work, we report for the first time the occurrence of Hirnantian sandstones, Silurian (Gorstian) graptolitic shales and probable Lower Devonian turbidites in the Massif. Comparison of our observations with regional stratigraphic data from literature allows us to suggest that the area was included in the distal part of the West African Craton passive margin during Cambrian-Devonian times.

Discussion on the contribution of graptolite to organic enrichment and gas shale reservoir: A case study of the Wufeng–Longmaxi shales in South China

The graptolitic shale of the Wufeng–Longmaxi Formations is widely deposited across the Ordovician and Silurian transition in South China, which is the target of shale gas exploration and development within China. The contribution of graptolites to organic enrichment and reservoir of gas shale is discussed below based on the statistics of nearly 1000 shale samples from the Wufeng Formation and the bottom part of the Longmaxi Formation in the southern and northern margins of the Yangtze plate. The assessment involves graptolites abundance, the total organic carbon (TOC) content analyses, and the different scales of scanning electron microscopy analyses of related samples. The TOC content of the Wufeng–Longmaxi graptolitic shales (including graptolites and non-graptolites, i.e., the host shale) is mainly controlled by that of its host shale, while less affected by the graptolites abundance, indicating that the graptolites barely influence the organic enrichment. Graptolites consist of a large number of organic matter with reticular biological tissue structure; they account for 20%–50% of the graptolitic area. The aforementioned also developed honeycomb-shaped pores with pore sizes ranging 110 nm-1.7 μm (an average of about 500 nm), which are higher than those of the organic pores in the host shale (108–770 nm, average 330 nm), proving that graptolites have an important contribution to shale gas storage space. Since there are a large number of graptolites within the shales from the Wufeng Formation and the bottom part of the Longmaxi Formation, the laminated and stacked local pattern of their distribution provides abundant storage space for shale gas. Moreover, the feature also serves as the predominant channel for shale gas flow. Therefore, the widely developed graptolites should be considered as one of the essential factors controlling enrichment and high productivity of shale gas in the Wufeng–Longmaxi Formations.

Oriented graptolites in the Uppermost Ordovician Wufeng black shale in southern China

Graptolites are elongated fossils that can be used to decipher the current direction to evaluate paleocurrent conditions. Abundant graptolite fossils are well-preserved and clearly exhibit strong alignment in the black shales of the Upper Ordovician Wufeng Formation on the southern edge of the Upper Yangtze Sea, South China. The orientations of these graptolites indicate that the remains of graptolites were deposited on the seafloor under the influence of currents, instead of under calm water conditions. Based on the measurements of the long-axis directions of graptolites, the distributions of graptolites preserved on the bedding planes display discernable orientations and exhibit systematic changes throughout the stratigraphic section. In the western region of the study area, the paleocurrent directions in the stratigraphic sections gradually change from NE–SW in the Dicellograptus complexus biozone to N–S in the Paraorthograptus pacificus and Normalograptus extraordinarius–N. ojsuensis biozones. However, in the eastern region of the study area, the paleocurrent directions are more variable, ranging from multi-directional (NNE–SSW and NNW–SSE) in the P. pacificus biozone to NW–SE in the N. extraordinarius–N. ojsuensis biozone. The changes in the orientations of graptolites in the D. complexus and the P. pacificus biozones are consistent with the southern configuration of the Upper Yangtze Sea and indicate that a coastal current existed. In addition, some of the contemporaneous deformation structures were caused by tectonic forces observed in the lower Graptolite Shale Member of the Wufeng Formation in the eastern region of the study area. Some of the changes in the orientations of graptolites in the N. extraordinarius–N. ojsuensis biozone can be interpreted to have resulted from a change in tectonic activity (the Caledonian orogeny) before the N. extraordinarius–N. ojsuensis biozone. Furthermore, Lingula brachiopods, which live on the seafloor in shallow water, have been reported in the black shale of the Wufeng Formation, which indicates that the paleoenvironment was at least partially oxic; this supported the growth, development and reproduction of planktonic graptolites and other organisms. These results support the interpretation that the remains of organisms were quickly buried and that the amount of exchange between pore water and the water column gradually decreased. H2S gas generated by the decay of organisms during deposition consequently generated a strongly reducing environment in these sediments, which resulted in the formation of black shales.

Microstructure of graptolite periderm in Silurian gas shales of Northern Poland

Core samples of Silurian graptolite shales from an exploration borehole within the Baltic Basin were examined by optical microscopy, Raman spectroscopy and high resolution transmission electron microscopy. Graptolite periderm is composed of poorly ordered carbonaceous material of mainly aromatic structures. The dimension of the coherent domains is 1–2 nm. They are composed of two to three stacked layers. Raman spectral parameters of graptolite periderm are correlated with the mean random (Rr) and mean maximum (Rmax) reflectance of graptolites. The spectral ID1/IG ratio predicts thermal maturity in a mean reflectance of graptolites (Rr) range between 1.30% and 1.80% (Rmax ≈ 1.40–2.00%; Vitrinite Reflectance Equivalent VRE ≈ 1.10–1.50%). It is possible to calculate VRERaman from the ID1/IG ratio.

Upper Ordovician and Silurian ash beds in the Holy Cross Mountains, Poland: preservation in mudrock facies and relation to atmospheric circulation in the Southern Hemisphere

Numerous volcanic ash beds (K-bentonite) are recognized globally in Ordovician to Early Devonian sedimentary successions, and are important for regional stratigraphic correlations. Relatively little, however, is understood of the origin and transport of these ash deposits. Here, we examine ash beds preserved in the Upper Ordovician and Silurian dark shales and mudstones of the Holy Cross Mountains, Poland, and discuss their origin in the context of contemporaneous tectonic configurations. The Ordovician ash beds occur in the upper Sandbian and Katian mudrock succession, whereas the Silurian K-bentonites form prominent beds in the upper Wenlock and lower Ludlow graptolite shales. Their various thickness and irregular distribution reflect fluctuation of volcanic activity and contribution of physical and biogenic factors in their preservation. Deposition under dysoxic or anoxic conditions and relatively rapid burial by muddy sediment seem to be dominant factors preventing physical and biological reworking and mixing of the volcanic ash. The Upper Ordovician ash beds in the Holy Cross Mountains, as well as the Baltoscandian K-bentonites, appear to be accumulated from pyroclastic material delivered by westerlies of the Southern Hemisphere from the Avalonian volcanoes. In the Silurian, the distribution of pyroclastic material was controlled by SE trade winds, and therefore the source area for the Wenlock and Ludlow K-bentonites in the Holy Cross Mountains appears to have been an active subduction zone developed close to the southeastern side of Baltica. However, the delivery of volcanic ash to the central part of Baltica might have been strongly affected by climatically driven latitudinal migration of the Intertropical Convergence Zone.

Dispositional characteristics of Ordovician Wufeng Formation and Silurian Longmaxi Formation in Sichuan Basin and its adjacent areas

According to data of gas wells and typical sections of Wufeng Formation and Longmaxi Formation in Sichuan Basin, shale of various graptolite zones were analyzed to determine depositional environment, lithology and thickness characteristics of the graptolite shale interval of WF2-WF3 in the lower part of Wufeng Formation, the graptolite shale interval of WF4 in Guanyinqiao Member of Wufeng Formation and the graptolite shale interval of LM1-LM4 in the bottom of Longmaxi Formation, and characteristics of shale horizontal distribution were also investigated. During the depositional period of the graptolite shale interval of WF2-WF3, the study area was less affected by the Guangxi movement, the depositional environment was the deep water of open sea, where black shale was mainly deposited; the sedimentation center was developed in northeast Guizhou-northeast Sichuan and south Sichuan, the maximum thickness was from 4 to 6 m in the sedimentation center. During the depositional period of the graptolite shale interval of WF4, the depositional environment in the study area changed greatly due to global sea level fall and enhanced Guangxi movement; the central Sichuan paleouplift, the central Guizhou paleouplift and the Jiangnan-Xuefeng palaeouplift were further expanded, and the area of the sedimentary basin decreased; the depositional environment was mainly carbonate bioclastic shoal of shallow sea, and partially deep sea which only was distributed in the Shizhu-Fuling-Wuxi area in east and northeast Sichuan and the Gongxian-Yongchuan area in south Sichuan; sediments of shallow water were dominated by limestone and argillaceous limestone with abundant Hirnantia, sediments of deep water were dominated by calcareous mudstone and shale with Hirnantia. During the depositional period of the graptolite shale interval of LM1-LM4, due to rise of global sea level and Guangxi movement, the sedimentary area was larger than that in the depositional period of Guanyinqiao Member, and the sedimentary environment mainly was stagnant deepwater; thickness of black shale in the graptolite shale interval of LM1-LM4 was large, and the maximum thickness was over 20 m. Furthermore, control of the central Sichuan paleouplift, the central Guizhou paleouplift and the Jiangnan-Xuefeng paleouplift on black shale was discussed, and control of the Zhiliujing underwater highland/uplift, Huayingshan highland and Dingshan highland as well as western Hubei-Hunan underwater highland/uplift on shale deposition and preservation was also investigated.

Silurian radiolarians from the Sepon Mine, Truong Son Terrane, central Laos and their palaeogeographic and tectonic significance

AbstractA Silurian radiolarian fauna based on 18 species is described from cherts from three sections in two quarries in the Sepon Mine area, central Laos. Each section contains distinctive assemblages butFutobari morishitai,F. solidus,Zadrappolus yoshikiensisandZ. tenuisare found in all three sections and are characteristic of the Ludlow–PridoliZ. yoshikiensisAssemblage. In addition, the Hoei Yang pit section containsZadrappolus hitoeganensis,Pseudospongoprunum sagittatum,Futobarisp. andSecuicollactasp. Section CQ containsPseudospongoprunum sagittatum,P. tazukawaensis,Praespongocoelia parva,P. sp.,Rotasphaerasp. cf.R. quadrata,Secuicollactasp. cf.cassa. Section QM also containsPseudospongoprunum parvispina,P.sp.,Zadrappolussp. andDevoniglansus(?)seponensisnew species Thassanapak is common. Evidence from Silurian and Devonian strata of the southern Truong Son Terrane indicates deepening towards the southern margin as defined by the Thakhek–Danang Shear Zone and exemplified by deep-marine Ludlow–Pridoli radiolarian cherts in the south, shallowing to fossiliferous, shallow-water sediments to widespread Old Red Sandstone type terrestrial facies 85 km further NE. The radiolarian cherts are in faulted contact with probable Llandovery limestone and Upper Ordovician – Llandovery graptolitic shales. The graptolitic shales are overlain with slight angular unconformity by the submarine fan, volcaniclastic-dominated, Namphuc Formation of Llandovery age. This fan received andesitic materials from the nearby Long Dai Volcanic Arc which extends eastwards to the Tamky Suture in Vietnam. This plutonic–volcanic arc was probably maintained by northwards subduction along the trend of the Permian–Recent Thakhek–Danang Shear Zone.

Stage-progressive distribution pattern of the Lungmachi black graptolitic shales from Guizhou to Chongqing, Central China

The Lungmachi Formation is widely distributed in Guizhou, Chongqing and the adjacent area. It is important for the study of Silurian biostratigraphy and shale-gas investigation. Based on those biostratigraphically well-studied sections from Guiyang to Huayingshan, we reveal the stage-progressive distribution pattern of the Lungmachi black shales. The distribution of the Lungmachi black shales in the studying area can be subdivided into four geographic belts from the south to the north, reflecting the joint effect of regional and global environmental changes. The graptolite depth zonation model was adopted herein to infer the water depth of major graptolite assemblages from the black shales. The changes in the water depth indicate two major stages. The first stage is named the transgressive distribution stage which ranged from the Persculptograptus persculptus Biozone (LM1, upper Hirnantian) to the Coronograptus cyphus Biozone (LM5, upper Rhuddanian), an interval mostly controlled by global sea-level rise. The second stage, ranging from the Demirastrites triangulatus Biozone (LM6, lower Aeronian) to the Spirograptus guerichi Biozone (LM9, lower Telychian), is named the regressive shrinking stage, during which the black shales were gradually replaced by mixed-facies or carbonate sediments from the south to the north, representing the effects of the persistent uplifting of the Central Guizhou Oldland.