Graptolite Biozones Research Articles

Vertical variations in planar lamination of marine shale: Elucidating hydrodynamic changes during the Ordovician–Silurian transition on the Upper Yangtze Block

The hydrodynamics and their evolution on the Upper Yangtze Block during the Ordovician–Silurian transition period remain unclear. The present study is an assessment of how regional and global events may have influenced the hydrodynamic evolution based on a planar lamination investigation of the shales from the Upper Yangtze Block. Analyses of large thin sections and argon-ion polished thin sections using field emission-scanning electron microscopy (FE-SEM) showed that there are four types of planar lamination, namely, silty graded planar lamination (SGPL), silt–clay graded planar lamination (SCGPL), silt–clay interlaminated planar lamination (SCIPL), and paper-like planar lamination (PPL). SGPL is formed by turbidity current with a flow speed less than 15 cm/s. SCGPL is formed by turbidity currents with a flow speed less than 15 cm/s for normal grading type and 15–25 cm/s for alternating grading type. SCIPL has a continuum of sparsely spaced type, closely spaced type, and alternating type, which is formed by bottom current with an increasing flow speed from 15 to 25 cm/s to above 25 cm/s. PPL can be divided into normal grading and composite grading types. The former is formed by vertical settling, while the latter is formed by bottom current with a flow speed of 5–15 cm/s. Vertically, types of planar lamination varied from SGPL to PPL and then SCIPL manifesting the waxing and waning of flow speed with a positive excursion at graptolite biozone Metabolograptus extraordinarius (WF4) and a negative excursion at graptolite biozone Persculptograptuspersculptus (LM1). The sudden decrease in flow speed across Linxiang and graptolite biozone Paraorthograptuspacificus (WF3) and the subsequent progressive increase from graptolite biozone Akidograptus ascensus (LM2) to graptolite biozone Demirastrites triangulatus (LM6) and to graptolite biozone Stimulograptus sedgwickii (LM8) during deposition of the Ordovician–Silurian transition succession on the Upper Yangtze Block were linked to the bulge uplift, the rapid subsidence, and the relaxation controlled by the Kwangsian orogeny. In contrast, the positive excursion at WF4 and the negative excursion at LM1 were strongly controlled by the Hirnantian Glaciation and global warming, respectively.

Ordovician chitinozoans of the Miaopo Formation at Zhenjin, Upper Yangtze Platform, South China

Chitinozoan investigations on the late Middle to early Late Ordovician in South China are limited. Documented chitinozoan occurrences are mainly from the Miaopo Formation on the Upper Yangtze Platform. The present study reports new data from the Miaopo Formation at the Zhenjin section in the Yichang area. In total, 13 genera and 33 species are recognised, and three new species are described: Spinachitina? coronifera sp. nov., Lagenochitina yichangella sp. nov. and Pellichitina confragosa sp. nov. The Baltoscandian index species, Laufeldochitina striata, is documented in the lowermost part of the formation. This is the first report of this species in South China. The L. striata Biozone is suggested for the basal part of the formation due to the presence of the eponymous species. The index species of the Laufeldochitina stentor Biozone, the Armoricochitina granulifera and Cyathochitina megacalix subzones adopted in the Jieling section, are also observed in the Zhenjin section. However, according to the new data obtained at Zhenjin, the first appearance datum of C. megacalix and A. granulifera coincides with that of L. striata. Therefore, the C. megacalix Subzone is kept but moved to the L. striata Biozone. Armoricochitina granulifera has stable occurrences in almost the entire Miaopo Formation, corresponding to the Nemagraptus gracilis graptolite biozone. It is slightly older than Baltic records but could be useful for recognising this time interval in South China.

A world-class source rock in southern China formed during the periods from Katian to Rhuddanian: Biostratigraphic distribution, depositional model and shale gas potential

Typical outcrops and shale gas wells from the upper Ordovician (Katian and Hirnantian) Wufeng Formation to the lower Silurian (Rhuddanian) Longmaxi Formation in the Sichuan Basin of southern China were selected to investigate the shale quality and shale gas potential. The chronostratigraphy was studied based on the graptolite biozones and well logging. The sedimentary environment, depositional facies, biostratigraphic distribution, depositional model and lithological characteristics were characterized. Organic-rich shale primarily developed in the late Katian to the early–middle Rhuddanian. Two organic-rich shale depocentres were identified in the southern and eastern-north-eastern Sichuan Basin. The spatial distribution of organic-rich shale of different graptolite biozones was determined and showed gradual migration northwest. The depocentre of the Dicellograptus complexus Biozone to the Cystograptus vesiculosus Biozone was controlled by paleo-uplift and a submarine high. The black shale typically lacks some graptolite biozones, and even if it is fully developed, the thickness of organic-rich shale is thin and has a poor shale gas source potential. The control effect of the graptolite biozone on shale-gas-enrichment helps to evaluate the shale-gas-enriched and productive intervals accurately. This study characterises the organic-rich shale distribution in the Sichuan Basin and helps in assessing organic-rich shale and identifying sweet spots of shale gas potential.

The Ordovician Tøyen Shale (Floian) and its graptolite fauna at Kinnekulle, Västergötland, Sweden – a regional overview

ABSTRACT The graptolites of the Tøyen Shale Formation of Kinnekulle in Västergötland, south-central Sweden, are described for the first time and their biostratigraphic distribution is documented from drill core and outcrop material. The faunas indicate an age of mid to late Floian (Billingenian), thus showing a reduced biostratigraphic range of the Tøyen Shale as compared to other areas where this rock unit occurs. A considerable stratigraphic gap is apparent at its base, from the late Tremadocian to mid-Floian, as the limestones of the Ceratopyge acicularis trilobite Zone (late Tremadocian Bjørkåsholmen Formation) are followed by shales with a fauna of the Baltograptus jacksoni graptolite Biozone. Unverified records suggest that mid–late Ottenbyan (Hunnebergian) strata (Megistaspis planilimbata trilobite Zone s.l.) locally may be present in the uppermost Bjørkåsholmen Formation as identified herein. There is no indication that the Tøyen Shale reaches into the Dapingian, as Dapingian graptolites have not been recognized. The overlying limestones of the “Lanna Limestone” belong to the Megistaspis polyphemus trilobite Zone and the Baltoniodus triangularis conodont Zone, suggesting a gradual contact of the lithological units.

Palaeoenvironmental Evolution Based on Elemental Geochemistry of the Wufeng-Longmaxi Shales in Western Hubei, Middle Yangtze, China

The organic-rich shales found in the Wufeng–Longmaxi Formation are typically deposited in oxygen-deficient reducing environments. One of the primary sources of debate revolves around the question of whether the anoxic bottom water found in these shales is either euxinic or ferruginous, and this matter remains unresolved. Previous studies have mostly focused on the Wufeng–Longmaxi Formation as a whole in order to understand the key factors that control organic matter accumulation (OMA). However, research on OMA for each member, including the Wufeng Formation (WF), the lower Longmaxi Formation (LLM), and the upper Longmaxi Formation (ULM), has been insufficient. This paper aims to investigate the palaeoenvironmental conditions and OMA mechanisms of the Wufeng–Longmaxi shales in western Hubei by integrating data on total organic carbon (TOC) content, mineral compositions, major and trace elements, and iron speciation. The results indicate that the Wufeng–Longmaxi shales were deposited under highly restricted hydrographic conditions, except for relatively open and upwelling conditions in the upper WF. Silica in the upper WF was primarily biogenic origin and not hydrothermal. Ferruginous conditions were the primary redox conditions for organic-rich shales except for minor formations in the lower LLM that were deposited under euxinic conditions. Due to the tectonic uplift caused by the Kwangsian Orogeny in the upper LLM, the palaeoenvironment was characterized by a warmer and wetter climate, high terrigenous influx, oxic conditions, and low productivity as the result of the insufficient nutrients caused by the weak upwelling, leading to the turnover of graptolite biozones from LM5 to LM6. The factors influencing OMA changed vertically. TOC contents have a highly positive correlation with Al content, indicating that terrigenous influx was the main factor affecting OMA in the WF, which significantly differed from patterns found in other regions. This suggests that the sedimentation rate of organic matter was higher than the terrigenous dilution rate during the WF stage. The combination of redox conditions and productivity were the main factors affecting OMA in the LLM, while terrigenous influx was the key factor controlling OMA in the ULM, resulting in the dilution of organic matter. Regions in the eastern Yiling block, which are close to the Qinling Ocean, show better prospects for shale gas exploration. This research will further facilitate the development of shale gas in this area.

The Ordovician System in Australia and New Zealand

Abstract The stratigraphic overview presented in this chapter substantially updates and revises the last major review of the Ordovician rocks of Australia and New Zealand published 40 years ago. In the western two-thirds of the present-day continent of Australia, Ordovician sedimentary rocks are restricted to intracratonic basins. The Canning Basin (Western Australia) and Amadeus Basin (central Australia) contain the best known Lower and Middle Ordovician shallow marine successions. The eastern third of the continent, known as the Tasmanides, comprises multiple orogens (i.e. Delamerian, Lachlan, New England, Thomson, Mossman) that formed along the convergent East Gondwana Margin. As a result, volcanic and intrusive rocks are much more common in these orogens than in the intracratonic basins. Their deep-water depositional environments span 31 graptolite biozones. Slope and basinal siliceous sedimentary rocks are constrained by a newly defined set of 12 conodont biozones, complementing the conodont biostratigraphic scheme refined for shallow-water environments from the basal boundary of the Ordovician to the latest Katian. In some places, these conodont biozones are integrated with radiometric ages from tuff interbeds (e.g. Canning Basin). Ordovician graptolitic strata in the Buller Terrane of New Zealand share palaeogeographic links with those in the Bendigo Zone of the western Lachlan Orogen.

Regional synthesis of the Ordovician geology and stratigraphy of China

Abstract China presently comprises several independent tectonic palaeoplates or terranes and parts of other blocks, which have been assembled over geological time. In the Ordovician, these blocks included South China, North China, Tarim, Qaidam, Junggar, Qiangtang-Qamdo, Lhasa and partially Himalaya, Sibumasu and Indochina, as well as the Altay-Xing'an and Songpan-Garze fold belts, which were discrete but near-adjacent. Twelve stratigraphic megaregions bounded by tectonic sutures or major fault zones can be recognized. Some of them are further differentiated into several regions according to the lithological and biotic facies or distinct stratigraphic sequences. Here, the palaeontologic features and biostratigraphic framework of these stratigraphic megaregions and regions are summarized. The unified biostratigraphic framework presented herein is supported by 33 graptolite biozones and 27 conodont biozones, together with supplementary biozones, communities or associations of brachiopods, trilobites, cephalopods, chitinozoans, acritarchs and radiolarians. With constraints of integrative chronostratigraphy, biostratigraphy, chemostratigraphy, cyclostratigraphy and magnetostratigraphy, along with some geochronologic data, our understanding of the temporal and spatial distribution of the Ordovician lithostratigraphic units on these major blocks has been significantly advanced. Vast amounts of new data accumulated in recent decades also constrain the major Ordovician geological and biotic events evident in China, such as marine anoxia, faunal turnovers and tectonic orogenies.

The Late Tremadocian (Ordovician) Graptolite Kiaerograptus from Central Hunan, China: Taxonomy and Biostratigraphy

AbstractWell‐preserved graptolites of the genus Kiaerograptus are first reported from the upper Tremadocian Yinchupu Formation in the Nanba section, Yiyang, Hunan Province, South China, including four species, i.e., Kiaerograptus lauzonensis (Erdtmann, 1966), Kiaerograptus stoermeri (Erdtmann, 1965), Kiaerograptus cf. supremus? Lindholm, 1991, and Kiaerograptus sp., which are robust in form. Based on these new specimens, the Kiaerograptus biozone is established for the first time in South China, increasing the late Tremadoc graptolite biozones in South China from four to five, in ascending order as follows: the Adelograptus tenellus biozone, the Aorograptus victoriae biozone, the Kiaerograptus biozone, the Sagenograptus murrayi biozone, and the Hunnegraptus copiosus biozone. A review of the occurrence records for robust Kiaerograptus species worldwide reveals that the distribution is restricted to the Aorograptus victoriae biozone and Kiaerograptus biozone in the late Tremadocian. This limited stratigraphical distribution makes Kiaerograptus a valuable taxon for precise biostratigraphical correlation at both the regional and global scales.

Latest Ordovician to earliest Silurian graptolites of northwest Peninsular Malaysia

AbstractGraptolites from the latest Ordovician to the earliest Silurian rocks of northwest Peninsular Malaysia are described and reviewed. The fossils were collected previously by C. R. Jones and presently by the authors inside the black mudstones from the basal section of Tanjung Dendang Formation in Pulau Langgun, Langkawi, which comprises assemblages from the Hirnantian Metabolograptus extraordinarius Biozone to the Rhuddanian Akidograptus ascensus–Parakidograptus acuminatus Biozone. The latest Ordovician strata also include a Hirnantia fauna bed between the Metabolograptus extraordinarius and Metabolograptus persculptus biozones, in which shelly fossils such as Mucronaspis sp. could be recovered. A revised graptolite biozonation is proposed for the latest Ordovician to the earliest Silurian succession of northwest Peninsular Malaysia. This interval is significant for understanding the extent of mass extinction events happening right at the end of the Ordovician period and subsequent faunal change in the region.

Using Constrained Optimization (CONOP) to examine Ordovician graptolite distribution and richness from the Central Andean Basin and their comparison with additional data from North America and Baltoscandia

Ordovician graptolite fossils represent one of the first abundant, well-documented records of marine zooplankton. An accurate assessment of their stratigraphic ranges, biogeography, and taxon richness is crucial to studies on the Great Ordovician Biodiversity Event. Unfortunately, our understanding of graptolite richness from high paleo-latitudes is poor and recent work has demonstrated that many high paleo-latitude taxa are often misidentified endemic species. Hence, high paleo-latitude localities likely contain a hidden graptolite richness.This work uses CONOP9 to construct a range chart and correlation model from the graptolite occurrences at thirty-eight Ordovician stratigraphic sections from the Central Andean Basin of South America, and parts of Laurentia and Baltoscandia. Thirteen graptolite biozones spanning the early Tremadocian (Tr1)–middle Dapingian interval (Dp2) are identified. Some taxa in the Central Andean Basin composite exhibit unusual ranges and occur in biostratigraphically unexpected positions in the composite range chart. In order to examine the possible causes of these unusual ranges, sections from Baltica and Laurentia were added to the analyses. These additional data facilitated results that were more consistent with the generally accepted positions of certain taxon.The first raw taxon richness curve of graptolites from the Central Andean Basin obtained with CONOP9 shows a diversification phase in the early Floian that continues to a peak in the late early Floian. A second peak occurs at the beginning of the middle Floian. A decreasing trend is observed from the top of the Didymograptellus bifidus Biozone to the “Isograptus victoriae” Biozone. These results generally agree with the graptolite richness curves published for the same interval in Baltica, Australasia, and the Yangtze region of SW China, but differs from the patterns observed for the Jiagnan and Yiyang regions of SE China. The rarified regional taxon richness curve, obtained with CONOP64, is also in agreement with global curves previously proposed by different authors.

Silica crystallinity: Characteristics and controlling factors in marine shale of the upper Yangtze area, China

The quartz crystallinity index (QCI) was applied to quantitatively characterise the crystallinity features of silica within the marine shale of the upper Yangtze area. A total of 67 samples, including 63 shale samples, 3 quartz vein samples, and 1 artificial quartz sample as reference, were applied in QCI calculation by X-ray diffraction (XRD). Furthermore, based on the identification and classification of graptolite biozones, a series of experimental analyses, including the total organic carbon (TOC) content, bitumen reflectance, and elemental composition, were conducted to determine the silica origin and controlling factors of crystallinity. The results indicated that biogenic silica in marine shale intervals had an obviously lower crystallinity than silica of other origins. The quartz vein in shale intervals presented the highest silica crystallinity. Silica that experienced a low diagenesis intensity exhibited the lowest crystallinity. Furthermore, biogenic silica in a state of overpressure had a much lower crystallinity than that under normal pressure. The crystallinity of silica in marine shale of the upper Yangtze area is affected by origin, diagenesis intensity, and overpressure. From the perspective of QCI, the Chengkou area is probably the next favourable area for shale gas exploration and development.

Uppermost Katian (Ka4, Upper Ordovician) conodonts in South China: Biostratigraphy, biofacies, and paleobiogeography

The conodont fauna from the Daduhe Formation in southwestern Sichuan and northeastern Yunnan, southwestern China, is analyzed and correlated with the uppermost Ordovician Amorphognathus ordovicicus conodont Biozone. This age assignment is further supported by the graptolites from the same sections, indicating a Ka4 Stage Slice, i.e., from the Dicellograptus complexus to Paraorthograptus pacificus graptolite biozones. Two common conodont species of the Upper Ordovician, Hamarodus brevirameus and Scabbardella altipes, are reported for the first time from the upper Katian in South China. The analysis of conodont biofacies indicates that the conodont fauna in the Daduhe Formation belongs to the cool-water Hamarodus brevirameus–Dapsilodus mutatus–Scabbardella altipes Biofacies. This discovery provides additional evidence for the cold tongue model during the Late Ordovician. The distribution of Yaoxianognathus yaoxianensis confirms the close biogeographical ties among South China, North China and Tarim. This distinctive species of eastern Gondwana might have originated in North China and dispersed to Tarim and South China.

Stratigraphic correlations between the Brabant Massif and the Stavelot, Rocroi and Givonne inliers (Belgium) and geological implications

The Caledonian basement crops out in the middle and southern part of Belgium in two major tectonic units: the Brabant Massif in the Brabant Parautochthon and the Stavelot-Venn, Rocroi, Givonne and Serpont inliers in the Ardenne Allochthon. The main aim of this work is to achieve a chronostratigraphic correlation between the Brabant Massif and the Ardenne inliers, from the lower Cambrian to the Middle Ordovician. Throughout his career, Michel Vanguestaine established an informal acritarch biozonation for this basement, which is only linked to the international stratigraphic scale in vigour at that time. Our first step was to correlate these informal biozones with the trilobite (Cambrian) and graptolite (Ordovician) biozonations which are currently well correlated with the chronostratigraphy. Then, compilation of the literature concerning each of these sedimentary units makes it possible to assign a chronostratigraphic position to their constituent formations. This work has permitted the establishment of a complete chart of the stratigraphic correlations between the Brabant Massif and the three main Ardenne inliers (Stavelot-Venn, Rocroi and Givonne). Geological implications are discussed: the Brabant Massif and the Ardenne inliers formed a single sedimentation basin with different and rheologically contrasting basements (rift and shoulder). New arguments confirm the presence of a Caledonian orogeny in the Ardenne.

Late Sandbian (Sa2) radiolarians of the Pingliang Formation from the Guanzhuang section, Gansu Province, China

AbstractA diverse, well-preserved radiolarian assemblage is reported from the Sandbian age Climacograptus bicornis Graptolite Biozone. This new assemblage, recovered from the Pingliang Formation in the Guanzhuang section, China, includes six new species along with 13 other previously described taxa. Geminusphaera new genus incorporates G. grandis n. sp. and G. kongtongensis n. sp. and is proposed for inaniguttids constructed from two distinct porous spheres bearing seven or more primary spines. Protopylentonema new genus is introduced to incorporate pylomate entactinarians with five-rayed initial spicules. It includes P. ordosensis n. sp. as well as P. aperta, P. rimata, and P. insueta that were formerly assigned to Kalimnasphaera. Micro-computed tomography investigation of skeletal microstructure supports establishment of the new genera together with other new spumellarian species: Haplotaeniatum implexa n. sp., Inanigutta quadrispinosa n. sp., and Kalimnasphaera pingliangensis n. sp. It also indicates that family-level reassignment of Etymalbaillella from the Proventocitidae to the Ceratoikiscidae is appropriate.Global distribution of Late Ordovician radiolarian occurrences highlights a strong preference for areas with equatorial to tropical sea surface temperatures. This must have led to ecological stress among radiolarian communities in adapting to global cooling in the Hirnantian.UUID: http://zoobank.org/3d3f55b8-0e70-4f9f-9738-265750d8ec3a.

Geochemical, petrographic and reservoir characteristics of the transgressive systems tract of lower Silurian black shale in Jiaoshiba area, southwest China

In recent years, lithofacies identification and classification have been a hot topic in shale gas research. Even if some previous works are considering the distribution characteristics of lithofacies in different system tracts (third-order sequences), only few studies systematically reveal the lithofacies distribution and reservoir characteristics. In this study, we focus on transgressive systems tract (TST) section which is characterized by high TOC content and highest gas content in the Long-1 member and divided the third-order TST into five Units (parasequences) according to graptolite biozonation, gamma ray (GR) and acoustic (AC) logging data. Twenty-six samples were analyzed for total organic carbon (TOC) content, mineral and elemental composition, porosity, pore type, pore size distribution and pore connectivity. The results show that Unit 1 is predominantly composed of S-3 and S-4 lithofacies, and Units 2–5 is predominantly composed of S-4 and M-2 lithofacies. The S-4 lithofacies was associated with high primary productivity of organic matter, and anoxic bottom water conditions. Redox conditions changed from anoxic to dysoxic condition and the productivity decreased through Unit 1 to Unit 5. Compared to other units, Units 1 and 2 are characterized by higher porosity and better pore connectivity. The S-3/S-4 and M-2 lithofacies exhibit different reservoir quality, especially in pore structure. Mineralogical composition is the key factor to determine lithofacies type and affects the characteristics of pore structure. Besides, TOC plays an important role in the development of organic matter pores. Anoxic bottom water conditions and high paleo-salinity contributed to the preservation of organic matter.

Quartz crystallinity index: New quantitative evidence for biogenic silica of the Late Ordovician to Early Silurian organic-rich shale in the Sichuan Basin and adjacent areas, China

Quartz crystallinity index (QCI) was used to reflect the crystallisation of silica in the Late Ordovician Wufeng (WF) and Early Silurian Longmaxi (LM) Formation shale, as well as the airborne volcanic ash-derived silica in the Lucaogou Formation tuffaceous shale, to distinguish the two types of silica. The silica in different graptolite biozones exhibited different crystallisation. The WF2–3, LM1–4 graptolite biozones showed obviously lower QCI values than the LM5–9 graptolite biozones and the Lucaogou Formation samples. The graptolite organisms played the role of adsorption, fixation, and precipitation in silicon accumulation and enrichment in stratum. The biogenic origin caused the poorest quartz crystallisation in WF2–3 and LM1–4 graptolite biozones samples. The airborne volcanic ash-derived silica in the Lucaogou Formation tuffaceous shale exhibited relatively poor quartz crystallisation because of weaker diagenesis intensity. Generally, although the WF2–3 and LM1–4 graptolite biozones underwent strong diagenesis and contained a small amount of detrital quartz, the silica still exhibited lower QCI values than the airborne volcanic ash-derived silica in the Lucaogou Formation tuffaceous shale. The biogenic silica crystallisation was much poorer than that of the airborne volcanic ash-derived silica. QCI is an effective quantitative index to demonstrate the biogenic silica in the organic-rich and silica-rich shale.

Geochemical characteristics and organic matter accumulation of Late Ordovician shale in the Upper Yangtze Platform, South China

The Ordovician shale (Wufeng Formation) in the Upper Yangtze Platform is now considered a successful development interval, independent of the Longmaxi Formation. However, accumulation mechanisms of organic matter (OM) within the Wufeng shale are rarely discussed and remain highly controversial. Petrological and geochemical characteristics were analyzed on 25 Wufeng samples from well X1 located in the Upper Yangtze Platform, and their implications for the OM accumulation mechanisms are further discussed. The Wufeng Formation is divided into three intervals based on graptolite biozones (named WF2, WF3, and WF4). Geochemical redox proxies (Mo, U, V, Ni, Cr, Co) suggest that WF2 and WF4 were deposited in oxic–suboxic conditions, whereas WF3 was deposited in suboxic–anoxic with episodes of euxinic conditions. Geochemical productivity proxies (Baxs, P, Sixs) indicate high paleoproductivity when WF2 and WF3 were deposited, which may have been controlled by upwelling as suggested by the high ratio of Cd/Mo. WF2 (averaged 1.42% TOC) is not as organic-rich because of poor preservation conditions and terrestrial dilution. High paleoproductivity and suitable preservation conditions were the controlling factors for the organic-rich WF3 interval (averaged 3.00% TOC). However, low paleoproductivity and poor preservation conditions caused low accumulation of OM in WF4 (averaged 0.82% TOC). OM accumulation within the Wufeng Formation reflects the interaction between the variations in redox conditions and paleoproductivity. The sedimentary environment during Wufeng deposition was mainly controlled by sea level, which, in turn, was controlled by global climate and global and regional tectonics.

Climate change in the subtropical Paleo-Tethys before the late Ordovician glaciation

The latest Katian (Late Ordovician) is marked by the third biodiversity climax of the Great Ordovician Biodiversification Event (GOBE) prior to the Late Ordovician mass extinction (LOME). In this study, the Chemical Index of Alteration (CIA) is used to determine the paleoenvironmental change recorded in the Arisu Member, the Siltstone Member, and the Sandstone Member of the Terekawat (= Tierekeawati) Formation (upper Katian) in the Tarim Basin, northwestern China. CIA values from these members are plotted in A–CN–K ternary diagrams respectively. Linear regression and 95% confidence interval estimation results indicate that these deposits were derived from the same parent rock, and the corrected CIA (CIA corr. ) values reveal significant paleo-climate changes during Late Ordovician. Sea-surface temperatures (SSTs) have been estimated by comparing 31 modern estuary systems. These data show that the CIA corr. values dropped from 77.41 to 64.83, corresponding to an interpreted temperature drop from 20.5 to 10.2 °C in the Dicellograptus complexus graptolite Biozone of the upper Katian (Ka-4). CIA corr. values varied within a range of 67.63–70.16, implying temperature fluctuations between 12.5 and 14.6 °C in the lower to middle Paraorthograptus pacificus Biozone. CIA corr. values rose again from 69.09 to 76.05, corresponding to a temperature rise from 13.7 to 19.4 °C in the upper Paraorthograptus pacificus graptolite Biozone of the latest Katian. This is interpreted to be a cooling and warming cycle, corresponding to the mid-Boda cooling and the late-Boda warming events. This study suggests that the climate change during late Katian was more intense than previous thought, especially in subtropical zones of Paleo-Tethys. This cooling event enhanced the thermohaline ocean circulation and became one of the major factors causing the well-known late Katian faunal dispersal (i.e. the Boda event). Hirnantian glaciation would have further promoted global oceanic cooling, over-stressed the ecosystems, and triggered the LOME. • Chemical Index of Alteration and confidence interval estimation are used to reconstruct climate change. • A temperature drop from 20.5 to 10.2°C and a subsequent rise are estimated in the Tarim Basin during the Late Ordovician. • This cooling event enhanced the thermohaline ocean circulation and contributed to the faunal dispersal (i.e. Boda event). • Hirnantian glaciation event doubled the cooling effect on ecosystem compared with this cycle.

Stratigraphic framework of the Wufeng-Longmaxi shale in and around the Sichuan Basin, China: Implications for targeting shale gas

Stratigraphic division and correlation are crucial for the identification of sweet spots and drilling design of shale gas. In this study, a stratigraphic division and correlation was carried out for the Wufeng-Longmaxi Formations in southern China from the prospective of lithostratigraphy, sea level changes, and biostratigraphy using data from seismic investigation, wells, and outcrops. The Wufeng and Longmaxi Formations were respectively divided into four members, Wu 1 and Wu2 for the former and Long 1 and Long 2 for the latter. Of the members, Long 1 was subdivided and its first subdivision (Long 11) was further divided into 4 layers (Long 111, Long 112, Long 113, and Long 114). Three eustatic cycles were recognized in the Wufeng-Longmaxi Formations. Cycle I corresponds to the Wufeng Formation with the maximum flooding surface at the top of Wu 1. Cycle II corresponds to Long 1, with the maximum flooding surface at the top of Long 113. Cycle III corresponds to Long 2. Furthermore, 4 graptolite biozones (WF1 to WF4) were identified in the Wufeng Formation and 9 graptolite biozones (LM1 to LM9) in the Longmaxi Formation. WF1-2 and WF3-4 correspond to Wu 1 and Wu 2, respectively; and LM1, LM2-4, LM5, LM6, and LM7-9 correspond to Long 111, Long 112, Long 113, Long 114, and Long 12 and Long 2, respectively. High-quality shales mainly occur in the Wufeng Formation and Long 11. The major intervals that should be investigated with regards to shale gas production include LM1–LM5 (10 m thick) in the Weiyuan Block and WF1–LM5 (20–35 m thick) in the Changning Block. Long 111 is believed to be an optimal target for drilling due to its high TOC content, siliceous content, porosity, microfracture density, and horizontal/vertical permeability ratio.

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.