Silurian Research Articles

Geochemistry of siltstones in the North Qilian Zhongbao–Mayinggou formations (Late Ordovician–Early Silurian): implications for paleoclimate, paleoweathering, and tectonic setting

The Late Ordovician–Early Silurian period witnessed the Phanerozoic mass extinction, glacial events, and volcanic events. Paleoweathering indexes chemical index of alteration (CIA), chemical index of weathering (CIW), and plagioclase index of alteration (PIA) indicated that the source area weathering changed from weak to moderate to intense. CIA values in the upper Zhongbao formation ranged from 66.71% to 73.97%, indicating a drier and colder climate. Upward, the CIA values on the bottom of the Mayinggou formation returned to the high value quickly (from 73.86% to 81.31%), suggesting that the ice age ended, the climate became warmer and wetter, and the sea level rose. The Al2O3–(CaO*+Na2O)–K2O triangular plots, Hf-La/Th, and SiO2–Al2O3/TiO2 bivariate plots inferred that the source of the siltstones in the two formations is mostly from the felsic igneous rocks. The samples from the Zhongbao–Mayinggou formations have chondrite-normalized rare earth element (REE) patterns similar to that of the North Qilian volcanic arc rocks. Geochemical discrimination plots displayed that the sediments of the Zhongbao–Mayinggou formations came from the active continental margin setting.

Geochemical Properties and Gas-Bearing Analysis of Lower Cambrian Black Shale in Western Hunan Province

Western Hunan province and its surrounding areas are significant targets for shale gas exploration and development in southern China, where the black shale of the lower Cambrian Niutitang Formation and Wunitang Formation is extensively distributed. Geochemical analysis was conducted on the lower Cambrian black shale from a new exploration well of XAD1 located at the southeast margin of the Yangtze paraplatform, followed by a discussion on gas-bearing properties using molecular dynamics simulation. The geochemical characteristics indicate that the black shale in well XAD1 was primarily deposited in a strongly reducing marine environment, with organic matter predominantly composed of type I kerogen derived from algae. Currently, it has reached a stage of high to over maturity with limited potential for liquid hydrocarbon generation. The recovery of the original hydrocarbon generation potential shows that they are excellent source rocks and have completed the main hydrocarbon generation evolution. Despite the favorable conditions for shale gas formation observed in well XAD1, the low measured gas content within the Niutitang Formation suggests that other geological factors may have contributed to a substantial loss of shale gas. Gas adsorption simulation reveals that the maximum methane adsorption capacity (15.77 m3/t) was achieved by Niutitang shale during the late Silurian period when there was an abundant source of natural gas without any influence from CO2, H2O or other molecules. However, due to a lack of natural gas replenishment and subsequent tectonic uplift and subsidence causing variations in temperature and pressure, the methane adsorption capacity gradually decreased (to 6.56 m3/t). Furthermore, water occurrence within the shale reservoir further reduced the methane adsorption capacity (below 2 m3/t), while tectonic activities exacerbated the loss of shale gas potential within this study area. The findings indicate that the dynamic alteration of gas-bearing properties in shale reservoirs due to tectonic movements is a crucial factor influencing the success rate of shale gas exploration in the study area, provided that there are sufficient gas resources and superior reservoir conditions.

Metals pre–enrichment during the syn–sedimentary period of Mengxing Pb–Zn Deposit, western Yunnan, SW China: Evidence on LA–ICP–MS trace elements and S isotopes of lamellar sphalerite

The Mengxing Pb–Zn deposit, which contains up to 1.02 Mt of Pb + Zn metal with an average Pb + Zn grade of ca. 12.13 %, is a representative of sedimentary–reworked deposit in western Yunnan. The orebodies, which are strata–bound and veined, occur in the Silurian Shangrenheqiao formation. The mineralization process has been divided into sedimentary stage and hydrothermal stage. Previous research has primarily focused on the economic ores enriched during the hydrothermal stage. However, the formation of syn–sedimentary ore, responsible for metal pre–enrichment, has been largely ignored. To solve the issue above, the in–situ trace elements and mapping, as well as in–situ S isotopes analyses, were conducted on lamellar sphalerite ore, which retains the characteristics of the syn–sedimentary ore. Analysis of the trace elements revealed that the sphalerite at Mengxing are rich in Ge, Cd, As, Sb and poor in Fe, Mn, In, Sn and Co overall, suggesting a metallogenic environment of medium–low temperature. The LA–ICP–MS trace elemental mapping graph indicated that colloidal sphalerite exhibits the obvious oscillation belts. The dark zonal band of sphalerite is more enriched in Fe, Mn, Cu, Ge, As, Ag, Sb, and Pb, and depleted in Ga, Cd, and Sn, compared to the light zonal band, suggesting the various formation temperature. Based on the equation of the first principal component (PC1*) relating trace elements to mineral formation temperature, it is estimated that ore in the sedimentary stage occurred at relatively lower temperatures (174.74℃ – 203.18℃), while that of the hydrothermal stage occurred at higher temperatures (209.78℃ – 240.18℃). The S isotopic composition of sphalerite indicated a significant distinction between the sedimentary stage (δ34SVCDT = 13.29 ‰ – 37.06 ‰, with an average value of 27.97 ‰) and the hydrothermal stage (δ34SVCDT = −6.45 ‰ – 31.57 ‰, with an average of 9.01 ‰). It is hypothesized that sulfur during the early stage originated primarily from thermochemical sulfate reduction (TSR) of seawater sulfate, while sulfur during the hydrothermal stage was likely influenced by hydrothermal fluid from the deep. In conclusion, the early lamellar ores formed during the Silurian period exhibit characteristics consistent with SEDEX type mineralization, leading to initial metal pre–enrichment, followed by further enrichment of ore grade by superimposed fluid during the hydrothermal stage.

Widespread Thalassinoides facies from the upper Silurian of South China: A case study from the northwestern Hunan Province, middle to upper Yangtze Block

The upper Silurian of South China records the first, and perhaps the only occurrence of widespread (spanning over 1400 km), thick to very thick bedded Thalassinoides facies (TVTFs) after the Late Ordovician mass extinction. However, the palaeobiological, palaeoecological features and potential significance of these facies are yet to be demonstrated. A case study at northwestern Hunan Province, middle to upper Yangtze Block, reports the occurrence of TVTFs in a storm dominated open marine succession of the upper Silurian Xiaoxi Formation, which can be further assigned to lower offshore, upper offshore, offshore transition to distal lower shoreface, and proximal lower to middle shoreface settings. The regular changes of bioturbation intensity and maximum penetrating depth from various settings suggest that local energy levels, especially frequency of storms could have controlled the development of Thalassinoides, while their burrow sizes were generally confined by population densities. The Thalassinoides facies could have been constructed by unknown crustaceans, which generally conducted a deposit-feeding strategy. The return of widespread TVTFs in the tropical oceans of South China during the late Silurian could reflect a restored primary productivity, an elevated oxygen level, and a final recovery of benthic ecosystem to pre-extinction level after the Late Ordovician mass extinction. TVTFs could have remarkably shaped the late Silurian benthic environment in view of their high abundance, large burrow sizes, nature of deep tiered burrow systems and a deposit-feeding strategy.

Construction of Regional Seismic Section Along Central Part of Iraq and its Geological Importance

A regional seismic section was constructed from fourteen local seismic lines from different surveys crossing the central part of Iraq from west to east. The seismic lines’ surveys were performed during the period 1974–1990. The length of the regional seismic section is 662.5 km. The considered seismic lines are unified to the sea level datum. The quality of seismic lines is relatively poor in the western part of the regional seismic section, but it is of good quality in the central and eastern parts. Seven reflectors were identified along the regional seismic section, corresponding to ages from the Ordovician to the Miocene. The formations are Khabour (Mid-Ordovician), Akkas (Upper Silurian), Kurra Chine (Upper Triassic), Najmah (Upper Jurassic), Mauddud (Mid-Cretaceous), Shiranish (Upper Cretaceous) and Fatha (Lower Fars) (Mid-Miocene). The Two-way times for these seismic reflectors from the west to the east are 1691 – 4000 ms, 750-4000 ms, 0-4000 ms, 208-4000 ms, 206 -3310 ms, 205 -2700 ms, and 216-2500 ms for the seven formations; starting from the oldest formation (Khabour Formation to the youngest one (Fath Formation ), respectively. The top depth values of the considered formations were defined from west to east for each formation (0-4.189) km, (0.456-4.880) km, (0.708-5.936) km, (0.802-8.5) km, (0-8.977) km, (0.5-9) km, and (1.8-9) km from the oldest to the youngest formation, respectively. The horizon generally dips towards the east. Two sedimentary basins were identified, the first in the western part of Iraq within the Paleozoic formation, while the eastern basin was within the Cenozoic and Mesozoic formations. These two basins are separated by an uplift at the central part of the regional seismic section. This uplift region was confirmed by gravity and magnetic high anomaly. Many faults were detected along the regional seismic profile. Most of these faults extend from the deeper to the shallower formations.

Geochemical and mineralogical characterizations of Silurian “Hot” shales: Implications for shale gas/oil reservoir potential in Jeffara basin-southeastern Tunisia, North Africa

The Silurian organic-rich hot shale deposits represent the origin of 80–90% Paleozoic sourced hydrocarbons on the entire Saharan Platform. As such, it is an area of significant interest for exploring its potential as an unconventional reservoir and gaining new insights into its properties. In this study, the geochemical and mineralogical analysis of eighty-two borehole samples from three studied wells (T, B, and M) located in Jeffara Basin, in southeastern Tunisa, distributed over a depth between 1200 m and 2200 m, show that the hot shale layer has significant potentials of an unconventional shale gas reservoir. According to the rock-eval and GCMS results, the total organic carbon (TOC) of the hot shale deposits is good to great, ranging between 0.54 wt% and 23.86 wt%, and the average hydrogen index (HI) is around ∼200 mg HC/g of TOC, which indicates that the hot shales were originally rich in type II/III organic matter (OM). The thermal maturity of the preserved OM is determined by the kerogen type with values varying from 0.4% to 1%, and Tmax values reaching 460 °C in T, B, and M wells which indicate that the hot shales preserved early mature to mature OM placed within oil/gas generation window. The calculated geochemical parameters reveal an average hydrocarbon generation potential (HGP) for the three studied wells of 20.37 kg HC/ton rock. In addition, the transformation ratios (TR) in T, B, and M wells are, respectively, 21.12%, 30% and 35.25%. However, only B and M wells show a cross-over layer, with oil saturation index (OSI) exceeding 100%, with significant potential as a hydrocarbon saturated source rock, making it a considerable target for unconventional oil exploration. Relevant results of Rock-Eval, biomarkers and XRD exhibit coherent features that reflect reducing marine conditions during the lower Silurian period. Moreover, the mineralogical composition was investigated to estimate the importance of frackable mineral fractions such as quartz (∼15%) and carbonates (calcite-dolomite). Mineral brittleness index (MBI) was calculated and compared to a well-known unconventional reservoir in the Neal shale member of the Floyd shale group in the Black Warrior Basin (US), which displays moderate to low MBI. The hot shales in the study area have a considerable MBI, which could be easily enhanced by hydraulic fracturing to liberate oil/gas from the potentially saturated unconventional shale reservoir.

Petrogenesis of Paleozoic trachyte and rhyolite in the Haidewula area, East Kunlun Orogenic Belt, and their implications for uranium mineralization

Zircon U–Pb-Hf isotopic compositions, mineralogical chemistry, and whole-rock geochemistry analyses were performed on rhyolite and trachyte from the Haidewula deposit in the East Kunlun Orogen Belt (EKOB). These analyses aimed to elucidate the genetic relationship between trachyte and rhyolite and to investigate the role of these volcanic rocks in uranium mineralization. Zircon U–Pb isotope dating indicates that the Haidewula rhyolite and trachyte were contemporaneous with emplacement ages of 421 Ma to 428 Ma, respectively. The trachyte samples exhibit high K2O/Na2O and Fe# [FeO/(FeO + MgO)] ratios, and belong to metaluminous, ferroan volcanic rock; the rhyolite samples display high A/CNK and Fe# ratios, elevated high field strength element contents, and can be classified as strongly peraluminous A2-type magma. Trace and major elemental geochemical signatures demonstrate that the Haidewula trachyte originated from the high-temperature partial melting of a mafic lower crust, followed by fractional crystallization of plagioclase, pyroxene, magnetite, and/or K-feldspar. On the other hand, the rhyolite was likely generated by high-temperature melting of granodiorite in a low water and mid-crust pressure environment, followed by fractional crystallization of feldspar and ilmenite. The occurrence of A-type volcanic rocks at Haidewula suggests that the initial timing of crustal extension in the EKOB was not later than 428 Ma. Overall, Paleozoic igneous rocks in the EKOB revealed that a post-collisional extensional regime, resulting from the rewelding of the microcontinents in eastern Gondwana, was responsible for intensive magmatism during the Late Silurian period. In comparison to other volcanic complexes hosting uranium deposits in eastern China, the relatively U-rich rhyolite and trachyte had the potential to contribute to the formation of U mineralization. In particular, the rhyolitic tuffaceous U-rich glassy volcanic rocks in the Haidewula deposit may represent favorable U-source rocks for mineralization, with uranium released through volcanic glass devitrification processes and leaching by oxidizing hydrothermal fluids.

Constraining reducing conditions in the Prague Basin during the late Silurian Lau/Kozlowskii extinction event

The Silurian was marked by repeated extinctions, carbon cycle volatility and significant intervals of climatic change. The most notable of these events were the Ludfordian Lau/Kozlowskii extinction and the associated Mid-Ludfordian Lau C isotope excursion, both of which have been linked to a period of global cooling and expanded reducing conditions in the global ocean. We present new data that characterize the marine palaeoredox conditions of the Prague Basin, a peri-Gondwanan terrane. We use I/Ca ratios to assess the local redox conditions in a shallow water carbonate succession and Fe speciation and redox-sensitive trace element concentrations to assess the local redox conditions of a deeper water sequence. Consistently low I/Ca values in the shallow water section suggest either persistent local low-oxygen conditions or possibly diagenetic overprinting. Fe speciation data suggest that the bottom water redox conditions in the deeper shelf setting were consistently anoxic with possible intermittent euxinia. Concentrations of redox-sensitive trace elements consistently higher than upper continental crust values also indicate persistent reducing conditions in the deeper part of the basin. These local redox proxy data from the Prague Basin, including trends in new pyrite S isotope ( δ 34 S pyr ) data, are consistent with previous findings of the expansion of anoxic and/or euxinic oceanic conditions. These data, derived from a mid-palaeolatitude marine setting, fill an important gap in our current global dataset for this interval of the late Silurian. Supplementary material: All the geochemical data reported and discussed herein are available in the Supplemental Data Tables at https://doi.org/10.6084/m9.figshare.c.7008107 Thematic collection: This article is part of the Chemical Evolution of the Mid-Paleozoic Earth System and Biotic Response collection available at: https://www.lyellcollection.org/topic/collections/chemical-evolution-of-the-mid-paleozoic-earth-system

Zircon U–Pb ages of the Paleozoic volcaniclastic strata in the Junggar Basin, NW China

Abstract A large set of Paleozoic volcaniclastic rocks is exposed in the northwestern margin of the Junggar Basin from the southern part of the Central Asian Orogenic Belt. The Carboniferous volcaniclastic strata in this area have been studied in depth, and an accurate chronostratigraphic framework of these strata has been established. However, there is a lack of sufficient geochronological data for the deposition times of the other Paleozoic volcaniclastic strata. In this study, zircon U–Pb dating of the Ordovician, Silurian, and Devonian volcaniclastic strata in the area reveals that the youngest age of the tuffite sample collected from the originally defined Ordovician strata is 398 ± 11 Ma, which represents the age of volcanic activity during the period of tuffite deposition. Based on this finding, the originally defined Ordovician strata are redefined as the Lower Devonian. The youngest ages of the silty tuff samples collected from the originally defined Silurian strata peak are 445–418 Ma, so its age is Upper Silurian. The youngest ages of the tuffaceous sandstone samples collected from the originally defined Devonian strata peak are 346–342 Ma, so these Devonian are redefined as the Early Carboniferous strata. Two Archean ages (2,501 ± 12 and 3,193 ± 8 Ma) were obtained in Silurian strata, thus confirming the existence of metamorphic rock basement in the provenance areas from which the sediments were derived.

Devonian intrusions of southwestern New Brunswick, Canada: U-Pb geochronology, petrogenesis and genetic constraints on gold mineralization

ABSTRACT The Clarence Stream intrusion-related gold deposit is hosted within metasedimentary and metavolcanic sequences of the Ordovician Cookson and Silurian Mascarene groups that were intruded by several phases of the Late Silurian to Early Devonian Saint George Plutonic Suite. Based on geochemical and geochronological data, the intrusive rocks in the deposit are categorized into three distinct suites, including gabbroic, granitoid, and Whittier Ridge Granite (WRG). The gabbroic suite is the same as the East Branch Brook metagabbro intrusion, which exhibits a subalkaline mafic composition fractionated from a depleted mantle in a suprasubduction zone setting. The granitoid suite shows the same geochemical characteristics and age as the metaluminous to slightly peraluminous Magaguadavic Granite and its fractionated phases. The WRG suite reflects characteristics of calc-alkaline, metaluminous to peraluminous I-type granites in the area, such as Sorrel Ridge and Mount Douglas granitic plutons. In situ U-Pb zircon geochronology yielded an age of 419.5 ± 3.8 Ma for a monzodiorite from the gabbroic suite. Porphyritic granite and alkali granite from granitoid suite yielded U-Pb zircon age of 402.4 ± 1.8 Ma and 394.3 ± 3.8 Ma, respectively. Geochronologic data from the WRG suite reflect two different crystallization ages of 383.8 Ma (382.4 ± 3.1 to 385.1 ± 4) and 372.4 Ma (367.4 ± 1.3 to 377.1 ± 1.9). Several pieces of evidence demonstrate that the gold mineralizing events occurred after the emplacement of the East Branch Brook metagabbroic dykes and are linked to the various intrusive phases of the nearby syntectonic Early Devonian Magaguadavic Granite. The absence of gold and sulphide mineralization in felsic dykes related to the WRG confirms that this plutonic body formed adjacent to the gold zones after the main gold mineralizing event.

LA-ICP-MS zircon U–Pb ages, geochemistry, and genesis of Paleozoic granites from Biezhentao Mountain, Western Tianshan, Xinjiang

The geologic history of the Western Tianshan region is important for understanding the evolution of the Central Asian Orogenic Belt and accretionary orogenesis. Paleozoic and Mesozoic igneous rocks are present along the northern margin of the Yili Block; however, studies of Paleozoic subduction and collisional events have been relatively limited. Published geochronologies of Middle Devonian magmatic rocks in this region are also lacking. Therefore, this study analyzed the zircon U–Pb ages and major and trace elemental compositions of three granite types collected from Biezhentao Mountain (Wenquan County, Western Tianshan). The medium-grained diorite (384.1 ± 3.6 Ma) and diorite–porphyrite (382 ± 3.2 Ma) are silica-rich, weakly peraluminous, alkali-rich, enriched in large-ion lithophile elements, depleted in high field strength elements, and belong to the calc–alkaline series, with A-type granite characteristics. The monzogranite (423.3 ± 9.4 Ma) exhibits A-type granite characteristics and belongs to the peraluminous calc–alkaline series. The findings suggest that the granitoids recorded two stages of tectonomagmatism during the Middle Devonian–Late Silurian. The Middle Devonian medium-grained diorite and diorite–porphyrite formed in a back-arc extensional setting, whereas the Late Silurian monzogranite formed in an active continental margin setting during the subduction of the North Tianshan Ocean. These results provide insights into the tectonic and magmatic processes that occurred during the evolution of the Western Tianshan region and the formation and evolution of the Central Asian Orogenic Belt.

Early Paleozoic tectonic affinity of the Lesser Xing'an–Zhangguangcai Range, NE China: A continental arc along the western margin of the Jiamusi Block

The Lesser Xing'an–Zhangguangcai Range (LXZGCR) in NE China was traditionally interpreted to be the eastern part of the Songliao–Xilinhot Accretionary Terrane, separated from the Jiamusi–Khanka Block to the east by the Heilongjiang Complex. However, recent studies have shown that the Heilongjiang Complex formed in the late Paleozoic–early Mesozoic. Therefore, the tectonic affinity of the LXZGCR during the early Paleozoic remains unclear. To constrain this key issue, we present new zircon UPb ages, Hf isotopic compositions, and whole-rock geochemical data for early Paleozoic igneous rocks in the LXZGCR. The new ages, along with previously published data, suggest that the early Paleozoic magmatism in the LXZGCR can be divided into four stages: Middle–Late Cambrian (523–486 Ma), Early Ordovician (485–469 Ma), Late Ordovician (464–443 Ma), and Late Silurian (433–423 Ma). Middle–Late Cambrian igneous rocks are dominated by calc-alkaline I-type granitoids and high-Mg andesite–dacite–rhyolite association. Early Ordovician igneous rocks consist of a suite of calc-alkaline diorites, granodiorites, monzogranites, alkali-feldspar granites, and rhyolites, with I-type affinities. Late Ordovician igneous rocks consist mainly of I-type granitoids with minor A-type granitoids and rhyolites. Late Silurian igneous rocks are composed of calc-alkaline I-type granitoids with minor dacites. The early Paleozoic granitoids and felsic volcanic rocks originated from partial melting of dominantly Paleoproterozoic–Mesoproterozoic lower crust, whereas the Late Cambrian high-Mg andesite–dacite–rhyolite association was probably derived from partial melting of a depleted mantle wedge metasomatized by subduction-related fluids. The rock associations and geochemical characteristics suggest that these early Paleozoic igneous rocks formed in active continental margin and back-arc extension settings. Comparing the early Paleozoic magmatism, zircon Hf isotopic compositions, Nd model ages, and Precambrian basement features of the LXZGCR and the Jiamusi Block suggest that the LXZGCR was part of the same tectonic unit as the Jiamusi Block during the early Paleozoic. We argue that the formation of the early Paleozoic igneous rocks in the LXZGCR was related to the eastward subduction of a branch ocean of the Paleo-Asian Ocean beneath the Jiamusi–Khanka Block.

Age of the basal ‘Lower Old Red Sandstone’ Stonehaven Group of Scotland: the oldest reported air-breathing land animal is Silurian (late Wenlock) in age

The basal upper Silurian–Lower Devonian ‘Lower Old Red Sandstone’ deposits of the Midland Valley of Scotland contain several important fossil biotas, including that from the Cowie Harbour Fish Bed. This biota is of great significance because it represents one of the oldest known examples of a fossilized terrestrial ecosystem and includes the oldest reported air-breathing land animal (the myriapod Pneumodesmus newmani Wilson and Anderson, 2004 ). Based on biostratigraphical evidence from dispersed spores this biota is dated as late Wenlock (late Silurian) in age. Recently, however, this age constraint was challenged, based on 238 U– 206 Pb radiometric analysis of zircons, and it was proposed that these deposits are much younger (Early Devonian: Lochkovian). This proposal has serious implications regarding (1) the composition and nature of early terrestrial ecosystems and (2) the geological setting with respect to the timing of terrane accretion and the onset of ‘Lower Old Red Sandstone’ sedimentation. We report on newly discovered dispersed spore assemblages and additional zircon data that confirm a late Wenlock (late Silurian) age. This age designation establishes the importance of the biota of the Cowie Harbour Fish Bed and is more compatible with both its biotic composition and the regional geological setting.

Late Silurian–Early Devonian tuffites from the Małopolska and Łysogóry blocks reflect arc-back-arc magmatic activity on the southern margin of Laurussia

Late Silurian–Early Devonian tuffites from the Małopolska and Łysogóry blocks reflect arc-back-arc magmatic activity on the southern margin of Laurussia

Silurian–Devonian construction of active continental margin in the southwestern Paleo-Asian Ocean witnesses the modification of fluvial system by early land plants

In accretionary orogeny, the intra-oceanic subduction-accretion could generate juvenile continental crustal materials and form new continental blocks by their lateral tectonic accretion, and the juvenile continental crust would provide a sialic basement for continental margin arc during the subsequent oceanic-continental subduction-accretion. These two distinct subduction-accretion processes occurred successively in the southwestern part of the Paleo-Asian Ocean during the Neoproterozoic–Late Paleozoic and created a multi-stage evolution of continental crusts in West Junggar. In this study, a Late Silurian landmass in West Junggar is revealed by the detailed geological mapping and comparison of the Late Silurian–Early Devonian angular unconformity in southern and northern West Junggar. This crucial unconformity confines the Middle Silurian termination of Early Paleozoic intra-oceanic subduction-accretion, as evidenced by the pre-Late Silurian emplacement of Cambrian–Ordovician ophiolitic mélanges, the Middle Silurian cessation of Early Paleozoic arc magmatism and the Late Silurian–Early Devonian sedimentary hiatus. The Late Silurian landmass was consequently constructed by amalgamations of island arcs, accretionary complexes and seamounts, and its break-up was marked by the Middle Devonian opening of the Karamay back-arc Ocean in central West Junggar. In contrast with the Early Paleozoic intra-oceanic arc systems, continental margin arcs on the sialic basement of rifted landmass emerged since the Middle Devonian, as suggested by our new chronological data and field observations. We further unravel that the increase in mudstone of Devonian successions on the active continental margin is contemporaneous with the emergence and boom of lycopsid forest in West Junggar, which is in agreement with a global trend in alteration of fluvial systems due to evolution of early land plants. In summary, this paper presents a typical record for two distinguished styles of subduction-accretion systems that developed successively in an accretionary orogeny, and sedimentary successions of the later active continental margin documented the interaction between terrestrial sedimentation and early land plants.

Encrustation of crinoid holdfasts and pluricolumnals from the Pridoli (upper Silurian) of Saaremaa Island, Estonia

The Pridoli (upper Silurian) of Saaremaa Island, Estonia contains crinoid holdfasts and pluricolumnals that are colonized by many epibionts (encrusters). They comprise Palaeoconchus tenuis, Cornulites sp., Anticalyptraea calyptrata, Favosites sp., as well as hederelloids, cystoporate and trepostome bryozoans, and stromatoporoids. The taxonomic composition of encrusters of crinoid pluricolumnals differs from that of crinoid holdfasts. The encrusters on pluricolumnals were numerically dominated by trepostome bryozoans and microconchids; the other taxa formed just a minor part of the sclerobiont association. The trepostomes dominate by the area of encrustation on pluricolumnals. The encrusters on holdfasts were numerically dominated by microconchids, followed by cystoporate and trepostome bryozoans; the other taxa formed a minor part of the sclerobiont association. The bryozoans dominated by the encrustation area on holdfasts. There is likely a negative correlation between potential substrate mobility and abundance of microconchids. The cystoporates colonized only relatively stable substrates such as crinoid holdfasts whilst trepostomes colonized also mobile substrates.

Geochemistry and Zircon U–Pb Geochronology of the Wugongshan Granites in the Northwestern Jiangxi Area, China: Implications for the Paleozoic Tectonic Development of South China

The properties of the Caledonian orogeny along the transition belt of the Yangtze and Cathaysia blocks have received much attention in recent years. The widespread Early Paleozoic granites provide critical geological clues for unraveling the tectonic evolution and geodynamic processes of the South China Continent (SCC). Here we present new zircon U–Pb chronology, whole-rock major and trace elements, in situ Hf isotopes for Paleozoic granites, i.e., the Wugongshan granites in the northwest Jiangxi province, and aim to explore the magmatism and properties of the Caledonian orogeny involved in their formation. Our new data show that the Wugongshan granites were emplaced during the Early Silurian Period (442–438 Ma). The Paleozoic Wugongshan granites belong to S-type muscovite-bearing peraluminous granites (MPG) and show a single origin. The Wugongshan granites exhibit negative εHf(t) values (−11.56 to −6.19) and TDM2 model ages of 2148–1809 Ma, indicating their derivation from an ancient crustal source, through partial melting of ancient crustal material. The Wugongshan granitic magmatism is probably being generated in an extensional environment related to an intracontinental orogeny setting. It is inferred that the Paleozoic tectonic–magmatic event in the Wugongshan area was associated with the oceanic–continental convergence of the Paleo-Tethys Ocean. The Wugongshan granites highlight the intracontinental magmatism in the Early Paleozoic orogeny in the SCC.

Evolution of Silurian phytogeography, with the first report of Aberlemnia (Rhyniopsida) from the Pridoli of West Junggar, Xinjiang, China

The development of phytogeography is an essential aspect of floral evolution. The Silurian is considered a pivotal period for the development of early land vegetation based on both macrofossil and microfossil evidence; however, the evolution of phytogeographic zonation at that time remains controversial. In this paper, we describe a new species of the rhyniopsid, Aberlemnia junggaria sp. nov., from the Pridoli (upper Silurian) of West Junggar, Xinjiang, China. The new plant consists of at least three orders of axes, an oval to reniform terminal sporangium bearing in sparse ultimate axes and sporangium with a nearly enclosed dehiscence line. A dataset of global Silurian macrofossil records was compiled, including 30 species within 24 genera from 38 localities covering 8 main paleogeographic blocks. Using these data, the spatial-temporal distribution of Silurian plant macrofossils was mapped. Biogeographic evolution is revealed for the first time by means of network analysis and cluster analysis, and two phytogeographic realms, the West Junggar-Vietnam and Laurussia-North Gondwana realms are provisionally recognized during the Pridoli Epoch. Considering the widespread microfossil evidence for land plant diversification and dispersal, this global phytogeographic zonation might have onset at an even earlier time.

Micropore Structure of Deep Shales from the Wufeng–Longmaxi Formations, Southern Sichuan Basin, China: Insight into the Vertical Heterogeneity and Controlling Factors

The microscopic pore throat structure of shale reservoir rocks directly affects the reservoir seepage capacity. The occurrence and flow channels of shale gas are mainly micron–nanometer pore throats. Therefore, to clarify the microstructural characteristics and influencing factors of the deep organic-rich shales, a study is conducted on the marine shale from the Upper Silurian to Lower Ordovician Wufeng–Longmaxi Formation in the southern Sichuan Basin. Petrographic lithofacies division is carried out in combination with petro-mineralogical characteristics, and a high-resolution scanning electron microscope, low-temperature nitrogen and low-temperature carbon dioxide adsorption, and micron-computed tomography are used to characterize the mineral composition and pore structure qualitatively and quantitatively, upon which the influencing factors of the microstructure are further analyzed. The results show that with the increase in burial depth, the total organic carbon content and siliceous mineral content decrease in the Wufeng formation to Long-11 subsection deep shale, while clay mineral content increases, which corresponds to the change in sedimentary environment from anoxic to oxidizing environment. Unexpectedly, the total pore volume of deep shale does not decrease with the increase in burial depth but increases first and then decreases. Using total organic carbon (TOC), siliceous mineral content showed a good correlation with total pore volume and specific surface area, with correlation coefficients greater than 0.7, confirming the predominant role of these two factors in controlling the pore structure of deep shales. This is mainly because the Longmaxi shale is already in the late diagenetic stage, and organic matter pores are generated in large quantities. Clay minerals have a negative correlation with the total pore volume of shale, and the correlation coefficient is 0.7591. It could be that clay minerals are much more flexible and are easily deformed to block the pores under compaction. In addition, the longitudinal heterogeneity of the deep shale reservoir structure in southern Sichuan is also controlled by the thermal effect of the Emei mantle plume on hydrocarbon generation of organic matter and the development of natural microfractures promoted by multistage tectonic movement. Overall, the complex microstructure in the deep shales of the Longmaxi Formation in the southern Sichuan Basin is jointly controlled by multiple effects, and the results of this research provide strong support for the benefit development of deep shale gas in southern Sichuan Basin.

New insights on the upper Silurian Fegaguira Formation shale-oil play in the Chotts Basin (southern Tunisia)

In Tunisia Chotts Basin, the upper Silurian (Ludfordian) Fegaguira Formation comprises organic-rich black mudstones deposited during a major anoxic event. It is a prolific source rock, having yielded a large volume of oil and gas from conventional reservoirs that reached about 69 MMboe, with around 45 MMboe as recoverable reserves still to be produced. Based on various investigations, the stratigraphy of the Fegaguira Formation is updated and its unconventional play potential is assessed. It is divided, in the present work, into three units (HSII.1, HSII.2 and HSII.3) characterized by gamma-ray values of up to 400° API, organic matter content (up to 17 wt% total organic carbon) and petroleum potential (up to 60 mgHC g −1 rock) with mature Type II marine kerogen. The first and the second units, which are dominantly organic-rich mudstones, can be compared to the Mississippian Barnett, Miocene Antelope and Cretaceous Tuscaloosa shales of the USA. Evaluation of the brittleness index shows that the HSII.1 and HSII.2 units are mostly ductile and comparable to tight oil and gas reservoirs, while the third HSII.3 unit, where organic-rich facies are juxtaposed to organic-lean limestone beds with natural fractures (porosity between 3 and 7%), may be compared to the Niobrara B Formation. Within the shale-oil fairway of the Chotts Basin, the estimated recoverable oil is around 1.3 Bbbl. It is comparable to the recoverable oil estimated volume for the Middle Member of the Bakken in the USA. This study demonstrates that the Fegaguira Formation source rock should be considered as an additional unconventional oil-shale target for Tunisia.