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Over the last two decades, there has been an intensive research effort focused on studying the climate fluctuations during the Ordovician period. In this study, we provide brachiopod and bulk carbonate rock stable isotopic data for oxygen (O) and carbon (C) from the Middle Ordovician succession from northern Estonia for evaluating the Middle Ordovician palaeotemperature and palaeoenvironmental history. We present δ18O curves from eight northern Estonian outcrops. This new data, together with the published results, enable compiling the first complete stable oxygen isotopic curve for Middle Ordovician to early Silurian for the eastern Baltoscandian Palaeobasin. Combining the published and new δ13Cbrac and δ18Obrac data allows us to address chemostratigraphic correlation in the Middle Ordovician. The Baltoscandian δ18O data generally confirm a cooling trend from Early to Middle Ordovician documented by previous studies in different palaeobasins. [ABSTRACT FROM AUTHOR]

With the continuous increase in exploration efforts in new zones and new strata, significant breakthroughs have been made in the natural gas exploration of the O1m56 to O1m4 formations in the Ordos Basin. Thus, the origin and exploration potential of subsalt natural gas have attracted much attention and urgently need to be addressed. On the basis of certain geochemical characteristics, genetic types, and sources of natural gas, a comprehensive study on the sedimentary environment, organic geochemical characteristics, and spatial distribution scale of source rocks are conducted in this paper by using geological and geochemical methods. The study shows that: (1) The Ordovician subsalt natural gas is mainly “pyrolysis dry gas,” among which the δ13C1 of Ordovician subsalt low sulfur (sulfur-free) natural gas is lighter, with an average value of −39.6‰; the δ13C2 ranges more largely from −35.6‰ to −25.8‰. In contrast, both δ13C1 and δ13C2 values are heavier in high-sulfur natural gas, revealing that different Thermochemical Sulfate Reduction (TSR) reaction stages have different degrees of influence on natural gas components and carbon isotope composition. (2) Subsalt natural gas is classified as “oil-type gas,” which is self-generated and self-accumulated, whose source rocks are mainly Ordovician subsalt marine deposits. (3) Three types of marine source rocks are developed in Ordovician subsalt, including black argillaceous rock, dark argillaceous dolomite (dolomitic mudstone), and dark micrite (bioclastic) limestone. In addition to micrite limestone, these rocks were mainly formed in a confined lagoon sedimentary environment with high salinity and anoxia. Sedimentary water was significantly stratified and the environment was highly reduced. The organic matter content of the source rocks is relatively high, with an average TOC value of 0.45%. The hydrocarbon-generating parent materials are mainly composed of bacteria and algae, and the organic matter evolution reaches high-over maturity stage. The total gas generation amount of the marine source rocks in Ordovician subsalt is approximately 43.8 × 1012 m3, which can provide hydrocarbons and accumulate for the subsalt favorable reservoir facies located far from Upper Paleozoic gas sources.

Abstract The tectonic of the Middle and Late Ordovician in the western margin of the north China Platform is complex, and the accumulation models of organic matter of the Wulalike Formation formed during this period are still unclear. Total organic carbon (TOC) content, mineral composition, organic carbon isotope composition, as well as the major and trace elements in the shale samples were all measured in this study. The Wulalike Formation was formed during a tectonic transition from a passive continental margin to an active continental margin. Hydrocarbon-forming organisms are mainly algae. Primary productivity, terrigenous input, and sedimentation rates influence input, dilution, and degradation of organic matter. The degree of degradation, redox, and basin restriction control the preservation of organic matter. The TOC of the Wulalike Formation is determined by the input and preservation degree of organic matter at different sedimentary periods. The organic matter accumulation model in the Wulalike Formation is controlled by regional structures, dominated by sedimentation rate and preservation conditions.

The "short-great-appendage" arthropods (Megacheira), such as Leanchoilia , have featured heavily in discussions of arthropod evolution, particularly related to the head and its appendages. 1,2,3,4 Megacheirans are subject to competing interpretations, either as a clade 4 or a grade, 5 in the stem group of Euarthropoda 6 or, alternatively, Chelicerata. 4 They are most diverse in Cambrian Burgess-Shale-type deposits, where the family Leanchoiliidae is represented by six genera, 7,8,9,10,11,12 characterized by the presence of three distal flagella on the great appendage with a presumed sensory function. We describe the first post-Cambrian member of this family, Lomankus edgecombei gen. et sp. nov, from the Upper Ordovician (Katian) Beecher's Trilobite Bed site of New York State—the first post-Cambrian megacheiran with the exception of the Silurian and Devonian Enaliktidae. Micro-computed tomography (micro-CT) scanning reveals the morphology of the short great appendage with elongate flagella, four biramous cephalic limbs, 11 trunk segments with biramous limbs and dorsal tergites, and an elongate telson unique within Leanchoiliidae. The great appendage is also unique: the long endites that bear the flagella in other leanchoiliids are absent (or at least greatly reduced) and each flagellum appears to attach directly to an individual podomere, suggesting a sensory rather than a raptorial function. The remarkable preservation of a well-developed ventral plate (epistome-labrum complex) anterior of the mouth reinforces a deutocerebral origin 2,13 of the short great appendages. Lomankus edgecombei unveils the three-dimensional (3D) head morphology of leanchoiliids in unparalleled detail and demonstrates that these iconic fossil arthropods ranged into dysaerobic environments in the Ordovician, where Lomankus occupied a deposit-feeding niche. • The youngest leanchoiliid arthropod described from the Late Ordovician of the USA • Iron pyrite preservation allows high-resolution 3D reconstruction by CT scanning • The head anatomy of leanchoiliids is resolved in unprecedented detail Parry et al. describe a new leanchoiliid arthropod from the Ordovician Period that is preserved in pyrite, allowing it to be reconstructed in 3D, revealing new anatomical details. This group of arthropods is well known from the preceding Cambrian, but this new discovery reveals that they survived long after the Cambrian explosion. [ABSTRACT FROM AUTHOR]

Based on logging, core, thin section and geochemical analysis, the tectonic-lithofacies paleogeographic pattern of first member to third member of Ordovician Majiagou Formation (O1m1–O1m3 for short) in Ordos Basin is reconstructed, and the tectono-sedimentary evolution characteristics and oil-gas geological significance are discussed. The results are obtained in four aspects. First, a set of marginal argillaceous dolomites with high gamma ray value developed steadily and diachronously at the bottom of Majiagou Formation, which distributed over the Huaiyuan Movement unconformity, with δ13C values positive drift characteristics comparable to global transgression of the Early Ordovician Floplian. Second, the global sea level rose and the ancient land was submerged into the underwater uplift in O1m1 to O1m2, and the central uplift was deposited for the first time in the Ordovician, forming a tectonic pattern of “one uplift and two depressions”. Subsequently, the subduction and extrusion outside the basin and the differentiation of uplift and depression in the basin of O1m3 resulted in the activation of the Wushenqi–Jingbian bulge. Third, the evolution of the tectonic pattern had a significant impact on the sedimentary paleoenvironment. The O1m1 overlaps westward, and saline lagoon is formed in eastern depression and influenced by the transgression. The transgression continued in O1m2 and resulted in communication with the wide sea, and the large-scale grain shoal developed around eastern depression, and the late dry shrinkage formed a small scale evaporite lagoon in upper part. Under the influence of highland sealing in O1m3, the water body gradually differentiated into dolomitic gypsum and saline lagoons to the east, and the grain shoal spread along the highs around sag. Fourth, the source rocks developed diachronously at the bottom of Majiagou Formation form a favorable source-reservoir assemblage with the shoal facies reservoir distributed around the slope of O1m2–O1m3, and they have certain exploration potential for natural gas.

The Lower Ordovician Tongzi Formation containing abundant shoal sediments is the most promising stratum for the petroleum exploration in the Sichuan Basin. However, the current studies mainly focus on the central part of the Basin, the systematic analysis of the southeastern part with well-developed shoal facies is lacking. This paper aims to clarify the characteristics and genesis of the Tongzi Formation reservoir in Southeastern Sichuan Basin, following an analysis of sedimentary facies within the sequence stratigraphic framework. The research shows that the main types of reservoir rocks are oolitic, intraclastic and bioclastic dolostones. And the reservoir spaces consist mainly of intergranular (dissolved) pores, intercrystalline (dissolved) pores, intragranular dissolved pores and fractures. Among them, intergranular pores account for the highest proportion, which is followed by intragranular dissolved pores. In addition, most throats are necking and flaky which mainly connect intergranular and intercrystalline pores, respectively. The diagenetic sequence shows that penecontemporaneous karst well improved the porosity of the reservoir in the early-stage although the cementation and compaction reduced parts of pores. The reservoir formation is associated with the tectonic-depositional settings, diageneses and terrigenous contamination. Paleohighlands and submerged uplifts, forming in the early Ordovician amalgamation between Yangtze and Cathaysia blocks, accumulated shoal sediments as the material basis for the reservoir formation. Penecontemporaneous karst forming intragranular dissolved pores and the dolomitization aiding grainstones to resist the pressure solution are the key to increasing porosity and preserving pores. The absence of terrigenous contamination prevented intergranular pores from being strongly cemented, which resulted in the reservoir difference between the central and southeastern Sichuan Basin. The study can be used as a reference for the further exploration of Ordovician petroleum in the Sichuan Basin and other regions owning similar geological settings.

The aulaceratid stromatoporoids first appeared in the late Middle Ordovician and had achieved a worldwide distribution in tropical to subtropical shallow-marine environments by the Late Ordovician; however, their presence in Baltica has not been documented. Here, we report Aulacera vohilaidia Jeon and Toom sp. nov. from the Upper Ordovician Adila Formation (Pirgu Regional Stage; late Katian stage) of Estonia, which is the first record of an aulaceratid stro­matoporoid in the Ordovician of Baltica. Aulacera vohilaidia Jeon and Toom sp. nov. is char­acterised by three distinct skeletal zones (axial, lateral, and outer), each of which consists of cyst plates with distinctive size and convexity. The axial zone is narrow, featuring large horseshoe-shaped cyst plates with a wavy, imbricated pattern in a line. The lateral zone consists of small cyst plates punctuated by sporadically developed pillars, and the outer zone comprises flat, parallel cyst plates. Ridged structures with cusps are formed by parallel cyst plates in the outer zone, and the growth surface displays a pustular reticulated structure with a polygonal morphology. The occurrence of Aulaceravohilaidia Jeon and Toom sp. nov. in Estonia extends the palaeogeographic distribution of aulaceratid stromatoporoids from peri-Gondwana, Laurentia, and Siberia to Baltica. The arrival of aulaceratid stromatoporoids in Baltica coincided with warming of the climate on the palaeocontinent and the expansion of the range of this group during the Late Ordovician.

The early Paleozoic tectonic evolution of the Qinghai-Tibetan Plateau remains not fully understood. The Longmu Co-Shuanghu Suture Zone within the Qiangtang block of northern Qinghai-Tibetan Plateau, records important evidence of the early Paleozoic evolutionary history of the plateau. In this study, we investigated early Paleozoic amphibolites exposed in the Taoxinghu area of central Qiangtang, using zircon U–Pb dating, mineral chemistry, whole-rock geochemistry, and zircon Hf–O isotopes. The objective was to identify the characteristics of the protoliths and the metamorphic processes. Two types of zircons with different characteristics occur in the amphibolites, yielding zircon U–Pb ages of 490 and 440 Ma attained using kernel density estimation, which represent the ages of the protolith and subsequent metamorphism, respectively. The amphibolites have SiO2 (42.4–55.7 wt.%), and Al2O3 (14.4–18.7 wt.%) and MgO (3.62–8.80 wt.%) contents. Their geochemical characteristics show large ion lithophile elements enriched and high field strength elements depleted. The zircon U–Pb age and rock geochemical data suggest the protoliths of the amphibolites were late Cambrian supra-subduction zone-type ophiolites that organized in association with Proto-Tethyan subduction. The subsequent Late Ordovician metamorphic event might be associated with the collision between continental blocks after oceanic closure. We favor that amphibolites provide essential constraints on the early Paleozoic tectonic evolutionary history of the Qinghai-Tibetan Plateau.

[Display omitted] • Cause of marine biodiversity decline following the GOBE investigated. • Hg isotopes indicate dynamic PZE fluctuations in slope and basinal facies. • Due to environmental instability tied to global cooling & stronger ocean circulation. • ∼23 % of atmospheric Hg sequestered by marine sediments during PZE events. The Darriwilian (Middle Ordovician) is marked by a striking peak of the Great Ordovician Biodiversification Event (GOBE) followed by an abrupt ∼50 % decline in invertebrate species. Understanding the ultimate driving mechanism behind this biotic turnover and its correlation with those proximate climatic-oceanic changes that directly result in elevated biotic mortality have garnered considerable attention. One hypothesis posits that oceanic redox evolution actively influenced biodiversification, and progressive expansion of marine anoxia caused the termination of this biotic event. In this study, we present sedimentological and geochemical profiles for three outcrops of different lithofacies across the Middle-Upper Ordovician transition, ranging from carbonate platform to deep-water slope-basinal facies of the Tarim and South China cratons. Mercury isotopes reveal that recurrent and spatially dynamic photic-zone euxinia (PZE) occurred in the slope-basinal facies while being absent in platform facies. Mass balance model results suggest that ∼23 % of local atmospheric Hg was sequestered in marine sediments due to elevated dissolved H 2 S in surface waters. This finding suggests that destabilized oceanic redox conditions developed during climatic cooling, particularly shoaling and upwelling of deep-marine euxinic waters into the photic zone, coupled with global carbon-cycle disturbances, resulted in biodiversity decline following the peak of the GOBE. [ABSTRACT FROM AUTHOR]

To clarify the genetic mechanism for phase change of the hydrocarbon in the ultra-deep reservoirs, a case study from the Ordovician hydrocarbon in the Halahatang–Shunbei area (HSA), Tarim Basin, NW China, was conducted. The results show that the Ordovician reservoirs in the HSA are characterized as multi-phase reservoirs with a lateral co-existence of condensates, volatile-oil reservoirs, normal oil reservoirs, and heavy oil reservoirs. From north to south, there are regular variations in the geochemical characteristics of the Ordovician hydrocarbon in different blocks of the HSA, showing an increasing trend in GOR, dryness coefficients, methane contents, methane carbon isotope values, and ethane carbon isotope values, while a decreasing trend in oil densities and wax contents. Because the same Cambrian–Lower Ordovician source for the Ordovician hydrocarbon is observed and the kerogen-cracking gas is dominated in the HSA, the regular variations of the hydrocarbon phases and geochemical characteristics can be interpreted as records of biodegradation and multistage oil–gas filling rather than controlled by the source rock organofacies, oil cracking, and gas invasion. The formation mechanism of the Ordovician multi-phase reservoirs in the HSA suggests that the deep strata of the Tarim Basin hold potential for the exploration of natural gas resources.

[Display omitted] • South Tibetan Detachment System, NW Himalaya as a Cambro–Ordovician terrane boundary. • C–O granites during Kurgiakh Orogeny between Sutlej and Dhauliganga valleys. • U-Pb zircon ages of granites between ∼ 499 and 487 Ma, remobilized from ∼ 34 to 13 Ma. • Extensional STDS controlled sediment supply into Tethyan Basin since early Cambrian. The South Tibetan Detachment System is an important extensional fault zone, separating the Greater Himalayan Sequence from the overlying Tethyan Himalayan Sequence, and is well exposed in the upper reaches of the Dhauliganga valley, NW Himalaya. This fault system is characterized by the occurrence of an extensive Cambro–Ordovician granite belt between Sutlej and Dhauliganga valleys, although only a few small granitoids intrude the high-grade mylonite gneiss of the Greater Himalayan Sequence in its immediate footwall. These bodies yielded U-Pb zircon crystallization ages between 498.92 ± 5.5 Ma and 486.54 ± 2.3 Ma. This work postulates that the South Tibetan Detachment System evolved as a major proto-tectonic marginal extensional terrane boundary during the Cambro–Ordovician Kurgiakh/Bhimphedian Orogeny, when it was the conduit for emplacement of the Cambro–Ordovician granite belt. Denudation of the Neoproterozoic Greater Himalayan Sequence and the Paleozoic granites on its footwall provided approximately ∼ 10 km thick sediments into the Tethyan Basin due to this fault system as a master growth fault. Reactivation of this fault system controlled further melting and emplacement of the Higher Himalayan Leucogranite belt during the Cenozoic. Zircon growth is observed in two distinct modes: pulsative from the Late Eocene to Early Oligocene, with peaks at 33.99 ± 1.07 Ma, 30.53 ± 0.32 Ma and 25.03 ± 0.54 Ma; and in the continuous mode from 23.68 ± 0.94 Ma to 13.30 ± 0.30 Ma, in the Miocene, for nearly 10.0 myr. These datasets reveal some of the oldest pulsative movements in the Late Eocene–Early Oligocene during crustal thickening, thrusting and associated metamorphism, followed by continuous extension during the Miocene. Data from the South Tibetan Detachment System are distinct in character, and do not support either its eastwards younging or diachronous movements along the Dhauliganga valley. [ABSTRACT FROM AUTHOR]

Real drilling near the upper Ordovician pinch-out line in the Tahe Oilfield shows that the drilling encountered karst reservoir. However, due to the transitional position between the denudation area and the overlying area and the special karst geological background, the existing drilling and completion data show that the reservoir space types in this area are complex and diverse. In this article, the classification of reservoir space near the Upper Ordovician pinch-out line and the extraction of logging response characteristics have been carried out based on drilling, logging, core, and crude oil quality data. Through this study, the classification scheme of karst reservoir space in the study area is proposed. The reservoir space types of karst reservoirs include fracture–cave, fracture–pore (light and low resistivity), fracture–pore (heavy and high resistivity), dissolved pore–pore (light and low resistivity), dissolved pore–pore (heavy and high resistivity), isolated pore (relatively isolated distribution of pores and fractures, weakly connected), and cave-type reservoir (sand and gravel filled or semi-unfilled). Furthermore, conventional logging parameters and five parameters sensitive to reservoir properties are extracted. The intersection maps based on the combination forms, fluid properties, and reservoir space effectiveness of different types of reservoir spaces are effective in distinguishing seven types of reservoir spaces and two types of stratified karst reservoirs. In this study, the reservoir space types and logging response characteristics of reservoirs near the Upper Ordovician pinch-out line are defined, which can provide a reliable geological basis for the quantitative identification and distribution evaluation of karst reservoirs.

The study of the ancient subterranean river system in the Tahe Oilfield in the Tarim Basin is still in its initial stage. Subterranean river caves are mainly classified based on the characteristics of the depth distribution and structural patterns of the subterranean rivers. However, there is a lack of comprehensive analysis regarding the spatial development patterns of complex subterranean river systems from geological perspectives such as structure, fault, paleogeomorphology, and water table. This hinders the understanding of primary and secondary relationships of subterranean rivers, spatial superposition patterns, and original connectivity relationships, thereby impeding comprehensive management research in the later stages of Tahe Oilfield development. To address this gap, an investigation was carried out in the S67 well block to clarify the developmental characteristics of the Ordovician ancient subterranean river system in the thrust anticline area of the main Tahe Oilfield. The study utilized methods like structural fault analysis, paleogeomorphology restoration, seismic attribute characterization, and vertical section interpretation to identify river types, classify systems, and examine geological origins. Noteworthy findings include the identification and analysis of a phreatic loop river for the first time. Results show that the S67 well block is located in a low-lying region at the southern edge of the karst platform within the main Tahe Oilfield area. This region features peak cluster depressions and karst hill depressions with minimal amplitude differences and shallow incision depths of surface water systems. The low-angle thrust structural style of the thrust anticline, combined with a network of faults, provides favorable dissolution channels for the multi-layered subterranean river system. In the study area, the Ordovician period is characterized by the emergence of distinct water-table subterranean river systems and phreatic loop river systems, all of which display a dendritic structure. The water-table subterranean rivers are divided into main, branch, and abandoned types, while the phreatic loop rivers are classified as ascending or symmetrical. The formation of ancient subterranean rivers in the thrust anticline area is primarily influenced by factors such as paleogeomorphology, water table levels, thrust anticline structure, and secondary fault networks.

A new species of bubble-headed trilobite, Staurocephalus oliveae sp. nov., is described from 250 specimens collected from a recently discovered locality in the Sholeshook Limestone Formation (upper Katian, Upper Ordovician; Ashgill, Cautleyan Zone 1 equivalent) in the village of Llanddowror, Carmarthenshire. Although no fully articulated specimens were found, the material is the most abundant and well-preserved of any Ordovician Staurocephalus species known from Britain and Ireland, and enables the complicated 3D structure of its cephalon to be reconstructed. Staurocephalus oliveae sp. nov. is the earliest representative of the genus recorded in Avalonia, following its presumed migration into the area from Laurentia or Baltica. Recognition of this Cautleyan species resurrects the potential usefulness of the long established, widely distributed, Staurocephalus clavifrons as a latest Katian (= Rawtheyan) indicator.

During the Late Ordovician period, changes in climate and mass extinctions were observed; however, the factors influencing these phenomena have not been fully understood. In order to understand the relationships among redox water conditions, climates, and mass extinctions in the Late Ordovician, this study analyzes the chemical index of alteration (CIA) in shales and 87Sr/86Sr in carbonate leachates as proxies of changes in chemical weathering intensity and chemical weathering rate in the Late Ordovician (mainly from Katian to Hirnantian). The results show that an enhanced chemical weathering rate (increased 87Sr/86Sr ratios) and decreased chemical weathering intensity (decreased CIA values) characterized the late Katian, which might be attributed to the global orogenesis and enhanced precipitation/runoff under the warming climate (late-Boda warming). This enhanced chemical weathering rate contributed to the CO2 drawdown in the P. pacificus biozone, corresponding to the initiation of cooling and further glaciation. Meanwhile, the enhanced weathering-induced high primary productivity could have contributed to the expansion of anoxic seawater in the Katian, which further caused the Katian extinction. The Hirnantian Glaciation was characterized by high 87Sr/86Sr ratios in carbonates and extremely low CIA values in shales, which were likely related to the exposure of continents during low sea level and the glacial grinding of unweathered rocks. This study shows that the highest denudation rate and lowest chemical weathering intensity in the Hirnantian stage might have resulted in enhanced CO2 release and contributed to the end of glaciation.

The diversification of Palaeozoic stromatoporoid sponges was interrupted by the Late Ordovician glaciation, which caused the first of the five major extinctions during the Phanerozoic. Glaciation and the subsequent expansion of anoxia during deglaciation are interpreted to have resulted in a persistent scarcity of stromatoporoids during the Hirnantian. Previously, Hirnantian stromatoporoids were only known from two regions: Estonia in Baltica and Anticosti Island in eastern Laurentia. Here we report stromatoporoids from the upper Hirnantian Shiqian Formation of South China, adding to the rare fossil record of Hirnantian stromatoporoids. Three new stromatoporoid taxa are identified: Ecclimadictyon ancipitum Jeon sp. nov., Camptodictyon contortus Jeon & Kershaw sp. nov., and Cystostroma rallus Jeon sp. nov. The appearance of these stromatoporoids in South China is stratigraphically later than solitary rugose corals, but coincides with tabulate corals in the aftermath of the end-Ordovician mass extinction. Compared to the wider palaeogeographical and environmental distributions of rugose and tabulate corals during the Hirnantian, stromatoporoids presumably had a narrower environmental tolerance. The stromatoporoid assemblage in South China during the Hirnantian is dominated by clathrodictyids, which exhibit similar faunal patterns to those found in Baltica and Laurentia during the same period. The rise of clathrodictyid-dominated assemblages began in the middle to late Katian of the Late Ordovician, particularly in peri-Gondwanan regions, and rapidly expanded worldwide during the subsequent Hirnantian and Silurian onwards. The Late Ordovician glaciation significantly hastened the global Ordovician–Silurian stromatoporoid transition, as evidenced by the disappearance of numerous labechiid-type stromatoporoids and the expansion of clathrodictyid-type stromatoporoids in the Hirnantian and the subsequent Silurian. [ABSTRACT FROM AUTHOR]