Late Ordovician Research Articles

Formation of Late Ordovician marine red beds: A case study of Sandbian deposits in the Tarim Basin, Northwest China

The late Ordovician was an important transition interval between the mid-Ordovician biological radiation and end-Ordovician extinction. Marine red beds (MRB) of Late Ordovician are distributed worldwide, including in the Americas, South Africa, England, and China. Among them, the conditions that contributed to the widespread Katian MRB was a combination of terrigenous input, cool-water settings, sediment oxidation, volcanic material input, and seasonal aridity. In contrast, Hirnantian MRB have a more limited distribution, and their occurrence might be related to regional tectonic settings and icehouse conditions. Sandbian MRB are distributed worldwide, but are relatively poorly studied. Even though Sandbian MRB are very similar in some characteristics to the Katian MRB, we don't know which factors were important to their formation and how these were related to marine environmental change and tectonic evolution. The Dawangou section near Kalping in the Tarim Basin of northwest China is an important global reference section for the auxiliary GSSP of the Middle–Late Ordovician boundary and for Sandbian through early Katian biostratigraphy and chronostratigraphy. Diffuse reflection spectroscopy, major and trace element analysis, rare earth element analysis, and sedimentology were employed to study the interval of Sandbian MRB in the Kanling Formation (within the N. gracilis graptolite zone) relative to underlying and overlying deposits. The red-colored strata have elevated total Fe and hematite contents, whereas the grey and grey-green samples have much a lower total Fe and a significant lack of hematite and goethite. The total Fe content for strata within the Sandbian MRB varies with the clay content. The AlFe correlation within the Sandbian MRB is 0.9, whereas it is 0.6 for the non-red sediments. The Th/U ratio is much higher in the red-colored beds (averaging 7.0) relative to the underlying and overlying sediments (averaging 4.4). However, a redox proxy of Ni/Co ratios does not exhibit a significant difference between the underlying and overlying sediments (averages of 2.6 and 2.8) and the reddish Sandbian MRB interval (average of 2.6). The results indicated that the Sandbian marine red beds of the Tarim were formed during conditions of marine oxygenation that helped form and preserve the iron oxide. However, the regional distribution and aspects of the sediment geochemistry imply that another essential process for the formation of these Sandbian MRB was the influx of terrestrial iron-rich sediments from the exposed uplifts into the mixed shelf regions; and that this influx was partly governed by the regional patterns of southward subduction of the South Tianshan ocean and of northward subduction of the Altyn to the margins of the Tarim block.

Multiple glacio-eustatic cycles and associated environmental changes through the Hirnantian (Late Ordovician) in South China

The Hirnantian glaciation, which was the first in the Phanerozoic to occur with elevated CO 2 levels, caused severe mass extinctions and disrupted biogeochemical cycling in the latest Ordovician. Previous research that aimed at exploring the triggers and consequences (both abiotic and biotic) of this glacial event have largely been based on condensed or incomplete sections. However, there is much debate over the detailed glacio-eustatic processes, and especially over the possible existence and characterization of multiple glacial cycles. Herein we conduct high-resolution geochemical analyses (via XRF core scanning and ICP-OES/MS) and a sedimentological study of three continuous upper Katian – lower Rhuddanian drill cores (Well Huangge-1, Well Shuanghe-1 and Well Yiwan-1), which represent a combination of nearshore and offshore facies in South China. The geochemical and sedimentological evidences reveal for the first time three glacio-eustatic cycles encompassed within a major sea level cycle through the upper Katian and Hirnantian. Each glacio-eustatic cycle comprises a combination of regressive and transgressive systems tracts, with the third one denoting the largest sea level fall and the glacial maximum. This pattern of sea level changes can be correlated with both the glacial sedimentary records and geochemical signals in North Gondwana, Laurentia and Baltica. These glacio-eustatic changes likely led to perturbations in regional marine nutrient cycling and formed a redox landscape that is depicted herein as somewhat different from previous models. We also present the glacio-eustatic effects at a basin scale, and a refined constraint for the tempo of polar ice sheet advances and retreats. It also calls for a re-evaluation of the mechanisms whereby a multi-cycle glaciation led to the Hirnantian biotic crisis, a relationship that may be much more complicated than the simplified cause-and-effect relationship proposed before. • Core scanning data revealed three glacio-eustatic cycles for the Hirnantian glaciation. • New pattern of glacial cycles correlates well with other records among different blocks. • Glacio-eustatic changes formed the redox landscape both temporally and spatially.

Nitrogen isotope evidence for a redox-stratified ocean and eustasy-driven environmental evolution during the Ordovician–Silurian transition

The Ordovician-Silurian (O/S) transition was a critical geological interval, during which significant biological events and dramatic environmental changes occurred. Extensive research has proven that changes in the ocean environment are one of the main causes of events that affect the biosphere. However, the redox structure and related geochemical evolution of the ocean during the O/S transition remain under debate. N isotopes are thought to have significant potential for further elucidating the oceanic redox structure and related geochemical evolution during the O/S transition, and are an effective supplement to the former environmental proxies of bottom sediments. This study presents high-resolution N isotope data from a well-sequenced drill core across the O/S transition from the Lower Yangtze Platform, South China, showing an interesting isotopic variation with high δ 15 N values (+1‰ to +4‰) during the Late Katian to Hirnantian ages of the Ordovician and much lower values (−1‰ to +1‰) during the Rhuddanian Age of the earliest Silurian. This δ 15 N variation is mostly controlled by ocean redox conditions and the corresponding changes in the nitrate reservoir. Combined with previously reported δ 15 N data, a redox-stratified ocean with a relatively deep redox transition zone was reconstructed according to the distribution of δ 15 N profiles across different palaeogeographic locations. The lower δ 15 N values and more reducing conditions in deep water sections demonstrate that glacio–eustatic fall, rather than increased upwelling, was likely the main driver of the global redox change during the Late Ordovician. Furthermore, a global δ 15 N decrease during the Late Katian may be associated with the debated Boda warm event, and the up to six positive δ 15 N excursions observed during the Hirnantian might suggest the cyclicity of the glaciation process. • δ 15 N can be correlated globally during the Ordovician-Silurian (O/S) transition. • Oceanic δ 15 N was controlled by water redox conditions during the O/S transition. • Positive δ 15 N excursions with a low δ 15 N baseline support a redox-stratified ocean. • δ 15 N records indicate Hirnantian redox evolution driven by eustatic sea-level fall. • Global δ 15 N variations correspond with a Boda warm event and multistage Hirnantian glaciation.

Tectonic evolution analysis constrained jointly by in-situ calcite U-Pb dating and apatite fission track for southwestern Ordos Basin

The lower Paleozoic strata of Ordos Basin is rich in oil and gas resources. The Lower Paleozoic marine carbonate in the southwestern Ordos Basin is characterized by large thickness and high abundance of organic matter. The source rocks are in dry gas window.However, the tectonic evolution has not been well evaluated. The timing of the earlier tectonic events is a key scientific problem and needs to be addressed timely.In this study, under the constraints of detailed field investigations, microstructure characteristics, in-situ trace element analysis and regional deposition-burial evolution history, the secondary calcite in-situ U-Pb dating is combined with apatite fission-track time-temperature simulation curves of the Lower Paleozoic carbonate strata and its fault facture zone to reveal four tectonic events including the Late Ordovician, Late Triassic, Late Jurassic and end of the Early Cretaceous-Cenozoic.A hydrothermal event occurred at the Late Ordovician 452±13 Ma, which might be linked to the subduction and closure of the Proto-Tethys Ocean during the Caledonian period.A limited Indosinian tectono-thermal event occurred at 214±22 Ma, which might be caused by the regional fault movement.The intense tectonic activities during the Yanshanian and Himalayan periods induced a strong uplift and denudation.The southwestern Ordos Basin experienced multi-stage complex tectonic evolution and the caprock was seriously damaged, leaving rather unfavorable preservation conditions for hydrocarbon.This study conducts a novel tectonic evolution analysis by combining low-temperature thermochronology with in-situ U-Pb dating of fault-related calcite, providing a new way to analyze tectonic evolution and later transformation process at the margin of complex superimposed basins.

Convergent evolution in planktic graptolites: independent origin of the dicranograptid morphology in the Hirnantian (latest Ordovician)

A new genus and species of planktic graptolite, Anjigraptus wangi gen. et sp. nov., is described from the Upper Ordovician (Hirnantian) sponge-dominated Anji Biota of Zhejiang Province, China. The new genus has a biserial proximal portion and two uniserial stipes distally, the same tubarium form as Dicranograptus. The new genus has probable pattern H development and lacks introverted thecae; it can thus be readily distinguished from the biostratigraphically important Late Ordovician graptolite Dicranograptus, the Late Ordovician Diceratograptus, and the Ordovician–Silurian Neodicellograptus. As Anjigraptus wangi gen. et sp. nov. appears to lack a nema in its uniserial portions and can be distinguished from all other biserial graptolites in the same assemblage, we conclude that the new taxon is not a teratomorphic form of a previously known species, but represents convergent evolution of the dicranograptid morphology. The holotypes of two species of Neodicellograptus (N. siluricus and N. spinosus) are re-illustrated. Lucy A. Muir [lucy@asoldasthehills.org ], Department of Natural Sciences, Amgueddfa Cymru – National Museum Wales, Cathays Park, Cardiff CF10 3NP, UK; Yuandong Zhang†* [ydzhang@nigpas.ac.cn ], CAS Key Laboratory of Economic Geology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, 39 East Beijing Road, Nanjing 210008, PR China; Joseph P. Botting‡ [acutipuerilis@yahoo.co.uk ], Department of Natural Sciences, Amgueddfa Cymru – National Museum Wales, Cathays Park, Cardiff CF10 3LP, UK; Xuan Ma† [mxnjues1990@126.com ], CAS Key Laboratory of Economic Geology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, 39 East Beijing Road, Nanjing 210008, PR China. †also affiliated with University of Chinese Academy of Sciences, Beijing 100049, PR China. ‡ also affiliated with Nanjing Institute of Geology and Palaeontology, 39 East Beijing Road, Nanjing 210008, PR China.

Nonmarine Mass Extinctions

A critical review of putative nonmarine mass extinctions associated with the so-called “Big 5 mass extinctions” of marine invertebrates (Late Ordovician, Late Devonian, end Permian, end Triassic and end Cretaceous) as well as a likely sixth mass extinction in the marine realm, the end-Guadalupian extinction, reveals little evidence of coeval marine and nonmarine mass extinctions. Little lived on land during the Ordovician other than a bryophyte-like flora that appears to have been diversifying, not going extinct, during the Late Ordovician. No case can be made for mass extinctions on land coeval with the marine extinctions of the Late Devonian-land plant diversity increased into the Carboniferous, and the tetrapod fossil record is inadequate to identify any mass extinctions. A case can be made for coeval plant/tetrapod extinctions and the end-Guadalupian marine extinctions, so this may be the first coeval marine-nonmarine mass extinction. However, problems of timing and questions about the extent of the nonmarine late/end-Guadalupian extinctions indicate that further research is needed. There were no mass extinctions of land plants, insects or tetrapods across the Permo–Triassic boundary. The Late Triassic was a time of low origination and high extinction rates on land and in the seas; there was no single end-Triassic mass extinction in either realm. The end-Cretaceous provides the strongest case for coeval land–sea mass extinctions, but there is no mass extinction of land plants, evidence of insect extinction is based on assumption-laden analyses of proxies for insect diversity and the tetrapod extinction was very selective. So, whether the nonmarine extinction at the end of the Cretaceous was a mass extinction is worth questioning. Part of the inability to identify nonmarine mass extinctions stems from taphonomic megabiases due to the relatively poor quality and uneven sampling of the nonmarine fossil record. Extinction resistance and resilience of terrestrial organisms is also a likely factor in the dearth of nonmarine mass extinctions, and this merits further investigation.

Early Paleozoic long-lived silicic volcanism in the Eastern Puna Magmatic Belt, Argentina

In several locations along the Eastern Puna Magmatic Belt (EPMB) porphyritic rhyolite facies are interbedded with thinly bedded mudstone and fine-grained sandstone deposited during deepening of the marine setting. Previous research mainly focused on the northern part of EPMB, identified four main syn-sedimentary rhyolitic-dacitic volcanic episodes from Early to Late Ordovician, the oldest being Tremadocian (earliest Ordovician). In this contribution, we describe coherent rhyolite facies associated with volcaniclastic breccias in the central part of the EPMB (Cajón, Tuzgle, Concordia and Agua de Castilla creeks), and we present new geochemical data and U/Pb zircon ages on coherent rhyolite facies (Cajón, Huancar and Opla). The studied rhyolites are syn-sedimentary sills with peperitic margins. The coherent rhyolites have SiO2 content of ∼65–71 wt%, peraluminous signature, enrichment in LILE (Cs, Rb, Ba) and marked negative Sr, Ti and P anomalies. The REE patterns normalized to chondrite show a higher content of the light REE and a negative Eu anomaly (Eu/Eu* between 0.44 and 0.52). The integration of the regional data with our results suggests that at least two rhyolitic-dacitic volcanic episodes occurred during the Ordovician evolution of the EPMB: The Early Ordovician (∼485-462 Ma) including Tremadocian and Arenigian facies (Rio Taique, Niño Muerto, Huancar, Opla, northern Pastos Chicos, Agua Cavada, Muñayoc, oldest Quichagua, Cordón de Escaya, Cajón, Tuzgle, Concordia and Agua de Castilla); and the youngest Late Ordovician facies (∼449-443 Ma; Oplita, southern Pastos Chicos, Cordón de Escaya and youngest Quichagua). Hence, sedimentation and silicic volcanism across the Eastern Puna Magmatic Belt spanned at least in two episodes along the Ordovician period with more volume and duration during the Early Ordovician (ca. 20 Ma).

Amsassia(calcareous alga) from the Lower Ordovician (Tremadocian) of western Newfoundland, and the biologic affinity and geologic history of the genus

AbstractModular coral-like fossils from Lower Ordovician (Tremadocian) thrombolitic mounds in the St. George Group of western Newfoundland were initially identified asLichenariaand thought to include the earliest tabulate corals. They are here assigned toAmsassia terranovensisn. sp. andAmsassia? sp. A from the Watts Bight Formation, andA.diversan. sp. andAmsassia? sp. B from the overlying Boat Harbour Formation.Amsassia terranovensisn. sp. andA.argentinafrom the Argentine Precordillera are the earliest representatives of the genus.Amsassiais considered to be a calcareous alga, possibly representing an extinct group of green algae. The genus originated and began to disperse in the Tremadocian, during the onset of the Great Ordovician Biodiversification Event, on the southern margin of Laurentia and the Cuyania Terrane. It inhabited small, shallow-marine reefal mounds constructed in association with microbes. The paleogeographic range ofAmsassiaexpanded in the Middle Ordovician (Darriwilian) to include the Sino-Korean Block, as well as Laurentia, and its environmental range expanded to include non-reefal, open- and restricted-marine settings.Amsassiaattained its greatest diversity and paleogeographic extent in the Late Ordovician (Sandbian–Katian), during the culmination of the Great Ordovician Biodiversification Event. Its range included the South China Block, Tarim Block, Kazakhstan, and Siberia, as well as the Sino-Korean Block and Laurentia, and its affinity for small microbial mounds continued during that time. In the latest Ordovician (Hirnantian), the diversity ofAmsassiawas reduced, its distribution was restricted to non-reefal environments in South China, and it finally disappeared during the end-Ordovician mass extinction.UUID:http://zoobank.org/ef0abb69-10a6-46de-8c78-d6ec7de185fe

Detrital Zircon Age Constraints on the Evolution of Paleo‐Tethys in NE Iran: Implications for Subduction and Collision Tectonics

AbstractThe timings of the onset of oceanic spreading, subduction and collision are crucial in plate reconstructions, but not always straightforward to resolve. The Paleo‐Tethys evolution dominated the Paleozoic‐Early Mesozoic tectonics of West Asia, but the timeline of events is still poorly constrained. We present new detrital zircon ages from the Binalud Mountains of NE Iran, in order to determine the timing of tectonic events in the region, and the wider implications for regional tectonics, paleogeography and climate change. Consistent age patterns in Paleozoic clastic rocks show a dominant provenance from the Neoproterozoic basement of northern Gondwana. We interpret this as deposition on a long‐lasting passive continental margin after the initial spreading of the Paleo‐Tethys Ocean. Central Iran‐Eurasia collision caused coarse clastic deposition of the Mashhad Phyllite in a peripheral foreland basin on the Paleozoic sequence of the northern Central Iran Block. The Mashhad Phyllite yields age clusters at 450–250 Ma and 1,900–1,800 Ma, with a clear provenance from a long‐lived continental arc at the active Eurasian margin since the latest Ordovician. Analysis of the age spectra allows us to constrain the maximum depositional age of the Mashhad Phyllite as no later than 228 Ma. This age provides a tight constraint on the timing of initial collision between Central Iran and Eurasia, which also coincides with the Carnian Pluvial Event (CPE). Our new timing evidence of collision reinforces the previous speculation on the Paleo‐Tethys closure as a causal mechanism for the CPE.

Volcanism-induced late Boda warming in the Late Ordovician: Evidence from the Upper Yangtze Platform, South China

Late Boda warming was the last warm episode in the Late Ordovician, and little is known about its cause. Volcanism was a hypothetical candidate that triggered this warming event owing to its potential to enhance the atmospheric CO2 level. Here, we conducted a high-resolution petrographic and geochemical study on the bentonite beds-bearing intervals from the Upper Ordovician Wufeng Formation in South China. This investigation enables us to find the detailed process of paleoenvironmental repercussions induced by volcanism from both short-term and long-term perspectives. The short-term effect was revealed by a 25-cm-long drill core bracketing three cycles of volcanic activity. Three nearly identical cycles have been identified, and each cycle is comprised of a bentonite bed, a siliceous clay-rich mudstone interval, a radiolarian bed, and a siliceous mudstone interval with intercalations of thin pyrite layer. During the eruption stage, a series of environmental and biogeochemical disruptions were brought to the local system, including ocean fertilization, the elevation of paleoproductivity, the onset of euxinia/intensified ocean anoxia, a warmer climate, and enhanced weathering. With the waning of volcanic activities, the volcanic impacts weakened, and a relatively stable water-column chemistry, microbial populations, diversity, and community structure were established until the onset of the subsequent volcanic event that broke equilibrium. The long-term effect was illustrated by the correlation between the distribution of bentonite beds and coeval global sea-level fluctuations. Our results show that intense and frequent volcanic activities in South China, probably contributed by coeval volcanic activities from other regions worldwide, exerted a cumulative heating effect on the global system, contributing to late Boda warming.

Geochemical Characteristics of Late Ordovician Shales in the Upper Yangtze Platform, South China: Implications for Redox Environmental Evolution

Changes to the redox environment of seawater in the Late Ordovician affect the process of organic matter enrichment and biological evolution. However, the evolution of redox and its underlying causes remain unclear. This paper analyzed the vertical variability of main, trace elements and δ34Spy from a drill core section (well ZY5) in the Upper Yangtze Platform, and described the redox conditions, paleoproductivity and paleoclimate variability recorded in shale deposits of the P. pacificus zone and M. extraordinarius zone that accumulated during Wufeng Formation. The results showed that shale from well ZY5 in Late Ordovician was deposited under oxidized water environment, and there are more strongly reducing bottom water conditions of the M. extraordinarius zone compared with the P. pacificus zone. Excess silica (SiO2(exc)) and substitution index of paleoproductivity (Y) indicated that the P. pacificus zone had higher paleoproductivity whereas the M. extraordinarius zone was lower. The high productivity level controlled O2 release in the shallow water area as well as the oxidation degree of the P. pacificus zone. The decrease of productivity and the relatively stagnant water mass of the inner Yangtze Sea controlled the formation of relatively reduced water conditions in the M. extraordinarius zone. The chemical index of alteration (CIA) results suggested that palaeoclimatic conditions changed from warm and humid to cold and dry climate from the P. pacificus to the M. extraordinarius zones in the study area. A comparative analysis of the published Fe-S-C data for the Xiushan Datianba section showed that in the P. pacificus zone of the inner Yangtze Sea, warm and humid climate conditions drove high productivity, sulphate flux and low reactive iron flux, which promoted the expansion of oxic ocean-surface waters and mid-depth euxinic waters. In the M. extraordinarius zone, the cold and dry climate with significant uplift of the Xiang’e Submarine High led to the relative sea level decline, resulting in low productivity, sulfate flux and high reactive iron flux, which promoted the expansion of the mid-depth ferruginous waters and the shrinkage of oxic ocean-surface waters. The results offered new insights into the co-evolution of continents and oceans, and explained the role of continental weathering and uplift of the Xiang’e Submarine High in the exchange of sulfate flux and nutrients in the redox environment change of inner Yangtze Sea during the Late Ordovician.

Genesis of the Bieyesamas Monzogranite of the Altay Mountain, Xinjiang, Northwestern China, and Its Rare Metal Resource Potential

Lying in the Altay Orogenic Belt in Xinjiang, Northwestern China, the Bieyesamas monzogranite pluton is located in the North Altay Terrane. It is one of the important granitic batholiths with a large amount of rare metal pegmatite dikes. According to LA-ICP-MS zircon U-Pb isotopic dating, the 206Pb/238U weighted average age of the Bieyesamas monzogranite is 451.1 ± 5.1 Ma ( MSWD = 6.0 ), indicating the formation age of Late Ordovician. The Bieyesamas monzogranite has secondary minerals such as garnet and tourmaline. The geochemical analysis shows that the pluton is characterized by high SiO2 (70.45%~75.44%), Al2O3 (14. 04%~17.14%), potassium and alkaline ( K 2 O = 4.20 % ~ 4.78 % , N a 2 O + K 2 O = 7.90 % ~ 8.99 % ), A/CNK (1.16~1.28), and high corundum (2.33%~5.08%) being found in CIPW standard minerals, belonging to high-K calc-alkaline peraluminous series. The pluton is enriched in LREE, depleted in HREE ( LREE / HREE = 5.99 ~ 9.65 ), with obviously negative Eu anomaly ( δ Eu = 0.44 ~ 0.60 ), while the trace elements are characterized by Rb, K, Nb, Ta, Hf, and U enrichment and Ba, Sr, Ti, and Zr depletion, as well as with high differentiation index ( DI = 93.24 % ). Zircon ε Hf t values range from 2.89 to 7.69, with the corresponding two-stage model ages ( T DM 2 ) of 941~1257 Ma. The mineral assemblage, geochemical characteristics, and zircon Hf isotope indicate the pluton experienced the highly fractionated process and belongs to highly fractionated S-type granite, which was formed by partial melting of the Meso- to Neoproterozoic crustal material. In the Bieyesamas monzogranite, the average contents of rare metals are obviously higher ( Li = 550 × 10 − 6 , Be = 10.18 × 10 − 6 , Nb = 18.91 × 10 − 6 , Ta = 2.14 × 10 − 6 , Rb = 500 × 10 − 6 , and Cs = 149.9 × 10 − 6 ) than the other rocks and Clark values of crust, which indicates that the Bieyesamas pluton has the enrichment potential of rare metals. The metallogenic geological conditions are superior in the Bieyesamas area of the Altay Mountain, and rare metal deposits and ore spots are widely distributed. In particular, the newly discovered rare metal deposits are characterized by large-scale mineralization, high grade and industrial utilization value, etc. It is preliminarily predicted that they have reached the medium-scale deposits. Therefore, the Bieyesamas area is one of the key areas for rare metal prospecting breakthroughs in the future, with great potential for rare metal mineral resources.

Carbonate shelf development and early Paleozoic benthic diversity in Baltica: a hierarchical diversity partitioning approach using brachiopod data

AbstractThe Ordovician–Silurian (~485–419 Ma) was a time of considerable evolutionary upheaval, encompassing both great evolutionary diversification and one of the first major mass extinctions. The Ordovician diversification coincided with global climatic cooling and paleocontinental collision, the ecological impacts of which were mediated by region-specific processes including substrate changes, biotic invasions, and tectonic movements. From the Sandbian–Katian (~453 Ma) onward, an extensive carbonate shelf developed in the eastern Baltic paleobasin in response to a tectonic shift to tropical latitudes and an increase in the abundance of calcareous macroorganisms. We quantify the contributions of environmental differentiation and temporal turnover to regional diversity through the Ordovician and Silurian, using brachiopod occurrences from the more shallow-water facies belts of the eastern Baltic paleobasin, an epicontinental sea on the Baltica paleocontinent. The results are consistent with carbonate shelf development as a driver of Ordovician regional diversification, both by enhancing broadscale differentiation between shallow- and deep-marine environments and by generating heterogeneous carbonate environments that allowed increasing numbers of brachiopod genera to coexist. However, temporal turnover also contributed significantly to apparent regional diversity, particularly in the Middle–Late Ordovician.

Petrogenesis of the Despen Volcanic Rocks of the Middle–Late Ordovician Volcanoplutonic Association of the Tannu-Ola Terrane (Southwestern Tuva)

Abstract ––We have studied the structure and composition of a volcanic unit in the valley of the Despen River, on the southern slope of the East Tannu-Ola Ridge. The unit was earlier assigned to the Lower Devonian Kendei Formation. The new geological and geochronological data show that it resulted from explosive volcanism at 460–450 Ma. The Despen volcanic rocks formed in association with granitoids of the Argolik complex at the end of the accretion–collision stage of evolution of the Altai–Sayan region, in particular, the Tannu-Ola terrane. These are predominantly felsic ferroan metaluminous and weakly peraluminous nappe volcanic rocks resulted from the differentiation of tholeiitic basalts. Their REE patterns, like those of the Argolik granitoids, are flat in the HREE, show a distinct Eu anomaly, and suggest magma generation at shallow depths in the upper crust. The magmatic source was of subduction origin, as evidenced by the negative Ta–Nb anomalies in the multielement patterns and by εNd(T) = +3.1 to +5.6, and has a Neoproterozoic model age, TNd(DM-2st) = 0.94–0.69 Ga.

Detrital zircon U–Pb geochronology of Xilin Group: Constraints for the early Paleozoic tectonic evolution of the Songliao Massif

The Xilin Group, composed of the Chenming, Laodaomiaogou, Qianshan and Wuxingzhen formations, is one of the Early Paleozoic terranes in the eastern Songliao Massif, mainly consisting of thick layers of fine clastic and carbonate rocks. This study presents LA-ICP-MS zircon U-Pb geochronological data for the Laodaomiaogou and Qianshan formations, further constraining their provenance and the Early Paleozoic tectonic evolution of the Songliao Massif on the eastern Central Asian Orogenic Belt. Most zircons from the Laodaomiaogou and Qianshan formations show magmatic oscillatory zoning and high Th/U ratios (0.26–2.41). Zircon U-Pb dating results indicate that the detrital zircons from the silty mudstone of the Laodaomiaogou Formation yield peak ages of 634 Ma, 775 Ma, 820 Ma, 880 Ma and 927 Ma, as well as multi-episodic Archean to Paleoproterozoic and Mesoproterozoic ages (1405–643 Ma), implying its deposition time is younger than ∼634 Ma. Furthermore, the occurrence of Early Cambrian fossils indicates that the Laodaomiaogou Formation was deposited during the late stage of the Early Cambrian (∼514 Ma). The zircons from the K-bentonite of the Qianshan Formation show four peak ages of 444 Ma, 471 Ma, 489 Ma and 518 Ma and the youngest age peak of 444 ± 4 Ma (n = 6) indicates that the Qianshan Formation was deposited during the Late Ordovician. In addition, the peak ages of the detrital zircons in the silty mudstone of the Qianshan Formation are 472 Ma and 498 Ma, as well as two other concordant points with 207Pb/206Pb apparent ages of 1824 Ma and 1985 Ma. The dating results in this study, together with published data, indicate the absence of Pan-African magmatic events in the Songliao Massif prior to the initial deposition of the Xilin Group, in contrast to those distributed widely in the Jiamusi Massif. Taken together, we conclude that the depositional provenance of the Laodaomiaogou and Qianshan formations was derived from the Songliao Massif. Furthermore, the characteristics of the detrital zircon age composition and rock associations indicate that the Laodaomiaogou Formation formed in a passive continental margin environment, in contrast to the Qianshan Formation, which formed in an active continental margin environment. The above results also imply that the Songliao and Jiamusi massifs might not have collided before the Late Ordovician.

The Truong Son, Loei-Phetchabun, and Kontum Terranes in Indochina: Provenance, Rifting, and Collisions

The three main regions of Indochina are defined as the Truong Son, Loei-Phetchabun, and Kontum terranes. The aim of this review is to integrate numerous petrological studies with sedimentary, palaeontological, and provenance studies in order to construct a preliminary tectonic model which shows the terranes docked in the earliest Carboniferous (Truong Son with Loei-Phetchabun) and in the Permian (Kontum). The Kontum Terrane is characterized by Proterozoic magmatism, mid-Ordovician to Early Devonian granites, and Permian charnockites. Major carbonate platforms developed in the Givetian to earliest Tournaisian on Truong Son and from the Visean to mid-Permian across Truong Son and Loei-Phetchabun terranes. The Truong Son has Silurian granites and a Late Ordovician to Silurian magmatic arc along its southern and western borders caused by subduction of oceanic lithosphere, the remnants of which are now partially preserved in the Loei and Tamky sutures. A region to the east of the Loei Suture in the Loei Foldbelt has a similar-age volcanic arc extending northwards into Laos and is included in Truong Son. A platform-margin coral-stromatoporoid reef developed on Truong Son in the Frasnian and was coincident with a gap in magmatism until the Pennsylvanian with granite magmatism became widespread until the Late Triassic. In the Loei-Phetchabun Terrane, Devonian siliciclastics, volcanics are overlain by Givetian limestones and patch reefs and in turn overlain by Late Devonian to earliest Tournaisian radiolarian cherts and siliciclastics. Tournaisian to early Visean siliciclastic turbidites were coincidental with initial uplift and deformation and were replaced by Visean terrestrial evaporites and coal and marine limestones. Arc magmatism along the Loei-Phetchabun Terrane indicates intermittent eastward subduction from mid-Devonian to mid-Triassic. Carboniferous-Triassic volcanism extends eastward along the northern margin of Cambodia and is probably an extension of Loei-Phetchabun. Ordovician within-plate basalts in Kontum and the transition from shallow marine Ordovician siliciclastics to Silurian deep-marine sediments and a volcanic arc in Truong Son suggest rifting of both terranes from Gondwana in the Late Ordovician. Initial docking of Truong Son with South China was probably in the late Silurian to Early Devonian and docking of Loei-Phetchabun and Truong Son was in the late Tournaisian to early Visean.

Peri-Gondwanan acritarchs, chitinozoans, and miospores from Paleozoic succession in the High Zagros Mountains, southern Iran: Regional stratigraphic significance and paleogeographic implications

Acritarchs and chitinozoans were documented from Ordovician strata and miospores from Lower Permian beds in the Zardkuh area. Ninety-one palynomorph species, comprising acritarchs (63 species of 31 genera), chitinozoans (16 species of 12 genera), and miospores (12 species of 10 genera) were recognized that form 10 local assemblage zones, comprising Zones A–F= acritarchs, G–I= chitinozoans, and J= miospores. Zone A occurs in the uppermost Ilebeyk Formation, indicating Late Cambrian; Zones B–E are present in the Zardkuh Formation, suggesting Early Ordovician (Tremadocian–Floian)–Middle Ordovician (Dapingian) and Zone F occurs in the Seyahou Formation, presenting Late Ordovician (Hirnantian). Likewise, three Gondwanan chitinozoan Zones G–I were established in this succession. Zones G–H occur in the Upper Shale Member of the Zardkuh Formation, supporting an Early-Middle Ordovician (Floian–Dapingian), and Zone I is restricted to the Seyahou Formation, suggesting Late Ordovician (Hirnantian) age. Finally, Zone J occurs in the Faraghan Formation, confirming Early Permian. Based on palynomorph zones, two hiatuses are present within the Paleozoic sequences of the study area. The first hiatus occurs between the Zardkuh and Seyahou formations, encompassing the Middle Ordovician (Dapingian) and much of the Late Ordovician (Sandbian and Katian). The second hiatus appears between the Seyahou and Faraghan formations, spanning the Silurian, Devonian, Carboniferous, and earliest Permian (Asselian). The palynomorph taxa, including Arbusculidium, Striatotheca, Coryphidium, Vavrdovella, Eremochitina, Velatachitina, and miospore taxa, confirm that the study area belongs to peri-Gondwana paleoprovince. Four new species are Vavrdovella mawlawii sp.nov., Vogtlandia zardkuhensis sp.nov, Pseudocoryphidium zagrosensis gen et sp.nov, and Pterospermella saadii sp.nov.

Kinematic evolution of fold-and-thrust belts in the Yubei-Tangbei area: Implications for tectonic events in the southern Tarim Basin

The Yubei-Tangbei area in the southern Tarim Basin is one of the best-preserved Early Paleozoic northeast-southwest trending fold-and-thrust belts within this basin. This area is crucial for the exploration of primary hydrocarbon reservoirs in northwestern China. In this study, we constructed the structural geometric morphology of the Yubei-Tangbei area using geophysical logs, drilling, and recent two- and three-dimensional (2-D and 3-D) seismic data. The Early Paleozoic fault-propagation folds, the Tangnan triangle zone, fault-detachment folds, and trishear fault-propagation folds developed with the detachment of the Middle Cambrian gypsum–salt layer. According to a detailed chronostratigraphic framework, the growth strata in the Upper Ordovician–Lower Silurian layer formed by onlapping the back limb of the asymmetric fault-propagation folds, which therefore defines the timing of deformations. The changes in kink band hinges and amplitudes in the Permian–Carboniferous and Cenozoic folding strata suggest that the evolution of the fold-and-thrust belts followed a sequential evolution process rather than a simultaneous one. Above the pre-existing Precambrian basement structure, the Yubei-Tangbei fold-and-thrust belts can be divided into four tectonic evolution stages: Late Cambrian, Late Ordovician to Early Carboniferous, Carboniferous to Permian, and Cenozoic. The northwestern-verging Cherchen Fault is part of the piedmont fold-and-thrust system of the southern Tarim foreland basin. We interpreted its strata as a breakthrough trishear fault-propagation fold that developed in three phases: Mid–Late Ordovician, Silurian to Middle Devonian, and Triassic to present. These tectonic events are responses of the Altyn-Tagh and Kunlun collisional orogenic belts and the Indian-Eurasian collision. The inherited deformation and structural modification in the southern Tarim Basin may be an indicator of the growth and evolution of peripheral orogens.

Sequence stratigraphy and carbon isotopes from the Trenton and Black River Groups near Union Furnace, PA: Constraining the role of land plants in the Ordovician world

Sea-level change influences carbon isotopic trends in both modern and ancient carbonate depositional environments. Generally, this relationship is manifested as a positive carbon isotopic excursion where the rising limb of the excursion is associated with transgression. These excursions exist because sea level can influence 1) local/regional/global carbon cycling, 2) basin restriction and meteoric influence, and 3) carbonate sedimentation. The first two processes are influenced, in part, by the large carbon reservoir represented by the terrestrial biosphere. Our goal is to explore the relationship between carbon isotopes and sea level prior to the evolution of vascular plants at a time when the terrestrial carbon reservoir was far smaller.This study focuses on the Upper Ordovician Union Furnace section in Pennsylvania, where a previously documented ~1‰ increase in δ13C values in the upper Sandbian (458.4–453.0 Ma; referred to as the “baseline shift”) has been interpreted to reflect a perturbation to the global carbon cycle due to the proliferation of non-vascular plants. By focusing on the interval deposited before the “baseline shift”, we were able to examine the relationship between sea level, carbon isotopes, and some of the earliest terrestrial ecosystems. We present a high-resolution δ13C record directly tied to the sedimentological and sequence stratigraphic framework. By sampling genetically related packages of rock we were able to document a relationship between carbon isotopes and sea level at the sequence and parasequence level. Our results show that carbon isotopic values decreased during transgressions and increased during regressions. This seemingly inverse relationship between δ13C values and sea level is interpreted to reflect increased terrestrial fluxes during regression in a world devoid of vascular plants. Our results suggest that the early colonization of land by non-vascular plants impacted regional carbon cycling in the Appalachian Basin and eventually the global carbon cycle in the Late Ordovician.

Cephalopod accumulations from the Late Ordovician of Cumbria: Mud mounds and stromatolites

Accumulations of cephalopods in the Late Ordovician (Katian [Ka4]) Troutbeck Siltstone Member of the Ash Gill Mudstone Formation at Skelghyll Beck in the Lake District and in the Keisley Limestone Formation of the Cross Fell Inlier contribute toward an understanding of their respective depositional environments as well as the palaeogeography of the southeastern margins of the Lakesman Terrane. The accumulation present in the Troutbeck Siltstone Member is interpreted as having been deposited in a shallow, near-shore lagoon that supported stromatolite growth. The shore lay to the north of this site. Geopetals infilling cephalopods forming concentrations in the Keisley Limestone Formation indicate that the conchs came to rest on angles of 30–40° to horizontal. The lithologies of the matrix suggest that they were deposited in cavities or fissures from which fine-grained carbonate was either excluded or winnowed away. Both observations accord with hypotheses that the Keisley Limestone Formation represents the remnants of a carbonate mud mound, and facilitate comparison with the contemporaneous Boda mounds of Siljan, Sweden. Comparison with local late Katian – Hirnantian successions indicate that the Keisley mound would have formed a prominent feature on the sea floor, becoming largely buried by argillaceous sediments during the Hirnantian.