Early Silurian Research Articles

Post-extinction diversification patterns of brachiopods in the early–middle Llandovery, Silurian

Network and frequency distribution analyses of global brachiopod occurrences in the earliest Silurian (Rhuddanian–Aeronian) revealed that brachiopod recovery from the end-Ordovician mass extinction during the early–late Rhuddanian was represented mainly by the reestablishment of Late Ordovician cosmopolitan holdover taxa. Brachiopods nearly doubled their generic diversity from Rhuddanian to Aeronian, owing to the radiation of both endemic and cosmopolitan taxa, associated with an overall post-glacial amelioration of global environment and increased habitat heterogeneity as a result of marine transgressions and expansion of epeiric seas. A drastic turnover from the Ordovician-type to Silurian-type brachiopod faunas took place in the Aeronian, several million years after the terminal Ordovician mass extinction. Compared with the orders of orthides and strophomenides that radiated and predominated in the Ordovician, the atrypides and pentamerides displayed pulses of drastic diversification from the Rhuddanian to Aeronian, mainly in tropically located paleoplates, to become the most abundant and diverse brachiopod orders in the Silurian. Atrypides recovered in the early Rhuddanian, whereas diversification of pentamerides was delayed until the Aeronian and becoming widespread in South China, Kazakhstan terranes, Baltica, Avalonia, and Laurentia.

Stratigraphy and depositional setting of the Silurian-Devonian Rockville Notch Group, Meguma terrane, Nova Scotia, Canada

The Silurian–Devonian Rockville Notch Group occurs in five separate areas along the northwestern margin of the Meguma terrane of southern Nova Scotia. In each area, the lowermost unit of the group is the White Rock Formation, which unconformably overlies the Lower Ordovician Halifax Group. Early Silurian U–Pb (zircon) dates from metavolcanic rocks in the White Rock Formation indicate that the unconformity represents a depositional gap of about 25 Ma. The U–Pb ages are consistent with early Silurian (Llandovery) trace fossils and sparse shelly faunas in metasedimentary rocks interlayered with the metavolcanic rocks. The metasedimentary rocks locally contain phosphatic ironstone and Mn-rich beds, and are overlain by mainly metasiltstone with abundant quartzite and metaconglomerate lenses. Some of the latter were previously interpreted to be Ordovician tillite. The White Rock Formation is conformably overlain by the slate- and metasiltstone-dominated Kentville Formation, which contains Upper Wenlock to Pridoli graptolites and microfossils. The overlying Torbrook Formation consists of metalimestone, metasandstone and metasiltstone, interbedded with phosphatic ironstone and minor mafic metatuff, and contains Pridoli to early Emsian fossils. It is in part laterally equivalent to the New Canaan Formation in the Wolfville area, which is dominated by slate, pillowed mafic metavolcanic rocks and fossiliferous metalimestone. Volcanic rocks in the Rockville Notch Group are alkalic and formed in a within-plate setting, probably related to extension as the Meguma terrane rifted from Gondwana. This process may have occurred in two stages, Early Silurian and Early Devonian, separated by a hiatus in volcanic activity. Stratigraphic differences suggest that the Meguma terrane was not adjacent to Avalonia before emplacement of the South Mountain Batholith.

Early Silurian to Early Carboniferous ridge subduction in NW Junggar: Evidence from geochronological, geochemical, and Sr-Nd-Hf isotopic data on alkali granites and adakites

The Central Asian Orogenic Belt (CAOB) evolved through a long-lived orogeny involving multiple episodes of subduction and accretion marking a major phase of continental growth during the Paleozoic. The northern part of the Western Junggar region (NW Junggar) offers a window into these processes, particularly to constrain the timing of closure of the Paleo-Asian Ocean. Here we report geochemical, geochronological, and isotopic data from K-feldspar granites and adakitic rocks from the NW Junggar region. Zircon U-Pb ages suggest that the granites were emplaced during Early Silurian to the Early Carboniferous (434–328Ma). The granites show geochemical characteristics similar to those of A-type granites, with high SiO2 (71.13–76.72wt%), Na2O+K2O (8.00–9.59wt%), and Al2O3 (12.28–14.08wt%), but depleted Sr, Nb, Ta and Eu. They display moderate to high positive εNd(t) and εHf(t) values (4.26–8.21 and 7.69–14.60, respectively) and young Nd and Hf model ages (T2DM-Nd=489–740 Ma and T2DM-Hf=471–845 Ma), suggesting magma derivation through partial melting of lower crust in the Boshchekul-Chingiz and Zharma-Saur arcs. The adakites are characterized by high Sr content (406.5–751.6ppm), and low Y (13.8–16.4ppm) and Yb (1.5–1.8ppm) content, yielding relatively high Sr/Y ratios (25.38–49.41) similar to those of modern adakites. They have high positive εNd(t) and εHf(t) values (7.85–8.25 and 13.23–15.97, respectively) and young Nd and Hf model ages (T2DM-Nd=429–535Ma and T2DM-Hf=355–550Ma), indicating that their source magmas were likely derived from partial melting of the oceanic crust beneath the Boshchekul-Chingiz arc. The petrogenesis and distribution of the A-type granites and adakites, as well as the tectonic architecture of the region, suggest that a ridge subduction event might have occurred during the Early Silurian to Early Carboniferous. In combination with previous studies in the Chinese Altai, we suggest a two-sided ridge subduction model for the Junggar-Altai region.

The Silurian stage boundaries in Bulgaria: the challenge of the Aeronian/Telychian (Llandovery) boundary

The upper Aeronian to lower Telychian (Llandovery Series, lower Silurian) strata in the parastratotype for the Saltar Formation (chert-shale succession) are documented in terms of their lithologies and graptolite biostratigraphy. The studied section Asaritsa сrops out in the western sector of the Stara Planina Mountains (Svoge Unit of the Srednogorie Zone). In section Asaritsa, the highest Aeronian graptolitic strata are separated from the lowest Telychian graptolitic strata by a graptolite-barren interval of pale-coloured shale. Based on the graptolites documented below and above this interval, as well as on its correlation to sections studied in detail from the Czech Republic and Spain, the immediate underlay of the pale-coloured shale is referred to the uppermost part of the sedgwickii Zone, the complete pale-coloured shale interval, or its greatest part, to the lower guerichi Zone, and the directly overlying it succession to the upper part of the guerichi Zone. The Aeronian/Telychian boundary is assumed to lie at the base of this pale-coloured shale package, or within its lower part. The study provides documentation of Metaclimacograptus undulatus, Parapetalolithus praecedens, Rastrites carnicus, Rivagraptus bellulus, and illustrations of Stimulograptus for the first time in Bulgaria.

A proposed new global stratotype for Aeronian Stage of the Silurian System: Hlásná Třebaň section, Czech Republic

The current Global Stratotype Section and Point (GSSP) for the Aeronian Stage (Llandovery Series, Silurian System), on Trefawr track in the Llandovery area of Wales, is an inadequate marker for precise, global, correlation. The International Subcommission on Silurian Stratigraphy has, therefore, undertaken the selection of a new GSSP for this level. The lowest occurrence of the graptolite Demirastrites triangulatus, 1.38 m above the base of the black-shale succession of the Želkovice Formation at the Hlasna Třebaň section in Central Bohemia, is proposed to mark the base of the Aeronian Stage. The section, which fulfils all formal requirements for stratotype of a chronostratigraphical unit, should be considered as a candidate for the new GSSP. An abundant, well-preserved, diverse graptolite fauna occurs through the section along with common chitinozoans, which indicate that the Spinachitina maennili Biozone spans the boundary interval. The section comprises the lower–middle Aeronian (D. triangulatus–Lituigraptus convolutus graptolite biozones) along with underlying Rhuddanian (Akidograptus ascensus–Coronograptus cyphus biozones) and Hirnantian strata. Several graptolite genera of primary importance in global correlation (Demirastrites, Petalolithus, Rastrites and Campograptus) first appear in the lower part of the triangulatus Biozone. The structurally simple section is somewhat condensed, but there is a uniform succession of black shale without any evidence of disconformity in the broad boundary interval. The Corg isotope record exhibits a minor positive excursion just above the base of the triangulatus Biozone, whereas TOC and N isotope and elemental geochemical records provide evidence for uninterrupted sedimentation under stable, anoxic conditions.

Pore structure, wettability and tracer migration in four leading shale formations in the Middle Yangtze Platform, China

Deposited in different sedimentary settings, four leading shale formations (Late Ordovician Wufeng, Early Silurian Longmaxi, Late Permian Dalong, and Early Jurassic Dongyuemiao Shales) are currently the most promising zones for shale gas development in the Middle Yangtze Platform of South China. Based on complementary tests [low pressure gas physisorption, mercury injection capillary pressure (MICP), contact angle measurement, fluid imbibition into initially dry shale, and tracer diffusion into initially fluid-saturated shale followed by tracer mapping with laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS)], this work investigates their pore structure (both geometry and connectivity) and wettability characteristics, as well as the coupled effects of these characteristics on fluid flow and tracer migration.Pores <50 nm account for the majority of the pore volume in these organic shales. The shapes of hysteresis loop from gas physisorption show that the pores are mostly inkbottle-shaped in Wufeng, Longmaxi and Dongyuemiao Shales, while they are mostly narrow plate- or slit-like shaped pores in Dalong Shale. These four organic shales are strongly oil-wetting and moderately water-wetting. According to the imbibition behaviors toward aqueous (deionized water) and oleic (n-decane) phases, hydrophobic pores are better connected than hydrophilic pore networks, which is consistent with the measured contact angles. Diffusion of nano-sized nonsorbing (perrhenate [ReO4−]) and sorbing (cesium [Cs+]) tracers into brine-saturated shales indicates a high spatial variability and limited pore connectivity in these organic shales. The effective diffusion coefficient values are on the order of 10−13 m2/s with an associated geometric tortuosity ranging from 11.1 to 41.4. Due to the limited edge-connected pore spaces and low diffusion coefficients, the migration of hydrocarbons will be very slow from the shale matrix to hydraulically created fractures, with the initially high production of hydrocarbons from connected pore spaces near the sample edge (e.g., fracture face).

Petrogenesis of the Early Palaeozoic granitoids from the Yunkai massif, South China block: implications for a tectonic transition from compression to extension during the Caledonian orogenic event

AbstractA comprehensive geochronological and geochemical study was carried out on the gneissic monzogranites, porphyritic granodiorites and charnockites in the Gaozhou complex of the Yunkai massif in the southern part of the South China block to better understand the Early Palaeozoic tectonic regime of the South China block. Laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) U–Pb dating of zircons indicates an age of 453.2 ± 5.1 Ma to the formation of the gneissic monzogranites, whereas the porphyritic granodiorites and charnockites were generated at 437.0 ± 1.5 Ma and 435.2 ± 2.2 Ma, respectively. The gneissic monzogranites show geochemical features consistent with the high-K, calc-alkaline rock series and are strongly peraluminous. They have SiO2contents ranging from 67.75 to 69.65 wt. % and relatively low CaO contents (1.66–1.94 wt. %). Their REE patterns are fractionated with enriched LREEs and negative Eu anomalies. The samples also show enrichment in LILEs (e.g. Rb and K) and Pb, and depletion in Sr, Ba and HFSEs (e.g. Nb, Ta, Ti and P). They haveεNd(t) values of −8.2 to −7.7. Conversely, the porphyritic granodiorites and charnockites are characterized as medium-K, calc-alkaline rock series and weakly to strongly peraluminous. They exhibit pronounced depletions in HFSEs and positive Pb anomalies. Compared to the earlier gneissic monzogranites, these rocks have relatively lower SiO2(65.50–69.36 wt. %), but higher CaO contents (3.34–4.05 wt. %), and have slightly lowerεNd(t) values (−9.1 to −8.4). Petrography and geochemical compositions of the gneissic monzogranites indicate that they are S-type granite and likely formed by partial melting of Neoproterozoic to Early Palaeozoic immature metagreywackes; whereas The porphyritic granodiorites and charnockites are A-type granite and likely derived from low degrees of partial melting of the dry, granulitic residue depleted by prior extraction of granitic melt. The new data for the Caledonian granitoids in the Yunkai massif suggest that they were formed in a post-collisional tectonic setting. They represent the earliest post-collisional alkaline magmatism reported so far in the Yunkai massif, and thus indicate a tectonic regime switch, from compression to extension, as early as the Late Ordovician to Early Silurian (~450–435 Ma).

Paleo-Ocean Environmental Fluctuations and Their Constraints on Micro/Nano Porous of Shale During Early Silurian in Upper Yangtze Region, Southwest China

Paleo-Ocean Environmental Fluctuations and Their Constraints on Micro/Nano Porous of Shale During Early Silurian in Upper Yangtze Region, Southwest China

Biodiversity patterns of Silurian Radiolaria

Data analysis of all published and well-dated Silurian radiolarian localities was conducted with the goal of revealing long-term patterns in Silurian radiolarian biological diversity. The chronostratigraphic distribution of 161 species was compiled from 25 publications, each selected because of their independent age control. The pattern and dynamics of changes in biodiversity are described using indices that allow for the assessment of changes in taxonomic richness through time. Following the Hirnantian (end-Ordivician) mass extinction, the Rhuddanian stage records very low levels of diversity, which then increase gradually throughout the Llandovery series and reach a maximum by the Sheinwoodian stage, before decreasing during the Homerian, and finally, rebounding during the Gorstian stage. The early Silurian (Llandovery) appears to be an interval during which few species went extinct, although the number of last occurrences increases progressively to reach a peak in the Ludfordian. At the same time first occurrences progressively decrease and are relatively low during the Silurian. Only the Gorstian is marked by a sudden increase in origination. The early Silurian is an interval of faunal recovery from the end-Ordovician mass extinction event. Similarly, the Gorstian stage is characterized by a recovery following the Homerian low in radiolarian diversity that may be correlated with the C. lundgreni extinction event, which affected a number of other pelagic groups (e.g. graptolites, acritarchs, conodonts, and chitinozoans), and with the Mulde event, which coincides with a 4.6‰ positive excursion of the δ13C curve.

Toward an integrated genetic model for vent-distal SEDEX deposits

Although genetic models have been proposed for vent-proximal SEDEX deposits, an equivalent model for vent-distal deposits has not yet appeared. In view of this, it is the object of this paper to present a preliminary integrated vent-distal genetic model through exploration of four major components: (i) nature of the ore-forming fluid, (ii) role of density of the unconsolidated host sediments, (iii) dynamics of sulfate reduction and (iv) depositional environment. Two sub-groups of SEDEX Pb-Zn deposits, vent-proximal and vent-distal, are widely recognized today. Of the two, the latter is by far the largest in terms of metal content with each of the 13 largest containing in excess of 7.5 M (Zn+Pb) metal. In contrast, only one vent-proximal deposit (Sullivan) falls within this size range. Vent-proximal deposits are characteristically underlain by local networks of sulfide-filled veins (commonly regarded as feeder veins) surrounded by a discordant complex of host rock alteration. These attributes are missing in vent-distal deposits, which has led to the widespread view that vent-distal ore-forming fluids have migrated unknown distances away from their vent sites. Because of the characteristic fine grain size of ore minerals, critical fluid inclusion data are lacking for vent-distal ore-stage sulfides. Consequently, hypothetical fluids such as those which formed MVT deposits (120 °C, 20% NaCl equiv.) are considered to represent vent-distal fluids as well. Such high-salinity fluids are capable of carrying significant concentrations of Pb and Zn as chloride complexes while the relatively low temperatures preclude high Cu contents. Densities of such metalliferous brines result in bottom-hugging fluids that collect in shallow saucer-shaped depressions (collector basins). Lateral metal zoning in several deposits reveals the direction from which the brines came. Relative densities of the ore-forming fluid and sediment determine whether the ore-forming fluid stabilizes on top of the sediment column or sinks into it. Metal sulfide precipitation occurs when bacterially produced H2S, diffusing upward from anoxic conditions within the sediment, reacts with metal-bearing chloride complexes in the ore-forming fluid. Since H2S is produced by bacterial sulfate reduction within the first 2 m of the sediment column even where overlain by oxic water, sulfide precipitation will always occur within the anoxic sediment regardless of where the ore-forming fluid comes to rest. Because of the high porosity of the sediment, replacement is precluded as a mechanism of sulfide emplacement in favour of void filling. Detailed textural analyses of the HYC and Howards Pass deposits have demonstrated the abundance of pre-exhalative framboidal pyrite and provide evidence for sulfate-reducing bacteria operating in these basins under normal steady-state conditions before arrival of the ore-forming fluids. The sudden presence of ore-forming fluid, however, dramatically changes the formerly steady-state situation of the local bacterial environment. A major result of this new condition is recorded in the sulfur isotope compositions of the sulfides. Whereas pre-exhalative framboidal pyrite is isotopically light, ore-stage sulfides are significantly heavier and display a reduced fractionation relative to contemporaneous seawater sulfate. Much of the reduced fractionation is linked to the increase in H2S production by sulfate-reducing bacteria. The major factor contributing to this increase is the life-saving action of sulfate-reducing bacteria during which the metal toxicity is mitigated by removal of the toxic ions by precipitating them out as sulfides. Several scenarios representing hypothetical thermochemical sulfate reduction (TSR) conditions convincingly demonstrate the extreme improbability that TSR played a role in formation of vent-distal deposits. A wide range of depositional environments is suggested by host rocks which range from impure carbonate to calcareous or dolomitic siliciclastics to normal siltstones and greywackes to calcareous and siliceous siliciclastics to highly siliceous (cherty) shales. Using the analyses of Mo concentrations as a proxy indicator of euxinia in ancient bottom waters in three vent-distal deposits ranging from Late Cambrian to Early Silurian, euxinia was excluded in all three cases. Regardless of the redox condition of the water column, however, the overriding necessary condition for vent-distal deposits to form is that the water column be quiescent to permit the establishment of a pre-exhalative sulfate-reducing bacterial community. The paper concludes with a six-stage genetic model beginning with exhalation of a dense brine and concluding with sulfide preservation in anoxic sediment.

Shoshonitic- and adakitic magmatism of the Early Paleozoic age in the Western Kunlun orogenic belt, NW China: Implications for the early evolution of the northwestern Tibetan plateau

The Western Kunlun orogenic belt in the northwestern margin of the Tibetan plateau contains two magmatic belts; early Paleozoic belt in the northern part of Western Kunlun Terrane (WKT), and early Mesozoic belt in the southern part of WKT. Both formed from northward subduction of the Paleo-Tethys. The early Paleozoic belt contains large Datong and Qiukesu igneous complexes and many smaller plutons. The Datong complex is mainly composed of dark-colored porphyritic syenite and monzonite with minor light-colored dykes of granite and monzonite. The dark-colored rocks are characterized by moderate SiO2 (58.2–69.3wt.%), and high Al2O3 (15.3–17.1wt.%), total alkali (Na2O+K2O=8.07–10.2wt.%) and ratios of K2O/Na2O (0.77–1.83). They plot in “shoshonite” field, and show high abundances of LILE including LREE ((La/Yb)n=15.4–26.2; mean 20.2) with pronounced negative anomalies of Nb–Ta–P–Ti in normalized trace elemental patterns and weak negative anomalies of Eu (δEu=2Eun/(Smn+Gdn)=0.68–0.80). The light-colored rocks contain slightly higher concentrations of SiO2 (60.3–72.0wt.%), similar Al2O3 (14.7–17.6wt.%), and slightly lower total alkalis (6.57–9.14wt.%) than dark-colored rocks. They show adakitic geochemical signatures with low Y (5.80–17.2ppm) and Yb (0.63–1.59ppm), and high Sr/Y (>40). U–Pb zircon dating indicates that shoshonitic rocks and adakitic dykes formed at ~444Ma to ~443Ma, and a separate small adakitic plug at ~462Ma. The mean εHf(t) values of zircon range from −1.6 to −0.94 (n=14) with TDM2 of ~1.5Ga for shoshonitic rocks and εHf(t) values from −1.8 to +0.72 (n=12) with TDM2 of 1.4 to 1.5Ga for adakitic rocks. Shoshonitic rocks show initial 87Sr/86Sr and εNd(t) of 0.7092–0.7100 and −3.9 to −3.2, respectively, and adakitic rocks yield initial 87Sr/86Sr and εNd(t) of 0.7099–0.7134 and −3.6 to −3.1, respectively. Similar Sr, Nd, and Hf isotope compositions for the shoshonitic and adakitic rocks suggest similar ancient rocks as their sources.Combined with the geological development of the area, we propose that a local extension in the WKT over the subducting Paleo-Tethys has induced the upwelling of asthenospheric mantle and partial melting in the lower crust during mid-Ordovician to early Silurian. The primary shoshonitic melt evolved into the parental magmas for the shoshonitic rocks and adakitic rocks through fractional crystallization of hornblende.

Trace elements indicating humid climatic events in the Ordovician–early Silurian

The chemical composition of the clay fraction separated from the carbonate rock of the north-eastern Baltoscandian Basin was analysed and interpreted. Increased contents of Rb, Zr, Nb, Ti and their Al2O3-normalised ratios were detected at several stratigraphical levels in the geological sections of the Middle Ordovician–Upper Llandovery. In the weathering areas, Rb, Zr, Nb, Ti and Al are sensitive to moist conditions in the clay-forming process. In the sedimentary basin, the contents of these elements in clay are preserved and allow to infer past climates. Humid events occurred in the Dapingian, Sandbian, early Katian and Hirnantian (Ordovician) and in the Middle and Late Llandovery (Silurian). Juxtaposition with the sea-level curve shows correlation of five humid climate intervals with eustatic transgressions, suggesting global causes for these climatic changes. The warm and humid events, lasting one to two million years, occurred as climaxes between ice ages. An exceptional humid event within the Hirnantian glacial time occurs during mid-Hirnantian transgression, i.e. at a time of relative warming, as well.

Organic geochemistry of the Silurian Tanezzuft Formation and crude oils, NC115 Concession, Murzuq Basin, southwest Libya

Thirty-six Silurian core and cuttings samples and 10 crude oil samples from Ordovician reservoirs in the NC115 Concession, Murzuq Basin, southwest Libya were studied by organic geochemical methods to determine source rock organic facies, conditions of deposition, thermal maturity and genetic relationships. The Lower Silurian Hot Shale at the base of the Tanezzuft Formation is a high-quality oil/gas-prone source rock that is currently within the early oil maturity window. The overall average TOC content of the Hot Shale is 7.2 wt% with a maximum recorded value of 20.9 wt%. By contrast, the overlying deposits of the Tanezzuft Formation have an average TOC of 0.6 wt% and a maximum value of 1.1 wt%. The organic matter in the Hot Shale consists predominantly of mixed algal and terrigenous Type-II/III kerogen, whereas the rest of the formation is dominated by terrigenous Type-III organic matter with some Type II/III kerogen. Oils from the A-, B- and H-oil fields in the NC115 Concession were almost certainly derived from marine shale source rocks that contained mixed algal and terrigenous organic input reflecting deposition under suboxic to anoxic conditions. The oils are light and sweet, and despite being similar, were almost certainly derived from different facies and maturation levels within mature source rocks. The B-oils were generated from slightly less mature source rocks than the others. Based on hierarchical cluster analysis (HCA), principal component analysis (PCA), selected source-related biomarkers and stable carbon isotope ratios, the NC115 oils can be divided into two genetic families: Family-I oils from Ordovician Mamuniyat reservoirs were probably derived from older Palaeozoic source rocks, whereas Family-II oils from Ordovician Mamuniyat–Hawaz reservoirs were probably charged from a younger Palaeozoic source of relatively high maturity. A third family appears to be a mixture of the two, but is most similar to Family-II oils. These oil families were derived from one proven mature source rock, the Early Silurian, Rhuddanian Hot Shale. There is a good correlation between the Family-II and -III oils and the Hot Shale based on carbon isotope compositions. Saturated and aromatic maturity parameters indicate that these oils were generated from a source rock of considerably higher maturity than the examined rock samples. The results imply that the oils originated from more mature source rocks outside the NC115 Concession and migrated to their current positions after generation.

Zirconology of lherzolites in the Nurali Massif

The age trend (SHRIMP U/Pb) of the evolution of zircon is obtained for the first time in lherzolites of the Nurali Massif. Zircons are subdivided into groups by the crystallomorphological and geochemical features. These specific features in zircon development are confirmed by the age dates. Precambrian dates (no younger than 1190 Ma) correspond to mantle sources of the lherzolite block. The Early Silurian (445–448 and 439–440 Ma) wass the time of lherzolite magmatism of 10–15 Ma in duration. The Middle Devonian (382.9 ± 8.7 Ma) corresponded to postmagmatic processes related to the effect of gabbro–diorite intrusions crowning in the Nurali Massif.

Two new early balognathid conodont genera from the Ordovician of Oman and comments on the early evolution of prioniodontid conodonts

Reports of Ordovician conodonts from the Arabian region of the Gondwanan margin are extremely rare. Here we provide a description of the apparatus of two new conodont genera and species, Aldridgeognathus manniki and Omanognathus daiqaensis, based on discrete elements recovered from the Am5 Member of the Amdeh Formation, Darriwilian, Ordovician of the Sultanate of Oman. The apparatuses contain 17 and 15 elements, respectively, and both possess three pairs of P elements. The apparatus structure of Omanognathus is similar to the bedding plane assemblage-defined genus Notiodella (= Icriodella) but differs in that as yet only 15 elements rather than 17 have been identified. Aldridgeognathus has similar P elements to the early Silurian apparatus Pranognathus but differs in the possession of a geniculate M element and a Baltoniodus-like S element array. Aldridgeognathus does not easily fit with either the 17-element Notiodella (Icriodella) or the 19-element Promissum templates and suggests that there may be other 17-element Ordovician apparatus templates with very similar or duplicated elements in the P element positions. A cladistic analysis based on the data set of Donoghue (2008) confirms that both new genera should be classified with the Balognathidae and suggests that they, along with another newly described three P element bearing genus Arianagnathus, are more derived than Baltioniodus and Prioniodus but ancestral to Icriodella, Sagittodontina, Promissum and Notiodella. The exact position of Aldridgeognathus is not well resolved in respect to the newly described Arianagnathus or Omanognathus. These new taxa add little to attempts to correlate the Arabian Peninsula with other palaeogeographic regions, but may prove useful for future correlation within the region and provide data to test the hypothesis of Dzik (2015) that the origins for prioniodontid conodonts lie in high latitudes during the Ordovician.http://zoobank.org/urn:lsid:zoobank.org:pub:84C8267D-76E8-4DD3-92F3-0B72CA976DEC

The climatic significance of Late Ordovician‐early Silurian black shales

AbstractThe Ordovician‐Silurian transition (∼455–430 Ma) is characterized by repeated climatic perturbations, concomitant with major changes in the global oceanic redox state best exemplified by the periodic deposition of black shales. The relationship between the climatic evolution and the oceanic redox cycles, however, remains largely debated. Here using an ocean‐atmosphere general circulation model accounting for ocean biogeochemistry (MITgcm), we investigate the mechanisms responsible for the burial of organic carbon immediately before, during, and right after the latest Ordovician Hirnantian (445–444 Ma) glacial peak. Our results are compared with recent sedimentological and geochemical data. We show that the late Katian time slice (∼445 Ma), typified by the deposition of black shales at tropical latitudes, represents an unperturbed oceanic state, with regional organic carbon burial driven by the surface primary productivity. During the Hirnantian, our experiments predict a global oxygenation event, in agreement with the disappearance of the black shales in the sedimentary record. This suggests that deep‐water burial of organic matter may not be a tenable triggering factor for the positive carbon excursion reported at that time. Our simulations indicate that the perturbation of the ocean circulation induced by the release of freshwater, in the context of the post‐Hirnantian deglaciation, does not sustain over sufficiently long geological periods to cause the Rhuddanian (∼444 Ma) oceanic anoxic event. Input of nutrients to the ocean, through increased continental weathering and the leaching of newly exposed glaciogenic sediments, may instead constitute the dominant control on the spread of anoxia in the early Silurian.

Paleozoic intrusive rocks in the Nalati mountain range (NMR), southwest Tianshan: Geodynamic evolution based on petrology and geochemical studies

A synthesis involving the data for the Nalati mountain region (NMR) in west Tianshan with a dataset including zircon U-Pb ages, Hf isotopic composition, major and trace elements of Paleozoic intrusions are presented to improve the understanding of regional geodynamic evolution. Paleozoic intrusive rocks in the NMR could be classified into four categories based on chronological and geo-chemical data: 480±5 Ma, 445-410 Ma, 345-320 Ma, and 295 Ma, which correspond to (1) closure of the Terskey Ocean and the opening of the south Tianshan back-arc basin, which was followed with the opening of the south Tianshan Ocean, (2) initial subduction of the south Tianshan oceanic crust, (3) major subduction stage, and (4) collision to post-collisional stage, respectively. Following the closure of the Terskey Ocean, the south Tianshan Ocean opened at Early Silurian and subducted under Yili-central Tianshan by the end of Early Carboniferous Period. The following breakoff of the subducted slab triggered partial melting of continental crust and formed voluminous granitic rocks in the NMR.

Early Paleozoic dioritic and granitic plutons in the Eastern Tianshan Orogenic Belt, NW China: Constraints on the initiation of a magmatic arc in the southern Central Asian Orogenic Belt

Early Paleozoic dioritic and granitic plutons in the Eastern Tianshan Orogenic Belt (ETOB) have been studied in order to constraint the initiation of a magmatic arc formed in this region. Zircon U–Pb dating indicates that two dioritic plutons in the northern ETOB were generated in the Late Ordovician (452±4Ma) and the Early Silurian (442±3Ma), respectively. Diorites from the two plutons are characterized by enrichments in large ion lithophile elements (LILE) and highly incompatible elements, with depletions in high field strength elements (HSFE) displaying typical geochemical features of a subduction-related origin. They have positive εNd(t) values (+5.08–+6.58), relatively young Nd model ages (TDM=0.71–1.08Ga), with Ta/Yb (0.05–0.09) and Nb/Ta ratios (12.06–15.19) similar to those of depleted mantle, suggesting a juvenile mantle origin. Their high Ba/La (13.3–35.9), low Th/Yb (0.72–2.02), and relatively low Ce/Th (4.57–14.7) and Ba/Th (47.8–235) ratios indicate that these diorites were probably produced by partial melting of a depleted mantle wedge metasomatized by both subducted sediment-derived melts and slab-derived aqueous fluids. Zircon U–Pb dating of a granitic pluton in the northern ETOB yielded a Late Ordovician intrusion age of 447±5Ma. Granites from this pluton show calc-alkaline compositions with geochemical characteristics of I-type granites. They also show positive εNd(t) values (+6.49–+6.95) and young Nd model ages (TDM=0.69–0.87Ga), indicating that the granites were most likely derived from juvenile lower crust. Our new dating results on the dioritic and granitic plutons suggest that arc-type magmatism in the northern ETOB began prior to or at the Late Ordovician (452–442Ma). In addition, north-dipping subduction of the Kangguertage oceanic lithosphere may account for the arc-type magmatism and the geodynamic process of the ETOB in the Early Paleozoic.

Marine redox conditions during deposition of Late Ordovician and Early Silurian organic-rich mudrocks in the Siljan ring district, central Sweden

The Late Ordovician Period witnessed the second largest mass extinction in the Phanerozoic Eon and the Hirnantian glaciation. To infer ocean redox conditions across the Ordovician-Silurian transition, we measured the U (as δ238U relative to standard CRM145=0‰) and Mo (as δ98Mo relative to standard NIST SRM 3134=+0.25‰) isotope compositions of 26 organic-rich mudrock samples from the Late Ordovician (Katian) Fjäcka Shale and the Early Silurian (Aeronian-Telychian) Kallholn Formation (Siljan ring district, Sweden). The magnitude of Re, Mo, and U enrichments, ReEF/MoEF and UEF/MoEF ratios, and sedimentary Fe speciation point to locally euxinic bottom water conditions during deposition of the Fjäcka Shale. The same proxies suggest that black shales of the Kallholn Formation were deposited under transiently euxinic conditions with the chemocline situated near the sediment-water interface, whereas gray shales stratigraphically equivalent to the upper Kallholn Formation were deposited from oxygenated bottom waters. These observations are consistent with higher δ98Mo and δ238U in the Fjäcka Shale compared with the Kallholn Formation.Because the Fjäcka Shale was deposited from persistently euxinic bottom waters, the Mo and U isotope compositions from these rocks can be used to estimate the extent of global ocean euxinia and ocean anoxia (euxinic plus ferruginous conditions), respectively. Elevated MoEF and Mo/TOC ratios in the euxinic Fjäcka Shale suggest no more than moderate basin restriction from the open ocean as well as non-quantitative removal of Mo from the euxinic bottom waters, thus pointing to Mo isotope fractionation between seawater and the euxinic sediments. Hence, we infer that even the highest δ98Mo (+1.28‰) preserved in the Fjäcka Shale is only a minimum estimate for the Mo isotope composition of coeval global seawater. Correcting for seawater-sediment Mo isotope fractionation, the δ98Mo of late Katian seawater may have been +1.4–2.1‰, which corresponds to ~10–70% Mo removal into the euxinic sink. The average authigenic δ238U of the Fjäcka Shale is −0.05‰ to +0.02‰ after correcting for a range of possible detrital δ238U values, thus yielding an overall average of ~0‰. Taking into account isotope fractionation during U removal to euxinic sediments, we infer that late Katian seawater δ238U was about −0.85‰ to −0.60‰. A steady-state U isotope mass balance model reveals that 46–63% of riverine U input was removed in anoxic settings. Based on the Mo and U isotope data, we infer that euxinic and anoxic waters may have covered <1% and at least 5% (potentially tens of percent) of the total seafloor area during the late Katian, respectively, based on previously published models that relate the magnitude of Mo and U burial fluxes to the areal extent of euxinic and anoxic seafloor. By comparison, only 0.21–0.35% and <1% of the total seafloor area was covered by anoxic waters today and during the Cenozoic, respectively. The difference between the estimated extent of ocean anoxia (euxinic plus ferruginous) and ocean euxinia points to an appreciable extent of ferruginous water masses during the late Katian. Integration of our data with previous studies thus supports the hypothesis that ocean oxygenation intensified during the subsequent Hirnantian glaciation (when seawater δ98Mo temporarily reached values similar to today). Hence, environmental stresses related to glaciation, not an expansion of ocean anoxia, may have triggered the first phase of the Hirnantian mass extinction.

First occurrence of Pterotheca (Gastropoda) from the Silurian (Aeronian) of Spain

Pterotheca hispanica nov. sp. is described from the basal beds (Stimulograptus sedgwickii graptolite Zone) of the Formigoso Fm., in the Cantabrian Zone of the Iberian Massif, north-western Spain. This specialized bellerophontoid gastropod is for the first time recorded in the Silurian of peri-Gondwana; it is the only Aeronian species known so far. Its most closely allied relatives are from higher beds placed in the Telychian of Scotland. Pterotheca is considered a highly derived genus within the Pterothecinae of the Carinaropsidae, with the strongly reduced to absent coiling and the growth of the septum being synapomorphies. In the present work it is shown that not only the apical angle of the septum can be used to distinguish species, but also the relative length and width of the septum. In some cases, the slit may extend as far back as the anterior part of the internal septum. Silurian species were hitherto only found in marginal Laurentian and Eastern Avalonian settings. Pterotheca is absent from central Laurentia and Baltica, where it could be expected to be present based on its wide Ordovician distribution. In the light of this, the occurrence of the genus in the Aeronian of Spain seems difficult to explain, albeit the genus was present in peri-Gondwana prior to the end-Ordovician extinction. The extreme scarcity of described taxa recorded from Avalonia, Baltica and Iberia during the Early Silurian may explain this patchy distribution.