Telychian Stage Research Articles

Chitinozoan biostratigraphy through the Aeronian–Telychian boundary interval on Anticosti Island, Canada

Abstract. The well-preserved Llandovery (lower Silurian) succession of Anticosti Island (Quebec, eastern Canada) contains an expanded Aeronian–Telychian boundary interval when compared to other coeval basins. This boundary interval on Anticosti Island also includes two of the most important Llandovery biogeochemical events, the late Aeronian and Valgu events. These two events were previously documented in the Jupiter and Chicotte formations through the study of conodont, graptolite, and brachiopod biostratigraphy and δ13C chemostratigraphy. Despite these multiple investigations, the exact position of the Aeronian–Telychian stage boundary on Anticosti Island has not been firmly established. Here we locally define and globally correlate chitinozoan biozones to refine the position of this stage boundary. The Ancyrochitina ramosaspina biozone, recognized in the Ferrum Member of the Jupiter Formation, correlates with the global Conochitina alargada biozone and indicates an Aeronian age. The Eisenackitina dolioliformis biozone suggests mostly a Telychian age for the Pavillon Member of the Jupiter Formation and the Chicotte Formation. Three new species are defined, namely Conochitina asselinae sp. nov., Spinachitina glooscapi sp. nov., and Ancyrochitina wilsonae sp. nov. (registration date: 4 December 2024, publication LSID: urn:lsid:zoobank.org:pub:11184506-F273-4D7A-BC28-A1BB4BC8FCD8, asselinae LSID: D282DA9D-6A9C-4B9E-AA6C-9ADC4473E95B, glooscapi LSID: 5801DC12-4EA1-439F-B7C8-FAF7C1B9D2C7, wilsonae LSID: 549EEF2F-7F19-4E6D-9F8E-384594B2FE65). Our new chitinozoan data, combined with previous studies, allow a comparison with the well-studied Baltic succession, confirming that limited unconformities mark the Aeronian–Telychian boundary interval on Anticosti Island, in contrast to the less complete coeval location. Our refined age model for the Aeronian to Telychian succession of Anticosti Island provides a solid baseline to study further Llandovery biochemical events in the aftermath of the Late Ordovician mass extinction.

Cross-basin chronostratigraphic correlation of carbonate succession (Llandovery, Michigan Basin, USA) using global carbon δ13Ccarb isotope excursions

Correlating shallow shelf carbonates and their deep basin equivalents is a perennial challenge in the geosciences, with wide-ranging implications. This hurdle is well illustrated in the Llandovery succession of the Michigan Basin, USA, a 40- to 265-m-thick carbonate interval represented by three lithostratigraphic units: the Cataract, the Burnt Bluff, and the Manistique groups. Although extensively studied at various localities within the basin and across the region, the chronostratigraphic relationships between these units remain unknown. The current study presents a cross-basin chronostratigraphic framework for the Llandovery succession utilizing globally documented carbon (δ13Ccarb) isotope excursions (CIEs). From 10 drill cores and three quarry sites throughout the Michigan Basin, five CIEs were identified and chronostratigraphically constrained using conodont biostratigraphy and conodont 87Sr/86Sr data. The five excursions are interpreted to be the global CIEs: the (1) Hirnantian Isotope Carbon Excursion (HICE; Hirnantian Stage), (2) Early Aeronian, (3) Late Aeronian (Aeronian Stage), (4) Valgu (Telychian Stage), and (5) Ireviken (Sheinwoodian Stage). Most importantly, the HICE and the Ireviken CIEs bracket the Llandovery strata preserved in the basin. The new high-resolution δ13Ccarb data suggest that CIEs can be effectively used to correlate among shallow marine shelf carbonates and their deeper water equivalents. The new chronostratigraphic framework shows that CIE-based time horizons across the Michigan Basin cut across lithostratigraphic unit boundaries, which indicates that these lithostratigraphic units are diachronous in the Michigan Basin. In addition to refining the stratigraphy of the Llandovery succession of the Michigan Basin, particularly the timing of various key sedimentary deposits, the new chronostratigraphic framework can be used to: (1) constrain the timing of various regional tectonic phenomena, (2) identify multiple tectonically driven siliciclastic sediment pulses in the basin, and (3) predict various stratal relationships that may result in previously unknown stratigraphic traps and, therefore, new hydrocarbon plays within the basin. The results of the current study also show that δ13Ccarb trends across the shelf-to-basin transect are spatially and temporally variable and do not match those reported in Modern carbonate settings, which possibly suggests that such δ13Ccarb trends, to some extent, reflect variations in water circulation and water mass heterogeneity during deposition.

GRAPTOLITES FROM THE UPPER ORDOVICIAN SECTION «VERKHNYAYA KARASU» IN THE CENTRAL PART OF GORNY ALTAI

Taxonomic complex of graptolites from the «Verkhnyaya Karasu» section have been studied. This section is located in the central part of Gorny Altai Mountains. The section covers a large stratigraphic interval: from upper Ordovician (Sandbian Stage) to Middle Silurian (Telychian Stage). For the Ordovician, the Bugryshikha, Khankhara and Tekhten' formations are distinguished. For the Silurian, the Vtorye Utyosy, Syrovaty, Polaty, Chesnokovka and Chesnokovka formations are distinguished. Previously, in this section no graptolites were found in the upper part of the Bugryshikha formation. There is a large and diverse complex of taxa collected in 2020. The graptolites assemblage from the upper part of the Bugryshikha formation consists of Reteograptus uniformis Mu et Zhang, Reteograptus geinitzianus Hall, Eoglyptograptus euglyphus Mitchell, Orthograptus apiculatus Elles et Wood, Orthograptus whitfieldi (Hall), Rectograptus truncatus (Lapworth), Rectograptus ex.gr. truncatus (Lapworth), Hustedograptus sp., Callograptus sp.

Age of the Silurian Lower Red Beds in South China: Stratigraphical Evidence from the Sanbaiti Section

The age of the Silurian Lower Red Beds in the Upper Yangtze region remains debatable. Twenty-four samples were collected for conodont biostratigraphical studies from the Paiyunan Formation in the Sanbaiti Section, Huaying, Sichuan Province. The conodont fauna from the Paiyunan Formation, together with the graptolites from the underlying Lungmachi Formation, indicates that the Lower Red Beds at Sanbaiti correspond to the lower Telychian. Comparative analysis indicates that most exposures of the Lower Red Beds in the Upper Yangtze region can be assigned, in general, to the Telychian Stage, except for several localities, where the Lower Red Beds can be roughly dated as an interval between the upper Aeronian and lower Telychian.

Geomagnetic polarity during the early Silurian: The first magnetostratigraphy of the Llandovery

Magnetostratigraphic studies in the Silurian are absent, and what is understood about the geomagnetic polarity during this time is based on polarity bias-type data from palaeopole-type studies. We provide the first composite magnetic polarity record through the Lower Silurian (Llandovery) from the magnetostratigraphy of six sections. These are integrated with graptolite biostratigraphy and some carbon isotope chemostratigraphy. The palaeomagnetic signal is carried by both haematite and magnetite, with haematite dominating in red-coloured mudstones and mostly magnetite in non-red lithologies. The influence of possible tectonic disruption of the fabric is assessed using anisotropy of magnetic susceptibility. Only the most thermally mature section at Backside Beck shows the imprint of initial tectonic fabric formation. The Llandovery is divided into 6 major normal-reverse-polarity chron couplets (referred to as LL1 to LL6). An additional longer, exclusively normal polarity interval (referred to as WE1n), beginning in Telychian Stage slice Te3, runs into the lower Sheinwoodian. Within these five polarity couplets there are 10 further submagnetozones, and 10 tentative submagnetozones. Average reversal frequency (including the tentative submagnetozones) was ca. 3.0 Myr−1 in the Early Silurian, which is probably an underestimate, due to insufficient sampling density in some parts of the Rhuddanian and Aeronian. This reversal frequency is similar to that in the late Cenozoic, indicating the future potential utility of magnetostratigraphy for high-resolution correlation and dating in the Early Silurian.

Graptolites from Silurian (Llandovery Series) Sedimentary Deposits Attributed to a Forearc Setting, Co to Formation, Co to Archipelago, Northeast Vietnam

Newly collected graptolites from the Co To Formation, Co To archipelago, NE Vietnam, comprise assemblages indicative of two biostratigraphical levels within the lower Silurian, Llandovery Series, Telychian Stage: the co-occurrence of Spirograptus turriculatus and Torquigraptus proteus? suggests an interval most likely within the upper part of the Spirograptus turriculatus Biozone or ‘Monograptus’ crispus Biozone, whilst Oktavites spiralis and Monoclimacis cf. subgeinitzi identify the Oktavites spiralis Biozone. The graptolites provide important biostratigraphical evidence for the age of the upper part of the lower Co To Formation, biostratigraphical ties between the NE Vietnamese succession of the Bac Bo Region and graptolite assemblages of the Long Dai Formation in the Viet-Lao Region of central Vietnam, and include the new species Monograptus hamulus sp. nov. co-occurring with S. turriculatus, which is perhaps an ancestral form to the later Telychian species Monograptus drepanoformis. We also report the first chitinozoans, including Belonechitina, from the Co To Formation.

The Silurian red beds of Tarim Basin: Signals of palaeoenvironment, palaeoclimate, and sea‐level change

The Silurian red beds of the Tarim Basin were formed in a coastal tidal and fluvial delta environment. They consist of red mudstone layers of the lower Tataaiertag Formation and separate the grey‐green muddy sandstones of the Kalpintag Formation from the red sandstones of the upper Tataaiertag Formation. These mudstones are regarded as the most important regional seal for Silurian oil reservoirs. They contain a significant record of palaeoenvironmental and palaeoclimate changes which is related to global sea‐level changes. Based on a systematic and continuous sedimentary study and multiple geochemistry analyses of samples taken from the Sishichang outcrop profile in west of the Tarim Basin, the Early–Middle Silurian palaeoclimate and sea‐level curve have been reconstructed. The Early–Middle Silurian Tarim Basin encountered five intervals of climate change, from humid transitioning to semi‐humid and dry–hot climate which shows signs of generally increasing oxidation. The reconstructed sea‐level curve indicates five sea‐level cycles throughout the Early–Middle Silurian succession. The most significant sea‐level drop occurred during the early Telychian stage, which represents the onset of the red mudstone section of the Tataaiertag Formation. This new sea‐level curve shows some differences compared with the earlier proposed global sea‐level curves. The dry–hot climate occurred during the late Telychian stage and the beginning of Wenlock age. The Early–Middle Silurian sea‐level change of the Tarim Basin was mostly synchronical to the eustacy. However, regional tectonic activities during the early Telychian stage further enhanced the effect of the global sea‐level fall and lead to a dry–hot climate trend which also caused the creation of the red beds of the Silurian period.

Onset of the middle Telychian (Silurian) clastic marine red beds on the western Yangtze Platform, South China

Marine red beds (MRBs) occur in many locations in the world during the Telychian Stage but it is unclear how they were related with the highly dynamic Silurian Earth system. To address this question, we conducted a case study involving geochronology, palaeogeographic reconstruction, facies analysis and sea-level history of the middle Telychian MRBs on the western Yangtze Platform of South China. Their distribution pattern indicates that the clastic MRBs owe their origin to the nearby oldlands. Facies analysis indicates that rocks during the study interval generally fall into three depositional belts: shallow subtidal, deep subtidal and shaly basinal belts. A chronostratigraphic framework constrained by both biostratigraphic data and eustatic events shows that, during the early Monoclimacis crenulata interval, olive-gray strata dominated both shallower (shallow and deep subtidal belts) and deeper (shaly basinal belt) settings, while distribution of the MRBs was restricted to deeper waters (shaly basinal belt) on the open-sea side of the platform. This situation changed after the middle M. crenulata interval when the MRB deposition was dominant in both shallower and deeper settings. The diachronous onset pattern of the MRBs coincided with prominent transitions in climatic conditions (from cooler to warmer), eustatic states (from regression to transgression), benthic faunas (decline of normal marine fauna and development of euryhaline assemblages) and reduced primary productivity in the shelf settings, most likely indicative of a change between different oceanic modes. Comparative studies suggest that, the widespread Telychian MRBs in both shallow and deeper water settings owe their origin to a very low organic carbon burial rate, which could be results of higher oxygen content in the ocean water and lower primary productivity.

Orbital control on cyclical primary productivity and benthic anoxia: Astronomical tuning of the Telychian Stage (Early Silurian)

The identification of Milankovitch cycles and the establishment of a high-resolution astronomical time scale for the Palaeozoic represent a research frontier in cyclostratigraphy and astrochronology. A continuous core drilled through the Telychian Stage (Llandovery, Silurian) of the Pasłek Formation in the Baltic Basin (Poland) is characterized by repetitive centimetre-scale alternations of homogeneous greenish-grey mudstones and faintly laminated dark-grey-to-black mudstones. Such lithological rhythmicity reflects a cyclical variation in redox conditions at the sediment-water interface, and therefore was used to construct a floating astronomical time scale (ATS) for the Telychian Stage. The cyclostratigraphic analysis was performed on a 5mm resolution greyscale scan based upon photographs of the slabbed core, with lower values associated with greenish-grey mudstones and higher values related to dark-grey-to-black mudstone facies. Spectral density estimation by means of the multitaper method (MTM) reveals significant peaks rising above the 95% red noise confidence level that we interpret as the 405-kyr long-eccentricity, short-eccentricity, obliquity and precession components. The MTM evolutive power spectral analysis (EPSA) shows a gradual increase in the velocity of sedimentation from 5.03m/Myr in the lowermost interval to 5.48m/Myr towards the uppermost part of the studied sequence. We postulate that the observed cycles reflect orbitally-driven climatic variations from stable wet conditions to monsoon-like high seasonal contrasts that affected weathering intensity, runoff and nutrient supply. These cyclical variations led to rhythmic variations in organic matter fluxes and benthic anoxic conditions. In the analysed record, orbital precession influenced the deposition of the greenish-grey mudstone and dark-grey-to-black mudstone couplets, occurring on average every 10cm. Furthermore, long-eccentricity modulation of precession controlled the relative depositional predominance of one facies over the other. Observed periodicities resemble those of Cenozoic and Mesozoic orbitally controlled records, thus suggesting that during the Silurian the orbital eccentricity forcing on the carbon flux acted in the same way as in the Cenozoic and Mesozoic. The orbital tuning of the studied interval, calibrated by means of accurate acritarch biostratigraphy, allowed for the estimation of an astronomically calibrated duration for the Telychian Stage of about 5.46Myr.

Carbon, oxygen, and sulfur isotope compositions and model of the Silurian rock formation in northwestern Belarus

The paper describes the results of study of the Silurian clayey–carbonate rocks ranging from the Telychian Stage (Llandovery) to the Gorstian Stage (Ludlow) recovered by the Borehole Davtyuny 3k in northwestern Belarus. Rocks of the Sheinwoodian Stage demonstrate a positive excursion of δ13C with amplitude of 4.7‰, marking the Ireviken biotic event recorded in the global chemostratigraphic curve. Values of δ18O for the carbonate material in the studied section (25.5–29.2‰ SMOW) are close to those for Silurian rocks from the Baltic region, Scandinavia, Ukraine, Poland, and Canada. The whole section contains postsedimentary gypsum as nodules and the infilling of fissures and fenestrae. Values of δ34S in gypsum (21.3–26.7‰ CDT) are close to those for the Silurian rocks on the Phanerozoic isotope plot. The formation of gypsum was related to a partial development of the supralittoral environment over the sublittoral and littoral clayey–carbonate substrate. The seawater accumulated in lowlands of the supralittoral plain after storms was intensely concentrated during arid conditions and accumulated in the clayey–carbonate sediment. The subsequent underground evaporation promoted the formation of gypsum as nodules in the unlithified sediments and the infilling of fissures and fenestrae in the lithified rocks.

Upper Llandovery (Telychian) graptolites of the Oktavites spiralis Biozone from the Long Dai Formation, at Lam Thuy village, Quang Binh Province, central Vietnam

A numerically small graptolite assemblage from the Long Dai Formation at Lam Thuy village, southwestern Le Thuy district, Quang Binh Province, central Vietnam, includes Oktavites spiralis (Geinitz). The relative abundance of this species in the assemblage, coupled to its occurrence with Oktavites bodentoerlensis Loydell, identifies the lower Silurian (Llandovery Series, Telychian Stage) spiralis graptolite Biozone. Identification of the new locality at Lam Thuy enhances the dataset of Silurian graptolite-bearing localities in central Vietnam and advertises the wide utility of graptolite assemblages for biostratigraphical correlation of lower Palaeozoic strata in northern and central Vietnam.

First appearance datum of the Silurian graptolite Oktavites spiralis, and its evolution on the northern margin of South China

Wang, X., Wang, J. & Zhang, J., May 2016. First appearance datum of the Silurian graptolite Oktavites spiralis, and its evolution on the northern margin of South China. Alcheringa 41, xxx–xxx. ISSN 0311-5518.Xin Wang [wx200315046@163.com], Early Life Institute and State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xian 710069, PR China & Research Center for Orogenic Geology, Xian Geological Survey Center, China Geological Survey, Xian 710054, PR China; Jian Wang [332891824@qq.com], Ju Zhang [710726657@qq.com], Research Center for Orogenic Geology, Xian Geological Survey Center, China Geological Survey, Xian 710054, PR China.Graptolites represent the standard tools for biozonation and correlation of Ordovician and Silurian strata. Oktavites spiralis, one of the index graptolite species of the Telychian Age (late Llandovery, Silurian), is of great significance in biostratigraphy. However, the broad definition of the species and the lack of data on its evolutionary changes have led to controversies regarding its stratigraphic range and, consequently, to difficulties in regional correlations. Recent investigations in multiple Telychian profiles in the Ziyang–Langao area, along the northern margin of South China, reveal that the first appearance datum of Oktavites spiralis is located at the base of the Spirograptus turriculatus Zone, and its last appearance datum lies at the top of the Cyrtograptus lapworthi Zone (Telychian stage). The thecae vary markedly up-section and such change is consistent across all studied sections. This morphological change can be used as the basis for accurate stratigraphic subdivision and, hence, for effective regional correlation. In addition, this species has a wide range in population size and population density relative to other coexisting graptolite species, and can be better distinguished by using a more precise set of criteria.

Early Silurian paleogeography and source-reservoir-cap rocks of the Middle-Upper Yangtze region in South China

The lithofacies paleogeography for the three successive ages (Rhuddanian, Aeronian and Telychian) in the Middle-Upper Yangtze region of South China in the Early Silurian (Llandoverian Epoch) is reconstructed according to the concept that structures control a basin, a basin controls facies, which, in turn, control the basic geology of oil and gas. The time-space distribution and sedimentology of source-reservoir-cap rocks are summarized. In the Rhuddanian age, the study area inherited and developed a semi-confinement stagnant basin resulting from tectonic compression started on Late Ordovician. The widely-covered organic-rich muddy shales occurred on deep-water shelves in a global-scale transgression after the Hirnantian glaciation and subsequent relatively slow regression. The sagged areas, formed mainly by tectonic tilting and plugging effect of rigid continental nucleus, are sedimentary centers of favorable source rocks. At the intermittent stage of tectonic compression during the Aeronian age, a mix-sedimented “carbonate ramp” formed by global sea-level drop, warm-humid climate and slight clastic sources. The reef-bank type carbonate reservoir rocks within Shiniulan Formation occurred in subtidal high-energy environments within inner ramp. In the subsequent Telychian stage, tectonic compression intensified, resulting in regression and overall uplifting of the basin, terminated the development of carbonate depositional environments. The eastern region was then dominated by shoreface and tide-dominated deltas where sand barrier type reservoir rocks in the Xiaoheba Formation occurred. The widespread muddy sediments of the Hanjiadian Formation could be cap rocks for the underlying potential oil-bearing layers.

EMPLACEMENT CONDITIONS OF A PORPHYRITIC FELSITE DYKE AND TIMING OF MOTION ALONG THE COOLIN FAULT AT BEN LEVY, CO. GALWAY

A NNE-trending porphyritic felsite dyke located on Ben Levy in Co. Galway cross-cuts the E-W trending Coolin Fault. The porphyritic felsite is an altered rhyolite containing ocellar feldspar with rare apatite inclusions, quartz phenocrysts as well as minor ilmenite, rutile and zircon, all within a ground-mass of feldspar and quartz. High whole-rock K/Na and Rb/Sr ratios and textural evidence from cathodoluminescence (CL) imaging indicate that the dyke underwent hydrothermal alteration; however, immobile Zr vs Ti02 for the Ben Levy Dyke is consistent with the trend of values for Galway Batholith dykes. Magmatic crystallization temperatures were calculated as 1067 + 14°C using Ti-in-zircon thermo metry. Quartz phenocrysts may have been affected by diffusive loss of Ti post-emplacement, and record an average temperature of < 574CC based on Ti-in-quartz thermometry. An age of 373.9±4.0Ma was determined from laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb isotopic analysis of zircon. The Ben Levy porphyritic felsite is similar in age and orientation to other late-stage dykes which cross-cut the Galway Batholith to the south, and may represent the northernmost extension of late-stage dyke activity. Motion along the Coolin Fault occurred during the Caledonian Orogeny, between deposition of the Lough Kilbride Formation (436 -428.2Ma) and intrusion of the Ben Levy Dyke at 373.9 ±4.0Ma. Introduction Coolin Fault. The Coolin Fault lies within a thr ust belt associated with the Caledonian Orogeny A felsic porphyritic dyke outcrops along the foot(Phillips 2001). Based upon field relationships in this wall scarp of a NNE-trending normal fault, location, motion along the Coolin Fault must have which transects Ben Levy near Ballard, Co. Galway occurred after deposition of the Kilbride Formation. (Fig. 1). The dyke has a strike of ~040°, dips steeply The Kilbride is constrained to the Telychian Stage to the W, and is about 2-3 metres wide in locations (436^428.2Ma) of the Llandovery Series (Holland where contacts can be clearly seen on both sides 1985; Cocks 1989) by the presence of the brachiopod of the porphyritic felsite (Locations A and C). The Eocoelia curtisi curtisi within the Annelid Grits porphyritic felsite extends through the Dalradian member in the lower Kilbride (Harper et al. 1995). Ben Levy Formation and into the Lower Silurian In this study, the timing of motion along the Lough Mask and Kilbride Formations on the north Coolin Fault is constrained using U-Pb age dat side of Ben Levy, cross-cutting the E-W-trending ing. Cathodoluminescence (CL) imaging, major and Irish Journal of Earth Sciences 29 (2011), 1-13 © 2011 Royal Irish Academy doi: 10.3318/IJES.2011.29.1 This content downloaded from 157.55.39.217 on Mon, 18 Apr 2016 08:03:01 UTC All use subject to http://about.jstor.org/terms 2 Irish Journal of Earth Sciences (2011 ) Ordovician-Silurian Sediments and Volcanics Carboniferous Oughterard Granite p Connemara Metamorphic Complex Galway Batholith

Strontium and carbon isotope stratigraphy of the Llandovery (Early Silurian): Implications for tectonics and weathering

A high-resolution 87Sr/ 86Sr curve and paired δ 13C carbonate-organic data set is generated for the Llandovery Series from the Ikla drill core in Estonia. A δ 13C carbonate curve is also presented from the Pancake Range in Nevada. Observed 87Sr/ 86Sr values in the Ikla drill core are at a minimum in the early Llandovery Rhuddanian Stage (∼ 0.7079 to 0.7080), and then trend to more radiogenic ratios in the basal part of the Telychian Stage. An 87Sr/ 86Sr high near ∼ 0.7084 is observed in the Telychian at the top of the studied section. The range of values is in general agreement with the data from previous sample sets of brachiopods and conodonts recovered from localities in North America and Europe that record a rising trend in the 87Sr/ 86Sr ratio throughout the Llandovery from approximately 0.7080 to 0.7084. The major increase in the 87Sr/ 86Sr ratio during the late Llandovery may be due to weathering of radiogenic source rocks that were uplifted during early Silurian continent–continent collisions. The Sr rise potentially coincides with the occurrence of an unusually thick sequence of K-bentonite beds representing large-magnitude ash falls in the early Telychian. A previously documented negative δ 13C excursion in marine carbonates in the lower Telychian interval of the Ikla core is quasi-synchronous with the increase in 87Sr/ 86Sr. Our new organic matter δ 13C data from the Ikla core confirm that this negative δ 13C carbonate excursion is not a result of diagenesis. Furthermore, a negative δ 13C excursion in carbonates from the early Telychian portion of the Pancake Range section in Nevada seems to confirm the global scope of this carbon cycle perturbation.

Implications of a new, biostratigraphically well-controlled, radio-isotopic age for the lower Telychian Stage of the Llandovery Series (Lower Silurian, Sweden)

Radio-isotopic analysis of single zircons from two early Telychian K-bentonites, one of which is among the most widespread Lower Paleozoic volcanic ash falls in northern Europe, yields overlapping weighted mean 206 Pb/ 238 U ages of 438.7 ± 1.0 Ma and 437.8 ± 0.5 Ma, respectively. The former age is from zircons of the Osmundsberg K-bentonite from the type locality at Osmundsberget in the Siljan area of central Sweden where it occurs in the lower part of the Spirograptus turriculatus Graptolite Zone and in the lower part of the Angochitina longicollis Chitinozoan Zone. Zircons giving the latter age are from a bed previously identified as the Osmundsberg K-bentonite at the Kallholn Quarry in the same area. Based on new biostratigraphic data, the latter bed is now considered to be slightly younger than the Osmundsberg K-bentonite. The dated stratigraphic level of the ash layers is slightly younger than the base of the Telychian Stage and thus represents a minimum age for the Aeronian/Telychian Stage boundary. A U-Pb age of > 438 Ma for the base of this stage, however, is older and in conflict with estimates in the most recent compilation of the Silurian time scale. In view of the fact that only three radio-isotopic dates from the entire Llandovery have been previously accepted, this new and biostratigraphically exceptionally well-controlled radio-isotopic date fills an important gap in the Lower Silurian geochronology.

Bohemian-type Silurian (Telychian) bivalves from Arctic Russia

The Bivalvia genera Ctenodonta, Dualina? and Sibirinka have been identified from the Silurian Telychian Stage, Llandovery Series of Severnaya Zemlya, Arctic Russia. This is the second record of the earliest epibyssate pteriomorphs previously reported from Siberia; they flourished later in the basins of central and southern Europe. Their distribution is consistent with surface ocean currents carrying abundant cephalopods, whose accumulation on the bottom surface together with ocean current activity represented a favourable environment for epibyssate bivalves.

Geochemistry and potential correlation of Silurian (Telychian) metabentonites from Ireland and SW Scotland

AbstractNew occurrences of metabentonites from the Telychian Stage of the Silurian successions in SW Scotland, Co. Down and Co. Mayo, Ireland, are described in terms of their biostratigraphical setting and geochemical characteristics. The Co. Mayo occurrence extends the known distribution of Telychian metabentonites in Europe by 280 km WSW from the coast of Co. Down.Comparisons between samples based on selected trace element ratios suggest three potential correlations. One is between a tuffaceous clay from the Finny School Member, Upper Kilbride Formation of Co. Mayo, and a metabentonite from the Tara Sandstone Formation, Gala Group, Co. Down. One pair of metabentonites from the Carghidown Formation of Kirkcudbrightshire can be related to one pair from the Ardglass Formation of Co. Down. In addition, a unique alkaline chemical composition is identified in two samples from the Kirkmaiden Formation of Kirkcudbrightshire and one sample from the Ballyquintin Formation of Northern Ireland, which implies the existence of a local extensional tectonic regime at that time, consistent with a back‐arc basin.These metabentonites extend across at least two Caledonian suspect terranes and a variety of different sedimentary facies. These data may assist future discoveries and be able to improve stratigraphic correlations across three terranes. Copyright © 2003 John Wiley &amp; Sons, Ltd.

New graptolite faunas from the Llandovery, lower Silurian of the Graveyard Creek Subprovince, Broken River region, Queensland, Australia

A continuous biostratigraphic sequence has been established, possibly from as low as the sedgwickii Biozone but certainly throughout the Telychian Stage (? sedgwickii, turriculatus, ?crispus, griestoniensis and crenulata biozones). Thirty-three taxa have been recorded, including the new genus Wandograptus wandovalensis sp. nov., the new species Pseudostreptograptus queenslandensis and the new subspecies Dictyonema paululum australis and Stimulograptus sedwickii loydelli . A number of species have been recorded from Australia for the first time: Dictyonema cf. geniculatum Bulman, Petalolithus kurcki (Rickards), Pristiograptus initialis (Kirste), Monoclimacis directa Zalasciewicz, Monograptus aff. distans (Portlock), Monograptus sartorius Tornquist, and Torquigraptus pragensis (Boucek). Nine forms have been left under open nomenclature. The type section for the Poley Cow Formation yields a griestoniensis biozone fauna close to the top, and this level can be correlated with a griestoniensis Biozone fauna in the Broken River crossing section, again just below the top of the formation. Further north, griestoniensis Biozone faunas have also been recognized, beyond Diggers Creek, and in the Quinton Formation at Top Hut, enabling a reliable time plane throughout the region. At Top Hut, the highest graptolitic strata yield a crenulata Biozone fauna; and stratigraphically lower sequences, referable to the ?sedgwickii, turriculatus and ? crispus biozones, have been established on the Broken River section. It is likely that a full Telychian (Upper Llandovery) sequence occurs in the Poley Cow and Quinton formations enabling accurate correlation with other parts of the world.

A chitinozoan morphological lineage and its importance in Lower Silurian stratigraphy

Margachitina margaritana is a distinctive, chain‐forming chitinozoan taxon of the uppermost Telychian Stage (Llandovery Series) to lower Homerian Stage (Wenlock Series). M. margaritana is shown to be the end member of a morphological lineage that developed from Calpichitina densa via the intermediate Margachitina banwyensis sp. nov. A local M. banwyensis Biozone, which correlates with the upper lapworthi graptolite Biozone, is proposed for the Banwy River section. The morphological lineage described provides a tool for the enhanced subdivision of the uppermost Llandovery Series, which may prove global in extent. This research suggests also that the base of the Wenlock Series may correlate with a level below the centrifugus graptolite Biozone and it also brings into question the systematic status of Calpichitina and Margachitina, although both genera are retained herein.