Middle Katian Research Articles

Литогеодинамическая эволюция внешней окраины североуральского палеошельфа Печорской плиты в позднем ордовике (бассейн р. Илыч, Северный Урал)

Upper Ordovician carbonate deposits in the Northern Urals are the most common in the Ilych River basin. At the same time, the sections drilled here, which are the objects of this study, have a fragmentary character and tectonic boundaries, which led to significantly different ideas about the age of some strata and, as a result, to the ambiguity of their sedimentation conditions. The lithological-facies, paleoecological and paleontological studies revealed that the section of the Upper Ordovician carbonate deposits in the Northern Urals was represented by two formations, the Bolshaya Kosya reef and four thickness covering three tiers of the International Stratigraphic Scale of the Upper Ordovician — Sandbian, Katian and Hirnantian. The analysis of the spatiotemporal relationships of 36 lithotypes isolated in these sediments showed that they were grouped into 10 paragenetic associations of rocks reflecting the change of sedimentation conditions from a deep-water ramp with the accumulation of terrigenous calcareous silts (Sandbian) to a carbonate shelf with a clearly defined brow and the formation of a reef shoal on it (Middle Katian) and further to a shallow ramp with the widespread development of erosion processes and the formation of sandbanks (Late Katian-Hirnantian).

Morphological variation in first-formed shells of the Ordovician Paucicrura–Diceromyonia brachiopod lineage of North America

Abstract First-formed shells of several species of Dalmanellidae (Brachiopoda) from the Ordovician (Katian) of North America were measured and compared: Cincinnetina multisecta, Diceromyonia tersa, Diceromyonia storeya, Paucicrura corpulenta, Paucicrura rogata, and Paucicrura sillimani. Sizes and structures of the first-formed shells suggest that members of this family had planktotrophic subadults, with some species showing indications of only an unshelled larval stage and others showing both a larval stage and a shelled juvenile stage. This differs from modern rhynchonelliformean brachiopods, which all possess only a lecithotrophic larval stage. The range of sizes of first-formed shells of most studied species are similar, but P. sillimani of Baffin Island, Canada (middle Katian), has a significantly larger first-formed shell that formed during an extended juvenile stage. This may have enabled the species to colonize newly exploitable habitats during an interval of rapid sea level rise in Laurentia during the Katian. This plasticity of developmental modes in the Dalmanellidae shows not only that using distantly related modern brachiopods as an analog for extinct Paleozoic lineages may be misleading, but also that development can vary within a single lineage and that timing of developmental stages should not be considered a reliable character for use in phylogenetic studies of brachiopods.

Ordovician plate tectonic and palaeogeographical maps

Abstract Plate tectonic and palaeogeographical maps are presented for nine time intervals during the Ordovician: early Tremadocian (485 Ma), late Tremadocian (480 Ma), early Floian (475 Ma), earliest Dapingian (470 Ma), early Darriwilian (465 Ma), late Darriwilian (460 Ma), middle Sandbian (455 Ma), middle Katian (450 Ma) and early Hirnantian (445 Ma). A plate model was used to predict the changing age of the ocean floor and, hence, the changing volume of the ocean basins and the resulting degree of continental flooding. The maps show the locations of active plate boundaries (i.e. subduction zones, mid-ocean rifts, transform faults, island arcs, back-arc basins and collision zones) and the age of the ocean floor. During the Ordovician there were six continental plates (Gondwana, Laurentia, Baltica, Cathaysia, Avalonia and Cuyania) and six oceanic plates (Iapetus, Ran, Kipchak, and Panthallasa 1, 2 and 3), as well as several poorly constrained back-arc basins (Zealandia, Alexander and Farwell). The palaeogeographical maps also illustrate the ancient distribution of mountains, land, shallow seas and deep ocean basins. Sea level was high during the Early Ordovician, fell during the middle Ordovician and rose again during the Late Ordovician. A precipitous fall in sea level occurred at the end of the Ordovician due to the short-lived Hirnantian Ice Age.

Stromatoporoids of the Katian (Upper Ordovician) Beiguoshan Formation, North China

North China was a key centre of Middle Ordovician stromatoporoid diversification. However, detailed studies of the subsequent Late Ordovician stromatoporoid development in this terrane are lacking; thus, the spatiotemporal development of North Chinese stromatoporoids is poorly understood. The Beiguoshan Formation (middle Katian age, Late Ordovician) contains the youngest Ordovician stromatoporoids in North China, comprising a unique stromatoporoid fauna, consisting of 12 species across 10 genera and three orders of traditional taxonomic groupings: labechiids Rosenella woyuense Ozaki, 1938, Pseudostylodictyon poshanense Ozaki, 1938, Pseudostylodictyon chunhuaensis (Jiang et al., 2011), Labechia sp., Labechiella gondwanense Jeon in Jeon, Liang, Kershaw, Park, and Zhang, 2022, Labechiella regularis (Yabe & Sugiyama, 1930a); clathrodictyids Clathrodictyon sp. cf. Cl. mammillatum (Schmidt, 1858), Camptodictyon amzassensis (Khalfina, 1960), Ecclimadictyon tiewadianensis (Jiang et al., 2011), Plexodictyon xibeiense Jeon sp. nov., Petriterastroma exililamellatum Jeon and Kershaw gen. et sp. nov.; and the stromatoporellid Simplexodictyon conspicus Jeon and Kershaw sp. nov. This assemblage is distinguished by: (1) succeeding Darriwilian species of North China, (2) few peri-Gondwanan species, and (3) stromatoporoid genera common in Siluro–Devonian assemblages. Plexodictyon and Simplexodictyon species (commonly found in middle Palaeozoic rocks) are here recorded in the Ordovician for the first time. Their stratigraphical occurrence in the middle Katian reveals an interval of absences before their faunal development in the middle Silurian, a stratigraphical gap caused by the Huaiyuan Epeirogeny, which largely shaped and controlled the regional stratigraphy of North China. This unique assemblage of Katian stromatoporoids is not known from other contemporary terranes, supporting the interpretation that North China was separated from the north-eastern peri-Gondwanan regions, thus constituting an independent palaeobiogeographical unit during the Late Ordovician. Early occurrence of these common Siluro–Devonian-type stromatoporoids indicates a prologue of the Ordovician–Silurian stromatoporoid faunal transition in North China, characterized by a decrease in labechiids and rapid diversification of clathrodictyids and related stromatoporoids of typical Silurian types. http://zoobank.org/urn:lsid:zoobank.org:pub:E8E8C006-D15D-490C-86B8-469E6F02A5A1

Small cornulitids from the Upper Ordovician (Katian) of Estonia

Two new cornulitid species, Conchicolites sutlemaensis n. sp. and C. parcecostatis n. sp., are here described from the Kõrgessaare Formation (middle Katian) of Estonia. The fauna of the Kõrgesaare Formation is especially rich in small cornulitids including Conchicolites sutlemaensis n. sp., C. parcecostatis n. sp., C. kroegeri, C. rossicus, Cornulites cf. shallochensis and C. sp. A. The muddy bottom cornulitid faunas appear to have been dominated by minute species of Conchicolites in the Late Ordovician. Among Conchicolites species, C. rossicus is most abundant in the Kõrgessaare Formation. The species of Conchicolites that are rare on hard substrates in limestones were likely adapted to life in soft bottom clay and muddy environments.

Reef Complexes of the Late Ordovician–Early Silurian Altai Basin: Classification, Structure, Paleobiota, and Paleogeography

Abstract —The paper presents data on the classification, structure, and biota of reef complexes from the Late Ordovician–early Silurian Altai Basin. The early Paleozoic history of the Altai Basin included the early–middle Katian, late Katian, early Hirnantian, late Telychian, and middle–late Sheinwoodian events of large-scale reef formation which produced framework reefs, mud mounds, and coral meadows. The early Paleozoic reefs of the area formed at rates of 40 to ~12 cm per thousand years. The late Telychian reef system, which formed in 0.51–1.0 Myr, occupied the largest area of the basin. The lateral extent of reef systems in the Late Ordovician–early Silurian basin of Altai depended more on the rates of vertical and lateral growth, as well as on the previous sea bottom topography and clastic inputs, than on the duration of reef formation. Carbonate deposition in the Altai basins of that time can be simulated with a general model implying formation of distal barrier reefs.

Oxygen Isotopes from Apatite of Middle and Late Ordovician Conodonts in Peri-Baltica (The Holy Cross Mountains, Poland) and Their Climatic Implications

This report provides oxygen isotopes from apatite of late Middle and Late Ordovician conodonts from the southern Holy Cross Mountains in south-eastern Poland. It was a unique time interval characterised by a significant change in the Ordovician climate, tectonic, and ocean chemistry. In the Middle and early Late Ordovician, the Holy Cross Mountains were located in the mid-latitude climatic zone at the southwestern periphery of Baltica; therefore, the δ18Oapatite values from this region provide new data on the 18O/16O budget in the Ordovician seawater reconstructed mainly from the tropical and subtropical realms. Oxygen isotopes from mixed conodont samples were measured using the SHRIMP IIe/MC ion microprobe in the Polish Geological Institute in Warsaw. The δ18Oapatite values range from 16.75‰VSMOW to 20.66‰VSMOW with an average of 18.48‰VSMOW. The oxygen isotopes from bioapatite of the studied section display an increasing trend, suggesting a progressive decrease in sea-surface temperature roughly consistent with an overall cooling of the Ordovician climate. Two distinctive positive excursions of δ18Oapatite have been reported in the upper Sandbian and middle Katian of the studied section and correlated with cooling events recognised in Baltica. They are interpreted as an isotope temperature proxy of climate changes triggered by a growing continental polar ice cap, but increased δ18Oapatite in the late Sandbian contradicts recently postulated climate warming during that time in subtropical Laurentia.

Different controls on the Hg spikes linked the two pulses of the Late Ordovician mass extinction in South China

The Late Ordovician mass extinction (LOME, ca. 445 Ma; Hirnantian stage) is the second most severe biological crisis of the entire Phanerozoic. The LOME has been subdivided into two pulses (intervals), at the beginning and the ending of the Hirnantian glaciation, the LOMEI-1 and LOMEI-2, respectively. Although most studies suggest a rapid cooling and/or oceanic euxinia as major causes for this mass extinction, the driver of these environmental changes is still debated. As other Phanerozoic’s mass extinctions, extensive volcanism may have been the potential trigger of the Hirnantian glaciation. Indirect evidence of intense volcanism comes from Hg geochemistry: peaks of Hg concentrations have been found before and during the LOME, and have all been attributed to global volcanism in origin. Here, we present high-resolution mercury (Hg) profiles in three study sections, from a shelf to slope transect, on the Yangtze Shelf Sea (South China) to address the origin of Hg anomalies across the Ordovician–Silurian (O–S) boundary. The results show Hg anomaly enrichments in the middle Katian, late Katian, the LOMEI-1 at the beginning of the Hirnantian glaciation, the LOMEI-2 in the late Hirnantian glaciation, and late Rhuddanian. The Hg anomaly enrichments during the middle–late Katian and late Rhuddanian would probably reflect a volcanic origin. We find two different controls on the recorded Hg anomalies during the extinction time: i.e., primarily volcanism for the Hg anomaly at the LOMEI-1 and euxinia for the Hg anomaly at the LOMEI-2. Expansion of euxinia at the LOMEI-1 would have been probably enhanced by volcanic fertilization via weathering of volcanic deposits during the Middle and late Katian, and combined with euxinia at the LOMEI-2 to finally be responsible for the two pulses of the LOME.

Katian (Late Ordovician) conodonts on the northwestern margin of the North China Craton

AbstractUpper Ordovician strata exposed from the Baiyanhuashan section is the most representative Late Ordovician unit in the northwestern margin of the North China Craton (NCC). In total, 1,215 conodont specimens were obtained from 24 samples through the Wulanhudong and Baiyanhuashan formations at the Baiyanhuashan section. Thirty-six species belonging to 17 genera, includingTasmanognathus coronatusnew species, are present. Based on this material, three conodont biozones—theBelodina confluensBiozone, theYaoxianognathus neimengguensisBiozone, and theYaoxianognathus yaoxianensisBiozone—have been documented, suggesting that the Baiyanhuashan conodont fauna has a stratigraphic range spanning the early to middle Katian. The Baiyanhuashan conodont fauna includes species both endemic to North China and widespread in tropical zones, allowing a reassessment of the previous correlations of the Katian conodont zonal successions proposed for North China with those established for shallow-water carbonate platforms at low latitudes.UUID:http://zoobank.org/7cedbd4a-4f7a-4be6-912f-a27fd041b586

Late Ordovician paleoceanographic change: Sedimentary and geochemical evidence from Northwest Tarim and Middle Yangtze region, China

Redox variation during the Late Ordovician were an important driver of palaeoceanographic and ecologic changes that may have eventually facilitated biotic extinction in the end of the Ordovician. Previously, the Late Ordovician paleoredox studies have focused mostly on the last 1–2 million years prior to the end of the Ordovician; redox conditions during the Sandbian–Katian time (early–middle Late Ordovician) are poorly understood. In order to fully evaluate the effects of palaeoceanographic variations on biotic evolutionary trends through the entire Late Ordovician, it is important to fully understand redox variations following the Great Ordovician Biodiversification Event (GOBE) and prior to the end-Ordovician mass extinction. The present study reconstructs paleoredox conditions from the Sandbian–Katian successions of the Dawangou section, northwestern Tarim Basin, northwest China and the Puxihe section of the Middle Yangtze region, South China using pyrite framboidal and redox-sensitive element analyses. The distribution of volcanic microspherules, fossil fragment content and elemental analysis were also applied to reveal possible volcanisms and chemical weathering conditions, and to evoluate their effects on palaeoxygenation and biotic variations. The new results indicate three distinct periods of fluctuating paleoredox conditions during the Late Ordovician in the northwest Tarim and the Middle Yangtze regions, China. Overall, redox conditions varied from mainly oxic but punctuated by short-lived anoxic–dysoxic episodes to rapid fluctuations between reducing and oxidizing conditions, to persistent euxinic conditions. The varition of oxygen minimum zone (OMZ) that developed in the midwater column might be the main cause of anoxic events during the Sandbian–early Katian. Moreover, abundant volcanic microspheres, enhanced chemical weathering, decreasing fossil fragments of benthos, and positive europium anomaly from the study sections collectively indicate that intermittent volcanism during the early–middle Katian may have triggered anoxic-euxinic events during this interval. If integrating the newly obtained redox data with previously published data for the Katian to Hirnantian from the Middle Yangtze area, the long-term, stepwisely accelerated oxyen-poor conditions during the Late Ordovician may have facilitated biotic extinctions in the end of the Ordovician.

The Upper Ordovician reef Bol’shaya Kos’yu, Ilych River, the Northern Urals (structure, paleobiocenosis, microfacies, model of formation)

Subject. The Upper Ordovician reef Bol’shaya Kos’yu, located on the western slope of the Northern Urals in the basin of the Ilych River is a unique object among coeval reefs due to the weak dolomitization of rocks and the ability to trace the phases of its development. Materials and methods. The article presents the results of paleontological, paleoecological and microfacial analyses of reef limestones selected by the author from 12 natural outcrops with a total capacity of about 150 m located along both banks of the Bol’shaya Kos’yu River. Results. The structure of the reef, morphology and taxonomic composition of the frame-forming biota were established, and the important role of sphinctozoal sponges was first identified. Among the reef limestones in the main genetic characteristics that reflect specific depositional environments, was identified three types of microfacies. Type 1 – reef microfacies – framestones and buflestones. Type 2 – microfacies of interbiohermic space and inter-carcass cavities exposed to wave and tidal action – bioclastic and peloid pakstones, biolithoclastic grainstones and lithoclastic rudstones. Type 3 – microfacies of the hidden microcosmic cavities, protected from the active wave impact – wackstone. Сonclusion. The composition of the microfacies and frame-forming biota characterize difficult differentiated, both on a section, and structure the reef, which was formed in the conditions of the carbonate platform-shelf margin with active hydrodynamics. Its growth occurred during transgression, which ended by the end of the middle katian a major regression that stopped its development. The revealed paleontological, paleoecological and microfacial data indicate that starting from the middle katian the first identified sphinctozoal sponges could settle in sublittoral environments with active hydrodynamics at depths up to 20 m.

Acritarch assemblages from the Middle–Upper Ordovician in the Aksu area, northwestern margin of the Tarim Basin, Xinjiang, China

Upper Darriwilian to middle Katian (Ordovician) outcrops are well exposed in the Kalpin (Aksu) area, in the northwestern margin of the Tarim Basin, Xinjiang, China. The Dawanxigou and Sishichang sections in the Kalpin area, near the Dawangou Auxiliary Stratotype Section and Point (ASSP), were palynologically investigated. Both sections provide well-preserved acritarch assemblages dominated by acanthomorph, sphaeromorph and netromorph morphotypes, attributed to 36 species and 20 genera. A comparison of the acritarch assemblages with coeval microfloras from other parts of the world confirms a wide cosmopolitism distribution, indicative of a breakdown of acritarch provincialism during the Late Ordovician. Several acritarchs from the Tarim Basin are restricted to the Late Ordovician, and as they are distributed worldwide, they present a potential for global bio-stratigraphical correlations. The acritarch assemblages from the different sections of the northwestern part of the Tarim Basin show slightly variable compositions, probably caused by different palaeo-ecological settings. The diversity and compositional changes of the acritarch assemblages, combined with the sedimentological evidence, suggest a transgressive sequence from the Sargan Formation to the Qilang Formation.

Ordovician successions in southern-central Xizang (Tibet), China—Refining the stratigraphy of the Himalayan and Lhasa terranes

The Ordovician stratigraphy of southern-central Xizang (Tibet) has been revised based on new conodont data recovered from 43 samples in four stratigraphic units and their integration with existing nautiloid and graptolite data. The Histiodella holodentata and Pygodus serra biozones have been identified respectively in the Alai and Jiaqu formations of the Chiatsun Group exposed near Alai village in Nyalam County within the Himalayan terrane, and the Yangtzeplacognathus foliaceus Subbiozone (lower part of the Pygodus serra Biozone) in the Sangqu Formation exposed at the Guyu section within Zayu County in the Lhasa terrane. Recognition of these biozones has increased the precision of correlation of the middle-upper Darriwilian strata in the region. Regional reassessment of the Ordovician stratigraphy permitted by new biostratigraphic data has allowed revised definitions for the Chiatsun and Keerduo groups and the Sangqu and Xainza formations. The Chiatsun Group is defined herein to include three lithologically distinctive formations in descending order, the Jiaqu, the Alai and the Adang formations. The stratigraphic age for the Jiaqu and Alai formations in the type area ranges from the middle Darriwilian (Histiodella holodentata Biozone) to middle Katian (Hamarodus brevirameus Biozone), but the age of the Adang Formation remains less certain.

Upper Katian (Upper Ordovician) trans‐Atlantic δ13C chemostratigraphy: the geochronological equivalence of the ELKHORN and PAROVEJA excursions and its implications

Since 2010 when the North American ELKHORN and Baltoscandic PAROVEJA isotope excursions were first described and named, their mutual age relations have remained uncertain, if not controversial. This was at least partly due to the incompleteness of the ELKHORN excursion in its reference section in western Ohio. The unexpected discovery of an apparently complete ELKHORN excursion in a drill core from St Marys in western Ohio has led to the conclusion that in terms of stratigraphical position and δ13C curve correspondence, the ELKHORN and PAROVEJA excursions are so similar that they apparently represent the same isotopic curve perturbation. The ELKHORN/PAROVEJA excursion occurs in the D. pacificus Graptolite Zone and uppermost A. ordovicicus Conodont Zone in the uppermost Katian Stage (Stage Slice Ka4 of Bergstrom et al. Lethaia 42, 97–197, 2009). Because the designation PAROVEJA was published two months before that of ELKHORN, it has priority as excursion designation. This excursion is particularly well represented in the carbonate successions in the Great Basin of western United States. Chemostratigraphy and biostratigraphy in that region show that the Richmondian transgression was contemporaneous with the beginning of the middle Katian WHITEWATER/MOE excursion. The onset of the Richmondian transgression has long been controversial but now available evidence suggests that it is of essentially the same age across large regions of the southern, western and central United States.

Ordovician carbonate mud mounds of the Baltoscandian Basin in time and space – A geophysical approach

Carbonate mud mounds developed within the Baltoscandian Basin, an epicratonic basin on the Baltica palaeocontinent, during the Ordovician. In the Upper Ordovician succession of the Baltoscandian Basin, a large number of mud mounds are present at three stratigraphic levels namely, the Kullsbergs mounds (late Sandbian-early Katian), Nabala and Rakvere mounds (middle Katian), and the Boda mounds (late Katian). These formed in a subtropical-tropical carbonate platform environment, covered by a shallow epicontinental sea. The mud mounds at these stratigraphic levels beneath and around Gotland have been characterized using a comprehensive seismic and well dataset acquired during a period of hydrocarbon exploration and exploitation which began more than 30 years ago. Interpretation of the largely unpublished seismic data in this study provides details on the distribution of mound complexes in the basin and constraints on the geometry of the mounds. Detailed structure contour maps of the top and the base of the Ordovician succession beneath Gotland based on the seismic interpretation are presented. The results give a comprehensive characterization of carbonate mud mound generation on Gotland which may help in understanding the distribution patterns of similar mound complexes in other parts of the Ordovician world formed in similar environments.

Geochemistry of Late Ordovician dalmanelloid brachiopods from Laurentia: testing the effects of paleolatitudinal gradient

Dalmanelloid brachiopod shells were collected from the Upper Ordovician Lexington Formation (lower Katian) of Kentucky, Sheguindah Shale (middle Katian) on Manitoulin Island, Ontario, and the Stony Mountain Formation (upper Katian) in the Winnipeg area, Manitoba. They were investigated to test the hypothesis of paleo-latitudinal zonation of the shelly benthos. A multi-technique approach was applied to evaluate the petrographic and geochemical (isotopic and elemental) preservation of the secondary layer of shells. Preliminary conventional microscopy, cathodoluminescence (CL), and scanning electron microscopy (SEM) confirmed the retention of primary shell ultrastructure (prismatic low-Mg calcite). The geochemical diagenesis proxies (e.g., Sr, Mn, Fe, and ΣREE) show insignificant correlations with the δ18O and δ13C values, thus supporting the preservation of at least near-primary geochemical compositions. Among the three lots of shells, the mean δ18O value is the highest in those from the Lexington Formation (–4.5‰ ± 0.3‰ VPDB), lowest from the Stony Mountain Formation (–6.8‰ ± 0.4‰ VPDB), and intermediate from the Sheguindah Shale (–6.0‰ ± 0.8‰ VPDB). The relative gradient in δ18O increase is in agreement with the paleo-latitudinal gradient, with Kentucky in subtropical, southern Ontario in mid-tropical, and southern Manitoba in subequatorial latitudes. The Lexington Formation shells also have the highest mean δ13C value (0.8‰ ± 0.2‰ VPDB) and relatively high P contents (170 ± 27 ppm), suggesting higher organic productivity, which is consistent with previous interpretation of frequent upwelling of nutrient-rich cool waters along the southeastern margin of Laurentia during the Katian. The Lexington shells also have a lower mean Th/U (0.6 ± 0.6), which is consistent with blooming organic productivity that likely led to more consumption of oxygen in the water column.

Characterization and discrimination of paleokarst breccias and pseudobreccias in carbonate rocks: Insight from Ordovician strata in the northern Tarim Basin, China

Ordovician carbonates in the central and southern Halahatang area of the Tarim Basin host highly productive hydrocarbon reservoirs. Previous studies suggest that the Yingshan and Yijianfang Formations (the primary producing intervals) have been extensively karstified. However, the current author has not observed significant karst features in the formations based on the detailed study of 380 m long cores from 16 wells. In the current study, true breccias are only identified in two wells in the Lianglitage Formation, including crackle and mosaic breccias as well as chaotic, clast-supported breccias. The breccias are interpreted as karst-related breccias that formed in near-surface environments during pre-Silurian subaerial exposure, and timing of karstification probably coincided with eustatic sea-level fall caused by the middle Katian glaciation. Widespread “karst features” of the Yijianfang and Yingshan Formations identified as breccias or sediment-filled cavities in many previous studies are actually only a mottled fabric (pseudobreccia) caused by selective diagenetic alteration and infiltration of bitumen in the deeper subsurface. The color difference between pseudoclasts and pseudomatrix does not reflect a mixture of clasts and cave sediments, but instead is characterized by distinct, nevertheless commonly subtle, variations in the limestone texture and whether bitumen is present. One the basis of core and thin section observations, SEM (scanning electron microscope) and XRD (X-ray diffraction) analyses, the author presents criteria to distinguish karst breccias and pseudobreccias that can be applied to other similar carbonate deposits. Discrimination between karst breccias and nodular limestones is also presented. Differentiation of karst and pseudokarst features serves not only an academic interest, but also has important applications for the exploration and development of carbonate reservoirs.

C, О, S, and Sr Isotope Geochemistry and Chemostratigraphy of Ordovician Sediments in the Moyero River Section, Northern Siberian Platform

The 87Sr/86Sr ratio in gypsum and limestones of the Ordovician section of the Moyero River decreases from the bottom upward from 0.7091‒0.7095 in the Irbukli Formation (Nyaian Regional Stage, ~Lower Ordovician Tremadocian Stage) to 0.7080 in the upper part of the Dzherom Formation (Dolborian Regional Stage, ~Upper Ordovician Katian Stage), which is well consistent with biostratigraphic subdivision of the section and existing concept concerning the strontium isotope evolution of the World Ocean. The most characteristic feature of the carbon isotope curve is decrease of δ13С values in carbonates from weakly positive values (0.5…1.1‰) in the Irbukli Formation (Nyaian Regional Stage) to sharply negative values (–5.4...–5.8‰) in the middle part of the Kochakan Formation (top of the Kimaian Regional Stage, ~end of the Dapingian–base of the Darriwilian Stage). Increase of δ18О from 20‒22‰ to 26‒28‰, the negative correlation of δ13С and δ18О, and decrease of δ34S in gypsum from 30‒32‰ to 22‒24‰ in this interval indicate that the 13С depletion of carbonates was not related to the sulfate reduction and oxidation of organic matter during diagenesis and that the negative δ13С excursion was of primary nature. The presence of negative δ13С anomalies at this stratigraphic level in Ordovician sections of the South and North America (Buggish et al., 2003; Edwards and Saltzman, 2014; McLaughlin et al., 2016) indicates the global or subglobal distribution of this event, which was possibly related to the emergence of the oldest ground vegetation. Against the general decrease of δ13С, the lower part of the section reveals three low-amplitude (1‒2‰) positive excursions, the position of which in general confirms the existing correlation scheme of the Moyero River section with the international scale. The upper part of the section is characterized by the alternation of low-δ13С intervals (from–2 to–3‰) and brief positive excursions with amplitude of 0.5‒1.3‰. The positive δ13С excursion terminating the Ordovician section of the Moyero River correlates with the δ13С excursion in the middle Katian Stage, while the δ13С excursion in the lower part of the Baksian Regional Stage correlates with the excursion marking the Katian–Sandbian boundary.

Paleobiogeography of the early Late Ordovician “Trentonian” (latest Sandbian to middle Katian) brachiopod fauna during a major marine transgression and colonization of the epicontinental seas in Laurentia

During the late Sandbian and early to middle Katian, a major marine transgression flooded the intracratonic basins and platforms of eastern North America, allowing brachiopods to disperse into the newly formed epicontinental seas. Two main brachiopod faunas can be differentiated using multivariate analysis of faunal data from localities across Laurentia during this time: a relatively diverse Scoto-Appalachian Fauna confined to the eastern margin of North America and peri-Laurentian terranes, and a less diverse but more widely distributed Epicontinental Fauna that expanded into the remainder of the flooded continent. The Epicontinental Fauna can be divided into four subfaunas with subtle compositional differences that correspond to different paleoenvironmental settings of eastern North America, influenced by such paleoecological factors as nutrient-rich cool-water upwelling onto the continent through the Sebree Trough, a greater range of habitable environments on the continental margin and in the adjacent deep Taconic foreland basin, and the interactions between pericratonic and intracratonic water masses of different physical and chemical properties. The Epicontinental brachiopod faunas dominated basins and platforms of continental interior until the Hirnantian extinction event.

Late Ordovician conodont biozonation of Australia—current status and regional biostratigraphic correlations

Zhen, Y.Y. & Percival, I.G. March 2017. Late Ordovician conodont biozonation of Australia—current status and regional biostratigraphic correlations. Alcheringa 41, xxx–xxx. ISSN 0311-5518.Seven conodont biozones are recognized in the Upper Ordovician of Australia. The Pygodus anserinus, Belodina compressa and Phragmodus undatus–Tasmanognathus careyi biozones are successively represented in the Sandbian. Although the Erismodus quadridactylus Biozone of the late Sandbian North America Midcontinent succession was previously recognized in the Stokes Siltstone of the Amadeus Basin and the Mithaka Formation of the Georgina Basin in central-north Australia, we argue for a middle–late Darriwilian age for these two units. Four conodont biozones, from oldest to youngest the Taoqupognathus philipi, T. blandus, T. tumidus–Protopanderodus insculptus and Aphelognathus grandis biozones, are established in the Katian of eastern Australia. Taoqupognathus species are particularly useful in correlation of the lower–middle Katian successions of eastern Australia with contemporary rocks in other parts of eastern Gondwana and peri-Gondwana, such as with the three major terranes of North and South China and Tarim. These regions, together with Sibumasu and eastern Australia, were part of the Australasian Superprovince during the Late Ordovician, with a strong palaeobiogeographic identity signalled by domination of Taoqupognathus, Tasmanognathus and Yaoxianognathus. Longstanding difficulties for precise correlation with the well-established North American Midcontinent or Baltoscandian successions in the Late Ordovician, owing mainly to strong endemism of the Australian faunas particularly from shallow-water settings, have been resolved by integration of regional conodont biostratigraphic schemes. The conodont biozonation of the Australian Upper Ordovician reviewed herein also provides a crucial chronological reference for better constraining the temporal and spatial range of Late Ordovician tectonostratigraphic events across the intracratonic basins of northern and western Australia and orogenic belts of eastern Australia.Yong Yi Zhen* [yong-yi.zhen@industry.nsw.gov.au] and Ian G. Percival [ian.percival@industry.nsw.gov.au], Geological Survey of New South Wales, W.B. Clarke Geoscience Centre, 947–953 Londonderry Road, Londonderry NSW 2753, Australia.