Early Ordovician Research Articles

Geology and genesis of the Early Paleozoic Zhengguang intermediate-sulfidation epithermal Au-Zn deposit, northeast China

Zhengguang is a partly eroded (about ≥500 m being eroded), structurally controlled intermediate sulfidation (IS) epithermal gold-zinc deposit, characterized by wide distribution of base metal sulfides, Fe-poor sphalerite and Mn-calcite, hosted by the Late Cambrian-Early Ordovician basaltic-andesitic volcanic and volcano-sedimentary rocks in northeast China. The deposit is located 4 and 8 km from the Early Ordovician Tongshan (including several IS veins about ~500 m away in the east called East Tongshan) and Duobaoshan porphyry copper deposits, respectively. A 484.8 ± 8.7 Ma (MSWD = 198) of Re-Os isochron age on pyrite indicates that it is the most ancient intermediate sulfidation deposit that has been dated.Four stages of veins have been discerned at Zhengguang. Early magnetite ± chalcopyrite ± epidote ± chlorite and K-feldspar ± pyrite ± chalcopyrite veins (stage I) were associated with restricted K-feldspar ± epidote (locally with chlorite) alteration and propylitic (epidote ± chlorite ± calcite) assemblages. Two primary epithermal gold stages developed at Zhengguang: pyrite-quartz ± Fe-poor sphalerite ± chalcopyrite ± galena ± electrum ± petzite ± hessite ± tetrahedrite/tennantite ± adularia veins (stage II) and (Mn-)calcite-quartz-pyrite ± Fe-poor sphalerite ± chalcopyrite ± galena ± hessite ± native gold (stage III), plus a late barren stage characterized by little sulfides (calcite ± chlorite ± hematite ± quartz, stage IV). Gold occurs chiefly as micro-inclusions of electrum and petzite in pyrite and sphalerite of stages II and III.Stable isotope analyses and fluid inclusion results indicate a magmatic-hydrothermal component in the ore fluids. Fluid inclusion data from stage I yielded the highest salinity (∼6.9 wt% NaCl) and homogenization temperature (average 264 °C). Calculated oxygen isotope composition of fluid (δ18Owater up to 4.9‰) equilibrated with quartz indicates a magmatic-hydrothermal source during stage I. A progressive addition of meteoric water component is suggested from stage I to IV of Zhengguang based on the average δ18Owater values equilibrated with quartz and homogenization temperatures (stage I: δ18Owater = 3.3‰, Th = 264 °C; stage II: δ18Owater = −1.4‰, Th = 182 °C; stage III: δ18Owater = −3.4‰, Th = 154 °C; stage IV: δ18Owater = −6.3‰, Th = 121 °C).Our new pyrite Re-Os dating result shows that Zhengguang formed at the same time with Tongshan and Duobaoshan within errors. Comparable sulfur isotopic compositions of pyrite from Zhengguang and Tongshan, analogous low-Fe content in sphalerite and similar mineral assemblage and alteration pattern between Zhengguang and East Tongshan IS veins collectively indicate a genetic connection between Zhengguang and Tongshan, which may derive from fluids exsolved from the same magma chamber. High-temperature assemblages (e.g., K-feldspr ± pyrite ± chalcopyrite) at Zhengguang indicate that there is potential for the discovery of porphyry copper–gold mineralization below the current level of diamond drilling. The confirmation of the Zhengguang as an Early Paleozoic epithermal deposit implies a good preservation and exploration potential of ancient epithermal deposits in eastern Central Asian Orogenic Belt.

Did hard substrate taxa diversify prior to the Great Ordovician Biodiversification Event?

AbstractThe Great Ordovician Biodiversification Event (GOBE) refers to one of the greatest increases in biodiversity during the Phanerozoic. Recent studies have shown that this taxonomic increase can be attributed to elevated origination rates around the Dapingian–Darriwilian boundary in the Middle Ordovician, while extinction rates stayed relatively constant throughout the Ordovician. Even though this global pattern of origination and extinction appears similar across diverse groups and geographical areas, earlier studies suggested that hard substrate taxa may have diversified prior to the GOBE, during the Early Ordovician. Here, we quantify Ordovician diversification dynamics of hard substrate taxa while simultaneously accounting for temporally varying sampling probabilities. Diversification rates of hard substrate taxa, both as a whole and when analysed as separate groups, appear to be very similar to those of free‐living benthic taxa. The observation that the diversification dynamics of many different taxonomic and ecological groups show the same temporal pattern, suggests a common cause of Ordovician diversification dynamics.

Two new early Asteroidea (Echinodermata) and early asteroid evolution

AbstractAerliceaster nexosusn. gen. n. sp. (Echinodermata), one of the oldest of known asteroids, is based on six specimens from the Floian (Early Ordovician) Garden City Formation of Idaho, andKolataster perplexusn. gen. n. sp. is based on two specimens from the Sandian (Late Ordovician) Mifflin Formation of Illinois. Although the asterozoan skeleton is subdivided into few ossicular categories, evolutionary derivations of all the categories are not fully established, and therefore published evaluations differ. Beginning with phylogenetic work placing asteroid ancestry within the Somasteroidea together with the new taxa described herein, aspects of early asteroid morphology are evaluated and ambiguities in need of further study identified. Uncertainties are considered to be founded in rapid early asterozoan diversification and the scanty fossil record.UUID:http://zoobank.org/b43d07cc-c8fb-4a84-bc6f-40aa6e0daea2

Processes and causes of Phanerozoic tectonic evolution of the western Tarim Basin, northwest China

This paper addresses the Phanerozoic tectonic evolution of the western Tarim Basin based on an integrated stratigraphic, structural and tectonic analysis. P-wave velocity data show that the basin has a stable and rigid basement. The western Tarim Basin experienced a complex tectonic evolutionary history, and this evolution can be divided into six stages: Neoproterozoic to Early Ordovician, Middle Ordovician to Middle Devonian, Late Devonian to Permian, Triassic, Jurassic to Cretaceous and Paleogene to Quaternary. The western Tarim Basin was a rift basin in the Neoproterozoic to Early Ordovician. From the Middle Ordovician to Middle Devonian, the basin consisted of a flexural depression in the south and a depression that changed from a rift depression to a flexural depression in the north during each period, i.e., the Middle–Late Ordovician and the Silurian to Middle Devonian. During the Late Devonian to Permian, the basin was a depression basin early and then changed into a flexural basin late in each period, i.e., the Late Devonian to Carboniferous and the Permian. In the Triassic, the basin was a foreland basin, and from the Jurassic to Cretaceous, it was a downwarped basin. After the Paleogene, the basin became a rejuvenated foreland basin. Based on two cross sections, we conclude that the extension and shortening in the profile reflect the tectonic evolution of the Tarim Basin. The Tarim Basin has become a composite and superimposed sedimentary basin because of its long-term and complicated tectonic evolutionary history, highly rigid and stable basement and large size.

Geological and geochemical implications of the complicated carbonate diagenetic process in the Lower Ordovician buried hills of the eastern Tazhong Low Rise, NW China, using Well M1 as an example

There are many structural buried hills in the east of Tazhong Low Rise that have significantly valuable for oil/gas exploration and research. In this work, geochemical and geological methods were used to research the complicated diagenetic process of Yingshan carbonate. The Yingshan carbonate underwent four diagenetic stages with three types of calcite, which diagenetic environment was complex and variable. During the Early Ordovician of syndiagenetic stage, the high-magnesium CI calcite develops along the micro-fracture and has no or very weakly cathodoluminescence color. In the following early diagenetic, the Yingshan carbonate subsided and developed the CIII calcite along the corroded fissure and pressolved sutures. After the M1 buried hill formed during the Late Silurian, the Yingshan carbonate experience the supergene stage, and suffered the weathering and denudation for a long period. During the Carboniferous karst, two cave horizons developed in the M1 buried hill, corresponding with two sets of Carboniferous carbonate rocks by the stratigraphic correlation. The cement CII calcite contains less Sr, Na, Mg, and more Mn and Fe than sea water, showing bright red in the cathodoluminescence. During the following middle and late diagenetic stage, M1 buried hill underwent a process of persistent burial and slight uplifting with CIII calcite cementing from the formation water. During the Permian, the hydrothermal fluid intensely influenced the buried hill, leaving the hydrothermal minerals.

Chapter 18 Tonian–Cryogenian rifting and Cambrian–Early Devonian platformal to foreland basin development outside the Caledonide orogen

Abstract Different parts of a Tonian–Early Devonian sedimentary succession, covering Proterozoic crystalline basement, occur along the erosional front to the Caledonide orogen, as outliers and coastal strips on land, and as more continuous strata in offshore areas. Rift-related Tonian–Cryogenian siliciclastic sedimentation preceded the break-up of the supercontinent Rodinia, the birth of Baltica and surrounding oceanic realms during the Ediacaran, and a marine transgression across Baltica during the Cambrian. An Ediacaran alkaline and carbonatite intrusive complex in central Sweden formed in connection with the extensional activity. Subsequently, during the Cambrian–Early Devonian, Baltica drifted northwards in the southern hemisphere to the equator, and six different lithofacies associations containing both siliciclastic and carbonate sedimentation were deposited in platformal shelf and Caledonian foreland basin settings. Bentonites in Ordovician and early Silurian successions were coupled to closure of the surrounding oceanic realms. Tectonic processes during the Caledonian orogeny around the margins to Baltica, the distance to different crustal components in this continent and climatic changes steered variations in lithofacies. Resultant fluctuations in sea-level gave rise to hiatuses and palaeo-karsts. Uranium and other metals in kerogen-rich black shales (Cambrian–Early Ordovician), hydrocarbons, stratabound Pb–Zn sulphide deposits in Cambrian (–Ediacaran?) sandstone, and limestone constitute the main resources.

Ediacaran, Early Ordovician and early Silurian arcs in the South Tianshan orogen of Kyrgyzstan

The oldest magmatic rocks from the South Tianshan orogen of Kyrgyzstan (STS) are important for better understanding of the Neoproterozoic and early Palaeozoic evolution of the southwestern Central Asian Orogenic Belt. Bulk rock major and trace element and Sm-Nd isotopic composition and zircon U-Pb ages of granitoids from mélange blocks reveal two previously unknown episodes of arc magmatism in the STS orogen of Kyrgyzstan, namely, the Ediacaran (ca. 624 Ma) and Early Ordovician (ca. 472 Ma) episodes. Moderately positive ɛNd(t) value of + 5.8 of the Ediacaran granodiorite indicates a mainly juvenile source for this rock. Negative ɛNd(t) value of −9.1 for the Early Ordovician granodiorite indicates melting of the predominantly Precambrian crustal material. The latter is also supported by the inherited zircon cores with an age range of 1.04–2.7 Ga. The geochemical signature of the Silurian (late Llandovery and Wenlock) basalts suggests formation in arc settings. Arc-related origin of the Ediacaran granodiorite suggests that mature oceanic basin with subduction system existed in the western STS since the late Neoproterozoic. The arc also evolved in the Ordovician and Silurian and was inactive in the middle Cambrian to the earliest Ordovician. Ediacaran granodiorite, old zircon cores and negative ɛNd(t) values of the Ordovician granodiorite suggest the existence of Precambrian microcontinents in the Kyrgyz STS. These crustal fragments were likely rifted off the Tarim and / or Karakum-Tajik microcontinents, where the rocks with similar ages occur. In the Palaeozoic, they traveled north and were incorporated in the STS orogen in the late Carboniferous.

Early Ordovician brachiopods from south-west Wales

The Early Ordovician brachiopods of south-west Wales (Pembrokeshire and Carmarthenshire) are identified, in most cases for the first time. That period is termed the local Arenig Stage in Britain, but it is now correlated with the Floian, Dapingian, and earlier half of the Darriwilian, which are more recently established international stages in the system of stratigraphical nomenclature. The rocks in south-west Wales were laid down as turbidites in a backarc basin which formed part of the margin of the relatively small independent microcontinent of Avalonia at that time. However, since the Rheic Ocean between Avalonia and the immense Gondwana continent was relatively narrow in the Arenig, the brachiopods reviewed here can be established to be part of the Mediterranean Province, which was also extensive in the higher-latitude sectors in and around Gondwana in the Early Ordovician. The 17 brachiopod species identified within the Arenig brachiopod faunas of south-west Wales are described, including the new species Aporthophyla prisca, Tritoechia bolohaulensis, Skenidoides pontyfennensis and Paurorthis? llangynogensis.

An Early Ordovician (Floian) asterozoan (Echinodermata) of problematic class-level affinities

AbstractFalloaster anquiroisitusnew genus new species (Asterozoa, Echinodermata) is described from the Floian (Early Ordovician) Garden City Formation of Idaho. The new taxon is known from a single small specimen. Because of weathering, remaining disk elements are incomplete. Dorsal surfaces of the ambulacral ossicles of two arms are available, one well preserved, whereas those of a third arm expose the ambulacrals essentially as they would appear in ventral view. Ambulacral ossicles were all but entirely lost on the remaining two arms.Albeit asterozoan,F. anquiroisitusis not assignable at the class level. It is suggestive of the Asteroidea in presence of a domal disk, five abruptly tapering triangular, arched arms, and ambulacral ossicles vaulted to form a furrow. Ambulacral morphology, including the presence of very large podial pores, is unlike that of early asteroids. In addition, no adambulacral or other virgal-series derivatives are present; ambital framework ossicles are absent; a single series of enlarged, plate-like arm ossicles, one series on each side of the arm, come together at the arm midline; and the plate-like series were supported laterally by recurved ambulacral margins. Only remnants of the disk near the periphery survive, these of overall expression unlike any other echinoderm, including asterozoans. The mouth frame is unknown.Falloaster anquiroisitusis argued to represent an extinct lineage apart from the four recognized asterozoan classes, thereby joining a limited number of other problematic early Asterozoa.UUID:http://zoobank.org/a13a5417-44b6-4ac3-90cf-9dc8fb5902bd

An Early Ordovician fossil seamount of the Hongguleleng–Balkybey Ocean in the northern West Junggar terrane (NW China) and its implications for the ocean evolution

Seamounts are common constituents in the ocean basins today, and can be accreted in the accretionary prism or carried into the subduction zone together with the slab. Here, we report a typical rock assemblage of a fossil seamount in the northern West Junggar terrane (NW China), which is composed of ocean island basalt (OIB), marble, siliceous mudstone, and chert. The OIB yields a zircon SHRIMP U–Pb age of 475 ± 4 Ma, and the detrital zircons of siliceous mudstone give a weighted average age of 472 ± 2 Ma (n = 50, MSWD = 1.09), showing the fossil seamount formed in the Early Ordovician. Geochemically, all the basalt samples manifest as high concentrations of TiO2 (mostly within 1.51–3.22 wt%), Na2O + K2O (4.6–10.65 wt%), and high εNd(t) values (+3.01–+3.65), with LREE enrichment ((La/Yb)N = 11–20.6) and no Nb, Ta negative anomalies, which suggests the signature of OIB. Moreover, the OIBs are inferred to have originated from a plume-related mantle source and to have been produced by ca. 1%–3% garnet + spinel lherzolite partial melting in an oceanic intraplate setting. The seamount from the Hebukesaier ophiolitic mélange (HOM) is suggested to have accreted in the accretionary complex with oceanic fragments during the Middle–Late Ordovician.

Chronological and geochemical constraints on the pre-variscan tectonic history of the Erzgebirge, Saxothuringian Zone

A combined U–Pb zircon geochronological and whole-rock isotopic and geochemical study has been carried out on high-grade orthogneiss, meta-basite, and meta-sediments from the Erzgebirge. The results indicate multiple pulses of Ediacaran–Ordovician magmatism in a transitional volcanic-arc to rift-basin setting. Orthogneiss from high-pressure nappes exhibit a step-like pattern of inherited zircon ages and emplacement ages of 500–475 Ma. In contrast, granite gneiss from the medium-pressure core of the Erzgebirge is characterised by three pulses of magmatism in the Early Cambrian, Late Cambrian, and Early Ordovician. A trend of decreasing Th/U ratios in zircon is observed to c.500 Ma, after which significant increases in the trend and variability of the data is inferred to mark the transition from arc-related to rift-related magmatism. Sediments deposited in the Early Cambrian have continental island arc affinity. Major detrital peaks in the Ediacaran and subordinate Tonian, Palaeoproterozoic, and Neoarchaean data are consistent with an Avalonian-Cadomian Arc and West African Craton derivation. The Early Cambrian sediments were locally reworked by a thermal event in the Ordovician resulting in leucocratic banding and recorded in Ordovician zircon rims characterised by systematically lower Th/U ratios. Ptygmatically folded leucocratic bands containing Ordovician zircon rims, associated with low Th/U ratios, are further observed in the granite gneiss core of the Erzgebirge. Variscan ages are rare, except in a fine-grained high-pressure micaschist, which contains exclusively small, structure-less, zircon with a weighted mean age of 350 ± 2 Ma. These data, along with a re-evaluation of previously published data, have been interpreted as the product of flattening subduction during the Early Cambrian; followed by the opening of slab windows in the Late Cambrian; and finally delamination in the Early Ordovician. Delamination of the orphaned slab led to asthenospheric upwellings triggering extension, bimodal magmatic pulses, recycling of fertile crust, high-temperature metamorphism, and cratonisation of relatively young crust.

Кокчетау-Иссыккульский террейн Казахстанского палеоконтинента и его палеоширотное положение в ордовике

Research subject. The Kazakhstan (Kazakh-Kyrgyz) paleocontinent attracts much research attention. In this article, we present data on the tectonic structure of the Early Paleozoic Kokchetau-Issykkul sialic terrain. Materials and methods . The geodynamic position of the Kokchetau-Issykkul terrain was determined by analysing samples of Ordovician rocks collected across 11 sites using the paleomagnetism method. Thus, we studied the paleomagnetism of red sandstones, siltstones, tuffaceous siltstones and basalts, as well as that of andesite and tuffaceous rocks in marine sediments, which affiliation to different Ordovician formations was established on the basis of trilobite, brachiopod, conodont and graptolite fauna. Results. Early Paleozoic rocks are exposed in the northern part of the studied terrain (Kokchetau district) and in the Issyk-Kul region, which covers southern Kazakhstan, northern Kyrgyzstan and western Xinjiang. The sutures of Chistopol-Terskei and Iradir-Ili ocean basins serving as the boundaries of the Kokchetau-Issykkul terrain are located in the allochthonius position. A multi-storey system of napes composed of pre-Paleozoic and Early Paleozoic rocks of the oceanic crust, volcanic oceanic island arcs and continental slope were studied near the Terskei margin of the terrain. This system of napes is covered by sediments containing graptolites of the Floian-Darriwilian age. Subduction-related volcanism near this margin of the terrain is of the Darriwilian and Sandbian age, while I-type granites are of the Darriwilian age. The over-thrusting of the Iladyr-Ili oceanic crust on the Ili border of the terrain also occurred in the middle Ordovician. Subduction-related mag-matism occurred here in the Late Darriwilian and Sandbian periods. Conclusion. The conducted studies testify to the geodynamics of the region. In the Early and Middle Ordovician, the Kokchetau-Issykkul terrain was a sialic volcanic arc separated from other sialic terrains by ocean basins. In the Middle Ordovician, the counter subduction of the oceanic crust under the Kokchetau-Issykkul island arc occurred. This process led to the closure of the Chistopol-Terskei and Iradir-Ili ocean basins in Middle Ordovician and to the formation the Kazakhstan continent by the collision of the Early Paleozoic sialic terrains and oceanic island arcs. According to the study of the paleomagnetism of Ordovician rocks in the Kokchetau-Issykkul island, the position of the middle part of this arc is determined to be at the paleogeographic latitude of 9.1 ± 5.4°.

Petrography and origin of the Lower Ordovician microbial carbonates in the Songzi Area of Hubei Province, middle Yangtze region, China

This study is the first systematic assessment of the Lower Ordovician microbial carbonates in Songzi, Hubei Province, China. This paper divides the microbial carbonates into two types according to growth patterns, namely nongranular and granular. The nongranular types include stromatolites, thrombolites, dendrolites, leiolites and laminites; the granular types are mainly oncolites and may include a small amount of microbiogenic oolite. According to their geometric features, the stromatolites can be divided into four types: stratiform, wavy, columnar and domal. Additionally, dipyramidal columnar stromatolites are identified for the first time and represent a new type of columnar stromatolite. The thrombolites are divided into three types: speckled, reticulated and banded. The grazing gastropod Ecculiomphalus and traces of bioturbation are observed in the speckled and reticulated thrombolites. This paper considers these two kinds of thrombolites to represent bioturbated thrombolites. These findings not only fill gaps in the field of domestic Ordovician bioturbated thrombolites but also provide new information for the study of thrombolites. Based on the analysis of the sedimentary characteristics of microbialites, the depositional environments of the various types of microbialites are described, and the distribution patterns of their depositional environments are summarized. The relationship between the development of microbialites and the evolution and radiation of metazoans during the Early to Middle Ordovician is discussed. Consistent with the correspondence between the stepwise and rapid radiation of metazoans and the abrupt reduction in the number of microbialites between the late Early Ordovician and the early Middle Ordovician, fossils of benthonic grazing gastropods (Ecculiomphalus) were found in the stromatolites and thrombolite of the study area. It is believed that the gradual reduction in microbialites was related to the rapid increase in the abundance of metazoans. Grazers not only grazed on the microorganisms that formed stromatolites, resulting in a continuous reduction in the number of stromatolites, but also disrupted the growth state of the stromatolites, resulting in the formation of unique bioturbated thrombolites in the study area. Hydrocarbon potential analysis shows that the microbialites in the Nanjinguan Formation represent better source rocks than those in the other formations.

Jaw elements in Plumulites bengtsoni confirm that machaeridians are extinct armoured scaleworms

Machaeridians are Palaeozoic animals that are dorsally armoured with serialized, imbricating shell plates that cover or enclose the body. Prior to the discovery of an articulated plumulitid machaeridian from the Early Ordovician of Morocco that preserved unambiguous annelid characters (segmental parapodia with chaetae), machaeridians were a palaeontological mystery, having been previously linked to echinoderms, barnacles, tommotiids (putative stem-group brachiopods) or molluscs. Although the annelid affinities of machaeridians are now firmly established, their position within the phylum and relevance for understanding the early evolution of Annelida is less secure, with competing hypotheses placing Machaeridia in the stem or deeply nested within the crown group of annelids. We describe a scleritome of Plumulites bengtsoni from the Fezouata Formation of Morocco that preserves an anterior jaw apparatus consisting of at least two discrete elements that exhibit growth lines. Although jaws have multiple independent origins within the annelid crown group, comparable jaws are present only within Phyllodocida, the clade that contains modern aphroditiforms (scaleworms and relatives). Phylogenetic analysis places a monophyletic Machaeridia within the crown group of Phyllodocida in total-group Aphroditiformia, consistent with a common origin of machaeridian shell plates and scaleworm elytrae. The inclusion of machaeridians in Aphroditiformia truncates the ghost lineage of Phyllodocida by almost a hundred million years.

A geochemical evaluation of the genetic relationship between Ouachita Mountains Paleozoic rocks and the Mississippi Valley-type mineralization in the southern Ozark Region, USA

The Pb and/or Zn deposits of the Tri-State and the Northern Arkansas Mississippi Valley-type (MVT) districts in the southern Ozark Region, located north of the Arkoma Basin and the Ouachita fold-thrust belt in North America, are genetically connected to the Pennsylvanian-Permian Ouachita orogeny. This orogenic event triggered a south-north topographic gradient flow of basinal brines, leaching metal-rich sediments en route. To establish whether the organic-rich shales of the Ouachita Mountains and the Ozark Region provided metals during the mineralization event, the Pb isotope compositions of the ores (sphalerite) have been compared to those of the sedimentary rocks (whole-rocks and leachates) of the Ouachita Mountains (Cambrian Collier Shale, Early Ordovician Mazarn Shale, Middle Ordovician Womble Shale, Late Ordovician Polk Creek Shale, Mississippian Stanley Shale, and Pennsylvanian Jackfork Sandstone) and the Ozark Plateau (Devonian/Mississippian Chattanooga Shale and Mississippian Fayetteville Shale). The Pb isotope compositions of ores in both districts show a broad and a linear range of isotopic values. This trend suggests mixing of Pb from two distinct end-member components. One end-member must be highly radiogenic, with Pb isotope ratios equal to or higher than the highest noticed value for ores. The other end-member must be less radiogenic, with Pb isotope ratios equal to or lower than the lowest value recorded for ores. The Pb isotope compositions of the analyzed whole rocks and leachates indicate that the best candidates for contributing the less radiogenic Pb to the ores are the sandstone members of the upper and middle Jackfork Sandstone and the lowermost section of the Chattanooga Shale.The depositional environment of the potential source rocks and their ability to sequester metals has been appraised using redox sensitive trace elements (Cd, Co, Cr, Cu, Mo, Ni, Pb, Th, U, V, Zn). Paleoredox proxies (e.g., enrichment factors of redox sensitive trace elements and V/V + Ni ratios) indicate deposition of the analyzed rocks mainly under anoxic and euxinic conditions, which are favorable for metal enrichment. However, oxic-suboxic conditions are also indicated by other geochemical proxies, suggesting a possible first order control of primary production that has resulted in an increased flux of organic matter. A disagreement in the redox conditions yielded by the paleoredox proxies has been noticed in several cases. This inconsistent behavior may be associated with several factors, including redox potential, pH, sulfur abundance, chemical availability, TOC values, and sediment provenance. The authigenic flux is the only fraction needed to unravel past redox conditions; however, the analyzed bulk sediments consist of authigenic, biogenic, and siliciclastic components, which may also have led to the observed disagreement in the redox conditions. Additional factors such as the basin geometry, the rates of sediment accumulation, the post depositional alteration processes associated with diagenesis and low-grade metamorphism during the Ouachita orogeny might have overprinted the depositional signatures.

Pre-Variscan tectonic setting of the south margin of Armorica: Insights from detrital zircon ages distribution and Hf isotopic composition of the St-Georges-sur-Loire Unit (S. Armorican Massif, France)

The Armorican Massif experienced a poly-orogenic evolution, represented by the Neoproterozoic Cadomian and the Devonian-Carboniferous Variscan tectonothermal events. In the southernmost part of the Armorican Domain (or Armorica microcontinent), the St-Georges-sur-Loire Unit consists of a block-in-matrix subunit, and a turbiditic subunit in the South and North, respectively. The St-Georges-sur Loire unit is interpreted as a remnant back-arc basin formed in Silurian-Early Devonian (ca. 440–400 Ma), and was deformed during the Variscan orogeny. Most of the sedimentary and magmatic blocks enclosed in clastic matrix are olistoliths derived from an assumed southern area called the “missing domain”, presently exposed as the Variscan Mauges Nappe composed of the Cadomian rocks and Early Paleozoic sedimentary cover. The “missing domain” was subducted beneath the Armorican massif during the Variscan collision. Detrital zircon age spectra from sandstones and siltstones from the olistostrome matrix reveal Neoproterozoic and Early Paleozoic age clusters, and less abundant Paleoproterozoic and Archean ages. Hf isotopic compositions of detrital zircons indicate dominantly negative εHf (t) values which are in agreement with a recycled continental source. Minor positive values correspond to a juvenile component that was formed during the Neoproterozoic Cadomian orogeny or Archean-Paleoproterozoic events. The Cambrian and Early Ordovician ages can be related to the pre-Variscan rifting of the Pannotia megacontinent that separated the Armorica microcontinent from Gondwana. Neoproterozoic ages imply a source of the magmatic rocks exposed in the Cadomian belt of the northern part of the Armorica microcontinent, and in the Mauges Nappe.

Conodont biostratigraphy and astronomical tuning of the Lower–Middle Ordovician Liangjiashan (North China) and Huanghuachang (South China) marine sections

In this study, systematic conodont biostratigraphy and cyclostratigraphy are carried out on Ordovician sections from the North China and South China blocks. The Liangjiashan section, situated on the North China Block margin, accumulated shallow marine carbonate during the Early Ordovician. The Huanghuachang section (GSSP for the Dapingian Stage), situated on the South China Block margin, accumulated thinly bedded carbonates and minor clastics in a deep shelf environment. Conodont biostratigraphy and 405-kyr long-eccentricity cycles interpreted from the Liangjiashan and Huanghuachang sections are used to establish an Astronomical Time Scale (ATS) for the Early–Middle Ordovician period. The ATS indicates that the duration of the Floian Stage is 7.08 ± 0.405 Myr. The temporal ranges of Lower–Middle Ordovician conodont biozones are also estimated: the durations of the Serratognathus bilobatus, Serratognathus extensus, Oepikodus communis, Oepikodus evae, and Baltoniodus triangularis zones are 1.0 Myr, 5.1 Myr, 0.2 Myr, 0.8 Myr, and 0.2 Myr, respectively. The evolution of the ratio of obliquity to total power and short-eccentricity power to total power depicts periodicities of 1 Myr for the s4–s3 term, and 1.9 Myr shifting to 1 Myr for the g4–g3 term. The shift in the g4–g3 term may indicate a chaotic transition in the Mars and Earth orbital motions from a (s4–s3)–2(g4–g3) to (s4–s3)–(g4–g3) resonance state during the Early Ordovician.

Deep‐time Paleogeographic Reconstruction Based on Database: Taking the South China T. approximatus Biozone (Early Ordovician) as an Example

Deep‐time Paleogeographic Reconstruction Based on Database: Taking the South China <i>T. approximatus</i> Biozone (Early Ordovician) as an Example

Ordovician conodonts and the Tornquist Lineament

The Holy Cross Mts. in southern Poland are generally believed to be split by a tectonic dislocation into two separate parts, a NE one being a part of the Baltic Craton and a SW part belonging to the Małopolska Terrane of a complex geotectonic history connected with the Trans-European Suture Zone (Tornquist Lineament). Unexpectedly, conodont assemblages of earliest Middle Ordovician (early Darriwilian) age from Pobroszyn in the northeastern Łysogóry region and from Szumsko in the southwestern Kielce region show virtually identical species composition. One of the dominant species both in Pobroszyn and Szumsko, Trapezognathus pectinatus sp. n., characterized by denticulated M elements, occurs elsewhere only on the northern margin of Gondwana. Separation of the Małopolska microcontinent from Baltica continued after the disappearance of Trapezognathus and an apparently allopatric speciation process was initiated by a population of Baltoniodus. Also in this case, denticulation developed in the M elements of the apparatus but the process of speciation of B. norrlandicus denticulatus ssp. n. was truncated by re-appearance of the Baltic lineage of Baltoniodus. Later conodont faunas from the region are of Baltic affinities, but remain distinct in showing a relatively high contribution from exotic species of Sagittodontina, Phragmodus, and Complexodus. The periodic North American Midcontinent tropical influences observable in the Baltic region did not reach the Holy Cross Mts. The Ordovician conodont faunas from the Holy Cross Mts. can be fit into a transect extending from Baltica to Gondwana, with several intermediate terranes. This new evidence disproves the long-held concept that the Łysogóry and Kielce regions had separate early Palaeozoic fates and that the Małopolska Terrane was unified with Baltica already in the earliest Ordovician. The Trans-European Suture Zone truly marks the presence of a wide Tornquist Sea in the early Palaeozoic.

Late Cambrian (Furongian) to mid-Ordovician euconodont events on Baltica: Invasions and immigrations

The stratigraphic distributions, taxonomy, diversity, evolutionary lineages and events of late Cambrian to Middle Ordovician euconodonts of Baltica are briefly reviewed and re-evaluated. The euconodonts of the late Furongian (Stage 10, late Cambrian) to Middle Ordovician successions of Baltica record innovation, immigration and two invasions of short duration and one major extinction. The innovation event with an origin on Baltica is the rise of the Proconodontus serratus lineage in the late Furongian (Stage 10). The immigration events comprise the Cordylodus immigration to Baltoscandia in latest Cambrian and this phylogenetic lineage persisted to mid-Tremadocian (Early Ordovician) and vanished at the global mid-Tremadocian extinction event. The late Floian – basal Dapingian transition is a significant event on Baltica. It is characterized by the immigration of the important genera Baltoniodus and Microzarkodina and upon their arrival these taxa remained on the Baltica palaeocontinent and stayed for a long period forming evolutionary lineages. The basal Darriwilian immigration event by the genus Lenodus and coeval prominent sea-level lowstand were caused by a short global icehouse stage related to the expansion of the polar ice sheets. The extinction event caused the disappearance of the characteristic Cordylodus fauna. Most events are related to changes in the palaeoclimate resulting from the movement of the Baltica palaecontinent and associated eustatic sea-level changes. However, the cause of the global early Tremadocian extinction event is not yet completely clear and remains unsolved. Perhaps abrupt and significant palaeoceanographic changes on a global scale (i.e., plate tectonic movements causing sudden change of ocean currents and climate from warm to cold) were the cause of this significant extinction in the Early Ordovician.