Ordovician Age Research Articles

Timing of deformation, metamorphism and leucogranite intrusion in the lower part of the Seve Nappe Complex in central Jämtland, Swedish Caledonides

ABSTRACT Recent studies in the context of the International Continental Scientific Drilling Project “Collisional Orogeny in the Scandinavian Caledonides” have focused on the importance of the Seve Nappe Complex (SNC) for understanding the subduction history of the Baltoscandian margin during closure of the Iapetus Ocean. In the classical Åre area of western central Jämtland, granulite facies migmatites and leucogranites of the Åreskutan Nappe provide evidence of Early Silurian (c. 440 Ma) high temperature metamorphism and a previous prograde, ultra-high pressure history, with microdiamonds. New LA-ICPMS zircon isotope age investigations of the underlying amphibolite facies Lower Seve Nappes, reported here, have also identified an Early Silurian tectonothermal history with pegmatitic leucogranite (c. 443 Ma) and, at lower structural levels, another felsic intrusion of earliest Middle Ordovician age (c. 469 Ma). The latter intrudes isoclinally folded host rock amphibolites and calcareous psammitic paragneisses and is itself tightly folded. Zircons in an amphibolite proved to be highly discordant but indicate Early Silurian metamorphism during isoclinal folding. Detrital zircons in a paragneiss are dominated by Sveconorwegian populations, but also include a range of younger Neoproterozoic grains down to the Early Ediacaran (c. 600 Ma). This new evidence of early Caledonian deformation and metamorphism indicates that, as farther north in the orogen, the Seve tectonothermal history in central Jämtland probably started early in the Ordovician, or before. Subduction and accretion along the Baltoscandian outer margin occurred prior to Scandian continent-continent collision, with Siluro-Devonian emplacement of the SNC across the foreland basins onto the Baltoscandian platform.

Palynological indications for Silurian – earliest Devonian age strata in the Netherlands

Abstract Knowledge of the stratigraphic development of pre-Carboniferous strata in the subsurface of the Netherlands is very limited, leaving the lithostratigraphic nomenclature for this time interval informal. In two wells from the southwestern Netherlands, Silurian strata have repeatedly been reported, suggesting that these are the oldest ever recovered in the Netherlands. The hypothesised presence of Silurian-aged strata has not been tested by biostratigraphic analysis. A similar lack of biostratigraphic control applies to the overlying Devonian succession. We present the results of a palynological study of core material from wells KTG-01 and S05-01. Relatively low-diversity and poorly preserved miospore associations were recorded. These, nonetheless, provide new insights into the regional stratigraphic development of the pre-Carboniferous of the SW Netherlands. The lower two cores from well KTG-01 are of a late Silurian (Ludlow–Pridoli Epoch) to earliest Devonian (Lochkovian) age, confirming that these are the oldest sedimentary strata ever recovered in the Netherlands. The results from the upper cored section from the pre-Carboniferous succession in well KTG-01 and the cored sections from the pre-Carboniferous succession in well S05-01 are more ambiguous. This inferred Devonian succession is, in the current informal lithostratigraphy of the Netherlands, assigned to the Banjaard group and its subordinate Bollen Claystone formation, of presumed Frasnian (i.e. early Late Devonian) age. Age-indicative Middle to Late Devonian palynomorphs were, however, not recorded, and the overall character of the poorly preserved palynological associations in wells KTG-01 and S05-01 may also suggest an Early Devonian age. In terms of lithofacies, however, the cores in well S05-01 can be correlated to the upper Frasnian – lower Famennian Falisolle Formation in the Campine Basin in Belgium. Hence, it remains plausible that an unconformity separates Silurian to Lower Devonian strata from Upper Devonian (Frasnian–Famennian) strata in the SW Netherlands. In general, the abundance of miospore associations points to the presence of a vegetated hinterland and a relatively proximal yet relatively deep marine setting during late Silurian and Early Devonian times. This differs markedly from the open marine depositional settings reported from the Brabant Massif area to the south in present-day Belgium, suggesting a sediment source to the north. The episodic presence of reworked (marine) acritarchs of Ordovician age suggests the influx of sedimentary material from uplifted elements on the present-day Brabant Massif to the south, possibly in relation to the activation of a Brabant Arch system.

Unraveling the origins and P-T-t evolution of the allochthonous Sobrado unit (Órdenes Complex, NW Spain) using combined U–Pb titanite, monazite and zircon geochronology and rare-earth element (REE) geochemistry

Abstract. The Sobrado unit, within the upper part of the Órdenes Complex (NW Spain) represents an allochthonous tectonic slice of exhumed high-grade metamorphic rocks formed during a complex sequence of orogenic processes in the middle to lower crust. In order to constrain those processes, U–Pb geochronology and rare-earth element (REE) analyses of accessory minerals in migmatitic paragneiss (monazite, zircon) and mylonitic amphibolites (titanite) were conducted using laser ablation split stream inductively coupled plasma mass spectrometry (LASS-ICP-MS). The youngest metamorphic zircon age obtained coincides with a Middle Devonian concordia monazite age (∼380 Ma) and is interpreted to represent the minimum age of the Sobrado high-P granulite facies metamorphism that occurred during the early stages of the Variscan orogeny. Metamorphic titanite from the mylonitic amphibolites yield a Late Devonian age (∼365 Ma) and track the progressive exhumation of the Sobrado unit. In zircon, cathodoluminescence images and REE analyses allow two aliquots with different origins in the paragneiss to be distinguished. An Early Ordovician age (∼490 Ma) was obtained for metamorphic zircons, although with a large dispersion, related to the evolution of the rock. This age is considered to mark the onset of granulite facies metamorphism in the Sobrado unit under intermediate-P conditions, and related to intrusive magmatism and coeval burial in a magmatic arc setting. A maximum depositional age for the Sobrado unit is established in the late Cambrian (∼511 Ma). The zircon dataset also record several inherited populations. The youngest cogenetic set of zircons yields crystallization ages of 546 and 526 Ma which are thought to be related to the peri-Gondwanan magmatic arc. The additional presence of inherited zircons older than 1000 Ma is interpreted as suggesting a West African Craton provenance.

Biostratigraphy of acritarchs, chitinozoans, and miospores from Upper Ordovician sequences in Kuh-e Boghou, southwest of Kashmar, eastern central Iran: Stratigraphic and paleogeographic implications

A siliciclastic succession is well exposed at Kuh-e Boghou, located in eastern-central Iran. To verify the exact age and paleogeographic position of this section of the Iranian Platform, a detailed, high-resolution palynological analysis of the Ghelli Formation was performed. Sixty-nine surface samples of the sedimentary succession were collected and analyzed. Most samples yielded abundant and well-preserved acritarchs (34 species of 24 genera), euglenids (3 species of a single genus), chitinozoans (36 species of 19 genera), and cryptospores (25 species of 10 genera). Based on the presence of well known chitinozoan species, a Late Ordovician (Katian–Hirnantian) age was assigned to this succession. The identified chitinozoan fauna allowed recognition of the Belonechitina robusta, Tanuchitina fistulosa, Armoricochitina nigerica, Ancyrochitina merga, Tanuchitina elongata, and Spinachitina oulebsiri biozones in ascending stratigraphic order as was previously established for the northern Gondwanan Domain. These results indicate that eastern-central Iran was part of the peri-Gondwana domain during the Late Ordovician. Seven morphotypes were recorded for the first time, and five of them were established as new species, namely Abditusdyadus khayyammii, Alhajrichitina spinifera, Dyadospora ferdowsii, Lophosphaeridium bulbosumensis and Moyeria hafezii, and two were left in open nomenclature (e.g. Eisenackitina sp. and Moyeria sp. A). The abundant cryptospore taxa in the Late Ordovician might be related to land-derived sediments acquired during the global sea level fall in the Late Ordovician glaciation or to the adaptation of primitive land plants in a wide range of climatic conditions. The association of terrestrial miospores with marine palynomorphs suggests a nearshore marine environment.

Age correlations for the Acoite Formation (Lower Ordovician) at Aguas Blancas Creek in the Cordillera Oriental of Jujuy Province, Argentina

Here, we use conodont and graptolite assemblages to age correlate the ‘Sepulturas limestones’ facies of the Acoite Formation exposed at Aguas Blancas Creek, near Altos de Lipán in the Cordillera Oriental of Jujuy Province, Argentina. An association of autochthonous conodont species is identified consisting of Drepanodus arcuatus, Drepanoistodus pitjanti, Erraticodon patu, Gothodus andinus, Paroistodus parallelus and Zentagnathus argentinensis. Tubaria referable to the graptolite genera Acrograptus, Baltograptus and Tetragraptus are also present. This assemblage corresponds to the middle interval of the Gothodus andinus Zone, together with the upper Tetragraptus azkharensis Zone, or possibly the lower Baltograptus cf. B. deflexus Zone, which collectively indicate an early to middle Floian (Early Ordovician) age. Lithological and faunal characteristics of the studied succession imply deposition in a shallow marine environment. We also suggest that the Aguas Blancas section is stratigraphically younger than the other upper outcrops exposed around the town of La Ciénaga, where the ‘Sepulturas limestones’ were originally defined. G.L. Guillermo [guillermo.albanesi@unc.edu.ar], CICTERRA (CONICET-UNC), CIGEA, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba, Av. Vélez Sarsfield 1611, X5016GCA, Córdoba, Argentina; E.K. Rueda [ruedaroballo@gmail.com ]; G. Ortega [gladyscortega@gmail.com ] Museo de Paleontología, CIGEA, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba, Av. Vélez Sarsfield 300, X5000JJC, Córdoba, Argentina; C.R Monaldi [crmonaldi@gmail.com ], Los Teros 197, 4401 Vaqueros, Salta, Argentina.

Lajishan Ayishan Formation Rhyolites: Implications for the Closure Time of the Proto‐Tethys Ocean

AbstractThe Qilian orogenic belt, as an important component of the central orogenic system, can be divided into three tectonic units from north to south: the North Qilian tectonic belt, the Central Qilian tectonic belt and the South Qilian tectonic belt. The Lajishan ophiolitic mélange zone is an important part of the South Qilian tectonic belt. The study of the formation age and tectonic attributes of the different rock units in the Lajishan is of great significance for reconstructing the tectonic evolution of the south Qilian tectonics belt and the Proto‐Tethys Ocean. The Ayishan formation is an important unit of the Lajishan mélange belt, but its stratigraphic age and formation environment are still not constrained, which limits the reconstruction of the early Palaeozoic tectonic evolution of the South Qilian tectonic belt and the closure time of the Proto‐Tethys Ocean. In recent years, a set of rock assemblages consisting of andesite, rhyolite, sandstone and pebbly sandstone have been identified in the Ayishan formation exposed in the Lajishan through detailed geological mapping in the field. They are in fault contact with the underlying ophiolite mélange.Regionally, a volcano‐sedimentary succession consisting of andesite, rhyolite, volcanic breccia, and volcaniclastic sandstone nonconformably overlies the Cambrian arc‐accretionarycomplex system in the Ayishan area. These rocks were originally assigned to the Ayishan formation and attributed an Early Ordovician age (BGMRQP, 1964, 1991). Our rhyolite samples from the Ayishan formation yielded a weighted mean 206Pb/238U age of 447 Ma using zircon U‐Pb dating, which can be interpreted as the eruption age of the volcanic rocks. These volcanic rock assemblages are distributed in sandstone, pebbled sandstone and conglomerate in the form of interlayers. Therefore, the zircon U‐Pb age of the volcanic rocks can constrain the age of Ayishan formation that it should belong to the late Ordovician rather than early Ordovician. Additionally, we propose that the Ayishan rhyolites were formed in collision‐related tectonic setting based on the geochemistry of the rhyolites, which means the initial continental collision between the Central Qilian block and the Qaidam block occurred at least in the Late Ordovician.Ayishan formation are in fault contact with the Cambrian Donggoumeikuang complex in the south. The Donggoumeikuang complex represents a Cambrian introceanic subduction system that formed in response to subduction of the Proto‐Tethyan Ocean (Yan et al., 2015; Fu et al., 2018). At 450 to 420 Ma, the Proto‐Tethyan Ocean closed and the Qaidam block collided against the central Qilian tectonics belt. Voluminous 450–440 Ma I‐type and S‐type granites (Yan et al., 2015; Tung et al., 2016) that straddle the Central and South Qilian belts formed a broad Andean‐type continental margin (Yan et al., 2019), which indicates that the transition from oceanic subduction to continental subduction occurred in 450–440Ma. At 440–420 Ma, the syncollisional and postcollisional granitoids extensively developed, accompanied by high‐pressure granulite‐facies metamorphism and anatexis in the South Qilian belt and the Qaidam block (Yu et al., 2014; Yan et al., 2015; Fu et al., 2018; Li et al., 2018).

Ordovician gastropods from pebbles in Cretaceous fluvial sandstones in south-east Disko, West Greenland

The gastropods Sinuopea sp. and Lecanospira cf. compacta (Salter 1859) of probable early Ordovician age are described from cherty limestone clasts within fluvial strata of the Cretaceous Atane Formation of south-east Disko, central West Greenland. The record of Sinuopea possibly suggests an earliest Ordovician (Tremadocian) age, slightly older than the Floian–Dapingian age suggested by the oldest known conodont assemblages described from West Greenland. The determinations provide supporting evidence for a former periodic cover of Ordovician strata in the Archaean terrane of south western Greenland, extending deep into the heart of the Laurentian landmass.

Late Cadomian rifting of the NW Gondwana margin and the reworking of Precambrian crust – evidence from bimodal magmatism in the early Paleozoic Moroccan Meseta

ABSTRACT The Meseta Goaïda magmatism, which occurs as gabbro, dolerite and rhyolitic intrusions in the Neoproterozoic basement and Cambrian platform sedimentary series of the Moroccan Hercynian orogen, has been ascribed to the Variscan orogenesis in the late Palaeozoic. Here, we provide new LA–ICP-MS U-Pb zircon ages – ranging from c. 519 Ma to c. 479 Ma – of this magmatism that reveal, for the first time in the Moroccan Central Meseta, the existence of early Cambrian to early Ordovician bimodal magmatism. Inherited zircons in these magmatic rocks yield dates that range from c. 1.2 to 1.0 Ga and c. 0.63 to 0.50 Ga. This inheritance suggests provenance from Precambrian crustal sources in the Cadomian‒Pan-African of the NW margin of the West African Craton, which is buried beneath the currently exposed Paleozoic Moroccan Meseta. The affinity of this early Palaeozoic magmatism is mainly tholeiite to transitional for the mafic rocks and high-K calc-alkaline for the silicic rocks. The Cambrian–Ordovician age and varying parental magma signature – as shown by trace element and Sr-Nd-Pb isotopic data from this magmatism and its Neoproterozoic granitoid basement rocks – indicate that the Moroccan Meseta early-Paleozoic magmatism might be associated with the weakening of the continental lithosphere during the early stages of rifting during the Cambrian–Ordovician along the NW Gondwana margin. This is supported by the presence of inherited zircons derived from reworking of the Precambrian crust in the early-Palaeozoic magmas.

Quantitative characterization of interlayer fractures in carbonate rocks based on finite element numerical simulation

Density, aperture and developmental position are key parameters of tectonic fractures in carbonate reservoirs for oil and gas exploration and development. The analysis of fracture development and distribution in interbedded bioclastic and biological limestone and mudstone of Carboniferous and Ordovician age in the Hetianhe area of the Bachu uplift, central Tarim Basin reveal that mudstone thickness, mechanical parameter properties and interlayer frequency are significant factors. A new simulative geo-mechanical modeling method was used to quantitatively calculate the 3D fracture distribution, development and to evaluate fracture penetration behavior through the interlayered mudstone in the vertical. Fractures were deduced from the stress and strain distribution. Interlayered limestone-mudstone models were established, and simulated results were compared with drill core and well logs. The results show that the mechanical parameters (Young's Modulus, Poisson's ratio) are primary factors controlling fracture development. Fracture development and distribution is mainly influenced by the limestone/mudstone thickness ratio where limestone was interbedded with multiple mudstone layers of variable thickness. Pressure is an important control on the generation of fractures, and the nature of lithological surfaces highly influences fracture penetration in interlayered limestone-mudstone. Where the limestone/mudstone thickness＞10 (a critical value), the limestone fracture easily penetrated along the contact surfaces. The simulated fracture linear density and aperture are consistent with the observations from drill core and geophysical well logs, and average accuracy was better than 80% in single wells.

REVIEW OF THE LOWER-PALAEOZOIC RESERVOIR PROSPECTIVITY OF W. IRAQ WITH REFERENCE TO JORDAN

The Palaeozoic basin of Jordan and Iraq occupies the NW part of the Arabian Plate. The Lower Palaeozoic succession is the most under-explored succession in Iraq, particularly in western Iraq where it is considered the most promising target for future exploration. The western Iraqi subsurface and Jordan geology are characterized by similar major chrono-stratigraphic rock units. In western Iraq, the Akkas-1 well is the only gas and condensate discovery in sandstone reservoirs of Early Silurian and Late Ordovician age. Lower Silurian sandstones of the Akkas Formation are an oil reservoir characterized by a mean porosity of 6.5% and permeability of 0.2mD. Underlying Upper Ordovician sandstones of the upper part of the Khabour Formation are also reservoir, but for sweet gas and condensate. The latter are comparable to both the Risha and Wadi Sirhan discoveries in Jordan. Several deeper potential sandstone reservoirs (of older Ordovician and Cambrian ages) are also present across the region. Equivalent Ordovician age Jordanian reservoirs are primarily developed in homogeneous, highly mature quartz arenites of storm-dominated clastic-shelf stratigraphies. Within the Risha Field and the Wadi Sirhan area, are also end-Ordovician (Hirnantian) glaciogenic and periglacial sandstones with more variable reservoir quality characteristics compared to the underlying quartz arenites. These often comprise highly immature arkoses (as diamictites) that provide a mineralogically much less stable assemblage in which there is a possibility of developing secondary porosity. Sedimentological and XRD mineralogical studies suggest that diagenesis is an important factor in developing of reservoir quality. Nevertheless, reservoir quality is a major exploration risk where burial was significant that led to quartz overgrowth and clay cementation. It is notable that some sandstone reservoirs have chlorite-clay coatings, which prevented formation of the quartz overgrowth and preserved good reservoir quality. The widespread occurrence of associated source and seal rocks together with the proven and potential reservoirs, suggests that Lower Palaeozoic prospects below the deserts of western Iraq are promising targets for future hydrocarbon exploration.

Neoproterozoic Windermere Supergroup Near Bayhorse, Idaho: Late‐Stage Rodinian Rifting Was Deflected West Around the Belt Basin

AbstractConflicting models of Rodinian rifting have been proposed to explain the recognized variation in the Neoproterozoic and early Cambrian tectonostratigraphic architecture of the western Laurentian margin. However, discrimination among rift models is hampered by limited exposure and metamorphism of the rocks. Southeastern Idaho preserves more than 6 km of Neoproterozoic and Cambrian strata. In contrast, along the inferred continuation of the margin in east central Idaho, correlative rocks are missing across the Lemhi arch. Our field mapping and U‐Pb dating studies, located approximately 50 km west of the Lemhi arch unconformity, focused on a succession of regionally extensive rocks that were previously assigned an Ordovician age. We show that ~1.5 km of strata here overlies a ~667 Ma reworked felsic tuff and was intruded by a 601 ± 27 Ma gabbro sill; we thus redesignate these rocks as Cryogenian and Ediacaran in age. These rocks are overlain by a ~1 km thick Ediacaran to middle Cambrian quartzite. Middle Ordovician quartzites overlie these middle Cambrian strata, indicating that though Neoproterozoic and lower Cambrian rocks are present west of the Lemhi arch, upper Cambrian and Lower Ordovician rocks are thin or absent. Comparison of this redesignated section to the closest correlative sections suggests an initial stage of symmetric rifting followed by later asymmetric rifting. We suggest that prerifting ~1,370 Ma magmatism within the Belt basin produced lithospheric rigidity that influenced the final stage of rifting and produced heterogeneity in the geometries of structural domains similar to those documented in other well‐defined, modern rift margins.

Paleozoic Age of Metaterrigenous Sequences of the Maksyutov Metamorphic Complex (Southern Urals): Results of U–Pb Dating of Detrital Zircons

The first U–Pb (LA–ICP–MS) isotope dating of detrital zircons from quartzites of two strata of the Maksyutov metamorphic complex (Southern Urals) was performed. Zircon grains from the Galeevo quartzites are well rounded. The concordant zircon ages fall into the ranges of 529–594, 956–2144, and 2709–2781 Ma; the main age peaks are 544, 551, and 1491 Ma. Zircon grains from the Yumaguzino quartzites are idiomorphic and weakly rounded. The concordant zircon ages fall into the ranges of 497–640 and 957–1027 Ma, forming distinct age peaks of 514, 548, and 605 Ma. The U–Pb ages obtained and their comparison with similar data on the other sequences of the Southern Urals and the Caspian Sea Region indicate the Ordovician age of the protolith of the studied quartzites of the Maksyutov complex.

Early Ordovician Age of Suprasubduction Ophiolites in the Northeastern Part of Central Kazakhstan: U–Th–Pb (SIMS) Dating of Plagiogranites

Suprasubduction zone ophiolites in the Bayanaul and Maikain–Kyzyltas zones in the northeastern part of Central Kazakhstan were studied. U–Pb dating of plagiogranites that constitute the plutonic complexes of the ophiolite sections was performed. The Early Ordovician age estimates of 480 ± 3 Ma for the plagiogranites in the Bayanaul zone and 477 ± 2 Ma in the Maikain-Kyzyltas zone were obtained for the first time. It was demonstrated that the formation time span of the plutonic and volcanosedimentary complexes in the studied ophiolites does not exceed 7–10 Ma.

The First Evidence of Cambrian Granitoid Magmatism during the Formation History of the Bureya Continental Massif of the Central Asian Fold Belt

The results of U–Pb geochronological studies of Early Paleozoic granitoids from the northern part of the Bureya continental massif are presented. It has been established that porphyroblastic hornblende–biotite granites of the Kivili massif referring to the complex of the same name are of 502+6 Ma in age, while weakly gneissic biotite granites of the Kivity massif, Sularin complex, are of 492+6 Ma in age. Thus, granitoids of both complexes are of Cambrian rather than Ordovician age, as was previously assumed. In combination with materials already obtained, these data indicate that intensive granitoid magmatism manifested itself in the Middle Cambrian–Early Ordovician within the Bureya continental massif. It is noteworthy that, in the structure of the Jiamusi–Khanka continental massif, which is adjacent to the Bureya one, Early Paleozoic magmatism manifested itself in a broader range, which is from the Early Cambrian to the Silurian. This may indicate both differences in the formation history of these continental massifs and insufficient knowledge of the Bureya massif.

U-Pb detrital zircon ages of Cambrian–Ordovician sandstones from the Taebaeksan Basin, Korea: Provenance variability in platform shelf sequences and paleogeographic implications

Abstract The early Paleozoic paleogeography of East Gondwanan terranes, including the North China Craton (NCC), is contentious, primarily reflecting the paucity of integrated geochronological, biogeographic, and tectonic data sets. Our new sensitive high-resolution ion microprobe data from 14 sandstones of the Taebaeksan Basin, Korea, indicate that its platform shelf sequences, typified by trilobite faunal assemblages diagnostic of the NCC, record the vestige of coeval arc magmatism. Detrital zircons analyzed from the sandstones yielded Eoarchean to Early Ordovician ages, which define three distinct types of distribution patterns characterized by: (1) double peaks at ca. 1.85 Ga and 2.50 Ga diagnostic of basement rocks in the NCC; (2) minor peaks at ca. 1.75, 1.6, and 1.2–1.1 Ga in addition to double peaks; and finally (3) a scattered array of late Paleoproterozoic to Neoproterozoic zircons lacking double peaks. The marked contrasts among the three types reflect significant changes in provenance, most likely linked to variations in paleo-water depths during the “Sauk” transgression. Longshore- or onshore-directed currents, associated with an increase in water depth, apparently brought outboard oceanic detritus and benthic trilobites into the relatively flat outer shelf of the Taebaeksan Basin. As a result, fine-grained sandstones received a large amount of detritus from distal sources, yielding mixed signatures in zircon age patterns and trilobite assemblages. Excluding the basal sandstone-conglomerate unit, five siliciclastic formations contain syndepositional zircon populations, and their weighted mean 206Pb/238U ages decrease upsection from 512 ± 5 Ma to 483 ± 2 Ma, indicating a sedimentary influx from contemporaneous volcanic activity. In conjunction with arc-related bulk-rock geochemistry and juvenile Nd isotopic signature, early Paleozoic detrital zircons likely represent the first-cycle detritus supplied for ∼30 m.y. from the proto-Japan arc that initially formed at ca. 520 Ma. Together with the occurrence of ca. 700–500 Ma detrital Pacific Gondwana zircons in fine-grained sandstones, Paleozoic arc-sourced detritus suggests that the Korean Peninsula was paleogeographically linked to an ancient convergent margin, perhaps extending from the Terra Australis orogen.

PRESENZA Dl TREMADOCIANO AD ACRITARCHI IN UNITA' TETTONICHE ERCINICHE A SUD DEL LAGO Dl MULARGIA (SARDEGNA CENTRALE)

The activity of the first Hercynian tectonic phase is documented in the Lake Mulargia area (central Sardinia) by the presence of numerous tectonic units. We have examined in particular the southern region of the lake, where one of the upper tectonic units (the Bruncu Maresusus Unit) outcrops. It has at its base a thick silicoclastic succession where we have found Tremadocian Acritarchs in its stratigraphically upper part. This formation is time—equivalent to the Solanas Formation and the Arenarie di San Vito Auct. Another tectonic unit, Bruncu Brevexilis, constitu ted from bottom to top of metavolcanities of probable middle Ordovician age and fossiliferous sediments of Caradocian, Silurian and Devonian age, is also present in the same area and lies under the Bruncu Maresusus Unit.

Source rock kinetics and petroleum generation history of the Upper Ordovician calcareous shales of the Hudson Bay Basin and surrounding areas

The current understanding of the Paleozoic petroleum system in Hudson Bay Basin remains far from complete due in no small part to the uncertainty in locally-calibrated generation and migration models underpinned with reliable source rock kinetic data. To improve the petroleum systems model, four representative immature organic-rich calcareous shales of Late Ordovician age from onshore outcrops surrounding the Hudson Bay were analysed to characterize their source rock generation kinetics. To constrain uncertainty due to laboratory measurement errors in kinetic parameter estimation, six heating rates were used in the pyrolysis experiments for each sample, resulting in 24 kinetic datasets that reduce uncertainty from both source rock heterogeneity and inherited laboratory measurement errors. Under the assumed geological time and geothermal framework, application of the kinetic models resulted in internally consistent generation histories, suggesting that the source rock started to generate oil when temperature reached 100 °C. It is likely where the maximum burial depth excessed 3000 m, the source rocks were in oil generation window with kerogen transformation ratio varying in a range between 52 and 78%. The modeled source rock maturity range from the immature outcrop samples is supported by bulk rock geochemistry data from cuttings of the source rock interval in offshore wells with high S1 peaks, large humps on the left shoulder on S2 curves, typical feature of FID pyrograms in the oil window. Comparison of the kinetic behavior of the samples in this study with those of typical marine Type I, Type II and Type IIS source rocks under laboratory condition and the assumed geological time/temperature framework suggests that kerogen converts to oil in a lower temperature range than those typical normal marine Type II source rock. We attribute the moderate sulfur content (likely high sulfoxide/thiophene ratio) and presence of kerogen component groups that are more prone to cleavage at low temperature as the cause for low temperature onset of hydrocarbon generation.

U-Pb ages on detrital zircons and geochemistry of Lula paragneiss from Variscan belt, NE Sardinia, Italy: implications for source rocks and early Paleozoic paleogeography

Chemical analyses of garnet-bearing metasediments and EMP and U/Pb analyses of detrital zircons of the Lula paragneisses in the Axial Zone of the Variscan belt (NE Sardinia) give significant contribution to the reconstruction of the early Paleozoic evolution of the northern Gondwana margin. The youngest middle Ordovician (465 Ma) age of detrital zircons indicate a derivation from a fore- arc sedimentary basin along an early Paleozoic convergent margin, which collected sediments from nearby emerged lands consisting of early Paleozoic volcanic arcs and pre-Paleozoic sequences. The chemical composition of metasediments is characterized by negative Sr anomaly and depletion in heavy rare earth elements (HREE) and Y as compared to the upper crust. Normalized to chondrite values, the paragneiss shows a steep REE pattern with light-REE enrichment, negative Eu anomaly and flat heavy-REE pattern. All these features suggest a derivation form older active continental margins or island arcs. Detrital zircon ages cover a very large time span (3151 + 97 Ma to 465 + 8 Ma) and reveal a complex history of inheritance and recycling. The oldest ages obtained on relic cores and/or magmatic stages, mostly enriched in Hf (Zr / Hf ratios: 45-24) and devoid of Y, indicate a contribution from granitoid rocks of mainly crustal origin. The ages of detrital zircons highlight a derivation of the sedimentary supply from the Sahara craton and/or the Arabian-Numidian shield along the northeastern margin of Gondwana. This fit well with derivation of early Paleozoic metasedimentary formations in southern Sardinia and northern Apennine, pointing out a common origin and location of these sedimentary basins along the northern margin of Gondwana at the early Paleozoic. As regards the Variscan orogeny, the common middle Ordovician age of Lula paragneiss and nearby Lodé orthogneiss highlight the role of Variscan tectonics in the assembly of different blocks of early-middle Paleozoic margin.

Geochronology of the Oliverian Plutonic Suite and the Ammonoosuc Volcanics in the Bronson Hill arc: Western New Hampshire, USA

Abstract U-Pb zircon geochronology by sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) on 11 plutonic rocks and two volcanic rocks from the Bronson Hill arc in western New Hampshire yielded Early to Late Ordovician ages ranging from 475 to 445 Ma. Ages from Oliverian Plutonic Suite rocks that intrude a largely mafic lower section of the Ammonoosuc Volcanics ranged from 474.8 ± 5.2 to 460.2 ± 3.4 Ma. Metamorphosed felsic volcanic rocks from within the Ammonoosuc Volcanics yielded ages of 460.1 ± 2.4 and 455.0 ± 11 Ma. Younger Oliverian Plutonic Suite rocks that either intrude both the upper and lower Ammonoosuc Volcanics or Partridge Formation ranged in age from 456.1 ± 6.7 Ma to 445.2 ± 6.7 Ma. These new data and previously published results document extended magmatism for >30 m.y. The ages, along with the lack of mappable structural discontinuities between the plutons and their volcanic cover, suggest that the Bronson Hill arc was part of a relatively long-lived composite arc. The Early to Late Ordovician ages presented here overlap with previously determined igneous U-Pb zircon ages in the Shelburne Falls arc to the west, suggesting that the Bronson Hill arc and the Shelburne Falls arc could be part of one, long-lived composite arc system, in agreement with the interpretation that the Iapetus suture (Red Indian Line) lies to the west of the Shelburne Falls–Bronson Hill arc system.

Fracture characterization and modeling of karsted carbonate reservoirs: A case study in Tahe oilfield, Tarim Basin (western China)

Abstract The characterization and modeling of fractures in karsted carbonate reservoirs are challenging due to the mechanical interactions between fractures and irregular karst elements. This study proposes an integrated approach for characterizing and modeling large-scale fractures within deeply buried fractured karst carbonates of the Yingshan Formation (Ordovician in age) located at the Tahe oilfield, Tarim Basin (western China). This investigation implements a promising workflow to stochastically model the fracture sets interpreted from borehole image logs and compute both fracture porosity and permeability tensor in karsted carbonates. For this case study, we have constructed a Discrete Fracture Network (DFN) by combining two structural seismic attributes (i.e. local structural dip and ant-tracking). We have also applied the automatic fault extraction (AFE) algorithm. The results indicate that this method detects a greater number of discontinuities associated with fractures compared to the conventional methods employed in this kind of reservoir (e.g. seismic coherence and curvature). The fracture set with higher values of fracture porosity and permeability coincided with the orientation of maximum horizontal stress, WSW-ENE. In the model, the highest values of fracture intensity correspond to the most productive oil wells. Finally, this study highlights the importance of fracture modeling for evaluating the distribution of the most favorable production areas, so-called sweet-spots, in karsted carbonates reservoirs.