Lower Middle Ordovician Research Articles

Early Cambrian hyoliths from the Brigus Formation of Avalonian Newfoundland

The Lower Cambrian (Placentian–Branchian Series) of Avalonian southeastern Newfoundland contains one of the most diverse and best-preserved assemblages of Early Cambrian hyoliths from the North American continent. Primary type, topotype and hypotype specimens from the Brigus Formation collected in the late nineteenth and early twentieth centuries are herein emended or newly recognized as Nevadotheca excellens, Levinoconus florencei gen. et sp. nov., Brigoconus greavesi gen. et sp. nov., Poutinella crispenae gen. et sp. nov., Toutonella chaddockae gen. et sp. nov., and Holmitheca quadricostata. Other less complete material is assigned to Aimitus sp., Dorsolinevitus? sp., Grantitheca? sp., Tizilites spp. 1 and 2, Hyolithidan spp. 1–8, and Opercula A–E. Biogeographical connections of the Avalonian species to Baltica, Laurentia, East Gondwanan South Australia, and the Siberian Platform are reinforced. The temporal distribution for most of these taxa remains unchanged except for Tizilites and Dorsolinevitus, which were previously known only from either the top of Cambrian Stage 4 or the Lower–Middle Ordovician, respectively. John M. Malinky [jmalinky@sbcglobal.net], Department of Physical Science, San Diego City College, 1313 Park Boulevard, San Diego CA 92101 (retired), Gerd Geyer [gerd.geyer@uni-wuerzburg.de], Lehrstuhl für Geodynamik und Geomaterialforschung, Institut für Geographie und Geologie, Bayerische Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany.

The controls of strike-slip faults on fracture systems: Insights from 3D seismic data in the central Tarim Basin, northwest China

The central Tarim Basin has gained wide attention for its petroleum reserves, especially the recent commercial discovery of carbonate reservoirs of the Ordovician age in the Shunbei oil and gas field. A systematic analysis is conducted based on 3D seismic interpretation in the central Tarim Basin. The results indicate the presence of several major strike-slip faults and associated fracture systems. The characteristics of major strike-slip faults indicate a lower positive or negative flower structure in the Lower-Middle Ordovician and faults with a normal sense of movement in the Upper Ordovician due to the regional stress field. The major strike-slip faults commonly cause fractures in multiple scales. The fracture systems in different segments of major strike-slip faults contain significant differences in characteristics. In addition, two models are established with regard to strike-slip fault-associated fracture systems in a compressional setting and an extensional setting. The development of fracture systems varies to a large extent in relation to the scales of a strike-slip fractured zone. The fracture systems in a small-scale shear zone or strike-slip fractured zone usually develop along the fracture plane. In contrast, the fracture systems in a large-scale strike-slip fractured zone commonly develop along the fault zone on both sides.

Stratigraphy, conodont biostratigraphy and correlation of the San Juan Formation (Lower-Middle Ordovician) in the Villicum Range, Eastern Precordillera, Argentina

Stratigraphy, conodont biostratigraphy and correlation of the San Juan Formation (Lower-Middle Ordovician) in the Villicum Range, Eastern Precordillera, Argentina

Peri-Gondwanan acritarchs and chitinozoans from the Lower–Middle Ordovician Lashkarak Formation in the Alborz Mountain Ranges, northern Iran: regional stratigraphical significance and palaeogeographical implications

The Lashkarak Formation (Lower–Middle Ordovician) in the Gerdkuh locality, 10 km west of Damghan city, northern Iran, has been found to contain acritarchs and chitinozoans. This study aimed to understand these chitinozoan and acritarch successions as well as brachiopods in this part of the Alborz Mountains, in a novel manner. Ninety-seven surface samples from this succession were collected and analysed. Thirty samples yielded well-preserved palynomorph taxa such as acritarchs, chitinozoans, and scolecodonts, as well as graptolite remains. In total, 53 taxa were identified among the acritarchs (38 species belonging to 21 genera) and chitinozoans (15 species belonging to 10 genera). Although scolecodonts and graptolite remains were also observed, they were not studied in detail. The palynological analyses revealed the presence of several diagnostic acritarchs in the Lashkarak Formation, including Vulcanisphaera simplex, Arbusculidium filamentosum, Coryphidium bohemicum, Dactylofusa velifera, Striatotheca mutua, Arkonia virgata, and Orthosphaeridium ternatum. These acritarchs allowed the recognition of five acritarch assemblage zones. Similarly, seven chitinozoan biozones were recognised: Eremochitina brevis, Desmochitina ornensis, Belonechitina henryi, Cyathochitina protocalix, Cyathochitina calix, Siphonochitina formosa, and Laufeldochitina clavata. These findings confirm the inclusion of the Alborz Mountains in the peri-Gondwana palaeoprovince during the Early–Middle Ordovician. The co-occurrence of the acritarch and chitinozoan taxa with previously identified brachiopods allowed for the establishment of a more detailed Early–Middle Ordovician biozonation, demonstrating their potential usefulness for global chronostratigraphy. Based on palynological and brachiopod assemblages, a shallow marine inner shelf setting is suggested for the Early–Middle Ordovician at the Gerdkuh locality. Moreover, in this study, four new morphotype species were erected, consisting of one new chitinozoan (Cyathochitina gerdkuhensis sp. nov.) and three new acritarchs (Baltisphaeridium razii sp. nov., Navifusa alborzensis sp. nov., and Orthosphaeridium iranense sp. nov.). However, Othosphaeridium cf. ternatum was left in open nomenclature.

Aryl isoprenoids from the Lower Paleozoic in the Tarim Basin, NW China: Insight into deep ancient hydrocarbon exploration

The deep ancient hydrocarbon accumulation in the Tarim Basin was extremely complex. It was characterized by large deep burial, multistage accumulations, multistage adjustments, and multistage secondary alterations, affording limited effectiveness of steroid/terpene biomarkers used widely in oil-source correlation. Aryl isoprenoids (AIs), as a series of diagnostic biomarkers, have attracted increasing attention. For a detailed investigation of AIs from the Lower Paleozoic strata and its internal crude oil in the Tarim Basin, various experimental techniques have been employed. The m/z 133 and 134 mass chromatograms of the AIs confirmed their identification. The anomalously heavy compound-specific δ13C (up to ∼ −16‰) indicated the unequivocal precursor of green sulfur bacteria that was only developed under photic zone euxinia (PZE) conditions. AIs were detected in highly mature source rocks and the crude oils suffered secondary alterations, indicating that AIs might have strong stability and anti-secondary-alteration ability. Based on their widespread distribution in the Lower–Middle Ordovician Heituao Formation (O1-2h Fm) and the Lower Cambrian Yuertus Formation (Є1y Fm) shales, the discovery of AIs in almost all crude oils suggests that the O1-2h Fm and Є1y Fm shales were the primary source rocks for the remarkably abundant Lower Paleozoic deep petroleum resources. This reverses the previously held belief that the Middle-Upper Ordovician source rocks were the primary source rocks. In addition, AIs might induce the formation of organic-rich source rocks (the TOC was up to 29.8%) by promoting high-quality preservation conditions (containing H2S). Furthermore, unlike the H2S poisoning mechanism for the end-Permian mass extinction explained by AIs, regionally distributed AIs triggered by H2S from non-volcanic eruptions could explain the biological radiations (such as the early Cambrian radiation). Therefore AIs might provide important insights into the research on deep ancient petroleum exploration and its contemporaneous biological events.

Coupled biostratigraphy and chemostratigraphy at Lanna, Sweden: A key section for the Floian–lower Darriwilian interval (Lower–Middle Ordovician)

The Lanna area in the province of Närke, south-central Sweden, hosts a succession of cool-water carbonate strata that are largely devoid of tectonic and diagenetic alteration, and span the Floian through lower Darriwilian Global stages (Ottenbyan–Kundan Regional stages). In this study we assess the integrated biostratigraphic and chemostratigraphic framework for the Ordovician sequence at Lanna, utilizing trilobites and conodonts coupled with bulk-rock carbon and oxygen isotopes (δ13Ccarb and δ18Ocarb). The local succession is thin but relatively complete; biostratigraphic details are similar to coeval successions in Baltoscandia. The new high-resolution δ13C and δ18O data are broadly consistent with other chemostratigraphic records at the regional scale, but notable differences in δ13C are obvious compared to global records. δ13C trends diverge markedly in the Dapingian and intra- and inter-regional differences become increasingly pronounced through younger strata. This may largely be explained by an overall low eustatic sea level, which led to more widespread oceanic restriction of epeiric seas. As a result, δ13C records became more influenced by local variables, muddling the global δ13C signal. This, in turn, suggests that sea level played a larger role than long-term changes within the global carbon cycle for regulating local–regional δ13C gradients in the Middle Ordovician. Regardless of the underlying cause(s), the documented variability poses significant challenges for using δ13C as a tool for detailed stratigraphic correlations. The limited expression of many named ‘carbon isotope excursions’ (CIEs) in the studied interval indicate that they are artefacts of local overprinting rather than reflective of changes to the global carbon cycle. Thus, these CIEs are of questionable correlative importance and validity. Although the scatter in numeric values is relatively large, long-term δ18O trends are overall consistent at the global scale, tentatively supporting the notion that climate cooled during the studied time interval.

Middle Ordovician (Whiterockian) gastropods from central Sonora, Mexico: affinities with Laurentia and the Precordillera

AbstractThe Lower–Middle Ordovician (Ibexian, Whiterockian) sedimentary rocks exposed at Rancho Las Norias includes the informally named Las Norias formation, which consists of an intercalation of carbonate and clastic sediments with abundant marine fauna. These deposits occur as the most austral sedimentary rocks of Ordovician age for Laurentia, providing a critical link to understand the distribution of Ordovician marine faunas of North America. An investigation of the gastropod fauna from the upper portion of the Las Norias formation, Sonora, Mexico, is undertaken for the first time. The gastropod assemblage includes Maclurites acuminatus, ?Monitorella sp., Lecanospira sp., Malayaspira aff. M. rugosa, Lophospira perangulata, and Hormotoma? sp. This assemblage indicates a paleogeographic relationship with Laurentia, including the USA (Nevada), Canada (British Columbia, Newfoundland), Greenland, and the Argentine Precordillera.

Fluid Inclusion, Isotopic, and Elemental Geochemistry Studies of Cave-Filling Calcite in the Lower–Middle Ordovician Yingshan Formation of Tahe Oilfield, NW China: Implication for Karstification in Non-exposed Limestone

Improving the recognition of paleo-fluid circulation history is of great significance to reconstruct pore evolution during carbonate diagenesis. Integrated petrography, fluid inclusion, isotopic and elemental geochemistry (laser ablation inductively coupled plasma mass spectrometry) studies, calcites generated in paleocaves, and fractures were investigated. This study aims to reveal the paleo-fluid origin and karstification event within the nonexposed limestone of the Lower–Middle Ordovician Yingshan Formation in the Tarim Basin. The only generation of blocky calcite growing along the karst paleocave and fracture walls [cave-filling calcite (CFC)] crosscuts burial stylolites. The secondary fluid inclusions obtained from CFC are characterized by the coexistence of liquid-only and liquid-dominated aqueous inclusions with low salinities values (0–2.4 wt%), suggesting that the CFC has experienced a low-temperature environment (<50°C). The depleted δ18O values (−15.32‰ to 12.45‰), seawater-like yttrium and rare earth element patterns, and low ΣREE (<0.65 ppm) have recorded the major episode of meteoric water leaching the Yingshan limestone. This view was further confirmed by the calculated δ18Owater values of parent fluids (−14.3‰ to 2.2‰). The relatively higher 87Sr/86Sr ratios (0.70942–0.70994) are interpreted as the result of meteoric water interacting with the overlying Silurian detrital rock when percolated downward. The geochemical evidence recorded by CFC indicates that the karstification event for Yingshan nonexposed limestone possibly took place in the Early Hercynian period during the late Devonian. Therefore, meteoric water percolating downward along the fractures penetrating insoluble strata and/or migrating laterally along the permeable strata is deemed responsible for the karstification event, although the overlying thick insoluble strata in the coverage area.

Characteristics and impacts on favorable reservoirs of carbonate ramp microfacies: A case study of the Middle—Lower Ordovician in Gucheng area, Tarim Basin, NW China

Based on the latest drilling core, thin section, 3D seismic, well logging data as well as exploration results, the sequence stratigraphy and sedimentary microfacies of the Middle—Lower Ordovician carbonates in Gucheng area, and their controlling effects on the development of reservoir were examined by the theories and methods of fine carbonate sedimentology. The results show that the Middle—Lower Ordovician in Gucheng area is a set of typical carbonate ramp deposits, which can be divided into 10 microfacies in 4 subfacies as follows: back ramp, inner shallow ramp, outer shallow ramp, and deep ramp. The back ramp subfacies consists of muddy-dolomitic flat and dolomitic lagoon microfacies; it is dominated by lamellar micrite dolomite tight in lithology. The inner shallow ramp subfacies includes dolomitic shoal and dolomitic flat in shoal top and dolomitic flat between shoals microfacies; it is mainly composed of crystal dolomite with metasomatic residual structure, as well as abundant karst vugs and intercrystalline pores. The crystal dolomite has an average porosity of 4.36%. The outer ramp subfacies includes medium—high and low energy grain shoal and inter-shoal microfacies; it is dominated by sandy limestone, oolitic limestone, and micrite limestone with few pores. The deep ramp subfacies is dominated by low-energy argillaceous deposits, with local presences of storm shoal microfacies. The Lower—Middle Ordovician has six third-order sequences from bottom to top, among which sequence (SQ3) represents the third member of Yingying Formation. The main reservoir has three fourth-order high-frequency sequences. Apparently, the favorable reservoir in the study area is jointly controlled by sedimentary microfacies and high frequency sequence in the carbonate ramp. The former controls the primary pore structure, and the latter controls the intensities of karstification exposure and dolomitization in the penecontemporaneous period. The dolomitic shoals and top dolomitic flats of different stages, superimposed and connected into favorable reservoirs of considerable scale like “platform margin”, are favorable exploration facies in the carbonate ramp.

Bioturbation enhanced petrophysical properties in the Ordovician carbonate reservoir of the Tahe oilfield, Tarim Basin, NW China

Bioturbation plays an important role in enhancing the reservoir capacity of tight reservoirs. This study aims to understand the alteration mechanism and effects of bioturbation on oil and gas reservoirs, to clarify the key control factors and constraints influencing the production of bioturbation. The petrophysical characteristics of bioturbation and host sediments in carbonate rocks, such as mineral composition, pore spaces, porosity and permeability, were studied based on the detailed observation and description of the Ordovician cores from the Tahe oilfield. The effect of bioturbation on petrophysical properties of carbonate rocks were carefully analyzed. The results show: (1) Two types of bioturbation, Thalassinoides-like burrows and Planolites-like burrows, mainly occur in the Ordovician cores of the Tahe oilfield, ranging from the Lower–Middle Ordovician Yingshan Formation to the Middle Ordovician Yijianfang Formation. The burrow-fills are mainly composed of dolomite with subhedral and euhedral crystals. The host sediments mainly consist of micrite. (2) The pores in the host sediments are poorly developed and are incapable of forming effective reservoir spaces. However, well-developed intercrystalline pores among dolomites and microfractures in bioturbated sediments with better connectivity can form effective pore spaces. The results of porosity and permeability show that the host sediments without bioturbation are characterized by poor porosity and permeability. However, with the increase of bioturbation intensity, the porosity of the bioturbated sediments firstly increases and then decreases, whereas the permeability increases all the time. (3) Multiple burrows overprinted in the Ordovician carbonate rocks, forming a large-scale bioturbated carbonate rocks with lateral continuity and vertical connectivity, due to the suitable sedimentary setting, ecological conditions, favorable spatiotemporal sediment matching, and abundant organism–substrate interaction. Subsequently, diagenesis (particularly dolomitization and dissolution) has played a positive role in altering the rock fabric and improving the petrophysical properties of bioturbated carbonates.

First record of lower – Middle Ordovician (Tremadocian – Dapingian) carbon isotope (δ13Ccarb) chemostratigraphy in the Canning Basin, Western Australia; calibrated with geochronology/biostratigraphy and implications for global correlations

New stable carbon isotope data (δ13Ccarb and δ18Ocarb) from the Lower–Middle Ordovician is presented from petroleum exploration well Olympic 1 in the Canning Basin, Western Australia. Olympic 1 continuously cored 319.53 m of sedimentary strata, from the basal part of the Willara Formation through the majority of the Nambeet Formation. This cored section comprises carbonate and siliciclastic deposits that constitute the primary fill sequence of the Canning Basin. Previous studies report CA-IDTIMS dates between 479.37 ± 0.16 Ma and 470.18 ± 0.13 Ma from bentonite beds within the Nambeet Formation and lower Willara Formation in Olympic 1. Four conodont biozones ranging from the Paroistodus proteus conodont Biozone (upper Tremadocian) to the Jumudontus gananda Biozone (upper Floian to lower Dapingian) have been identified from the same interval. The new carbon isotope data from the Olympic 1 core are superimposed on this robust framework to provide context for Tremadocian to Dapingian stable isotope trends for Australia.Three positive δ13Ccarb excursions are identified in the Olympic 1 δ13Ccarb isotope curve, which are correlated to the global generalized δ13C curve. The highest of these occurs in the basal Willara Formation, immediately above a CA-IDTIMS radioisotopic date of 470.18 ± 0.13 Ma within the upper Floian to lower Dapingian J. gananda conodont Biozone. The second falls in the upper Nambeet Formation, immediately above a middle Floian CA-IDTIMS radioisotopic date of 472.82 ± 0.13 Ma, and within the Oepikodus communis conodont Biozone. The oldest positive excursion occurs in the lower Nambeet Formation, immediately above a CA-IDTIMS radioisotopic date of 479.37 ± 0.16 Ma, and within the upper Tremadocian P. proteus conodont Biozone.Correlation of the Olympic 1 δ13Ccarb curve to Lower-Middle Ordovician δ13Ccarb curves from the Great Basin USA, Argentine Precordillera, Western Newfoundland and South China confirms the global distribution of dissolved inorganic carbon preserved in the geological record. This integrated record may be a useful reference for constraining the timing of major oceanographic events, and a key to establishing better links between the sedimentary successions of Western Australia and the larger Ordovician world.

Preservation of the Cadia Valley porphyry Au–Cu district, NSW, Australia: Silurian basin formation and subsequent inversion

Preservation of pre-Cenozoic porphyry Cu deposits requires geological events that bury the deposits before significant exhumation. Burial may be due to basin development over the top of the porphyry system or to burial beneath thrust sheets. The porphyry Au–Cu deposits in the Cadia district of central New South Wales underwent both events, and thus this metal-rich part of the crust survived the overprinted tectonic events that shaped eastern Australia. The Cadia porphyry Au–Cu deposits are associated with the Upper Ordovician to lower Silurian Cadia Intrusive Complex emplaced into the Upper Ordovician to lower Silurian Forest Reefs Volcanics and Middle Ordovician Weemalla Formation during the final phase of Macquarie Arc magmatism. Published stratigraphic evidence suggests west-northwest-striking faults influenced stratigraphic facies and thicknesses in the Forest Reefs Volcanics. Generally north- to east-northeast-striking faults that developed early in the district’s history are likely a product of exhumation of the porphyry Au–Cu deposits and their host rocks during and after the Benambran Orogeny. Alteration halos of several of the porphyry Au–Cu and skarns were exhumed at this time. Later during the Silurian (Wenlock to early Ludlow), the partially exhumed porphyry systems were buried beneath sediments, correlated with the Waugoola and Mumbil groups, in a rift–sag basinal sequence deposited initially in local fault-bounded basins that must be connected regionally to formation of the Cowra and Hill End troughs to the west and east, respectively. This basin geometry was controlled by the reactivation of the extant basement faults. Shortening of the Silurian sedimentary succession during younger Paleozoic orogenies was accommodated in thin-skinned fold and thrust faults forming a complex network of bedding-parallel detachments, ramps and anticlines that are linked to steeply dipping reverse and oblique-reverse slip faults in the underlying Forest Reefs Volcanics and the Cadia Intrusive Complex. KEY POINTS The world-class porphyry Au–Ci deposits were initially exhumed for a relatively brief interval during the early Silurian as a result of the Benambran Orogeny. The porphyry Au–Cu deposits were subsequently buried beneath Silurian sediments of the Waugoola Group accumulating in rifted basins formed during the Wenlock and Ludlow epochs of the Silurian, with sedimentary deposition continuing into the Pridoli (latest Silurian) in the lower part of the Mumbil Group. The Silurian basin was inverted with thin-skinned fabrics in the Silurian sedimentary rocks and thick-skin fabric localised along pre-existing faults, burying the deposits.

Carbon isotope (δ13Ccarb) stratigraphy of the Early–Middle Ordovician (Tremadocian–Darriwilian) carbonate platform in the Tarim Basin, NW China: implications for global correlations

AbstractGuided by conodont biostratigraphy and unconformities observed in the field, stable carbon isotopic analysis (δ13Ccarb) was performed on 210 samples from Lower–Middle Ordovician (Tremadocian to Darriwilian) sections and wells in the Tarim Basin, NW China. The δ13C trend in the Tarim Basin sections has three distinct characteristics: (1) from the Tremadocian to the Floian, a positive shift from −1.9 ‰ to −0.2 ‰ is observed near the boundary between the Penglaiba Formation and the Yingshan Formation; (2) from the Floian to the Dapingian, a positive shift in δ13C from −3 ‰ to −0.7 ‰ occurred under large-scale sea-level rise and a change in the sedimentary environment from a restricted platform to an open platform. Changes in the conodont type are also observed in the Tabei region; and (3) from the Dapingian to the Darriwilian, δ13C first decreased and then increased, showing a negative shift at the Dapingian–Darriwilian boundary. During the Floian, δ13C decreased in the study area, while it first decreased and then increased in other regions, which may reflect local sea-level movements in response to isostatic crustal movements. Two types of positive shift were identified at the Floian–Dapingian boundary, which likely show the effects of local factors, including a disconformity, dolomitization, and platform restriction, superimposed on the global signal of the carbon isotope. Some conodont zonations and recurrent negative excursions in Tremadocian, Floian and Dapingian stages appear to be truncated by unconformities, which are accompanied by short-term subaerial exposure due to sea-level fall and local tectonic uplift.

Ordovician (Floian-lower Darriwilian) conodont biofacies of the San Juan Formation in the Cerro Viejo of Huaco, Argentine Precordillera

The conodont biofacies from the San Juan Formation at the Cerro Viejo of Huaco, Central Precordillera of Argentina, are analyzed for the identification of faunal dynamics and sea-level changes. The Prioniodus elegans and Oepikodus evae zones, Floian in age (Lower Ordovician), are identified in the lower San Juan Formation at the inner part of the Huaco anticline. In the western flank of the anticline the succession of the carbonate stratigraphic unit extends from the Floian/Dapingian boundary (Lower-Middle Ordovician) up to the lower Darriwilian (Middle Ordovician) Lenodus variabilis Zone. For the interpretation of conodont biofacies through the Los Gatos creek section, 5044 conodonts from rock samples of the Floian to the lower Darriwilian are studied. Conodont abundance and generic diversity graphs, and cluster analysis, reveal the following biofacies. The Juanognathus-Bergstroemognathus, Protopanderodus-Reutterodus-Drepanodus, Protopanderodus-Oepikodus, Juanognathus-Semiacontiodus and Rossodus-Periodon-Protopanderodus biofacies are determined, which indicate various deposits stacking from inner to middle and outer carbonate ramp environments of the San Juan Formation. The associated analysis of biofacies and lithology allow for the recognition of two transgressive events in the San Juan Formation at the Los Gatos creek section, which could be related to transgressive systems tracts (TST) that occurred during the Lower and Middle Ordovician. The recovered conodonts have a CAI that varies from 2 to 2.5, indicating a burial paleotemperature between 60 and 155 °C. Furthermore, the variation of CAI in conodonts of the same sample, the presence of conodonts with corroded surface lamella, recrystallization and crystal overgrowths, and the presence of stylolites in limestone beds of the San Juan Formation indicate the circulation of hydrothermal fluids.

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.

Depositional facies and stratal cyclicity of carbonate successions in the Yingshan and Yijianfang Group (Lower-Middle Ordovician) in Yuejin-Tuoputai Region, Tarim Basin, NW China

The Lower-Middle Ordovician Yingshan and Yijianfang Group in the Tarim Basin are overwhelmingly composed of cyclic carbonates. Based on microscopic observation and facies analysis from two borehole sections in Yuejin-Tuoputai area, two main types of facies are recognized: semi-restricted subtidal, open-marine subtidal, and these are further subdivided into six and nine lithofacies in the Yuejin and Tuoputai area, respectively. In general, these facies are vertically arranged into shallowing-upward, meter-scale cycles. These cycles are commonly composed of a thinner basal horizon reflecting abrupt deepening, and a thicker upper succession showing gradual shallowing upwards. Based on the vertical facies arrangements and changes across boundary surfaces, two types of cycle: semi-restricted subtidal and open subtidal, are further identified. The semi-restricted subtidal cycles, predominating over the middle-upper Yingshan Group and the lower Yijianfang Group, commence with algal bindstone in the bottom and are capped by pelletoid grainstone and bindstone with peloidal grains. In contrast, the open subtidal cycles, dominating the upper Yijianfang Group, are dominated by clastizoic grainstone or clastizoic Bindstone. Based on vertical lithofacies variations, cycle stacking patterns, and accommodation variations revealed by Fischer plots, nine larger scale third-order depositional sequences (Sq3-Sq11) are recognized. These sequences generally consist of a lower transgressive and an upper regressive systems tract. The transgressive tracts are dominated by thicker than-average cycles, indicating an overall accommodation increase, whereas the regressive tracts are characterized by thinner-than-average cycles, indicating an overall accommodation decrease. The sequence boundaries are characterized by transitional zones of stacked thinner-than-average cycles, rather than by a single surface. These sequences can further be grouped into lower order sequence sets: the lower and upper sequence sets. The lower sequence set, including Sq3–Sq7, is at the rising–descending cycle in Fisher Curve plots. And this indicates the accommodation space keeps steady. The upper sequence set, including Sq8–Sq11, some features can be found that these Fisher Curves of sequences flunctuate strongly in the middle-lower Yijianafang Group while keep steady in the upper part. This indicates a phenomenon, where the accommodation space varies from violent to gentle.

Плюмы - новое слово в геологии Урала

Subject. The ideas of plume tectonics became active and developing only several years later than the theory of plate tectonics. But in the Urals, the question of plume tectonics, contrary to plate tectonics, was not raised, with rare exceptions, until the latest decade. It is explained by great difficulties in revelation of plumes in any ancient foldbelt, because seismic tomography is not applicable, and magmatic complexes are partly covered by younger sediments, partly eroded, and, additionally, undergone distortion, connected with continental break-ups and drift, subduction and fold-and thrust deformations. Materials and methods. However in recent years, among the magmatic complexes of the Urals (mostly in its western areas), Pay-Khoy and Novaya Zemlya, thanks to precise definition of magmatic formations ages, study of their geodynamically conditioned petrochemistry and their correlation with other regions, the author identified ten levels of probable manifestations of plume/superplume events. Results. It is assumed that in the modern Urals the relics of the following plumes: Navysh (Lower Riphean), Mashak (Middle Riphean), Arsha and Kiryabinka (Terminal Riphean), Mankhambo (Cambrian), Kidryas (Lower-Middle Ordovician, Ushat (Late Ordovician-Lower Silurian), Kola-Dniepr (Devonian), Stepninsk (Permian), Ural-Siberian (Lower-Middle Triassic) had remained.

Bioestratigrafía y provincialismo de conodontes del tramo medio-superior de la Formación San Juan en el cerro Viejo de Huaco, Precordillera, Argentina

The present study deals with the conodont biostratigraphy from the middle and upper parts of the San Juan Formation (Lower-Middle Ordovician) exposed at the Los Gatos creek section, west of the cerro Viejo de Huaco, Central Precordillera of San Juan Province. The numerous conodonts recovered, corresponding to 55 species, allow to recognize a series of biozones in the studied section. The biostratigraphic analysis carried out herein allows determining associations of conodonts assignable to the Oepikodus evae, Oepikodus intermedius, Baltoniodus triangularis-Tripodus laevis and Lenodus variabilis zones that correspond to the middle Floian to lower Darriwilian. The Baltoniodus navis and Microzarkodina parva zones were not identified due to insufficient information provided by this stratigraphic section. In general, the levels corresponding to the Baltoniodus triangularis-Tripodus laevis Zone contain more conodonts, than the records of other localities from Precordillera. These specimens are well preserved with a color alteration index (CAI) of 2-2.5, indicating overburden paleotemperatures ranging from 60° and 155° C. The large presence of fragmented conodonts, with recrystallized surfaces and crystal overgrowth, could indicate the effect of distinctive diagenetic fossil processes on the bearer limestone. The analysis of the diversity and abundance of genera and species of conodonts by chronostratigraphic intervals presents a great percentage of cosmopolitan taxa, 18% and 38% in common, at species level, only with the North-American Midcontinent Province (NAMP), and with a small percentage with the North-Atlantic Province (NAP) and the Precordillera, which allows to approximate a greater paleobiogeographic affinity of the conodonts recovered with those of the NAMP than those of the NAP for the entire interval studied. On the other hand, the Precordillera is verified as a province with its own characteristics as identified by several authors.

Identification and characteristic analysis of carbonate cap rock: A case study from the Lower-Middle Ordovician Yingshan Formation in Tahe oilfield, Tarim Basin, China

Accurate description of heterogeneous distribution and complex sealing capacity of carbonate cap rock is essential for prospecting carbonate reservoirs. Based on core and thin sections, well drilling, fullbore microscan imaging (FMI) and conventional logs, qualitative and quantitative identifications of cap rock were performed within the Yingshan Formation (O1-2y) in Tahe oilfield. Porosity and permeability measurements, scanning electron microscope (SEM) analysis, and displacement pressure calculations were used to characterize the reservoir-cap assemblages. After calibration based on cores and wireline logs, four recognition patterns of cap rock, including blocky-bearing, layered-bearing, porphyritic-bearing and linear-bearing, are developed on FMI. Four sensitive logging parameters (deep-shallow dual lateral resistivity (RD-RS), natural gamma ray (GR), caliper (CAL)) were chosen to construct the quantitative identification criteria, with the higher value being RD-RS ≥ 1000 Ω m and lower being GR ≤ 15 API. Four lithologic types and eleven subtypes were identified in carbonate cap rock, and micrite was dominant lithology. The cap rock is characterized by low porosity ranging from 0.04 to 1.4% (averaging 0.568%) and low permeability ranging from 0.004×10−3 to 0.95×10−3μm2 (averaging 0.07×10−3μm2). Five sets of cap rock have a significant bottom-up difference in diverse members, and the thickness of individual cap rock ranges from several meters to tens of meters. The stability parameter (S) was adopted to reflect the distribution differences, and superior cap rock defined as the Order I (S > 3.84) occurs in the middle part of the 3rd member. Capillary seals control the sealing capacity, and the displacement pressure difference of effective sealing among the cap rock and underlying oil-bearing intervals should be 3 MPa and higher. Strong vertically superimposed and poor laterally connected direct carbonate cap rock plays a decisive role in controlling hydrocarbon preservation and production of carbonate reservoirs.

Characteristics and genetic types of the Lower Paleozoic natural gas, Ordos Basin

The source of the Lower Paleozoic natural gas in the Ordos Basin has been a highly controversial issue. Using data obtained from newly drilled wells, we present the geochemical characteristics of Lower Paleozoic natural gas in various regions of the Ordos Basin. The Lower Paleozoic natural gas is dominated by hydrocarbons, of which methane accounts for 40.17%–97.24%, and the heavier gaseous hydrocarbon (C2+) ranges from 0.01% to 9.72%. Non-hydrocarbon gases in the Lower Paleozoic gas reservoir are mainly CO2 and N2, 2%–10% for CO2 and 0.04%–48.56% for N2, higher than those in the Upper Paleozoic gas reservoir. This observation showed that the high CO2 and N2 content was related to the Ordovician marine carbonate rock deposits. The carbon and hydrogen isotopes of the methane, ethane, and propane in the Lower Paleozoic natural gas of the Ordos Basin showed various degrees of reversal, indicating the mixing origin of natural gas. In the central–eastern Ordos Basin, coal-derived gas from the Carboniferous–Permian (C-P) coal measure and oil-associated from the Lower Ordovician Majiagou Formation marine source rocks gas coexist, as well as mixtures of these two gases. For the Lower Paleozoic natural gas from the western margin of the basin, coal-derived gas coexist with oil-associated gas, but no mixing gas, in which coal-derived gas originated mainly from the C–P source rocks and oil-associated gas from the Lower Ordovician Kelimoli Formation and Middle Ordovician Wulalike Formation source rocks in the western margin of the basin. The Lower Paleozoic natural gas in the southern part of the Ordos Basin was mixing gas, and the oil-associated gas is the dominant one. The natural gas was originated from the Lower Ordovician Majiagou Formation source rocks in the central–eastern Ordos Basin and the Pingliang Formation mudstone and argillaceous limestone at the southern margin of the basin.