Visean Age Research Articles

Visean palaeoincisions as centers of fluid-generation in the Kama coal basin

The paper considers palaeoincisions in the Turnean limestones of the Volga-Ural basin, made by alluvial-deluvial sediments of the Visean age and containing interlayers of coals. Processes occurring in these sediments (catagenic reduction of thickness of coal beds and coal-bearing mudstones, aggressive influence of products of defluidization of coal organic matter on the host rocks, etc.) strengthen the fluid dynamic heterogeneity of intracrustal deposits and contribute to emigration of hydrocarbons (HC). The main oil-and-gas-generating strata in the Carboniferous section include rocks of the Tournaisian stage and the Bobrikovsky horizon of the Viseian stage. Palaeoincisions, along with the area distribution of Viseian coals, can be considered as centers of fluid generation, including liquid and gas HC.

Geology of the recently discovered massive and stockwork sulphide mineralization at Semblana, Rosa Magra and Monte Branco, Neves–Corvo mine region, Iberian Pyrite Belt, Portugal

AbstractThe recently discovered massive and stockwork sulphide mineralization of Semblana-Rosa Magra and Monte Branco, situated ESE of the Neves–Corvo volcanogenic massive sulphide (VMS) deposit in the Iberian Pyrite Belt (IPB) is presented. Geological setting and tectonic model is discussed based on proxies such as palynostratigraphy and U–Pb zircon geochronology. The mineralization is found within the IPB Volcano-Sedimentary Complex (VSC) Lower sequence, which includes felsic volcanic rocks (rhyolites) with U–Pb ages in zircons of 359.6 ± 1.6 Ma, and black shales of the Neves Formation of late Strunian age. Massive sulphides are enveloped by these shales, implying that felsic volcanism, mineralization and shale sedimentation are essentially coeval. This circumstance is considered highly prospective, as it represents an important exploration vector to target VMS mineralization across the IPB, in areas where the Lower VSC sequence is present. The Upper VSC sequence, with siliciclastic and volcanogenic sedimentary rocks of middle–late Visean age, shows no massive mineralization but a late Tournaisian (350.9 ± 2.3 Ma) volcanism with disseminated sulphides was also identified. Nevertheless, stratigraphic palynological gaps were found within the Strunian and in the Tournaisian sediments, between the Lower and Upper VSC sequences, reflecting probable erosion and uplift mechanisms linked with extensional tectonics. The Semblana and Monte Branco deposits and the Rosa Magra stockwork are enclosed by tectonic sheets that dismembered the VSC sequence in a fold-and-thrust tectonic complex, characteristic of the NE Neves–Corvo region. The methodologies used allow a geological comparison between Neves–Corvo and other IPB mine regions such as Lousal–Caveira, Herrerias, Tharsis and Aznalcollar.

The La Bellière gold and antimony district (French Armorican Massif): A two-stage evolution model controlled by Variscan strike-slip tectonic

The La Bellière gold district, in the Ligerian domain of the Armorican Massif is the third one of France in term of production. It shows gold-bearing quartz veins neighboring with a complex network of Sb veins, hosted in the Neoproterozoic metasedimentary basement of the Mauges Nappe. New structural data, mineralogical, textural and geochemical assays have been carried out to clarify the gold and Sb veins relationships. Our results favor of two-step hydrothermal event with an early deep-seated gold event followed by a late shallower Sb event. The gold-bearing quartz veins are hosted by N60°E to N90°E sinistral strike-slip faults. These veins exhibit a four-stage paragenesis: 1) early barren quartz filling, 2) arsenopyrite-pyrite with minor scheelite assemblage, 3) gold and base metal sulfides crystallized after a ductile/brittle deformation stage, and 4) pyrite-carbonate coeval with a late brittle stage. These results support a continuous evolution model from a deep emplacement level (12 to 9 km) to a shallow one (7 to <5 km) with a brutal change in the P/T conditions that triggered the deposition of gold (stage 2) controlled by a seismic-valve mechanism. On the contrary, Sb veins correspond to open space vein types, hosted in a N20°W to N20°E conjugated strike-slip fault network arranged in a Riedel shear model, controlled by N130-140°E regional dextral strike-slip shear zone associated to the South Armorican Shear Zone system. A two-stage evolution is recognized, the first one consists of early pyrite-arsenopyrite with microcrystalline quartz, followed by a late stibnite stage formed after geodic quartz deposition. Quartz texture argues for a shallow emplacement (<5 km) of veins controlled by a suction-pump mechanism. Chronologically, the gold vein opening complies with a NE-SW maximum shortening direction different from the late-Variscan stress field responsible for the dextral Armorican shear zone during late Carboniferous, and constrains an indirect dating. Thus, a Visean age is proposed for the formation of gold veins. The overall features of gold deposits are close to those of mesozonal orogenic gold deposits in Europe. Conversely, Sb-bearing veins are generated by a N-S shortening during the late Carboniferous. The Sb event, distinct in time from the earlier gold one, might correspond to a different expression of the so-called “epizonal” type in the orogenic gold model. It is proposed that the Late Carboniferous “Or 300” event is not the unique metallogenic period for gold deposition in the French Variscan domain. Our study highlights the role of the sinistral strike-slip shearing of possible Visean age, in the gold metallogenesis of the Ligerian domain.

Two generations of Variscan garnet: Implications from a petrochronological study of a high-grade Avalonia-derived paragneiss from the Drosendorf unit, Bohemian Massif

Garnets in metapelitic paragneisses from the southern Drosendorf unit in the Austrian part of the Bohemian Massif exhibit two episodes of growth during the Variscan orogeny, which can be distinguished on textural and chemical grounds. The first garnet (grt1) records evidence of high-grade metamorphism in the Late Devonian (Frasnian–Famennian), while the second garnet (grt2) formed by a second high-grade event in the Early Carboniferous (Visean). Both garnet generations contain abundant inclusions, of which monazite, rutile and crystallised melt droplets are particularly useful for reconstructing P–T–t conditions. The Late Devonian age (373 ± 9 Ma) for the first episode of garnet growth was obtained from chemical dating of monazite inclusions in grt1. Metamorphic conditions during the first episode of garnet growth are estimated to have been between 0.7 and 0.8 GPa at 680–700 °C and 0.95–1.10 GPa at 745–785 °C. There followed a phase of cooling and exhumation, after which the second garnet (grt2) were formed beginning under amphibolite facies conditions and continuing prograde to peak conditions of 0.95–1.10 GPa and 745–785 °C, which are similar to those of the first garnet forming event. Subsequently, the rocks experienced near isothermal decompression to 0.5–0.8 GPa. Chemical dating of both monazite inclusions in grt2 and the matrix provide a Visean age (343 ± 3 Ma).A study of detrital zircons in these paragneisses revealed zircon forming events at around 1.2, 1.5 and 1.8 Ga, suggesting an Avalonian provenance. The lack of zircons younger than 1 Ga and the presence of Cadomian metamorphic monazite relics (652 ± 15 Ma) indicates an Early Neoproterozoic deposition age for the sedimentary protolith likely. Our documentation of a Late Devonian high-grade metamorphic event in rocks derived from Avalonian corroborates tectonic models which assume that frontal parts of the Armorican terrane had already docked with Avalonia by this time.

A new early Visean coral assemblage from Azrou-Khenifra Basin, central Morocco and palaeobiogeographic implications

A new early Visean coral assemblage has been recorded from turbidite facies in the southern part of the Azrou-Khenifra Basin, northwest of Khenifra, central Morocco. The newly discovered Ba Moussa West (BMW) coral fauna includes Siphonophyllia khenifrense sp. nov., Sychnoelasma urbanowitschi, Cravenia lamellata, Cravenia tela, Cravenia rhytoides, Turnacipora megastoma and Pleurosiphonella crustosa. The early Visean age of the coral assemblage is supported by foraminiferal and conodont data, with the recognition of the basal Visean MFZ9 Zone. This confirms that the first transgression in the Azrou-Khenifra Basin was during the earliest Visean. The allochthonous coral assemblage was recovered from coarse-grained proximal limestone debris flow and turbidite beds within a fault-bounded unit, lying to the west of a thrust syncline containing upper Visean limestones. No evidence exists of the former early Visean shallow-water platform from which the corals were derived. All other in situ platform carbonate rocks around the southern margin of the Azrou-Khenifra Basin are probably of late Visean (Asbian–Brigantian) age. The early Visean Ba Moussa West coral fauna can be compared with that at Tafilalt in eastern Morocco, as well as in other Saharian basins of Algeria. Many of the genera and species in the Ba Moussa West assemblage are identical to those in NW Europe, with which it must have had marine connections. The new rugose species described, Siphonophyllia khenifrense, is probably endemic to North Africa. Its ecological niche in NW Europe was occupied by S. cylindrica or S. aff. garwoodi.

Tectonic Features and Stages of Evolution of the Baltic–Mezen Shear Zone in the Phanerozoic, Northwestern Russia

The general tectonic features of the Baltic-Mezen zone developed along the border of the Fennoscandian shield and the Russian Plate in the north of the East European platform, are considered. The geophysical remote methods and structural analysis were used. It has been established that the zone has a length of more than 1000 km with a width of 10–40 km and can be traced from the Gulf of Finland in the sub-latitudinal direction along the southern shores of the Lake Ladoga and the Lake Onega in the valley of the River Onega.The Baltic-Mezen zone is represented by a system of folds and faults, fracture zones and brecciation, complicating the complexes of the Archean–Paleoproterozoic basement and the Vendian–Phanerozoic cover. Throughout the zone, changes in the configuration and displacement of the axes of the magnetic anomalies are noted; on seismic sections, the systems of discontinuities penetrating the basement and cover complexes are decrypted. On the surface, the discontinuous structures of the zone are expressed in the various forms of relief, often violate and displace glacial morphostructures. Based on the analysis of materials on the nature of the paleoseismites distribution and surfaces of unconformity in the sediments of the cover, as well as the structural features of various rock complexes, a long period of development of the Baltic-Mezen zone was established during the Caledonian, Hercynian and the latest stages of deformation. The Caledonian stage was associated with the formation of paleoseismites horizons of Kukruz phase (~ 460 Ma), as well as the folded-fault structures formed in the Late Silurian–Early Devonian. The Hercynian stage was manifested by several seismic and tectonic events in the Late Devonian (Frasnian age), in the Early (Visean age) and Late (Kasimovian century) Carboniferous. The newest stage was associated with Neopleistocene glaciodynamic processes and two stages of seismic events in the late and post-glacial period, which were accompanied by activation of ancient faults and local development of folded structures. The developed scale of paleoseismic and deformation events of the Baltic-Mezen zone can be used for tectono-dynamic reconstructions based on additional analysis of materials of structural-kinematic research.

Two Early Carboniferous Brachiopods, Levitusia humerosa (Sowerby, 1822) and Grandispirifer mylkensis Yang, 1959, from the Arisu Formation of Okuhinotsuchi, South Kitakami Belt, Japan

Two early Carboniferous brachiopod species, Levitusia humerosa (Sowerby) and Grandispirifer mylkensis Yang, are described from Japan for the first time. These species, from the middle part of the Arisu Formation in the Okuhinotsuchi area, South Kitakami Belt, indicate an early Visean age. Therefore, the middle part of the Arisu Formation is correlated with the lower Visean. In terms of palaeobiogeography, occurrence of the two species suggests that South Kitakami was probably the eastern extension of the North China Province, and located near and to the east of North China (Sino-Korea) during the early Visean.

The main reserve of accelerated effective opening of oil and gas fields in Ukraine

Economic hardship in Ukraine during the years of independence led to a sharp reduction of exploration work on oil and gas, a drop in hydrocarbon production, a decrease in inventories and a sharp collapse of research work to ensure the growth of hydrocarbon reserves.The hydrocarbon potential of various sources of Ukrainian subsoil is quite powerful and can provide future energy independence of the country. Potential hydrocarbon resources in traditional traps of various types are exhausted by only 25 %. Ukraine has recently experienced so-called “shale gas boom”. The experience of extraction of shale gas in desert areas of the United States can not be repeated in densely populated Ukraine in the absence of such powerful shale strata, resource base, necessary infrastructure, own technologies and techniques and economic, environmental and social risks.Taking into account the fuel and energy problems of the state, we constantly throughout the years of independence oriented the oil and gas industry and the authorities on the active use of our own reserves and opportunities for accelerated opening of new oil and gas fields.The results of geological exploration work in the old oil and gas basins at the high level of their study indicate that deposits in non-structural traps dominate among open deposits.A complex of sequence-stratigraphical, lithology-facies and lithology-paleogeographical studies is being successfully used to forecast undeformational traps in well-studied oil and gas bearing basin of the Ukraine – the Dniprovsko-Donetsky basin. The authors predict wide development of stratigraphic, lithologic, tectonic and combined traps in terrigenous sediments of Tournaisian and Visean age, reef-carbonate massifs of the lower Tournaisian, lower and middle Visean age and others. They should become the basis for exploration of oil and gas fields for the near and medium term and open the second breath of the basin.

The oldest known tetrapod (Temnospondyli) from Germany (Early Carboniferous, Viséan)

A unique skull roof fragment of a relatively large-sized tetrapod of Visean age from Chemnitz-Glosa, Saxony, is described. The specimen consists of three bones, an elongated supratemporal with a radially arranged dermal sculpture and the sulcus of the otical part of the infraorbital line, the medial portion of the squamosal which is sutured with the anterolateral supratemporal, and a small, strip-like tabular bone. A deep “otic notch” is indicated. This new tetrapod was predominantly aquatic, as indicated by the deep and relatively broad lateral line sulcus. The type of dermal sculpture and the configuration of the bones indicate that the specimen is probably an adult temnospondyl, with the course of the lateral lines resembling those of dvinosaurians. Together with Balanerpeton from Scotland, this is the geologically oldest temnospondyl and the oldest known tetrapod record in Germany up to now.

On some Mississippian (Carboniferous) brachiopods from neptunian dykes of the Harz Mountains (central Germany)

Different generations of Mississippian neptunian dykes developed in the drowned Iberg–Winterberg Devonian seamount (Harz Mountains, Germany) yielded poorly diverse and generally small-sized brachiopod faunas dominated by spire-bearers (athyridides, spiriferides) and including several homeomorphic spiriferides. Four limestone levels are distinguished: Erdbach-II Limestone (Scaliognathus anchoralis Zone and basal Pseudognathodus homopunctatus Zone, late Ivorian and basal early Visean), Actinotheca Limestone (Pseudognathodus homopunctatus Zone, Arundian–Holkerian–early Asbian), Goniatite Limestone (Gnathodus bilineatus and Lochriea nodosa zones, late Asbian–early Brigantian), Ibergirhynchia Limestone (Lochriea nodosa Zone, early Brigantian). The faunas (dominant cephalopod facies) of the traditional ‘Erdbach Limestone’ in all older literature related to the Iberg–Winterberg Massif are a mixture of the two older horizons (light crinoidal limestones; late Ivorian to early Asbian), of which the age of their fossil content (especially the well-studied trilobites) needs to be revised by conodonts. New spiriferide genera are described, namely Roemerithyris gen. nov. and Felsithyris gen. nov. with Spirifer macrogaster Roemer 1852 and Felsithyris hercynica gen. et sp. nov. as type species, respectively. The latter species is the most common element of an almost monospecific assemblage of early–middle Visean age. Regeneration traces after fish attacks, developed on a spiriferide shell, are illustrated.

Integrated analysis of fault sealing capabilities when studying and mapping hydrocarbon traps in the Dnieper-Donets Depression

This paper demonstrates how a set of advanced technologies provided a better understanding of the geological structure of the Solokha gas condensate field and underground gas storage located in onshore Ukraine. In particular, these technologies helped us improve the process of hydrocarbon trap delineation and subsequent exploration well planning in this area. The study area is structurally complex because it is located in the central part of the axial zone of the Dnieper-Donets Depression (DDD). This part of the DDD is characterized by active halokinesis that has produced morphologically developed salt bodies and associated with them plicative and disjunctive deformations of the host rocks. The structure of the area has to a large extent been formed under the impact of one of such bodies, the closely located and actively developed Bakeyski diapir. The workflow used consists in synergic integration of high-quality 3D seismic data processing, detailed structural and stratigraphic interpretation and seismic inversion combined with pore pressure measurements in wells, lithotype clustering using Kohonen self-organizing maps, and fault seal analysis. A ramified network of faults, which plays an important role in the study area, formed independent tectonic traps in the Middle Jurassic sediments of the Bayosian age and led to the compartmentalization of one of the stratigraphic traps in the Lower Carboniferous sediments of the Visean age. To analyze sealing capabilities of these faults, Allan diagrams and Shale Gouge Ratio are used. The first of these methods is based on the juxtaposition of permeable and impermeable rocks on both sides of the fault, and the second, more advanced, is based on the calculation of the percentage of the clay component of all rocks that have slipped along the fault past a given point.

The significance of early Permian and early Carboniferous U–Pb zircon ages in the Bossòst and Lys‐Caillaouas granitoids (Pyrenean Axial Zone)

The Axial Zone of the Alpine Pyrenees exposes Ediacaran‐Palaeozoic metasediments and orthogneisses intruded by abundant Variscan granitoids that resulted in elongated HT metamorphic domes separated by tight synforms with low‐grade metasediments. Yet, the age of the granitoids within the metamorphic domes remains debated, hindering the correlation of Variscan deformation stages and the HT metamorphism between the different domes. Here, we dated two granitoids located within classical metamorphic domes using U–Pb LA‐ICP‐MS zircon geochronology: the Bossòst and the Lys‐Caillaouas granitoids in the Garona and Lys‐Caillaouas domes, respectively. The results indicate that the leucogranitic facies of both granitoids have Early Permian crystallization ages at 295 ± 2 Ma and 297 ± 2 Ma, respectively. Furthermore, we found a small population of inherited zircon with Serpukhovian (331–323 Ma) ages in the Lys‐Caillaouas granitoid, reinforcing the existence of an elusive but widespread early Carboniferous magmatic event in the Pyrenees. These ages discard two previous hypotheses explaining the age disparity observed in the different domes: either that the HT‐LP metamorphism is of Visean age only or that the dome formation was diachronous along the Axial Zone, ranging from Visean in the central part to late Carboniferous in the eastern Axial Zone. The early Permian age found in the leucogranitic facies of the Bossòst granitoid questions the proposed Visean age for the formation of the Garona dome and the peak HT‐LP metamorphism and points to an Early Permian age as most likely in line with what is observed in the Lys‐Caillaouas Massif.

Reliability of very low-grade metamorphic methods to decipher basin evolution: Case study from the Markstein basin (Southern Vosges, NE France)

Low- and very low-grade metamorphic studies investigating the alteration and reaction progress of clay minerals are powerful tools to decipher the thermal evolution of sedimentary and inverted meta-sedimentary basins. Sheet silicates such as illite and chlorite are very common in sedimentary basin sequences. They can be used to determine the grade of diagenesis and low-temperature metamorphism as measured through the XRD: illite Kübler-Index (KI; illite “crystallinity” in older literature) and the chlorite Árkai-Index (ÁI; chlorite “crystallinity” in older literature), respectively. Although the ÁI method is considered to be slightly less sensitive than the KI method, a reliable correlation between both methods is often observed in metamorphic domains with a uniform heat-flow history and minor tectono-structural complexity. Complementary to these methods, the K-white mica b cell dimension provides a robust estimate of pressure facies reached in very low- to low-grade temperature domains.Here, we present a case-study from the Markstein basin located in the Southern Vosges. The lithostratigraphic units in the basin are characterized by deep marine flysch sequences of Upper Devonian to Upper Visean age and volcano-clastic sediments, respectively. The Markstein basin is surrounded by granitoids with intrusion ages between 340 and 326Ma. A previous study showed orogenic deformation characterized by regional folding, and a contact metamorphism found in an outer halo of the granitoids up to 1500m away from the contact (delineated by the occurrence of biotite). Here we present a multi-disciplinary study combining mineral assemblages, illite and chlorite “crystallinity indices”, and K-white mica b cell dimension. Our approach allows to (i) map in (great) detail the areal extent of both regional/burial metamorphic and contact metamorphic domains; (ii) reveal the metamorphic zonation within both domains; and (iii) better constrain regional/burial and contact metamorphic history. The contact metamorphic domain is characterized by the occurrence of biotite and/or actinolite and low K-white mica b cell dimensions, whereas the zone of incipient orogenic metamorphism yields KI and ÁI values of the high-grade diagenesis and anchizone with intermediate K-white b cell dimensions.

Paleomagnetism of the Carboniferous Rouchel Block, Tamworth Belt, and pole path revision for the New England Orogen, eastern Australia

ABSTRACTStructural, magnetic and gravity trends of the southern New England Orogen (SNEO) indicate four oroclinal structures, none conclusively confirmed paleomagnetically. Curved structures of the Tamworth Belt (TB)—a continental forearc exposed across six tectono-stratigraphic blocks with interlinked Carboniferous stratigraphies and extensive ignimbritic rocks known to retain primary magnetisations despite prevalent overprinting—are prospective to oroclinal testing through comparison of Carboniferous pole paths for individual blocks. Pole paths (a) have been established for the Rocky Creek and Werrie blocks (northwestern/western TB), (b) are described herein for the Rouchel Block (southwestern TB), and (c) are forthcoming for the Gresford and Myall blocks (southern/southeastern TB). The Rouchel path derives from detailed paleomagnetic, rock magnetic and magnetic fabric studies. Thermal, alternating field and liquid nitrogen demagnetisations show a low-temperature overprint, attributed to late Oligocene weathering, and high-temperature (HT) primary and overprint components in both magnetite and hematite carriers, showing slight, systematic, directional differences with hematite providing the better cleaned site poles. Seven primary mean-site poles of Tournaisian and mainly Visean age and three overprint poles show six positive fold tests, five at 95% or higher confidence levels. Two dispersed groupings of intermediate (IT) and HT overprint site poles of Permian and Permo-Triassic age are attributed to early and late phases in oroclinal evolution of the SNEO. HT and IT/HT overprint site poles of mid-Carboniferous age are attributed to Variscan Australia–Asia convergence. Individual pole paths for the Rocky Creek, Werrie and Rouchel blocks show no noticeable rotation between them, indicating primary curvature for the southwestern TB. Their integrated SNEO pole path establishes a reference frame for determining rotations of the southern and southeastern TB.

Early Visean paleogeography of northern Siberia: New evidence of rift to drift transition along the eastern margin of Siberia

In the Late Devonian–Early Carboniferous the eastern part of Siberia was affected by multi-stage rifting, which led to formation of a Late Paleozoic–Mesozoic passive margin. Four sections of the Lower Visean strata were studied in the northern part of the Kharaulakh Ridge, along the Lena River and Bykov Channel (north-east of Siberia). The studied sedimentary succession comprises two lithologically contrasting units: coarse-grained turbidites and hemipelagic cherts. Deposition occurred in a marine basin bounded by steep slopes, which is a key geomorphological feature facilitating deposition of coarse-grained turbidites. Within the studied sandy turbidites, several different zones of submarine sand-rich fans have been recognized. Integrated sedimentological and petrographical analyses allow us to interpret that the nearby uplifts composed of Proterozoic–Middle Paleozoic rocks were the main provenance areas for the Visean clastic rocks. Those uplifts were formed as a result of the latest stages of rifting in the study region that occurred in the earliest Visean, following termination of much of the tectonic activity during the Tournaisian. A similar rock association has been described previously from the central and southern part of the eastern margin of Siberia suggesting that the rift to drift transition occurred approximately at the same time across East Siberia and is of Early Visean age.

Lower Carboniferous (middle Visean) foraminifers and algae from an interior sea, Southern Carnarvon Basin, Australia

A moderate diversity biota of foraminifers, algae and calcareous microproblematica is recorded from the Lower Carboniferous Yindagindy Formation within the intracratonic Southern Carnarvon Basin in Western Australia. Very shallow metahaline to hypersaline conditions are suggested. The palaeobotanical components are sporadic calcitarcha, tuberitinaceans, codiaceans Orthriosiphon? sp. 1, dasycladaleans? Koninckopora tenuiramosa, and algosponges Issinella devonica, Serrisinella cf. serrensis, Issinella? sp. 1, Kamaena cf. awirsi, Proninella? sp. and Stacheoides spp. The foraminiferal assemblage is dominated by a new koktjubinid taxon: Praekoktjubina yindagindyensis nov. gen., nov. sp. Subordinate associated foraminifers are more sporadic Salpingothurammina? sp. 1, Plectinopsis michelseni nov. gen., nov. sp., Rectopravina multifida nov. gen., nov. sp. (with 4 morphotypes), and scarce Endothyra spp. Despite many of these taxa being endemic, a middle Visean age is probable, based on: (1) the maximum global geographic extent of K. tenuiramosa reached during the middle Visean, (2) the disappearance of Plectinopsis nov. gen. in Palaeotethys and the Urals, and (3) the first occurrence of Praekoktjubina nov. gen. in Palaeotethys. An age no older than the middle Visean is supported by sparse conodonts from the Formation and comparison to episodes of carbonate deposition in the Bonaparte Basin, further north in the East Gondwana rift system. Absence of middle Visean archaediscoids may be due to high salinity. The following algal and foraminiferal families and superfamilies are emended: Scribroporellaceae (a lectotype is selected for Orthriosiphon saskatchewanense); Palaeospiroplectamminidae (due to the creation of Plectinopsis nov. gen.); Haplophragminoidea (due to the creation of Rectopravina nov. gen.); and Globivalvulinoidea and Koktjubinidae (due to the creation of Praekoktjubina nov. gen.).

Zircon U-Pb ages of granitoid apophyses in the western part of the Kłodzko-Złoty Stok Granite Pluton (SW Poland)

Granitoids from the Graniec–Bardo and Myszak apophyses of the Klodzko–Zloty Stok (KZS) Granite Pluton that intrude the Upper Paleozoic flysch of the Bardo Unit have zircon U-Pb ages, measured by SHRIMP, of 341.6 ± 2.8 Ma and 341.4 ± 2.2 Ma, respectively. These results augment our previous dating that recorded a short period of Middle Mississippian (Visean) hypabyssal magmatism that produced a variety of KZS igneous rocks of different compositions between ca. 341 and 331 Ma. The Graniec–Bardo and Myszak apophyses belong to the earliest stage of the pluton emplacement. Geochemical and petrographic studies of the dated samples indicate that they are biotite- and hornblende-rich tonalite and syenogabbro of diverse composition and typical of rocks originating from hybrid magmas formed in the geotectonic transition from an early stage collisional granitoid emplacement to its fast orogenic uplift. The polymetallic auriferous ore mineralisation of contact metasomatic type found in intimate contact with the Graniec–Bardo apophysis near Bardo Śląskie may also be of Visean age.

EXPLORATION OPPORTUNITIES OF THE TECTONICALLY INDUCED MISSISSIPPIAN HYDROTHERMAL DOLOMITE RESERVOIRS IN THE DNIEPER-DONETS BASIN

Exploration for oil and gas in the Lower Carboniferous (Mississippian) Carbonate Platform of Tournaisian - Early Visean age in the Dnieper-Donets basin (DDB) meets in many cases with particular problems caused by lack of detail knowledge about the spatial reservoir morphology and origin of the carbonate reservoirs. Application of the Trenton-Black River exploration play concept based on searching for structurally-controlled hydrothermal dolomite reservoirs (HTD) helps to solve the above task. Complex multistage hydrocarbon prospects in the Tournaisian - Lower Visean carbonate platform combining reservoirs of biohermal/reefal bodies, massive limestones with intense intraformational fracturing (“Mississippi lime” type), clastic and karstified rocks of the erosional residuum at intra-Visean unconformity («Mississippi chat» type), and structurally-controlled hydrothermal dolomites (“Trenton - Black River” type) are indentified in this study and create new promising exploration trend in the DDB. New strategy will shift exploration focus from limited portfolio of anticline and reefal traps to tectonically disturbed peripheral areas and monocline slopes of local depressions in the margin zones of the DDB paleorift thus extending its exploration life.

Advances in the research on Carboniferous deep‐water marine deposits in western Junggar, northwestern China

Carboniferous deep‐water marine strata have been insufficiently studied in western Junggar, NW China where the deep‐water facies successions have long been disputed in terms of age constraints, sequence and palaeoenvironmental reconstruction. This paper introduces some views in the light of new materials obtained from this region in recent years. The presence of the Visean plant fossils from the upper Ta'erbahatai Formation in the Tarbgatay Mountains indicates that the formation can be extended to the Early Carboniferous epoch in age. This unit also displays obvious diachroneity, which is of Late Devonian to Early Tournaisian age in the Saur Mountains and Late Devonian to Visean age in the Tarbgatay Mountains. The Xibeikulasi, Baogutu and Tailegula formations are widely distributed in northwestern Karamay areas. The scouring structures and graded bedding near the boundaries between the three formations confirm the stratal sequence that they were originally assigned, namely the Xibeikulasi, Baogutu and Tailegula formations in ascending order. The ‘fossil chaos’ of the three formations is due to mistaking fossils of other stratigraphic units for fossils of these three formations. After revision, only the Early Carboniferous fossils are considered reliable, and combined with the newly found plant fossils, the Xibeikulasi, Baogutu and Tailegula formations are re‐assigned to the early Visean, late Visean, and latest Visean to Serpukhovian ages, respectively. An extension of the lower Hala'alate Formation was recognized in the southwestern Hala'alate Mountains. The presence of the latest Early Carboniferous brachiopods constrains the Hala'alate Formation as late Serpukhovian to Bashkirian in age, bearing the mid‐Carboniferous boundary. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Geology of the Rosário–Neves Corvo antiform, Iberian Pyrite Belt, Portugal: new insights from physical volcanology, palynostratigraphy and isotope geochronology studies

The lithostratigraphic sequence in the Rosário–Neves Corvo antiform comprises the Phyllite–Quartzite Group, whose top is of Famennian age, the Volcanic Sedimentary Complex, of Strunian to upper Visean age, and the Mértola Formation (the lower unit of the Baixo Alentejo Flysch Group) of upper Visean age. The volcanic sedimentary complex comprises a lower sequence of Strunian (Late Famennian) age and an upper sequence of lower to upper Visean age. Detailed mapping of the antiform towards NW of the Neves Corvo mine, supported by palynological dating, identified two new lithostratigraphic units: the Barrancão member (upper Famennian) ascribed to the Phyllite–Quartzite Group and made up of laminated dark shales with siliceous lenses and nodules, and the Ribeira de Cobres Formation of the Volcanic Sedimentary Complex, containing shales, siltstones and fine volcaniclastic rocks. Based on zircon U–Pb isotope dating, five discrete felsic magmatic events were identified at approximately 354, 359, 365, 373 and 384 Ma. This suggests that the volcanic activity in the area has extended for about 30 Ma, in a context of high regional heat flow as indicated by the geochemical signatures of the felsic volcanic rocks. The characteristics of magmatism and the depositional environment indicated by the sedimentary record should therefore have been highly favourable for massive sulphide formation. However, evidence of massive sulphide mineralization in the study area is still to be found. Moreover, reconstruction of the volcanic facies architecture demonstrated that the volcanic units in the Rosário area are strongly dominated by coherent facies typical of the inner part of thick lavas/domes. In fact, most of their external part, the more favourable location for possible massive sulphide mineralization, is missing. Palynological dating indicates a significant hiatus, recognised between the lower and upper sequences of the volcanic sedimentary complex, which implies erosion of the top of the volcanic centre, where VHMS deposits could possibly have formed. However, lateral areas of this volcanic centre, eventually preserved at depth, have good potential to host massive sulphide mineralization.