Visean Age Research Articles

The Fish Swimming Trace Undichna from the Mississippian Mauch Chunk Formation, Eastern Pennsylvania

Undichna, the swimming trace of fish, was recovered from the middle member of the Mauch Chunk Formation in eastern Pennsylvania in strata of Late Mississippian (Visean) age. These traces represent the only evidence of fish known from the Mauch Chunk Formation and the Carboniferous of Eastern Pennsylvania. The Mauch Chunk Formation in the study area is characterized by an ephemeral fluvial depositional environment that yields a diverse invertebrate ichnoassemblage of the Scoyenia ichnofacies and a tetrapod footprint assemblage dominated by the tracks of temnospondyl amphibians. The Undichna specimens are preserved on fine-grained, mudstone-draped, rippled sandstone. Two specimens, assigned to U. Britannica, consist of pairs of well-defined, narrowly incised, sinusoidal wave traces that are out-of-phase. Two specimens, assigned to U. quina, consist of two pairs of in-phase sinusoidal waves intersected by an additional single wave with a greater amplitude. Two specimens, each composed of a single wave, are assigned to U. unisulca. The slabs of rock with the Undichna traces also preserve tetrapod undertracks assigned to Batrachichnus, Matthewichnus, and Hylopus. This trace fossil association indicates the presence of small fish, some with caudal and/or anal fins and some with pectoral, pelvic, and anal and/or caudal fins, that touched the sediment during periods of intermittent flooding of subaerial surfaces walked upon by early tetrapods. Examples of Undichna from Mississippian or older deposits, especially from a fluvial paleoenvironment, are rare; few have been reported from the Devonian and the Lower Carboniferous.

Amphibian Body Impressions from the Mississippian of Pennsylvania, USA

Temnocorpichnus isaacleai is a new ichnogenus and ichnospecies of temnospondyl amphibian body impression from the Mississippian (Visean) Mauch Chunk Formation of eastern Pennsylvania, United States. The shovel-shaped head, robust limbs, relatively short trunk and smooth integument diagnose the ichnotaxon and readily distinguish it from Hermundurichnus and Sauropleura, the only other named body impressions of Paleozoic tetrapods. Temnocorpichnus is a compound ichnogenus in which the footprint ichnogenus Batrachichnus is a behaviorally distinct component. The temnospondyl identity of Temnocorpichnus adds to the sparse and earliest records of temnspondyls, which are of Visean age. The smooth integument of the ichnogenus does not support the presence of ventral scales or armor in the earliest temnospondyls, but body proportions of the Mauch Chunk body impressions indicate a relatively terrestrial temnospondyl not matched by any taxon now known from bones. Three closely associated impressions of Temnocorpichnus on a single bedding plane suggest some sort of gregarious behavior in Mississippian temnospondyls and may support speculation that internal fertilization and associated courtship behavior evolved independently in one group of amphibians more than 300 million years ago.

A juvenile Early Carboniferous (Viséan) coelacanth from Rösenbeck (Rhenish Mountains, Germany) with derived postcranial characters

Abstract. A small coelacanth specimen of Visean age from a newly described locality near Rosenbeck at the northern margin of the Rhenish Mountains (North Rhine-Westphalia, Germany) is described. The head and pectoral girdle are not preserved, however, the specimen can be distinguished from all other known Carboniferous coelacanths by derived characters of the articulated postcranial skeleton. Derived characters include: (1) The slender first and second dorsal fins that carry only seven to eight and six fin rays, respectively. (2) Both the pelvic and anal fin have a broad base and are unusually weakly lobed. (3) The fin rays of the second dorsal fin are much more robust than those of the first dorsal fin. (4) The second dorsal and anal fins are longer than the first dorsal and pelvic fins. The Rosenbeck coelacanth is interpreted as a juvenile specimen, since the basal plates that support the fins are not ossified. doi: 10.1002/mmng.201000004

Fossil floras from the Emma Fiord Formation (Viséan, Mississippian) of the Canadian Arctic Archipelago and their paleoenvironmental context

Abstract Fossil floras from three exposures of the Emma Fiord Formation from Ellesmere Island, Axel Heiberg Island, and Devon Island in the Canadian Arctic Archipelago fill a gap in our knowledge of the regional distribution of Visean age plant assemblages. Plant fossils occur as compressions in two lithologies, calcareous shale and black shale, representing two different lacustrine depositional environments within a broader system of rift lakes that developed prior to the beginning of widespread Sverdrup Basin sedimentation. Calcareous shales from the Grinnell Peninsula of Devon Island preserve an in situ forest dominated by articulated arborescent lycopsid compression fossils similar to Lepidodendron veltheimii , while black shales from the Kleybolte Peninsula of Ellesmere Island and the Svartevaeg Cliffs of Axel Heiberg Island preserve a more diverse assemblage consisting of fragmentary lycopsid compressions and disarticulated foliage taxa similar to species of Rhodea , Aneimites , and Fryopsis . The Emma Fiord macroflora is comparable to those found in similar tectonic and sedimentary settings in Alaska, Greenland, and Spitsbergen. These assemblages are not as diverse as contemporaneous or roughly contemporaneous tropical lowland floras from North America and Europe.

Bol’shakovskii gabbro massif as a fragment of the Southern Urals zone of Early Carboniferous rift

In this paper new data on the absolute age and geochemistry of rocks of the Bol’shakovskii massif, situated in the central part of the Aramil-Sukhteli zone of the Southern Urals, are given. The obtained values are evidence for its Visean age. By the geological-petrographic and petro- and geochemical features, the rocks of the Bol’shakovskii complex differ sharply from ophiolite-type gabbroids, although they reveal a substantial similarity with the gabbro-granite formation of the Magnitogorsk megazone. The Bol’shakovskii massif is situated in the northern branch of the South Urals zone of Early Carboniferous riftogenesis; and its formation is most probably associated with magmatism events during the rift regime in the died_out island arc.

The South Millevaches Middle Carboniferous crustal melting and its place in the French Variscan belt

AbstractSeveral episodes of crustal melting are now well identified in the Variscan French Massif Central. Middle Devonian (ca 385-375 Ma) migmatites are recognized in the Upper and Lower Gneiss Units involved in the stack of nappes. Late Carboniferous migmatites (ca 300 Ma) are exposed in the Velay Massif only and Middle Carboniferous migmatites crop out in the Para-autochthonous Unit and southern Fold-and-Thrust Belt. In the SW part of the Massif Central, the South Millevaches massif exposes migmatites developed at the expense of ortho- and paragneiss. They form kilometer-sized septa within the foliated Goulles leucogranitic pluton, which is in turn intruded by the non-foliated Glény two micas granite pluton. Monazite grains extracted from these three rock-types have been dated by the EPMA chemical method. Three samples of migmatite yield a late Visean age (ca 337-328 Ma), the Goulles and Glény granitic plutons yield ages at 324-323 Ma and 324-318 Ma, respectively. These new results enlarge the evidence of a Middle Carboniferous crustal melting imprint that up to now was only reported in the eastern part of the French Massif Central, in the northern Cévennes and in the Montagne Noire axial zone. At the scale of the French Variscan massifs, the Visean crustal melting event is conspicuously developed since it is recognized from the Massif Armoricain (Vendée and south coast of Brittany) to the Central Vosges. This episode is synchronous with the huge thermal event responsible for the “Tuffs anthracifères” magmatism of the northern Massif Central and Vosges, and took place immediately after the last thickening phase recorded both in Montagne Noire and Ardennes, that is on the southern and northern outer zones of the Variscan Belt, respectively. However, the geodynamic significance of this major event is not fully understood yet.

U-Pb geochronology of felsic volcanic rocks hosted in the Gafo Formation, South Portuguese Zone: the relationship with Iberian Pyrite Belt magmatism

AbstractFelsic volcanic rocks exposed in the Frasnian Gafo Formation, in the Azinhalinho area of Portugal, display very similar geochemical signatures to volcanic rocks from the Iberian Pyrite Belt (IPB). located immediately to the south. The similarities include anomalously low high field-strength elements (HFSE) concentrations, possibly caused by low-temperature crustal melting, which translate into classification problems.A geochronological study, using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of zircon grains from these rocks, has provided concordia ages of 356±1.5 Ma and 355±2.5 Ma for two samples of rhyodacite porphyry, and 356±1.4 Ma for a granular rhyodacite. These results show that volcanism at Azinhalinho was broadly contemporaneous with IPB volcanism, widely interpreted as being of Famennian to Visean age. Considering that the host rocks of the Azinhalinho volcanic rocks are Frasnian, and therefore deposited synchronously with the Upper Devonian Phyllite-Quartzite Group sedimentation in the IPB basin, the radiometric ages imply that the Azinhalinho felsic rocks are intrusive and likely represent conduits or feeders to the volcanism of the IPB.

THE FACIES TYPES AND ZONATION OF THE VISEAN OIL AND GAS BEARING DEPOSITS OF THE CENTRAL PART OF DDB

The main facies types of the Visean deposits of the Dnieper-Donetsk basin have been distinguished on the foundation of combined lithology-facies researches. The character of the facies distribution on area and in section and features of the sedimentation at the Visean age have been determined.

Age of supraore sequence at the syrostan talcomagnesite deposit in the southern Urals

The first information on findings of fossils in the supraore sequence in the open pit at the Syrostan talcomagnesite deposit in the southern Urals is reported. The remains of colonial corals (Tabulata) indicate the Visean age of the coaly-carbonate sequence that overlies talcomagnesite.

Variscan lamprophyres in the Lower Penninic domain (Central Alps): age and tectonic significance

Abstract Lamprophyre dykes have been recently discovered in blocks of gneiss embedded in a calcschist formation of wildflysch type that forms the top of the Mesozoic-Tertiary metasedimentary cover of the Antigorio nappe (the Teggiolo zone) in the Val Bavona (Lower Penninic, NW Ticino, Switzerland). The presence of the lamprophyres gives a clue to the possible source of these blocks. Similar dykes occur in the N part of the Maggia nappe where they are intruded into the Matorello granite and the surrounding gneisses. We studied these lamprophyres at two localities in the Teggiolo zone (Tamierpass and Lago del Zött) and at one locality in the Maggia nappe (Laghetti). Detailed mineralogical and geochemical investigations confirm their great similarity, particularly between the Tamier and Laghetti dykes. They all recrystallized during Alpine metamorphism under amphibolite facies conditions and lost their primary mineral assemblages and textures. The chemistry reveals a calc-alkaline affinity, a limited differentiation range, features of mineral accumulation and intense remobilization in some cases. The lamprophyres are characterized by a high mg# and relatively low contents in REE and other incompatible elements. In situ SHRIMP and LA-ICPMS U-Pb zircon dating yielded ages of 284.8 ± 1.7 Ma (Tamier), 290.0 ± 1.3 Ma (Zött) and 290.5 ± 3.7 Ma (Laghetti). These ages are compatible with the general late- to post-Variscan magmatic evolution of the Helvetic and Lower Penninic domains. The lamprophyres are considered as melts derived from the lithospheric Variscan mantle, variously hybridized and differentiated at the contact with crustal material during late- to post-orogenic extension. These lamprophyres are chemically distinct from earlier lamprophyres of Visean age, emplaced together with their associated granites in transcurrent fault zones during the Variscan orogenic compression. The similarity of these different dykes suggests that the front of the Maggia nappe is a likely source of the gneissic blocks embedded in the calcschists at the top of the Teggiolo zone. They would have been provided by the advancing Maggia nappe during its thrusting over the Antigorio nappe and simultaneous closure of the Teggiolo sedimentary basin.

A NOTE ON CARBONATE-FILLED NEPTUNEAN DYKES IN THE OX MOUNTAINS GRANODIORITE, NW IRELAND

This paper documents the first record of carbonate-filled neptunean dykes within the Ox Mountains Granodiorite. A post-Carboniferous age for the formation of the dykes is rejected and a Visean age favoured. Differential uplift and tilting of segments of the Ox Mountains inlier could have contributed to a complex sub-Visean unconformity and Visean deposition. Field relationships and petrology of the neptunean dykes Neptunean dykes within the Ox Mountains Grano diorite crop out on a peninsula of the isthmus near Pontoon (G 209 005) between Lough Conn and Lough Cullin, Co. Mayo (Fig. 1) at a height of 15m above sea level. They are formed mainly of horizon tally laminated micritic limestones. Seven neptunean dykes occupy eroded vertical joints with a mean ori entation of 098? (Taylor 1966, Figs 32A and 32D; Crane 1984. Fig. 32), extend to a depth of less than a metre, and are less than 2cm in width (Figs 2a and 2b). No evidence of movement along the contact walls has been observed. The dyke infills have sharp contacts with the granodiorite (Figs 2a-2c) and con sist of alternating clean light grey (N7, Geological Society of America 1970) and silty dark grey (N3) sub-parallel bands that are usually less than 3mm in thickness (Fig. 2b). Some bands exhibit small-scale load/erosional structures (Fig. 2b). Since the bands are approximately horizontal they are probably in their formation position. The mineralogy of the infills is predominately micro crystalline calcite (>96%) with clastic fragments of main ly silt-sized sub-angular quartz (rarely exceeding 0.5mm in diameter) and rare altered feldspar and micas (Fig. 2c). Most likely the clastic fragments have been derived from the adjacent granodiorite. Palaeontological material has not been observed and attempts to isolate microfos sils from the infill have proved unsuccessful. Hence, the age of formation for the neptunean dykes has not been resolved with any certainty.

Discovery of Cathaysiodendron Lee from the Early Carboniferous of Jingyuan, Gansu, western China *

Abstract As one of the representative members of the Late Palaeozoic Cathaysian flora, Cathaysiodendron has been previously recorded from the Early Permian Taiyuan and Shanxi Formations in north China. The species of Cathaysiodendron presented here are from the lower member of Chouniugou Formation of Ciyao, Jingyuan, Gansu. The overlying marine beds yield fusulinids that are of Visean age in western Europe and conodonts of the Datang Stage in south China, so that the age of the bed containing Cathaysiodendron as Visean, Early Carboniferous, is well constrained. Such a record of Cathaysiodendron is significant in terms of Carboniferous phytogeography and the evolution of the Cathaysian Lepidophytes. A new species, Cathaysiodendron jingyuanense Wang & Wu sp. nov. is described and a second one mentioned.

Mississippian (Lower Carboniferous) microfossils from the Chios Mélange (Chios Island, Greece)

The Chios Mélange is a thick Paleozoic wildflysch sequence that crops out on the Greek island Chios. It is composed of chert, limestone and volcanic blocks floating in a siliciclastic turbiditic matrix. New data suggest that the youngest blocks within the Chios Mélange are clasts of a breccia from the Kouramia–Nenitouria area that contain conodonts of late Visean or early Serpukovian age. The conodont fauna from the breccias is characterized by the genera Gnathodus and Lochriea, which favored deeper-water, open-marine facies. Elsewhere on the island, in the Papalia-Nagos area, calcareous microfossils of middle to late Visean age have been recovered from lime grainstone beds within the Chios Mélange. Allochems making up the grainstones are interpreted to have been transported into deeper-water turbiditic facies, but there is no evidence of stratigraphic reworking. Age-diagnostic calcareous microfossils include the alga Koninckopora inflata and foraminifers in the genus Paraarchaediscus. In situ Mississippian microfossils indicate that the Chios Mélange is older than Pennsylvanian (Upper Carboniferous) – Permian, as previously thought. The revised age suggests that the origin of the Chios Mélange may be related to the development of an accretionary prism during the Hercynian Orogeny. © 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.

Showa Shell reduces CO 2 produced in hydrogen generation

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.

The Shaimerden Supergene Zinc Deposit, Kazakhstan: APreliminary Examination

The Shaimerden supergene zinc deposit in the southern Urals Mountains is located in the province of Kostanai in northwest Kazakhstan. It lies at the southern end of the Kostanai megasyncline, a north-northeast–trending, structurally controlled area of lower Paleozoic clastic and carbonate sedimentary rocks and volcanic rocks. A zinc-lead resource estimated at 4,645,100 tonnes at 21.06 percent Zn has been defined. The deposit is hosted within a sequence of intertidal to open-marine carbonates and evaporites of Visean (Early Carboniferous) age. Although drilling to date has not intersected a fault, significant faulting in the area is suggested by the presence of polymict debris flows comprising a wide range of carbonate facies and by large variations in micropaleontologic dates. Sulfide deposits replaced hydrothermally dolomitized carbonates and were subsequently reworked into polymict conglomerates of probable Carboniferous age that were deposited in a marine environment. Weathering of the sulfide mineral deposits took place during the Triassic Period, following uplift during the late Paleozoic. The weathering occurred in situ, and small intervals of relict sulfides were preserved in the center of the deposit. The degree of weathering increases outward from the center of the deposit, which passes from massive sulfide to massive hemimorphite-smithsonite to weathered clays with hemimorphite-smithsonite fragments. The supergene minerals are overlain by bauxitic clays of Cretaceous age and Quaternary silty soils and sands.

40Ar/39Ar Geochronologic Constraints on the Timing of Massive Sulfide and Vein-Type Pb-Zn Mineralization in the Western Meseta of Morocco

The Meseta domain of Morocco is located on the southern margin of the late Paleozoic Hercynian orogenic zone. 40Ar/39Ar ages of igneous and hydrothermal minerals (biotite and illite) were analyzed from a representative massive sulfide deposit at Hajar (1.5 Mt Zn, 0.45 Mt Pb, 0.11 Mt Cu, and 900 t Ag) and a vein-type deposit at Tighza (1.0 Mt Pb, 0.3 Mt Zn, and 1,000 t Ag) in the western Meseta. A weighted mean age of hydrothermal biotite of the Hajar deposit is 300.9 ± 2.6 Ma (2 σ uncertainty), identical to a hydrothermal illite age of 305.4 ± 4.8 Ma. These two ages are calculated from about 93 percent of the released 39Ar. Another illite age, calculated from 74 percent of the released 39Ar, 313.4 ± 9.1 Ma, overlaps with or is slightly older than these concordant ages. These 40Ar/39Ar results indicate that the biotite and illite alteration of the Hajar deposit formed almost simultaneously at 304 to 300 Ma. This age range is younger than the Visean age previously assumed on the basis of geologic setting. These 40Ar/39Ar ages are interpreted as the age of hydrothermal activity subsequent to massive sulfide mineralization. A magmatic intrusion beneath the deposit may have triggered this hydrothermal activity. Three 40Ar/39Ar weighted mean ages and one plateau age from the Tighza deposit range from 280.6 to 267.7 Ma. Igneous biotite of the Kaolin granite and hydrothermal illite of the Ighrem Aouss and North veins in the deposit indicate ages of 280.6 ± 5.5 Ma, 280.6 ± 3.1 Ma, and 275.5 ± 3.4 Ma, respectively. These three ages are identical within 2 σ uncertainties. The age of the hydrothermal biotite from the Mispickel granite, 267.7 ± 2.3 Ma, is slightly younger. This younger age may have resulted from two periods of magmatic-hydrothermal activity or from resetting of hydrothermal biotite by Ar loss. Regardless, these ages indicate that granite intrusion and hydrothermal alteration associated with galena- and sphalerite-bearing veins occurred mainly at 280 to 275 Ma in the Tighza deposit. The identical ages of the magmatic intrusion and hydrothermal alteration suggest that the vein-type Pb-Zn mineralization, as well as W mineralization, was associated with the magmatic intrusion. The influence of the late deformation and metamorphic overprint may be related to the lowest-temperature steps of the analytical data. The dates obtained from the lowest-temperature steps from both the Tighza and Hajar deposits concentrate between 235 Ma and 220 Ma. This time range is compatible with the geologic constraints on the deformation, and these dates are thus attributed to the post-Visean regional metamorphism.

Sedimentology and diagenesis of the Dinantian succession in the Vinalmont borehole

The dolomites of the Grande Dolomie de Namur which make up the lower part of the Vinalmont borehole are considered to be eogenetic, i.e. relating to reflux of saline to hypersaline water or due to seawater circulation adjacent to the development of a mixing zone. Their depleted δ18O (-13.50‰ to -13.90‰) is interpreted in terms of recrystallisation due to interaction with high temperature fluids. Their δ13C-composition (-1.68‰ to +1.50‰ PDB) roughly falls within the interval of the original Lower Carboniferous marine calcite, and thus has not been reset. Columnar calcites, which fill up many of the cavities within the upper part of the former formation, are interpreted as palaeo-speleothems of Upper Tournaisian/Lower Visean age. These dominantly non-luminescent columnar calcites with growth bands and microsparite intercalations of detrital origin display stable isotopic signatures supporting their meteoric origin (δ18O- and δ13C-values of -6.49‰ to -10.32‰ PDB and -2.47‰ to -4.79‰ PDB respectively). In the Vinalmont borehole the ‘Encrinite de Flémalle’ and the ‘Limestone of Avins’ are missing. This is explained by non-deposition or by erosion in relation to the just cited karstification. Within the overlying Terwagne Formation mud- to wackestones and peloidal pack- to grainstones display many features indicative of restricted sedimentation conditions with episodic emergence as testified by the development of palaeosols and subaerial crusts. The stable isotope values of these horizons support soil-related processes (δ18O between -8.55‰ and -6.55‰ PDB and δ13C between -5.95‰ and -1.03‰ PDB). An evolution towards more open marine subtidal sedimentation conditions in the Vinalmont Limestones (dominantly pack- to grainstones) is apparent based on the varied fauna assemblage and sedimentary characteristics. The original marine stable oxygen signature has also been reset due to interaction with meteoric water (-9.45‰ and -9.70‰ PDB) but their δ13C (+0.27‰ to +1.07‰ PDB) reflect a clear rock buffering with preservation of the original marine Lower Carboniferous signal. The study of late diagenetic features is helped by the development of bed-parallel compactional and steeply inclined tectonic stylolites. Late diagenetic zebra-dolomite formation has been observed in the Grande Dolomie de Namur. They formed out of hot (mean Th of 115 °C; δ18O of -9.1‰ to -9.4‰ PDB) suprahydrostatic fluids circulating along steep faults. The slightly depleted δ13C-signal (varying between -2.08‰ and -3.09‰ PDB) might also reflect high temperature precipitation conditions. The development of mono-crystalline non-luminescent columnar calcites in these dolomites, filling up large cavities is a second important late diagenetic process. They are interpreted to be the product of hydrothermal karstification (δ18O-values of -16.2‰ to -16.7‰ PDB and δ13C-values of +0.07‰ to -1.17‰, PDB). Furthermore several fracturation events followed by cementation have been recognised of which the later generations typically are characterised by bright yellow luminescent calcite. They are thought to relate to thermal sulphate reduction and thus could be time equivalent with the late diagenetic pyrite which preferentially crystallised along the stylolites.

New data on Lower Carboniferous stratigraphy and palaeogeography of the Malaguide Complex (Betic Cordillera, Southern Spain)

The Malaguide basement is formed by a thick, strongly deformed but weakly metamorphosed sedimentary succession of ?Ordovician–Carboniferous age, mainly made of basinal mudstones and turbidites, which includes a thin but conspicuous Lower Carboniferous chert–limestone interval (Falcoña formation). The chert member (ribbon radiolarites) yielded, for the first time in Southern Spain, Tournaisian radiolarians. The Visean age of the limestone member is refined by conodonts. This formation is related to a period of generalized pelagic sedimentation, caused by relatively high sea level, low clastic input and high equatorial productivity, which preceded the closure of the Palaeotethys basins due to the Variscan orogeny.

Stabilization of early-formed dolomite: a tale of divergence from two Mississippian dolomites

Abstract Several large hydrocarbon accumulations in Alberta, Canada are hosted in dolomitized successions of stacked, thin sabkha-capped cycles of Visean age. Porosity is micro-intercrystalline and occurs in dolomitized restricted subtidal and intertidal muds that have their fine primary fabric preserved. Two such fields are here considered, that illustrate divergent and contrasting modes of dolomite stabilization despite initial similarities in facies and textures. The dolomite in the upper Debolt Formation of the Dunvegan Field (NW Alberta) forms planar-e or microsucrosic fabrics with crystals in the 1–20 μm range. The dolomite is non-ferroan, Ca-rich (average of 58 mol% CaCO 3 ), and poorly ordered. Its stable isotopic signatures range from −0.12 to +3.4‰ VPDB for δ 18 O (mean=+1.3‰) and +0.9 to +4.3‰ VPDB for δ 13 C (mean=+2.6‰). The average radiogenic 87 Sr/ 86 Sr ratio for this dolomite is 0.7077. Both sets of values are consistent with dolomite precipitation from Mississippian marine or modified marine (evaporated) seawater. These parameters are strongly reminiscent of Holocene protodolomites and hence suggestive of a sabkha dolomitization process (shallow seepage reflux or evaporitive pumping). This dolomite with its high associated porosity (average of 15%, and up to 38%), relatively unaltered mineralogical and chemical signatures, both preserved despite 4 km of burial depth, suggests a very unique set of relatively non-reactive physico-chemical conditions during burial (likely a closed system). In contrast, the dolomite from the Mount Head Formation of the Shell Waterton Field (SW Alberta) has undergone measurable neomorphic alteration in several stages in deeper burial environments (open system). Such alteration has affected its crystal size (range δ 18 O ranges between −1.5 in the least altered dolomite and −13.2‰ VPDB; δ 13 C ranges from +3.9 to −1.5‰ VPDB; and 87 Sr/ 86 Sr ranges from 0.7078 to 0.7090). The dolomite has retained a degree of non-stoichiometry with an average Ca content of ∼55 mol% CaCO 3 . The Mount Head dolomites clearly indicate ongoing reactivity between the rocks and basinal fluids during burial, and are in this respect representative of the norm for dolostones in the Western Canada Sedimentary Basin.

Présence d'un magmatisme d'âge Viséen moyen dans le versant sud du massif du Pilat, Massif central oriental; conséquences

Dating of a microgranular granite, which intrudes the granito-migmatitic complex linked to the Velay Upper Carboniferous granite, gives a Middle Visean age (342 Ma). The relationships between these various granites and the low pressure metamorphic series from Pilat are re-examined.