Montagne Noire Research Articles

Paleogeographic patterns of the Cambrian-Ordovician transition in the southern Montagne Noire (France) : preliminary results

Abstract The Cambrian-Ordovician transition in the southern Montagne Noire records a major siliciclastic regressive trend of prograding shoaling complexes (the La Dentelle Formation), separating two transgressive storm-dominated sedimentary systems of mixed (carbonate-siliciclastic) deposits. The latter comprise the underlying La Gardie and Val d’Homs Formations, and the overlying Mounio Formation, all of them displaying evidence of an important synsedimentary tectonic activity. Isolated settings of carbonate productivity, located on intra-shelf ramps and horsts, contain the richest and diversified faunistic communities comprising trilobites, echinoderms, conodonts, carbonate- and phosphate-shell brachiopods, sponge spicules, etc. Although the Cambrian-Ordovician transition of the southern Montagne Noire did not record volcanic events but a rather distensive regime inducing paleotopographies, the latter may reflect a distinct extensional regime recorded in other earliest Ordovician platforms of the French Massif Central, involving oceanization and a major magmatic activity.

Adequacy of the Early Ordovician trilobite record in the southern Montagne Noire (France): biases for biodiversity documentation

ABSTRACTThe litho- and biostratigraphical subdivisions of the Tremadocian-Arenigian succession of the southern Montagne Noire are hereby revised. The Lower Ordovician diversity patterns are estimated through statistical analysis of 27 families, 64 genera and 132 species of trilobites across nine interval zones; from bottom to top: the Proteuloma geinitzi, Shumardia (Conophrys) pusilla, Euloma filacovi, Taihungshania miqueli, Taihungshania shui landyranensis, Colpocoryphe maynardensis, Neseuretus (Neseuretus) arenosus, Apatokephalus incisus and Hangchungolithus primitivus zones. Maximum trilobite diversity occurred within the E. filacovi and A. incisus zones, alternating with two sharp declines (S. (C.) pusilla and T.shui landyranensis zones), in both cases drastically related to transgressive-regressive trends. Trilobites attained their highest diversity in muddy outer-platform settings, decreasing in fine-grained sandstones and siltstones of shoreface environments, where trilobites could still be numerically abundant. Four major trilobite turnovers are recognised at the bottom of the P. geinitzi, S.(C.)pusilla, E. filacovi and T. shui landyranensis zones, in which survivors from previous intervals are not reported at specific level, and important generic replacements took place. Despite the relative incompleteness of the Lower Ordovician trilobite record, envisaged after analysing the high proportion of monotypic taxa and discontinuous ranges of some families, the trilobite record is representative enough to estimate diversity patterns, although a better understanding of the palaeoenvironmental control will greatly enhance biodiversity resolution.

Adequacy of the Early Ordovician trilobite record in the southern Montagne Noire (France): biases for biodiversity documentation

Adequacy of the Early Ordovician trilobite record in the southern Montagne Noire (France): biases for biodiversity documentation

ORDOVICIAN [DOBROTIVIAN (LLANDEILLIAN STAGE) TO ASHGILL] CRINOIDS (PHYLUM ECHINODERMATA) FROM THE MONTES DE TOLEDO AND SIERRA MORENA, SPAIN WITH IMPLICATIONS FOR PALEOGEOGRAPHY OF PERI-GONDWANA

Seven crinoid species and one very unusual column are recognized from the Ordovician, Dobrotivian (Llandeillian Stage) to Ashgill of Spain. Three previously described species, Heviacrinus melendezi Gil Cid, Domínguez, and Silván, 1996; Coralcrinus sarachagorum Gil Cid, Domínguez, and Silván, 1998; and Ortsaecrinus cocae Gil Cid, Domínguez, Torres, and Jiménez, 1999, are considered, and four new species assigned to three new genera are described herein, including Visocrinus castelli, Fresnedacrinus ibericus, Morenacrinus silvani, and Merocrinus millanae. Previous reports of Ramseyocrinus from Spain have been revised, and we agree that this disparid is not presently known from Spain. These crinoids represent the largest Ordovician crinoid fauna from Gondwana, including these localities and those from the Montagne Noire along peri-Gondwana during the Ordovician. The crinoids reported here, from the Montes de Toledo and Sierra Morena, and the Ordovician crinoids from Montagne Noire have very little in common with Ordovician crinoids from the Prague Basin. Thus, these new data support paleogeographic plate positioning that allows for either paleoclimatic, paleoenvironmental, or paleogeographic isolation between the Prague Basin and peri-Gondwana.

Ordovician [Dobrotivian (Llandeillian Stage) to Ashgill] crinoids (phylum Echinodermata) from the Montes de Toledo and Sierra Morena, Spain with implications for paleogeography of Peri-Gondwana

Seven crinoid species and one very unusual column are recognized from the Ordovician, Dobrotivian (Llandeillian Stage) to Ashgill of Spain. Three previously described species, Heviacrinus melendezi Gil Cid, Domínguez, and Silván, 1996; Coralcrinus sarachagorum Gil Cid, Domínguez, and Silván, 1998; and Ortsaecrinus cocae Gil Cid, Domínguez, Torres, and Jiménez, 1999, are considered, and four new species assigned to three new genera are described herein, including Visocrinus castelli, Fresnedacrinus ibericus, Morenacrinus silvani, and Merocrinus millanae. Previous reports of Ramseyocrinus from Spain have been revised, and we agree that this disparid is not presently known from Spain. These crinoids represent the largest Ordovician crinoid fauna from Gondwana, including these localities and those from the Montagne Noire along peri-Gondwana during the Ordovician. The crinoids reported here, from the Montes de Toledo and Sierra Morena, and the Ordovician crinoids from Montagne Noire have very little in common with Ordovician crinoids from the Prague Basin. Thus, these new data support paleogeographic plate positioning that allows for either paleoclimatic, paleoenvironmental, or paleogeographic isolation between the Prague Basin and peri-Gondwana.

Facies and depositional setting of the Lower Cambrian archeocyath–bearing limestones of southern Montagne Noire (Massif Central, France)

Abstract Archaeocyath-bearing limestones of the Lower Cambrian Pardailhan Formation of southern Montagne Noire are restricted to few intervals (H1, H2, H3) of the mixed carbonate-detrital succession exposed in several superposed tectonic units affected by severe deformation. The comparative analysis of the archaeocyath assemblages together with a detailed sedimentological investigation leads to the understanding of the depositional settings and of the building style of the calcimicrobial-archaeocyath groups and to a better definition of the paleogeographic relationships of the different tectonic units. Interval H1: small, low-relief ‘pioneer reefs’ built by Epiphyton bushes and Girvanella crusts, associated with clusters of small stick-shaped archaeocyaths, colonized the detrital sand of the bottom. They are frequent in southern Minervois and more randomly distributed in northern Minervois and northern and southern Pardailhan and represent short-lived attempts of carbonate colonization on the sandy, mobile substrate of a wide continental shelf. Interval H2: Platy and bioclastic grainstones form the substrate and apron of small crust/cement reefs and associated clusters of ribbon-like and conical archaeocyaths in southern Minervois, or of mud-rich calcimicrobial mounds in southern Pardailhan. Displaced cups of stick-shaped archaeocyaths similar to those of Interval H1, occur in the granular facies, while in place saucer-like cups of Anthomorpha margarita are associated to the mud-rich mounds. This facies association records the repeated attempt at instauration and discrete development of a carbonate platform made of low-relief banks, the margins of which were colonized by Girvanella crust-buildups whereas in the more protected deeper zones Epiphyton/Renalcis mud-mounds dominated. Interval H3: Epiphyton/Renalcis, mud-rich mounds with solitary, large saucer-like cups of Anthomorpha margarita, represented by long, ribbon-like fragments, are dominant in the upper part of the platform. They rest on reduced lenses of grainstone and Girvanella crust boundstone and in southern Pardailhan are interbedded with nodular, marly mudstones containing bioclastic debris partly derived from the buildups. In this area they mark the transition from platform to shallow basin.

Phyllocarid remains (Malacostraca, Crustacea) from the basal Famennian (Late Devonian) of the Montagne Noire (Southern France)

From the early Famennian of the Montagne Noire 19 different morphotypes of probably phyllocarid (Crustacea) affinities are described. Apart from formerly unknown enamel-like structures that veil the uppermost tips of the denticles on most of the gnathal lobe morphs, the stratigraphic implications concerning extinction behaviour across the Kellwasser mass extinction event and during early Famennian diversification are discussed. In addition, phyllocarids in the Famennian seem to form a useful stratigraphic tool for biostratigraphical correlation. Therefore, three basal Famennian phyllocarid assemblage zones are distinguished.

Ordovician palaeogeography with new palaeomagnetic data from the Montagne Noire (Southern France)

A joint palaeomagnetic and 40Ar/39Ar study has been performed on two olistolithic blocks from the Cabrières Wildflysch in the Montagne Noire region of the Massif Central in France. There, andesitic volcanic and volcaniclastic rocks of Llanvirn–Early Caradoc age (ca 470–458 Ma) occur. Despite extensive secondary alteration, destruction of the dominant magnetic mineral phase and 40Ar/39Ar whole rock experiments that demonstrate that the volcanic rocks suffered significant argon loss, a positive fold test and the presence of dual polarities suggest that a primary, Ordovician magnetisation has mostly survived. This is one of the few documented cases where the argon system was substantially reset whilst a subordinate set of small, relatively unaltered magnetite grains, probably hosted in silicates, still carry the original, in this case Ordovician, remanence.The new data show that the Montagne Noire region was located at high southerly latitudes (68° +17/-15) during the Mid-Ordovician. This latitude represents the location for NW Gondwana of which the Massif Central was a part. Palaeomagnetic data from all the Central European massifs and terranes demonstrate a close link to the Gondwana Margin during the Lower and Middle Ordovician.

Small shelly fossils from the Lower Cambrian Lastours Formation, southern Montagne Noire, France

The paper presents the first description and illustration of archaeocyaths and small shelly fossils from the lower member of the Lower Cambrian Lastours Formation (southern Montagne Noire). The outcrops of the Caunes–Minervois sheet thrust (Pardailhan nappe) represent alternation of relatively low-energy, shallow subtidal substrates (highly bioturbated mottled limestones), and reef flanks or inter-reef settings surrounded by winnowed archaeocyath-spiculate shelly pavements. The archaeocyathan assemblage (Botoman in age) is dominated by the genera Anthomorpha and Inessocyathus, from which the species Inessocyathus levis is reported for the first time in the southern Montagne Noire. Phosphate-shell microfossils comprise hyolithelminths ( Hyolithellus and Torellella) and conodont-related sclerites ( Yunnanodus), the latter having previously only been reported from the Meishucunian of south China.

Magnetostratigraphy susceptibility of the Frasnian/Famennian boundary

Magnetosusceptibility event and cyclostratigraphy (MSEC) is used to establish a non-polarity-based magnetostratigraphy susceptibility (MSS) between the stratotype region for the Frasnian/Famennian (F/F) boundary sequence in the Montagne Noire of southern France, the eastern Rheinisches Schiefergebirge of Germany, Belgium, the eastern Anti-Atlas of Morocco, and the Arbuckle Mountains of southern Oklahoma (USA). Despite differences of depositional environment, the MSS is remarkably consistent and can be described in the context of a hierarchy of magnetozones that allow the extension of correlation away from the Montagne Noire reference section. The nature of the controls on the influx of iron into the marine system produces a natural hierarchy of at least seven orders or magnetozones designated MSZ1, MSZ2, MSZ3, MSZ4, MSZ5, MSZ6, and MSZ7. These are characterized in terms of the magnitude of their duration. The MSS reference section of choice for the F/F boundary is the well-known Trench C at La Serre (LSC) in the northeastern Montagne Noire of southern France. The F/F boundary is located at the lower boundary of magnetozone La Serre Iα3 b at La Serre as well as in all other sections studied. MSZ4 and MSZ5 magnetozones are used to establish intra- and inter-regional correlation between the reference sequence and the other regions. The base of the Upper Kellwasser event is as variable within the MSS zonation as it is within the biostratigraphic zonation, as is the position of the Lower Kellwasser event.

Rugose corals and associated carbonate microfossils from the Brigantian (Mississippian) of Castelsec (Montagne Noire, southern France)

The disused quarry east of Castelsec offers a view of shallow-marine carbonates of the poorly known Uppermost Mississippian of the Montagne Noire. At Castelsec, sections are studied in two characteristic facies types (bioclastic wackestone and microbial dominated boundstone) of the Upper Mississippian. The succession is rich in rugose corals and carbonate microfossils. Six genera with seven species belonging to a rugose coral fauna consisting of at least eight genera with several species are described herein; Dibunophyllum castelsecensis sp. nov. is described as new. Twenty-seven carbonate microfossils of different groups have been identified. The Castelsec succession is Brigantian in age, based on the stratigraphic occurrence of rugose corals, foraminifers, and calcareous algae observed in both sections. The rugose coral fauna shows relationships with the well-known fauna of northwestern Europe and the Ouralian–Asian Province. Typical elements of northwestern Europe are missing at Castelsec and vice versa. This differentiation between north and south is interpreted as responses to different palaeolatitudes and tectonic settings.

Response of aquatic hyphomycete communities to enhanced stream retention in areas impacted by commercial forestry

1. Aquatic hyphomycetes are an important component of detritus processing in streams. Their response to enhanced stream retentiveness was tested by manipulating three streams located in Kielder Forest (northern England), a large plantation of exotic conifers, and two streams in Montagne Noire (south‐west France) dominated by native broadleaf woodland. Treatment was by placement of logs or plastic litter traps into a 10–20 m stream section. Fungal spores were collected from stream water upstream and downstream of the treated sections over 1–2 years.2. The average concentration of fungal spores in reference sections was nearly 10× greater in the French streams than in the English streams. The number of hyphomycete species was also higher in the French streams. These differences between regions were probably a consequence of the much lower standing stock and diversity of leaf litter in the English streams.3. Despite these large regional differences, the treatment had a clear effect in all streams. Detrital standing stocks were enhanced in treated sections by up to 90% in French streams and 70% in English streams.4. Mean spore density below treated sections increased by 1.8–14.8% in French streams and 10.2–28.9% in the naturally less retentive English streams. The number of fungal species increased significantly below the treated sections of the English streams, although not the French ones.5. In biologically impoverished plantation streams, input of woody debris can increase detritus retention and enhance hyphomycete diversity and productivity. This may have consequent benefits for detritus processing and macroinvertebrate production.

Stratigraphie et sédimentologie des « calcaires à Productus » du Carbonifère inférieur de la Montagne noire (Massif central, France)

Stratigraphy and sedimentology of the Lower Carboniferous 'Calcaires a Productus' of the Montagne Noire (Massif Central, France). The 'Calcaires a Productus' of the Montagne Noire are microbial build-ups. Two formations are defined and dated respectively as Uppermost Visean (Upper Warnantian-Brigantian) and Serpukhovian on the basis on corals. That makes these limestones out to be younger than previously stated (Lower and base of Upper Warnantian-Asbian and base of Brigantian) and indicates that the development of the olistoliths and thrusts including them, due to the Variscan orogeny, was at least as young as the Upper Serpukhovian. The Serpukhovian limestones of the Montagne Noire are correlated with the Lanet Limestone (Mouthoumet Massif, Corbieres) and Ardengost Limestone (central Pyrenees). To cite this article: E. Poty

Thrust-related, diapiric, and extensional doming in a frontal orogenic wedge: example of the Montagne Noire, Southern French Hercynian Belt

The Montagne Noire, which is situated at the toe of the orogenic wedge of the French Massif Central South European Variscides, appears to be a well-suited area for studying the origin and evolution of middle to upper crustal domes adjacent to foreland basins. The data reported in the present paper show that the Montagne Noire dome is a particular type of basement-involved frontal culmination in an orogenic wedge and foreland basin system. This frontal culmination is characterized by a syn-contractional HT decompression recorded by clockwise PTt paths and widespread strata overturning in thrust and fold structures, which controlled the sedimentation in the adjacent foreland basin. These unusual characteristics are interpreted to be a result of the succession of thrusting, diapirism and extensional collapse. Antiformal stacking of syn-metamorphic thrust sheets controlled the first stages of the foreland basin development. Diapirism was essentially responsible for the HT decompression and widespread strata overturning. Extensional doming was a result of late- to post-metamorphic collapse acting on the pre-existing high-amplitude dome. Diapirism and associated isothermal decompression metamorphism, which constitute the essential difference between the Montagne Noire and ‘ordinary’ frontal ridges in orogenic wedges, were probably enhanced by a local partial melting of the upper to middle crust. It is suggested that the occurrence of these phenomena in front of an orogenic wedge was related to local over-thickening due to the superposition of an upper crustal antiformal stack on top of a lower crustal ramp anticline.

Sedimentary patterns across the Lower–Middle Cambrian transition in the Esla nappe (Cantabrian Mountains, northern Spain)

Abstract In the carbonate platforms of the western Gondwana margin, the extinction recorded at the Lower–Middle Cambrian boundary is accompanied by a profound change in the style of carbonate deposition. The Lancara Formation of the Esla nappe (Cantabrian Mountains, northern Spain) contains a distinct sedimentary turnover due to a combination of tectonism, eustatic fluctuations, and immigration and colonization of new benthic communities, such as the youngest archaeocyathan assemblage of the entire Iberian Peninsula. During latest Early Cambrian times, a regressive trend is recorded in the Lancara Formation. This regression was recorded on a peritidal-dominant, homoclinal ramp that is topped by a tectonically induced discontinuity (D1). The latter surface marks the beginning of a last prograding, regressive tendency recorded on an intra-shelf ramp with ooidal/bioclastic shoals protecting archaeocyathan-microbial patch reefs. The overlying discontinuity (D2) corresponds to a major erosive unconformity, which coincides with the Lower–Middle Cambrian boundary in the Cantabrian Mountains. The subsequent, long-term, earliest Middle Cambrian rise in relative sea-level allowed deposition of low-relief, bioclastic shoals bearing a diverse and cosmopolitan assemblage of benthic fauna. Finally, the previous evolution is bounded by a third discontinuity (D3), which marks the beginning of a rhythmic sedimentation indicative of a major phase of tectonic breakdown and drowning of platforms recognised throughout southwestern Europe. Two associations of calcimicrobes occur in the latest Early Cambrian regressive trend of the Lancara Formation: (i) Proaulopora and Subtiflora are identified in peritidal, high-energy settings, lacking self-supported structures, whereas (ii) intergrowths of Epiphyton, Renalcis and Girvanella encrusted branching colonies and solitary archaeocyaths in protected (back-shoal) patch reefs. The latest Early Cambrian regression is correlated in southwestern Europe in both siliciclastic (Iberian Chains and Ossa–Morena) and carbonate-dominant platforms (Cantabrian Mountains, Montagne Noire and Sardinia). Its tops are recognised as diachronous unconformities ranging in age from early Bilbilian to the Bilbilian–Leonian or Lower–Middle Cambrian boundary.

Extensive microbial mats and their influences on the erosional and depositional dynamics of a siliciclastic cold water environment (Lower Arenigian, Montagne Noire, France)

Abstract Lower Arenigian (Ordovician) rocks of the Montagne Noire, France, represent a shallow-marine environment of a high latitude position along the northern margin of Gondwana. Within the weakly metamorphosed siliciclastics six depositional units were recognized: (1) outer shelf; (2) foreshore zone, below wave base; (3) foreshore zone, above wave base; (4) sand bars of shoreface zone; (5) intertidal deposits; and (6) lagoonal zone. Wrinkle structures and various other phenomena were mediated by coherent (cyano-)bacterial mats. The structures can be related to the different facies zones of the paleoenvironment. Whereas the muddy outer shelf, and the high-energetic sand bars were not overgrown by any mat-constructing microbial populations, fine sands of the foreshore zone, the tidal flats and lagoonal areas were widely colonized. Restriction of mats due to competition of space by endobenthic macroorganisms, or by grazers was low. The microbially induced structures are composed of organic material, pyrite, clay minerals (illite, chamsonite, chlorite), and chert. The minerals precipitated in situ during degradation of the organic layers by the activity of heterotrophic bacteria at low temperatures. Because the microorganisms formed a dense organic carpet covering extensive areas of the ancient sea-bottom, they influenced significantly the erosional and depositional dynamics of the sedimentary system of the local Arenigian. Biostabilization counteracted erosion, and baffling, trapping and binding enriched mineral particles. Additionally, the in situ formed minerals contributed to the total amount of sediment. The biotic influence lead to increased accumulation of sediment within the depositional area. The study shows that microbial mats of great extension occur within Phanerozoic siliciclastics of cold paleoclimate zones, and that preservation of the mat fabrics was possible. The significant mats influenced the local sedimentary system, an aspect scarcely taken into account for such paleoenvironments.

Early Late Cambrian Trilobites OF Australo‐Sinian Aspect From The Montagne Noire, Southern France

Ten species of Late Cambrian trilobites occur in a limestone lens at the base of the Val d'Homs Formation in the Cesse Brook section at Ferrals‐les‐Montagnes in the Montagne Noire. Associated in this lens are species referred to Ammagnostus (Ammagnostus), Kormagnostus?, Olentella, Stigmatoa, Shengia, Abharella, Proceratopyge, (Proceratopyge), ProchuangiaPalaeadotes and Paraacidaspis. Of these, Stigmatoa courtessolei and Paraacidaspis ultima are described as new species. The Ferrals fauna has a decidedly Australo‐Sinian palaeobiogeographical relationship, with the implication that previously developed models suggesting a high latitudinal position for southern France during the Late Cambrian should perhaps be revised. key words: Late Cambrian, trilobites, southern France, Ferrals‐les‐Montagnes, Montagne Noire, biostratigraphy, biogeography.

Genesis of polymetallic and precious-metal ores in the Western Mediterranean province (Cévennes, France–Sardinia, Italy)

An attempt has been made at correlation between the Lower Palaeozoic Iglesiente domain of southwest Sardinia and the southern Cévennes-Montagne Noire Variscan domain of France, with particular regard to the Cambrian-hosted economic lead-zinc concentrations and the spatially associated precious-metal (Au) occurrences. Geological, lithogeochemical and lead isotope investigations led to the following conclusions. (1) The major lead-zinc deposits of Iglesiente and southern Montagne Noire have a Mississippi Valley-type origin, which involved remobilization of pre-existing Cambrian syngenetic mineralization during the Ordovician distensive 'Sardic phase'. Lead isotope evidence suggests mixed crustal sources for the Iglesiente lead. (2) The southern Cévennes stratiform Minerai Zéro of the Malines district is not syngenetic with the Cambrian host rocks. For this ore type a synto late-tectonic Variscan formation model is proposed, similar to the model indicated for the genesis of the Salsigne gold deposit in the Montagne Noire. A reevaluation of the Vigan gold occurrences, regionally associated with the Minerai Zéro economic lead-zinc bodies, is therefore suggested. (3) The newly discovered Tertiary epithermal gold province of Sardinia is genetically quite distinct from the Palaeozoic lead-zinc province of Iglesiente.

Evaporitic constraints on the southward drifting of the western Gondwana margin during Early Cambrian times

Lower Cambrian evaporites and carbonates are reported from nearly all the platforms of the western Gondwana margin, which comprises the Souss, Ossa–Morena, Cantabro-Iberian, Armorican and Montagne Noire–Sardinian Basins. Both lithologies were deposited in climatically restricted belts and their changing palaeogeographic distributions, according to recent biostratigraphic correlations, are used to infer the latitudinal motion of this margin. As a result, the time span involved in the Cordubian–Ovetian–Marianian interval (8–10 m.y.) requires a relative high rate of drifting, which supports the high apparent polar wander path rates (defined by palaeomagnetic data) proposed for Early Palaeozoic times. The varied and abundant relics of primary to early diagenetic evaporites (gypsum, anhydrite and halite) demonstrate that extensive evaporitic conditions were associated with carbonate and mixed platform systems in an Early Cambrian arid subtropical belt. Evaporites were originally more abundant than suggested by the reported remains because the deposits have undergone a multistep diagenesis that erased most of the former morphologies. Some petrographic criteria are proposed for recognizing silica pseudomorphs after evaporites, such as the development of ‘chicken-wire’ and enterolithic structures, and the presence of lenticular to lozenge-shaped crystals of gypsum and anhydrite relics.

Evidence and effects of fluid circulation on organic matter in intramontane coalfields (Massif Central, France)

Recent evidence for a Late Carboniferous hydrothermal event responsible for Au–As mineralization within the Variscan belt of the French Massif Central adds a supplementary episode to the already rather complex thermal history of this area. To better understand this history, 45 coal samples from various sites in the Massif Central were studied petrographically (reflectance analysis) and geochemically (Rock-Eval pyrolysis). The results of this study suggest that the studied coal was buried to 1500 m and that the coalification took place within 25 Ma, probably ending at the boundary between the Early and Late Permian (marked by the Saalic orogeny). Two thermal end-members basins were identified: (i) the Carboniferous of Bosmoreau-les-Mines (Limousin) and West Graissessac (Montagne Noire) showing geothermal paleoflow values between 150 and 180 mW m −2, and (ii) the Stephanian of Argentat and Détroit de Rodez (SW Massif Central) with values estimated at between 100 and 120 mW m −2. By plotting the T max and R o values on a diagram, the samples were grouped into two populations, the first showing a positive correlation between R o and T max and the second with higher T max values than expected after R o values. Selected samples of the second group are also characterized by a high Oxygen Index (OI) that increases with T max. These divergence between R o and T max associated with a high OI may be the result of the circulation of slightly oxidizing hot fluids subsequent to coalification. The other kind of R o– T max divergence seems to be linked to local, particularly high thermal activity, especially in Graissessac and Bosmoreau basins. It is interpreted as being due to a difference in response of these two maturity indicators, respectively to the intensity and duration of the thermal events (e.g., short-lived hydrothermal circulation and thermal domes of regional extent).