Sedimentary and biofacies records in calciturbidites at the Devonian-Carboniferous boundary in Moravia (Moravian-Silesian Zone, Middle Europe)

Summary At the Devonian/Carboniferous boundary, major climatic and oceanographic changes influenced sedimentation on carbonate platforms and in peri-platfrom asreas. Three deep-water carbonate successions in Moravia, which were selected to represent different paleotectonic settings, have been studied with the aim of testing the influence of eustatic, climatic and tectonic controls on sedimentation and conodont paleoecology and taphonomy. On the slopes of the wide carbonate platforms of the Moravian Karst Development (Lesní lom and Grygov sections), an exemplary highstand shedding systems developed in the upper Famennian (expansa Zone), marked by a pronounced thickness of their respective calciturbidite successions and an abundance of shallow-water skeletal grains.Palamatolepis— andBispathodus-dominated conodont assemblages contain an admixture ofPolygnathus representing a transported, near-shore component. The eustatic sea-level fall in the praesulcata Zone and the lowstand conditions at the D/C boundary resulted in a decline of carbonate platform production and condensed deposition or nondeposition. In the Lesní lom section, a condensed sequence of turrbiditic calcarenites and shales (Middle praesulcata—lowermost sulcata Zone) was followed by lime mud calciturbidites (sulcata and duplicata Zones). In the conodont assemblages, the first event in the Lower praesulcata Zone was associated with the reduction of ‘mesopelagic’Palmatopic and a bloom of epipelagicPolygnathus communis. The second event in the Middle praesulcata Zone corresponds to the onset of polygnathidprotogranthodid biofacies, indicating a carbonate slope environment. In the Grygov section, a pronounced thickening and upward-coarsening succession of tubiditic calcilutites through calcarenites and intraclast breccias, with poor palmatolepid-bispathodid connodont assemblages (expansa Zone), indicates a progradation of the calciturbidite system associated with sea-level highstand. After a break in sedimentation, covering the interval from the Lower praseulcata to the base of Lower crenulata Zone, thick-bedded, fine-grained calciturbidites were deposited in the Lower crenulata Zone, and are associated with poor, mixed assemblages where siphonodellids and polygnathids predominate. At the isosticha-Upper crenulata/Lower typicus boundary, coasre grained, turbiditic calcarenites and breccias rich in clastic quartz grains and mixed conodont assemblages with reworked Frasnian and Famennian conodonts indicate a deep erosion of the source area, presumably due totectonic uplift (relative lowstand). In the Jesenec section, on the flanks of the volcanic seamount (the Drahany Development), a deep-water Upper Famennian condensed succession of calciturbidites and presumably winnowed pelagic limestones is marked by conodont assemblages of palmatolepid-bispathodid biofacies. More proximal calciturbidites with mixed deep-water and shallowwater conodonts prograde at the top of the Upper Famennian succession (Middle to Upper expansa Zone). A striking hiatus, covering the interval from the Early preaesulcata to the base of Lower crenulata Zone, resulted from extreme condensation and submarine bottom current erosion due to sea-level lowstand in the late Famennian and early Tournaisian. The renewed middle Tournaisian calciturbidite sedimentation with strong evidence of erosion at the source area indicates global eustatic rise and tectonic uplift of the Drahany Development seamounts (relative lowstand). The earlier occurrence of the uplift in the Jesenec area, relative to the Grygov section, shows the advance of tectonic processes over time in the Moravian-Silesian basin (orogenic polarity) as a consequence of Variscan orogenic movements.     

Famennian conodonts from the Hongguleleng Formation at the Bulongguoer stratotype section,western Junggar,Northwest China

The Hongguleleng Formation, the highest and most important Devonian marine carbonate horizon in western Junggar, contains an endemic shallow-water Icriodus–Polygnathus conodont fauna with rare palmatolepids and other genera. The conodont faunas from the Bulongguoer and the Wulankeshun sections are similar, with high abundance (about 40%) of endemic taxa (13 taxa), indicating isolation of the Junggar Basin during early Famennian. Non-endemic species in the faunas suggest that the Lower Member of the Hongguleleng Formation is assignable to the Pa. rhomboidea Zone to the Pa. marginifera marginifera Zone of early Famennian, not including the Frasnian–Famennian boundary. The Upper Member may be of late Famennian–early Tournaisian in age on the basis of our preliminary faunal analysis.     

The history of Devonian-Carboniferous reef communities: Extinctions,effects, recovery

Summary Analysis of the taxonomic composition, diversity and guild structure of five “typical” reef and mud mound communities ranging in age from Late Devonian-Early Carboniferous indicates that each of these aspects of community organization changed dramatically in relation to three extinction events. These events include a major or mass extinction at the end of the Frasnian; reef communities were also effected by less drastic end-Givetian and mid-late Famennian extinctions of reef-building higher taxa. Peak Paleozoic generic diversities for reef-building stromatoporoids and rugose corals occurred in the Eifelian-Givetian; reef-building calcareous algal taxa were longranging with peak diversity in the Devonian. These three higher taxa dominated all reef-building guilds (Constructor, Binder, Baffler) in the Frasnian and formed fossil reef communities with balanced guild structures. The extinction of nearly all reef-building stromatoporoids and rugose corals at the end of the Frasnian and the survival of nearly all calcareous algac produced mid-late Famennian reef communities dominated by the Binder Guild. Despite the survival of most calcareous algae and tabulate corals, the mid-late Famennian extinction of all remaining Paleozoic stromatoporoids and nearly all shelf-dwelling Rugosa brought the already diminished Devonian reef-building to a halt. These Devonian extinctions differ from mass extinctions by the absence of a statistically significant drop in taxonomic diversity and by their successional and cumulative effects on reef communities. Tournaisian mud mounds contain communities markedly different from the frame-building communities in Late Devonian and Visean reefs. Mound-building biotas consist of an unusual association dominated by erect, weakly skeletonized members of the Baffler Guild (chiefly fenestrate Bryozoa; Pelmatozoa) and laterally expanded, mud-binding algae/stromatolites and reptant Bryozoa. The initial recovery to reefs with skeletal frameworks in the Visean was largely due to the re-appearance of new species of abundant colonial rugose corals (Constructor Guild) and fenestrate Bryozoa. This Frasnian-Visean evolution in the taxonomic composition and structure of the reef-building guilds is also expressed by abrupt changes in biofacies and petrology of the reef limestones they produced. Thus, “typical” Frasnian reef limestones with balanced guild structures are framestones-boundstones-bafflestones, Famennian reefs are predominantly boundstones, Tournaisian mud mounds are bafflestones and Visean reefs are bafflestones-framestones.     

Progressive drowning of carbonate platform in the Moravo-Silesian Basin (Czech Republic) before the Frasnian/Famennian event: facies,compositional variations and gamma-ray spectrometry

The Moravo-Silesian Basin (MSB; eastern Czech Republic and southern Poland) hosted an extensive shallow-water carbonate platform in the Middle Devonian to Frasnian interval. The platform drowned in a stepwise fashion from the Palmatolepis hassi to the Pa. linguiformis zone. Three types of drowning successions were revealed from conodont biostratigraphy, facies, microfacies and gamma-ray spectrometry data: (A) drowning to periplatform turbidite setting; (B) drowning to (hemi)pelagic seamount setting and (C) drowning associated with the stratigraphical gap. In the lower Pa. hassi zone, rapid subsidence caused the platform to drown locally along the N–S to NW–SE trending faults (type A drowning). In the upper Pa. rhenana to the Pa. linguiformis zone, the drowning accelerated in the western part of the MSB due to locally higher subsidence rates combined with the Late Frasnian biotic crisis (type B). In the southern part of the basin, the platform emerged shortly before the Frasnian/Famennian (F/F) boundary and drowned in the Early to Late Famennian (type C). The primary cause of drowning was differential subsidence at the Laurussian passive margin. Eustatic sea-level fluctuations, if any, contributed only to a minor extent to the Late Frasnian drowning, but were effective in type C drowning during the Famennian. The drowning boundaries are associated with increased contents of K and Th, reflecting the deceleration of carbonate production. Uranium contents display isolated peaks that roughly correlate with the drowning boundaries or the stratigraphic gaps associated with the F/F boundary. The uranium contents are considered to reflect local depositional conditions and are not suitable for stratigraphic correlation. On the other hand, from the K and Th contents, we can infer Late Frasnian sea-level fluctuations with duration on the order of 1 Myr. These cyclic variations in K and Th contents proved to be useful in platform-to-basin stratigraphic correlation.     

Compositional variations in calciturbidites and calcidebrites in response to sea-level fluctuations (Exuma Sound, Bahamas)

The compositional variation of Pleistocene carbonate gravity deposits from the Exuma Sound Basin, Bahamas, was determined. Two types of gravity deposit were present in the cores of ODP Leg 101, Site 632A, i.e., calciturbidites and calcidebrites. In analogy with earlier studies, the compositional variations in the calciturbidites could be linked to different sources on the carbonate margin, i.e., platform interior, platform edge, and platform slope. Calciturbidites deposited during interglacial, sea-level highstands show a dominance of non-skeletal grains, largely derived from the platform interior, while calciturbidites of glacial, sea-level lowstands, show a dominance of skeletal platform-edge to platform-slope-derived grains. Thus, the calciturbidite composition can be used to reconstruct the position of absolute sea level. In addition, the mud content of the calciturbidites increased after Marine Isotope Stage 11. In contrast, the composition of the calcidebrites remained unaltered through time and showed a clear dominance of platform-edge-derived sediments during varying sea-level positions. The Bahamian carbonate platform is located in a tectonically stable passive-margin setting and the gravity-flow deposits were laid down in an environment exclusively controlled by eustatic sea-level fluctuations. This study shows that all types of gravity-induced carbonate deposits, calciturbidites, and calcidebrites, were deposited in response to global eustatic sea-level variations. The sediment composition could be linked directly to sediment input from specific facies realms along the carbonate platform margin. Hence, sediment composition analysis is a strong tool that may be used to discriminate between gravity-induced deposition triggered by eustatic sea-level changes and that related to tectonic events, when analyzing resedimentation processes in sedimentary basins.     

Uppermost Devonian–Lower Carboniferous conodonts from the Indert Formation in the Shine Jinst area,South Mongolia

In southwestern Mongolia, conodonts from the Indert Formation at its type locality in Shine Jinst were investigated to improve regional and global correlation of the uppermost Devonian–lowermost Carboniferous. The abundance and diversity of the conodont fauna is quite low in general, whereas representatives of Siphonodella, Protognathodus, Pseudopolygnathus, Polygnathus and Bispathodus have been collected from the Indert-Uul section. Five conodont biointervals, from the Bi. costatus-Pr. kockeli Interregnum (ckI) to the Siphonodella sandbergi Zone, could be recognized from the top Heermorit Member and the lower part of the Shombon Member, while the middle and upper parts of the Shombon Member cannot be zoned based on the conodonts. Absence of index fossils Si. sulcata, Pr. kuehni and Pr. kockeli hampered precise recognition of the Devonian–Carboniferous boundary (DCB) at Indert-Uul, but this transition is well delineated by typical uppermost Famennian and Tournaisian conodont assemblages. The DCB is tentatively placed at the base of the Shombon Member, consistent with the lithological boundary and the resuming of carbonate production during the DCB transitional interval.     

Revision of Famennian-Tournaisian (Late Devonian – Early Carboniferous) conodont biostratigraphy of Sardinia,Italy

Conodont biostratigraphy of the Famennian and Tournaisian of Sardinia is provided on the basis of all data available in the literature. More than 130 conodont taxa documented from this time-interval allow recognition of nineteen conodont biozones. The zonation scheme adopted here is broadly similar to the standard zonation schemes used in the Upper Devonian and Lower Carboniferous, though a few variations are needed to better accommodate the species occurrences in Sardinia: the velifer Zone and the styriacus Zone have been reintroduced, even though with slightly different meanings than previously used; the Lower praesulcata Zone is expanded to include the Middle praesulcata interval. The Upper praesulcata Zone and the sulcata Zone are missing. All zones are described and discussed and a complete list of bioevents is provided.     

Biostratigraphic subdivision and correlation of Uppermost Carboniferous/Lower Permian sediments in the Southern Alps: Fusulinoidean and condont faunas from the Carnic Alps (Austria/Italy), Karavanke Mountains (Slovenia), and Southern Urals (Russia)

Summary In order to establish a refined biostratigraphic subdivision and correlation of the Uppermost Carboniferous/Lower Permian deposits of the Southern Alps (Carnic Alps, Karavanke Mountains; Austria/Italy/Slovenia), two major microfossil groups (fusulinoideans, conodonts) were investigated within the same sample. The fusulinoidean species diversity (71 species, including five new species and three new subspecies) and generic composition were reviewed and complemented. Additionally, the data on fusulinoidean assemblages were supplemented by co-occurring conodont faunas (seven species). Accompanying studies on material from the type sections of the Southern Urals (Russia) were made to improve the biostratigraphic correlation with the Russian standard zonation and to discuss paleobiogeographical aspects of the faunal associations. An integrated microfacies analysis of the sampled material in the Southern Alps serves to evaluate the relationships between certain genera and specific microfacies types. The fusulinoidean fauna of the Lower “Pseudoschwagerina” Limestone is of late Gzhelian age. The Carboniferous/Permian boundary is close to the base of the Grenzland Formation, which covers the entire Asselian and a part of the Sakmarian. The Upper “Pseudoschwagerina” Limestone and Trogkofel Limestone are Lake Sakmarian to Artinskian. The studies sequences in the Karavanke Mountains. formerly known as “carbonate and clastic Trogkofel beds”, correlate to the Lower “Pseudoschwagerina” Limestone, respectively with parts of the Grenzland Formation. Due to the lithologic differences, new formation names (Dolzanova Soteska Fm., Born Fm.) were introduced for the so-called “Trogkofel” Limestone along the Dolzanova Soteska. Whereas late Gzhelian/Asselian fusulinoidean faunas of the Southern Alps correspond to the Southern Uralian faunas to a large extent, Sakmarian and Artinskian faunas reveal an increasing divergence in species and genus composition. Climatic as well as geographic barriers may have prevennted the dispersal of Paleotethyan taxa into the Southern Urals. Biostratigraphic correlation of Sakmarian to Artinskian deposits is therefore possible only on the basis of the sparse conodont faunas.     

Late Devonian Conodonts from the Zeravshan-Gissar Mountainous Area,Uzbekistan

Late Devonian conodonts, recovered from the siliceous and carbonate rocks of the upper part of the Akbasay Formation on the left side of the Kule Gorge, Zeravshan-Gissar mountainous area (Uzbekistan), are studied. Seven conodont associations are defined for the Frasnian–Famennian interval.     

The middle–late Tournaisian crisis in conodont diversity: a comparison between Northeast Laurussia and Northeast Siberia

The middle–late Tournaisian (Hastarian–Ivorian) transition is marked by isotopic and sedimentological evidence of climatic cooling and glaciations accompanied by a sea level fall and changes in global ocean circulation. At this time, the extinction among conodonts was followed by its gradual recovery in the late Tournaisian–early Viséan. Siphonodellids, representing an important part of conodont assemblages in the early–middle Tournaisian, became extinct. This study aims to compare conodont diversity dynamics in Northeast Laurussia and Northeast Siberia. The materials used range in paleolatitudinal gradients from 20°N (the north of the Urals and Pechora Craton) up to 45°N (Northeast Siberia), and in paleoclimatic gradient from an equatorial to a warm subtropical climate. The middle and late Tournaisian conodont associations of these regions demonstrate a high similarity in taxonomic composition and diversity dynamics, which may suggest that the glaciation and the following decrease in temperature probably was not the direct cause of conodont extinction; instead, the changes in the food web implied by the carbon isotope compositions of conodont elements and host carbonates could have been the main driver of the crisis in conodont diversity     

New Middle Devonian conodont data from the Dong Van area,NE Vietnam (South China Terrane)

Middle Devonian conodonts from the Si Phai section in NE Vietnam are described. The section ranges from the Middle Devonian ensensis to timorensis conodont zones to the Late Devonian rhomboidea conodont Zone. A rich overall assemblage is described, including 27 taxa of species or subspecies rank and 11 taxa described in an open nomenclature. Among the dominant Polygnathus forms, four new taxa are described: Polygnathus linguiformis saharicus subsp. nov., Polygnathus linguiformis vietnamicus subsp. nov., Polygnathus rhenanus siphai subsp. nov., and Polygnathus xylus bacbo subsp. nov. Conodont assemblages are attributed to polygnathid, polygnathid-klapperinid, and klapperinid conodont biofacies representing hemipelagic to pelagic environments. The klapperinid biofacies, unreported in the previous literature, are here attributed to offshore areas of the external shelf. The taxonomic compositions of the studied conodont assemblages, as well as their CAI characteristics (CAI 4–5), suggest a palaeogeographic affinity of the studied strata to the Chinese Devonian Guangxi Basin, and the South China Terrane in general. Furthermore, the conodont biofacies and the palaeogeographic distribution of the fauna are discussed.     

Conodont color (CAI) versus microcrystalline and textural changes in upper triassic conodonts from Northwest Germany

Summary Scanning Electron Microscopy on conodonts from the Middle Triassic “Trochitenkalk” from Northwest Germany with CAI values ranging from 1 to 5 revealed structural alterations during progressive diagenesis. Conodonts with low CAI values of 1.5 already show syntaxial recrystallization of apatite on the conodont surface. Aggrading crystallization of the internal structure occurs in conodonts with CAI=5. The usually short-prismatic and blocky syntaxial crystallites become increasingly longer and an overall increase in crystallite size takes place. Conodonts from the telemagmatically heated sediments overlying the intrusive body of the Vlotho Massiv are often corroded, i.e., they have pocked, frosted to pitted surfaces with dolomite rhomb overgrowths, indicating contact with ascending dolomitizing brines.     

Step-wise morphological trends in fluctuating environments: Evidence in the Late Devonian conodont genus Palmatolepis

Long-term morphological changes were investigated in the conodont genus Palmatolepis, using a Fourier analysis of the outline of platform elements. Three time-slices and four Late Frasnian - Early Famennian sections on North Gondwana were studied. No difference existed between locations, but shape variations were recorded associated with the successive Kellwasser events. The Middle Famennian period was analyzed in a section located at the Baltica margin across the annulata event. Some morphological changes may be related to the deposition of the corresponding black shales, but the main feature is a heterogeneity of the populations, which can be split into two morphological types. Latest Famennian conodonts from another North Gondwanan section show only minor shape changes during this time interval, which is environmentally stable prior to the global Hangenberg event. Based on synthetic shape variations across the whole period an morphological trend towards narrower elements is observed. This morphological change happens step-wise in response to global events. The amplitude of the shape shift is proportionally related to the extend of the successive Late Devonian global events.     

The Frasnian-Famennian mass extinction: insights from high-resolution sequence stratigraphy and cyclostratigraphy in South China

Two sequences (SFr, SFa), each 1-1.2 Myr in duration, are recognised in the strata across the Frasnian-Famennian (F-F) transition both in carbonate platform and interplatform basinal successions in South China. The sequence boundary between the two sequences is placed a little below the top of the Frasnian. The sequences are basically composed of coarsening-upward/bed-thickness increasing-upward cycles and shallowing-upward cycles (parasequences) in basinal and platform deposits respectively, which stack into cycle-sets (typically six to eight cycles). 10 and 12 cycle-sets are identified in sequences SFr and SFa respectively. These cycle-sets can be further grouped into larger-scale composite cycle-sets (herein termed mesocycle- and megacycle-sets with two and four cycle-sets respectively). This vertical cycle-stacking pattern and the hierarchy of cyclicity suggest a Milankovitch style of forcing such that the cycles and cycle-sets were formed in response to the orbital perturbations of precession (16-18 kyr) and eccentricity (∼100 kyr in duration), respectively. In the basinal cycles, smaller-scale rhythmic stratification beds (typically six to eight beds in a cycle) are extensive, and were likely caused by millennial-scale climatic forcing. In the lower sequence, SFr, the latest highstand deposits consist of calciturbidites and debrites in deep-water strata and fenestral limestones in shallow-water strata, representing a major (third-order) sea-level fall. Within these deposits, four cycle-sets are further identified in both coeval deep-water and platform successions. Succeeding deeper-water organic-rich facies, within which three cycles occur, are the transgressive deposits of the overlying Famennian sequence (SFa). These cycles represent three higher-frequency (16-18 kyr) sea-level fluctuations and accompanying anoxia, superimposed on a major third-order sea-level rise. The F-F boundary is placed at the top of the first cycle, based on conodont data. Thus, a major sea-level fall and then a rise occurred in the F-F transitional period. Faunal and sedimentological data reveal a massive biotic decline in concert with the major sea-level fall, and a further biotic demise coinciding with the major sea-level rise and its three superimposed higher-frequency sea-level fluctuations and accompanying anoxia. The F-F biotic crisis was therefore characterised by two episodes of step-down extinction. On the basis of Milankovitch orbital rhythms, the first major biotic extinction took place over ∼400 kyr, and the subsequent event was ∼50 kyr in duration, i.e. ∼450 kyr for the entire event. At the same time as the massive decline of normal-marine fossils during the latest Frasnian sea-level fall, there was widespread cyanobacterial growth and a thriving of planktonic calcispheres, suggesting eutrophic conditions. This situation could have caused a severe biotic loss, as a result of the deterioration of surface water clarity and formation of anoxic bottom waters due to over-consumption of oxygen through respiratory demands and decomposition by the cyanobacteria and phytoplankton. The subsequent rapid sea-level rise with superimposed higher-frequency sea-level fluctuations and accompanying anoxia could have caused rapid elevation of anoxic bottom waters and expansion of eutrophic surface waters over shallow-water platforms due to enhanced upwelling ocean currents and improved ocean circulation. This situation would have exerted further stresses upon the already-weakened biota, leading to a further biotic demise. However, a small number of organisms such as pelagic tentaculitids, small mud-adapted brachiopods and gastropods did survive into the Famennian, although with very low diversity.     

Upper Eocene larger foraminiferal–coralline algal facies from the Klokova Mountain (southern continental Greece)

Within the Gavrovo–Tripolitza area (southern continental Greece), marine carbonate platforms existed from the Late Triassic to the Late Eocene. The Middle–Upper Eocene marine shallow-water carbonates of the Klokova Mountain represent remnants of the large volumes of sediment that were produced on a middle ramp sedimentary system which culminated in the Lower Oligocene terrigenous deposits. Facies analysis of Bartonian–Priabonian shallow-water carbonate successions and the integration with palaeoecological analysis are used to produce a detailed palaeoenvironmental model. In the proximal middle ramp, porcelaneous foraminiferal packstone facies is characterised by larger foraminifera such as Praturlonella and Spirolina. These forms thrived in a shallow-water setting with low turbidity, high-light intensity and low-substrate stability. The foraminiferal packstone facies, the thin coralline wacke–packstone facies and the rhodolith packstone facies deposited approximately in the same depth range adjacent to one another in the middle-ramp. Nummulitids (Nummulites, Assilina, Pellatispira, Heterostegina and Spiroclypeus) increase in abundance in the middle to distal mid-ramp together with the orthophragminids. Coralline algae, represented by six genera, are present in all facies. Rhodoliths occur in all facies but they show different shapes and growth forms. They develop laminar sub-ellipsoidal shapes in higher turbulence conditions on mobile sand substrates (foraminiferal packstones and rhodolith rudstones), whilst sub-discoidal shapes often bound by thin encrusting coralline plants in lower hydrodynamic settings. The distinctive characteristics of the palaeoecological middle-ramp gradient are an increase in dominance of melobesioids, a thinning of the encrusting coralline plants and a flattening of the larger benthic foraminiferal shells.     

Hydraulic sorting of microbiota in calciturbidites— A dinantian case study from the rheinische schiefergebirge,Germany

Summary A calciturbidite bed from the lower part of the Kieselkalk Formation (late cd II) at Wallau, eastern Rheinisches Schiefergebirge, displays ideal grading of reworked calcareous shallow-water microbiota, ranging from plurimillimetric agglutinated foraminifers and fragments of calcareous algae (Koninckopora sp.) to plurimicronic calcispheres, radiolarians and sponge spicules. Microbiota derived from all levels of the platform. Correspondingly, several carbonate microfacies types could be discerned. The early diagenetic micrite base of the turbidite preserved the anoxic basinal facies. The turbidite bed belongs to Foraminiferal Zone 15 (V3bα), theAlbaillella cartalla Zone (radiolarian chronology), and is the Lower and part of the Uppertexanus Zone (standard conodont zonation). From the few published data on foraminifers, the Kieselkalk is thought to range from Mid Viséan V2b to Late Viséan V3b gamma.     

Carboniferous stratigraphy and depositional environments in the Ahnet Mouydir area (Algerian Sahara)

An up to 3,000-m-thick pile of Carboniferous rocks covers the northern fringe of the Precambrian Hoggar Massif (Touareg Shield) in southern Algeria, thus terminating the depositional history of the Palaeozoic in this sector of the North African Craton. The previous Devonian (Eifelian to Frasnian) palaeogeographic configuration of the area, characterized by ridges and shallow basins, is leveled by a widespread Famennian playa and lower Tournaisian delta sedimentation on a largely undifferentiated shelf. Tournaisian to Moscovian strata were deposited under open-marine, deltaic, shallow-subtidal, fluvial, and continental environments. The Carboniferous sequence can be subdivided into 12, largely interfingering, lithostratigraphic formations, which were dated by conodonts, ammonoids, foraminifers, and brachiopods, yielding a modified biostratigraphic framework of the area. The formations are stacked in four transgressive–regressive cycles, which include two major gaps, one during the middle Tournaisian, the other during the middle Visean to Serpukhovian. The oscillations of sea level can be traced into neighboring areas and seem coeval to early pulses of the Late Carboniferous/Early Permian glaciation of Gondwana. To a lesser degree they may reflect more local tectonic effects of the Variscan orogeny.     

Eustatic trends in conodont diversity across the Frasnian–Famennian boundary in the stratotype area, Montagne Noire, Southern France

Trends in generic diversity of successive conodont communities are analysed in sections of different environmental settings across the Frasnian–Famennian (F–F) boundary in the stratotype area, Montagne Noire, France. The evolution of conodont biofacies and abundances matches the overall pattern already observed in many sections elsewhere in the world and supports the interpretation of an important eustatic sea-level fall during the Upper Kellwasser event. The change from late Frasnian deep-water palmatolepid–polygnathid biofacies to shallower-water polygnathid-icriodid biofacies during the Upper Kellwasser event occurred in all sections studied. The shallowing trend culminated at the end of the Kellwasser Event as indicated by the substantial increase of formerly poorly represented icriodids, whereas palmatolepids concomi-tantly diminished. This event occurred earlier on oxygenated outer platform submarine rises than in oxygen-depleted depressions. The sudden sea-level fall prior to the Frasnian–Famennian boundary was followed, at the beginning of the Famennian, by a deepening trend when palmatolepids dominated again. These changes in conodont generic associations and abundances occurred rapidly and synchronously. As a result, the stratigraphic resolution obtained with the evolution of biofacies is higher: it permits not only a more accurate location of the base of the Upper Kellwasser event in environments where it cannot be distinguished lithologically, but it also allows the recognition of intrazonal gaps.     

Conodont biostratigraphy of Upper Devonian–Lower Carboniferous deposits in eastern Alborz (Mighan section), North Iran

The Devonian/Carboniferous (D/C) transition is characterized by a major transgressive/regressive cycle which led to a widespread ocean anoxia known as the Hangenberg Black Shale Event (HBSE), as well to a major sea-level fall (Hangenberg Sandstone Event, HSSE), recognized around the world. Both events are known as the Hangenberg Crisis. In order to examine the D/C transition in shallow water environment, the Mighan section in eastern Alborz was studied in terms of conodont biostratigraphy and stable isotope geochemistry. Twenty-five conodont species belonging to seven genera were identified and 5 conodont zones discriminated; namely, the Bispathodus aculeatus aculeatus Zone, Bispathodus costatus Zone, Bispathodus ultimus Zone, Siphonodella praesulcata Zone, costatus-kockeli Interregnum, and the sulcata Zone. Below the Devonian–Carboniferous boundary (DCB), the Hangenberg Black Shale and Hangenberg Sandstone equivalents were recognized, representing the Hangenberg Crisis that highly affected trilobite, ammonoid, brachiopod and conodont faunas at Mighan and worldwide. The kockeli Zone of the latest Famennian is missing at Mighan due to the lack of conodonts, probably related with the major environmental changes linked with the Hangenberg Crisis recognizable worldwide. Carbon isotopes measured of micrites from Mighan indicate a proximal depositional environment of a shallow shelf with terrestrial input and the oxygen isotope values from conodont apatite suggest warm seawater temperatures of tropical and subtropical setting in the study area.