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The Upper Cretaceous carbonates along the Zagros thrust-fold belt “Harir-Safin anticlines” experienced extensive hot brine fluids that produced several phases of hydrothermal cements, including saddle dolomites. Detailed fluid inclusion microthermometry data show that saddle dolomites precipitated from hydrothermal (83–160 °C) and saline fluids (up to 25 eq. wt.% NaCl; i.e., seven times higher than the seawater salinity). The fluids interacted with brine/rocks during their circulation before invading the Upper Cretaceous carbonates. Two entrapment episodes (early and late) of FIs from the hydrothermal “HT” cements are recognized. The early episode is linked to fault-related fractures and was contemporaneous with the precipitation of the HT cements. The fluid inclusions leaked and were refilled during a later diagenetic phase. The late episode is consistent with low saline fluids (0.18 and 2.57 eq. wt.% NaCl) which had a meteoric origin. Utilizing the laser ablation U-Pb age dating method, two numerical absolute ages of ~70 Ma and 3.8 Ma are identified from calcrete levels in the Upper Cretaceous carbonates. These two ages obtained in the same level of calcrete indicate that this unit was twice exposed to subaerial conditions. The earlier exposure was associated with alveolar and other diagenetic features, such as dissolution, micritization, cementation, while the second calcrete level is associated with laminae, pisolitic, and microstromatolite features which formed during the regional uplifting of the area in Pliocene times. In conclusion, the hydrothermal-saddle dolomites were precipitated from high temperature saline fluids, while calcrete levels entrapped large monophase with very low salinity fluid inclusions, indicative for a low temperature precipitation from water with a meteoric origin.

Petrographic studies, geochemistry, δ13C and δ18O isotopes have been used to characterize the well-preserved carbonate depositional environments of the Bobassa and Ndimba unit in the 640-565 Ma Ombella-M'poko Formation, Central African Republic. The carbonate facies are mainly composed of calcite, dolomite and quartz with some accessory minerals (opaque minerals, muscovite and pyrite). The Ca/Mg ratios show that the Bobassa carbonates are slightly metamorphosed dolomitic limestones, while those of Ndimba correspond to slightly metamorphosed highly dolomitic limestones and cipolins. The Bobassa unit recorded a depositional evolution from fluvial to peritidal environments in a possible tectonically active platform as evidenced by changes of lithology and slump structures. The Ndimba was deposited in a tidal-influenced fluvio-lacustrine environment under dry conditions. Geochemistry revealed that the Bobassa and Ndimba units recorded variable redox conditions from anoxic to oxic. The PAAS-normalized REE + Y distributions indicate that the both units were slightly altered by freshwaters and detrital materials in reduced and acidic environments. The lowest δ18O and highest δ13C values suggest warm surface waters and deeper water column respectively. The carbon isotope profiles of Bobassa and Ndimba units show values varying between −0.89 and −0.34‰ that are not stratigraphic coeval with the cap carbonates δ13C profiles deposited in the aftermath Marinoan Glaciation, but rather are correlated to the middle to upper part of the Schisto-Calcaire Group in Gabon and Republic of Congo and the time-equivalent Lukala Subgroup in DRC.

Jeribe Formation in two subsurface sections were studied using samples from two wells of Jambour oil field, Kirkuk area-northern Iraq to determine its petrographical characteristics, microfacies, d...

The Rio del Rey Basin belongs to the coastal basins of Cameroon with Cretaceous to Cenozoic deposits. The Cretaceous (Cenomanian and Campanian-Maastrichtian) and Cenozoic (Paleocene) sediments were studied using major and trace elements geochemistry associated with mineralogical data. The studied samples are mainly made up of Fe-shale and shale with rare intercalations of Fe-sand in the Cenomanian deposits, Fe-shale and shale in the Campanian-Maastrichtian deposits and sub-litharenite, Fe-sand, shale and Fe-shale in the Paleocene deposits. These samples contain high LREE/HREE ratios (9.27 -24.23), no to slight positive Eu anomalies (Eu/Eu*= 1.02 – 1.44), moderate to high Al2O3/TiO2 (10.42 – 112.93) ratio and their position in the Ce vs. La/Yb, Zr vs. TiO2 and La/Sc vs. Th/Co diagrams indicate that they derivate from mainly felsic associated with subordinate intermediate igneous rocks. The CIA (Chemical Index of Alteration; 61.69 – 92.76), PIA (Plagioclase Index of Alteration; 62.16 – 99.47), K2O/Na2O ratios (0.08 – 22.125) and Zr/Sc vs. Th/Sc diagram suggest the source rocks have experienced low to high recycling and weathering (Paleocene deposits), high chemical weathering (Campanian-Maastrichtian) and moderate to high chemical watering (Cenomanian deposits). The SiO2 vs. Al2O3 + K2O + Na2O plot suggests a predominant semi-arid to arid climate with periodically semi-humid climate during the Paleocene, attested by the evolution of the clay minerals composition with association of illite-chlorite/vermiculite/kaolinite in the Cenomanian sediments and periodic appearance of siderite in the Campanian-Maastrichtian and Paleocene deposits. The Sr/Ba (0.41 – 2) and U/Th (0.16 – 0.28) ratios indicate brackish to shallow marine environment of deposition. The Arc-Rift-Collision of two sets of high and low silica rocks and SiO2 vs. K2O/Na2O diagrams indicate a Collisional or Active Continental Margin tectonic setting similar as the Cretaceous (Lower Mundeck and Logbadjeck Formation) and Paleocene (Nkapa Formation) deposits from the Douala sub-basin, which could reflect the Precambian basement history linked to the Neoproterozoic orogeny.

Bullslaughter Bay in southern Pembrokeshire, UK, exposes sections of Upper Mississippian limestone strata. In many places, the rock suffered an isovolumetric alteration during a period of sea-level oscillations. We used multiple approaches to study the weathered rocks, combining sedimentological, petrographic and isotopic compositions (δ18O and δ13C values). Two main microfacies are recognized: (i) packstones/grainstones, characteristic of an open marine shallow subtidal/intertidal environment, with a high degree of agitation, slightly elevated salinity and temporary subaerial exposure; and (ii) mudstones/wackestones in a lagoonal setting and intertidal or supratidal environments, with a pedogenetic influence. In both cases, a complex diagenetic story, which started early in a meteoric environment, induced a strong alteration producing loose sediments in place of the parent rock. Calcretization, at or near the sediment surface in the vadose zone, was one of the most widespread diagenetic modes. It could be associated with beachrocks. Carbon and oxygen stable isotope analyses from more or less weathered limestones support the petrographic data: they show non-marine values with δ13C ranges of from −2.13 ‰ to 1.75 ‰ and δ18O from −6.05 ‰ to −4.66 ‰. These values are systematically lower than those of the middle Carboniferous seawater. Some periods of low sea level and subaerial exposure allowed gypsum to form. Neoformation of euhedral quartz by probable replacement after sulfate, and halite pseudomorphs after gypsum in a hypersaline environment are documented for the first time in southern Pembrokeshire. The studied weathered limestones present a complex diagenetic evolution related to sea-level oscillations in a range of hot and contrasting seasonal climates.

Utilizing sophisticated tools in carbonate rocks is crucial to interpretating the origin and evolution of diagenetic fluids from the Upper Jurassic carbonate rocks along the Zagros thrust-fold Belt. The origin and evolution of the paleofluids utilizing in-situ strontium isotope ratios by high resolution laser ablation ICP-MS, integrated with stable isotopes, petrography and fieldwork are constrained. Due to the lack of information on the origin of the chemistry of the fluids, the cements that filled the Jurassic carbonate rocks were analysed from the fractures and pores. This allowed us to trace the origin of fluids along a diagenetic sequence, which is defined at the beginning from the sediment deposition (pristine facies). Based on petrography and geochemistry (oxygen-, carbon- and strontium-isotope compositions) two major diagenetic stages involving the fluids were identified. The initial stage, characterized by negative δ13CVPDB values (reaching −10.67‰), involved evaporated seawater deposited with the sediments, mixed with the input of freshwater. The second stage involved a mixture of meteoric water and hot fluids that precipitated as late diagenetic cements. The late diagenetic cements have higher depleted O–C isotope compositions compared to seawater. The diagenetic cements display a positive covariance and were associated with extra- δ13CVPDB and δ18OVPDB values (−12.87‰ to −0.82‰ for δ18OVPDB and −11.66‰ to −1.40‰ for δ13CVPDB respectively). The distinction between seawater and the secondary fluids is also evident in the 87Sr/86Sr of the host limestone versus cements. The limestones have 87Sr/86Sr up to 0.72859, indicative of riverine input, while the cements have 87Sr/86Sr of (0.70772), indicative of hot fluid circulation interacting with meteoric water during late diagenesis.

In this paper, we have tested the application of magnetic susceptibility measurements in Cu-Ag-Zn-Pb-(Fe)-mineralized carbonates of the BIe subgroup (Democratic Republic of Congo) as an efficient tool for regional and global high-resolution stratigraphic correlations in the Neoproterozoic marine carbonates. To achieve this goal, we integrate the low-field magnetic susceptibility (XLF) data with facies analyses, geochemistry and isotope stratigraphy. The microfacies analyses of two cores, Tshinyama#S70 and Kafuku#15, drilled in the early Neoproterozoic carbonates of the Mbuji-Mayi Supergroup reveal a deep carbonate ramp setting associated with a microbial/stromatolitic mid-ramp environment. High-resolution stratigraphic correlations using magnetic susceptibility and C-isotope curves established for both cores, 190 km apart, suggest a sedimentary hiatus at the base of the Tshinyama#S70 succession. C-O and Sr isotopes and Sr/Ca and Fe abundances show that a diagenetic meteoric overprint affected the series of the Tshinyama#S70 core and a thermal effect related to mineralizing fluids affected the Kafuku#15 core carbonates.

International audience; The Neoproterozoic Schisto-Calcaire Group (630 to ca. 580 Ma) was deposited on an extensive carbonate shelf in the margin of the Congo Craton in the Niari-Nyanga and Comba subbasins (Gabon and Republic of the Congo). It consists of three carbonate-dominated subgroups (SCI to SCIII, up to 1300 m-thick) recording relative sea-level changes. The SCIc Formation, at the upper part of the SCI Subgroup, is a carbonate succession of meter-scale shallowing-upward cycles, composed of a standard sequence of 7 facies grouped in 5 facies associations recording the evolution of a marine ramp from distal carbonate muds and giant stromatolitic bioherms (F1–F2) and extensive ooid shoals (F3), to proximal settings submitted to evaporation near a sabkha (F7). Fifth-order ‘meter-scale’ (or elementary parasequences) packages are grouped into fourth-order sequences (parasequence sets), which are not correlative in the whole basin. Two categories of fifth-order elementary parasequences are recognized, on the basis of physical bounding surfaces: (i) subtidal cycles bounded by marine flooding surfaces across which subfacies deepen; and (ii) peritidal cycles bounded by subaerial exposure surfaces. These cycles are the result of the interplay of relative sea-level changes due to eustatic variations related to periodic extensional tectonic events affecting the whole basin. The Niari-Nyanga and Comba subbasins experienced basin tectonics in the general context of the rifting of Rodinia creating changes of relative sea-level in the different parts of the shelf. The SCIc cycles are enclosed into a third-order sequence with two major transgressive-regressive phases, related to the deposition of the SCI Subgroup. The most typical sedimentologic feature of the SCIc Formation is the deposition of giant stromatolitic bioherms (stacked up to 20 m) topped by ooid shoals (up to 75 m thick) deposited during high systems tract prograding and forced regressive systems tract phases that ended with a lowstand systems tract phase with evaporitic and karstic conditions at the top of the SCIc Formation. The elementary parasequences and parasequence sets are probably the result of the migration of lateral environments related to the variation of the energy in relation to tectonic setting. As a result, a regional sea-level increase is for the first time highlighted in transgressive systems tract phase (composed of microbial induced sedimentary structures Facies) in the lower part of the SCIc Formation. The third-order succession can be followed more than 100 km in the Republic of the Congo and several hundred meters from South of Gabon to the Lower Congo in the Democratic Republic of the Congo. Tentative detailed sequence stratigraphy correlations between both Congo's highlight the role of tectonics affecting both areas.

We report the discovery of 715 to ca. 810 million–year–old fungal fossils through molecular characterization of vestigial chitin., Precambrian fossils of fungi are sparse, and the knowledge of their early evolution and the role they played in the colonization of land surface are limited. Here, we report the discovery of fungi fossils in a 810 to 715 million year old dolomitic shale from the Mbuji-Mayi Supergroup, Democratic Republic of Congo. Syngenetically preserved in a transitional, subaerially exposed paleoenvironment, these carbonaceous filaments of ~5 μm in width exhibit low-frequency septation (pseudosepta) and high-angle branching that can form dense interconnected mycelium-like structures. Using an array of microscopic (SEM, TEM, and confocal laser scanning fluorescence microscopy) and spectroscopic techniques (Raman, FTIR, and XANES), we demonstrated the presence of vestigial chitin in these fossil filaments and document the eukaryotic nature of their precursor. Based on those combined evidences, these fossil filaments and mycelium-like structures are identified as remnants of fungal networks and represent the oldest, molecularly identified remains of Fungi.

A field, petrographic and geochemical study of two Triassic&ndash, Jurassic carbonate successions from the Maritime Alps, SE France, indicates that dolomitization is related to episodic fracturing and the flow of hydrothermal fluids. The mechanism governing hydrothermal fluids has been documented with the best possible spatio-temporal resolutions specifying the migration and trapping of hydrothermal fluids as a function of depth. This is rarely reported in the literature, as it requires a very wide range of disciplines from facies analysis (petrography) to very diverse and advanced chemical methods (elemental analysis, isotope geochemistry, microthermometry). In most cases, our different recognized diagenetic phases were mechanically separated on a centimetric scale and analyzed separately. The wide range of the &delta, 18OVPDB and 87Sr/86Sr values of diagenetic carbonates reflect three main diagenetic realms, including: (1) the formation of replacive dolomites (Type I) in the eogenetic realm, (2) formation of coarse to very coarse crystalline saddle dolomites (Types II and Type III) in the shallow to deep burial mesogenetic realm, respectively, and (3) telogenetic formation of a late calcite cement (C1) in the telogenetic realm due to the uplift incursion of meteoric waters. The Triassic dolomites show a lower 87Sr/86Sr ratio (mean = 0.709125) compared to the Jurassic dolomites (mean = 0.710065). The Jurassic calcite (C1J) shows lower Sr isotopic ratios than the Triassic C1T calcite. These are probably linked to the pulses of the seafloor&rsquo, s hydrothermal activity and to an increase in the continental riverine input during Late Cretaceous and Early Cenozoic times. This study adds a new insight into the burial diagenetic conditions during multiple hydrothermal flow events.

The upper carbonate-rich parts of the West Congo Supergroup (~ 1000–560 Ma) from the Democratic Republic of the Congo have hitherto been considered as a record of abrupt eustatic and climatic events accompanying glaciation and deglaciation of a Snowball Earth-type Marinoan ice age that was of global extent. These strata have however never been investigated in detail. Results of new sedimentological work at key outcrops over a 1300 km outcrop belt show that pre- and post-Marinoan carbonates are respectively, storm-influenced sediments deposited principally in a mid/outer-ramp setting, and deep-water slope carbonates (calicturbidites) representing a lobe-fringe or levee-overbank setting. The Upper Diamictite Formation held previously by some to be a subglacial tillite, comprises gravity flows (debrites) deposited in deep water below wave base along the unstable margins of a carbonate ramp. A direct glacial influence on sedimentation for diamictites or any accompanying facies cannot be readily identified. Sedimentary facies reported here primarily record the presence of deep-water submarine to alluvial fan systems related to extensional tectonic processes of the central-southern Macaubas Basin (now located in Brazil) between 700 Ma and 660 Ma followed by the 630-Ma onset of the pre-collisional magmatic arc in the Aracuai-West Congo Orogen. No extreme short-lived climatic or eustatic events of a Snowball Earth-type ice age are recorded in the studied succession, which primarily reflects long-term overriding regional tectonic controls resulting in diachronous sedimentation along the western margin of the Congo Craton.

A moderately rich ostracod fauna is reported from the upper part of the St. Joseph Formation (Fm), the Eau Noire Fm and the lower part of the Couvin Fm in the Eau Noire section located nearby Couvin. The section that crosses the Emsian/Eifelian boundary belongs entirely to the Couvinian historical stage. The ostracod fauna observed appertains to the Eifelian Mega-Assemblage and is indicative, in the Eau Noire Fm and in the Couvin Fm, of continuous shallow open-marine environments close to the fair-weather wave-base. The sampling and the number of ostracods extracted from the St. Joseph Fm are not sufficient to make environmental inferences and the study does not demonstrate an abnormal change in the ostracod fauna neither in relation with the Eau Noire Fm/Couvin Fm boundary, nor in relation with the Emsian/Eifelian boundary. The ostracods present near the Emsian/Eifelian boundary are mentioned for the first time in the southern border of the Dinant Synclinorium and they display close relations with the Eifel Mountains (Germany) and the Holy Cross Mountains (Poland).

This study concerns the absolute LA ICP-MS U-Pb dating of hydrothermal cementation, particularly in tectonically controlled fractures filling saddle dolomites and blocky calcites in the carbonate succession of Bekhme Formation (Campanian–Maastrichtian). Dating of multiphase hydrothermal cementation combined with δ18O and δ13C analyses, petrography and field observations shows two major episodes of hydrothermal fluid circulation. The first was measured at ~73.8 Ma and is associated with the Zagros folding-faulting tectonic event (Late Cretaceous) and with the depositional age of the Bekhme Formation (75.1 Ma), and the second was at 30.3 Ma and is associated with the second folding-faulting tectonic event during the Tertiary. The δ18O and δ13C values grouped into two populations corroborate the two episodes of hydrothermal fluid flow. Late Cretaceous folding and faulting tectonics and reactivation during the Tertiary in the study area provided conduits for the hydrothermal fluids along deeply seated faults. This work shows for the first time through U-Pb dating that the hydrothermal diagenetic processes in Upper Cretaceous Bekhme Formation are directly linked with the timings of the major regional tectonics in the Zagros Fold-Thrust Belt (ZFTB) through the flow of hot fluid fluxes in a fractured sedimentary basin.

The focus of this study is based on a detailed analysis of the hyperthermal events of the Paleocene / Eocene limit of 56 Ma and the lower Eocene (for the 54-52 Ma interval, Figure 1). This example will show that the Earth has experienced many times much higher temperatures than today, with warmer, sometimes more acidic oceans and an atmosphere much richer in CO2 (or CH4) than the current one. Are these past events precursors of the current situation?

The study of 3430 ostracods collected in the Lomme, Hanonet and Trois-Fontaines formations (Late Eifelian/Early Givetian) in four sections of the Dinant Synclinorium (Ardenne) allowed the recognition of more than 75 taxa. Their study displays the progressive evolution from an external mixed siliciclastic-carbonate ramp to a carbonate platform around the Eifelian/Givetian boundary. Ostracods from the Lomme Fm are published here for the first time. The leperditicopid ostracods are probably indicative of brackish water environments during the deposition of the Trois-Fontaines Fm, and consequently point to wet and warm climatic conditions during Early Givetian times in the Dinant Synclinorium. Their absence in widespread lagoonal environments in the upper part of the Givetian may be on the contrary related to very arid climate conditions. That important climatic change is probably in relation with the Taghanic Biocrisis.

In the Lower Congo region, the Ediacaran Schisto-Calcaire Subgroup consists of five carbonate-dominated formations (C1 to C5). They record tectono-eustatic sea-level fluctuations controlled by several short-time extensional tectonic events occurred in the whole basin, followed by the development of the Aracuai-West Congo Orogen between 630 Ma and 560 Ma. The uppermost units of the C2 Formation, i.e. C2d and C2e members, consist of open marine to peritidal/sabkha cycles of 1–4 m in thickness formed during in a Highstand Systems Tract (HST). The unexposed transition between the C2 and C3 formations is interpreted as a ‘final’ HSTphase which initiated the burial of the carbonate ramp by prograding siliciclastics or an early Transgressive Systems Tract (TST) phase. The carbonates of the C3 Formation represent open marine shallowing-upward cycles of 3–8 m in thickness, with deposition at the top of massive oolitic barrier shoals during a TST which flooded the entire the Neoproterozoic West Congo Basin. During the highstand, contributions of river water and land-derived material inputs occured, intermittently according to the semi-arid to arid conditions that prevailed in the restricted inner ramp and in the sabkha facies belts. In term of geochemistry, the disturbed δ13C trends of the post-Marinoan C2 and C3 carbonates rather reflect early diagenetic variations related to (i) the mixing of carbonate rocks with 13C and 18O depleted fluids including decarboxylation during early organic diagenesis and deep burial, or (ii) the meteoric alteration during carbonate stabilization, than temporal signals of the global ocean chemistry. This observation does not negate the stratigraphic utility of δ13C ratios for intrabasinal correlations.

New detailed lithological, sedimentological, chemostratigraphic data were obtained from exploration drilling samples on the C5 carbonate-dominated formation of the Neoproterozoic Lukala Subgroup (former Schisto-Calcaire Subgroup) from the West Congo Belt (WCB) in the Democratic Republic of Congo. This formation records the last post-Marinoan sea-level events that occurred in the whole basin, followed by the development of the Araçuaï-West Congo Orogen between 630 and 560 Ma. The C5 Formation consists of back-reef lagoonal and peritidal/sabkha cycles of ∼2.0 m in thickness, that record a short-time marine regression, rapidly flooded by a marine transgression with deposition of organic-rich argillaceous carbonates or shales under dysoxia and anoxia conditions. These dysoxic/anoxic waters were rapidly followed by a regional-scale marine transgression, favouring mixing with well-oxygenated waters, and the development of benthic Tonian to Cambro-Ordovician Obruchevella parva-type ‘seagrasses’ in the nearshore zones of the lagoons. New δ13C and 87Sr/86Sr isotopic data in the C5 Formation of the Lukala Subgroup are used in the frame of a correlation with the Sete Lagoas Formation in Brazil. Relatively comparable negative to positive δ13C excursions point to marine flooding of the whole basin and allow extension of the debatable Late Ediacaran age of the uppermost Sete Lagoas and C5 formations. Sr isotope “blind dating” failed due to low Sr concentration related to a dolomitization event close 540 Ma. Several tentative datings of the C5 Formation converge to a Late Ediacaran age ranging between 575 and 540 Ma. As the overlying Mpioka folded Subgroup, the C5 series suffered the Pan African deformation, dated at 566 ± 42 Ma. Unlike the previously generally accepted interpretation, our data suggests that the Mpioka Subgroup was deposited in the Early Cambrian.

The Ediacaran Schisto-Calcaire Group is a ∼1300 m-thick succession belonging to the West Congo Supergroup in Central Africa. In the Comba Basin, it consists of three carbonate-dominated units defined as formations (SCI to SCIII) that are unconformably overlain by clastic deposits (Mpioka Group) interpreted as a molassic formation associated with the Panafrican Orogen. The underlying Upper Tillite and Cap Carbonate (SCIa) units, considered as markers of the Snowball Earth event were studied in three sections. We investigated the carbonates of the Schisto-Calcaire Group by defining new microfacies (MF1-MF7) and we performed C and O isotopic analyses in order to constraint the depositional and diagenetic events directly after the Marinoan interval. Stratigraphic variations of the stable isotopes are important in the series with lighter δ18O values (>1.5‰) than those of the Neoproterozoic ocean in the SCIc unit. According to regional stratigraphy a temperature effect can be dismissed and a freshwater surface layer is the origin of such negative δ18O values in this unit. The negative δ13C anomaly (−3.5‰ on average) of the Cap Carbonate is similarly to the δ18O values (−6.4‰ on average) in the range of the marine domain during postglacial sea level rise. The sample suite as a whole (SCII and SCIII formations) displays heavier δ18O and δ13C than those of the lower part (SCI unit) of the Schisto-Calcaire Group. The comparison with the Lower Congo (Democratic Republic of Congo) and Nyanga (Gabon) basins shows that the meteoric flushing in SCIc unit of the Schisto-Calcaire Group was regional and not local, and could be derived from a climatic evolution. Although an overall overprint is present, our isotopic relationships argue against overall diagenetic resetting of primary compositions and suggest that with careful examination combined with detailed petrographic analysis general depositional and diagenetic controls can be discerned in oxygen and carbon isotopic data in the Schisto-Calcaire Group.

This study presents a preliminary assessment of the petroleum potential of the Meso-Neoproterozoic Mbuji-Mayi Supergroup in the Sankuru-Mbuji-Mayi-Lomami-Lovoy Basin in the southern-central Democratic Republic of Congo. This basin is one of the least explored in Central Africa and is a valuable resource for the evaluation of the petroleum system in the greater Congo Basin area. Highly altered carbonates (potential reservoir rocks) and black shales (potential source rocks) are present in the Mbuji-Mayi Supergroup, which can be divided into the BI and overlying BII groups (Stenian and Tonian, respectively). For this study, samples of the BIe to BIIe subgroups from five boreholes and two outcrops were evaluated with petrographic, petrophysical and geochemical analyses. Carbonates in the BIe to BIIe subgroups with reservoir potential include oolitic packstones and grainstones, stromatolitic packstones and boundstones, various dolostones, and brecciated and zoned limestones. Thin section studies showed that porosity in samples of these carbonates is mainly vuggy and mouldic with well-developed fractures, and secondary porosity is up to 12%. Black shales in the BIIc subgroup have TOC contents of 0.5–1%, and the organic matter is interpreted to have been derived from precursor Type I / II kerogen. The thermal maturity of asphaltite in carbonate samples is indicated by Raman spectroscopy-derived palaeo-temperatures which range from ∼150 to ∼260°C, which is typical of low-grade metamorphism. Raman reflectance (RmcRo%) values on asphaltite samples were between 1.0 and 2.7%, indicating mature organic matter corresponding to the oil and wet gas windows. Source rock maturation and primary oil migration are interpreted to have occurred during Lufilian deformation (650–530 Ma). The solid asphaltite present in fractures in the dolostones of the BIIc subgroup may represent biodegraded light oil from an as-yet unknown source which probably migrated during the Cambrian-Ordovician (∼540–480 Ma). This migration event may have been related to the effects of the peak phase of Lufilian deformation in the Katanga Basin to the SE. This study is intended to provide a starting-point for more detailed evaluations of potential hydrocarbon systems in the Sankuru-Mbuji-Mayi-Lomami-Lovoy Basin and the adjacent greater Congo Basin area.

During the Cryogenian-Ediacaran, tectonically- and climatically-driven perturbations created sea level changes . Snowball Earth-type glaciations suggest ~500–800 m sea level fall or at least 1,000–1,500 m in eustatic sea-level change as a result of severe climate changes. However, in Central Africa, geologic evidence of such processes is lacking. In the Lower Congo region (Democratic Republic of the Congo), detailed facies analysis and sequence stratigraphy of the Neoproterozoic uppermost Haut Shiloango-Lukala carbonate ramp system allowed to reconstruct the multiple order relative sea-level changes. Two hundred twenty-three fifth-order elementary parasequences , grouped into eleven fourth-order depositional sequences or parasequence sets, recorded a severe sea-level fall up to 15 m for slope-outer-to-outer ramp facies of the uppermost Haut Shiloango Subgroup-Upper Diamictite Formation followed by two distinct sea-level rises for the slope-outer-to-inner ramp facies of the Lukala Subgroup. First marine transgression shows a cumulative ~60 m of sea-level rise throughout C1 to C3 (Lukala Subgroup) sediment accumulations. Second marine transgression shows a cumulative ~50 m sea-level rise throughout C4 to C5 sediment accumulations. The transition from uppermost Haut Shiloango to Upper Diamictite ramp system points to an isostatic rebound and uplift of the rift flanks of the Congo Craton , creating paleoreliefs, potentially allowing local glaciation and periglacial sedimentation. This rebound can be ascribed to a diachronous far-field effect of ~660 Ma Macaubas Basin opening in Brazil. First marine transgression is interpreted as a consequence of syn-to post-rifting related to sea-floor spreading, which initiated the subsidence of the basin. A ~30 m of sea-level rise drowned the C1 formation carbonate ramp, which turned on the oversupply and progradation of offshore to nearshore marine deposits on the wedges of the basin. An abrupt change from both marine transgressions points to ~25 m of sea-level rise that is interpreted as the consequence of the development of ~585-560 Ma Aracuai-West Congo Orogen, which significantly increased the siliciclastic supply in the basin. Our results show that no anomalous climatic or eustatic events such as those proposed in the Snowball Earth model are recorded in DRC. On the contrary, relative sea-level changes result from long-term overriding regional tectonic processes controlling diachronous sedimentation along the western passive margin of the Congo Craton.

The Meso- and Neoproterozoic Mbuji-Mayi Supergroup (1155 Ma to ca. 800 Ma) was deposited in the SE–NW trending siliciclastic-carbonate failed-rift in the Sankuru-Mbuji-Mayi-Lomami-Lovoy Basin. Drillcore- and outcrop-derived microfacies, isotope (C, O and Sr) compositions of carbonates and REE + Y distributions are integrated to unravel the paleoenvironmental and chemical conditions prevailing during deposition and alteration (or contamination) of the Mbuji-Mayi carbonates. The carbonate succession (BIe subgroup and BIIa to BIIe subgroups), composed of 11 microfacies (MF), records the evolution of a marine ramp submitted to evaporation, with basinal and low-energy outer-ramp environments (MF1–MF5), biohermal mid-ramp (MF6) and restricted tide-dominated lagoon inner-ramp (MF7–MF9) settings, overlain by lacustrine (MF10) and sabkha (MF11) environments. The ramp margin is characterized by thick stacks of stromatolitic bioherms. δ13C and δ18O relationships in the Mbuji-Mayi carbonates allow discrimination between meteoric (δ13C: −7.5‰ to +0.0‰, δ18O: −7.0‰ to −1.0‰) and burial lithifications (δ13C: −1.5‰ to +0.0‰, δ18O: −15.1‰ to −7.0‰), that overprinted a primary marine signal (δ13C: −1.5‰ to +2.0‰, δ18O: −3.0‰ to +0.5‰) partially preserved in the subgroups. Unaltered pristine signals are found in the Mbuji-Mayi carbonates with 87Sr/86Sr ratios (0.7065–0.7082) similar to those of the marine-preserved strontium signatures of the early Neoproterozoic oceans. The PAAS-normalized REE + Y distributions indicate that the BIe carbonates were altered by Fe-oxide-rich hydrothermal fluids. BIIb and BIIe carbonates exhibit uniform light REE depleted patterns suggesting inputs of detrital river material whereas a marine seawater, highlighted by the REE + Y distributions is preserved in BIIc and BIId carbonates. The pattern of carbon, oxygen and strontium isotopic variations in the Mbuji-Mayi carbonates reflects deposition and early diagenesis in variety domains in marine, evaporitic and meteoric conditions. Almost all Mbuji-Mayi carbonates display discrete seawater REE + Y distributions, reflecting influences of particulate and colloidal materials derived from riverine inputs or hydrothermal fluids. Our systematic REE + Y approach allows also to infer the nature of the dolomitization processes operating in each carbonate subgroup, i.e. dolomitization may be attributed to evaporative reflux of groundwater or mixing zones of freshwater lenses. The internal architecture and evolution of the carbonate Mbuji-Mayi succession are similar to many Phanerozoic ramps submitted to sealevel variations, climatic changes and episodic detrital inputs.

The carbon, oxygen and strontium isotope geochemistry is the most widely applied chemostratigraphic tool in the reconstruction of paleoenvironments and indirect regional or global correlations for the Neoproterozoic times. Relatively good preserved carbonate rocks of the West Congo Supergroup, i.e. the Schisto-Calcaire Subgroup of the Democratic Republic of Congo and Gabon, and the Mbuji-Mayi Supergroup, i.e. the BII group from the Democratic Republic of Congo, make these Neoproterozoic successions unique for chemostratigraphical studies. In this paper, we propose to discuss on the fidelity of d13Ccarb and Sr signatures in Neoproterozoic carbonates on the basis of a severe diagenetic control, using trace and major geochemistry, combined with C and O stable isotope analysis. Our result highlights that the d13Ccarb fluctuations of the Schisto-Calcaire Subgroup reflect (i) facies variations, (ii) exchanges between isotopically light carbon in meteoric waters and carbonate during lithification and early diagenesis, and/or (iii) regional metamorphism grades rather than temporal signals of ocean chemistry, while in the unmetamorphosed carbonates of the Mbuji-Mayi Supergroup, reported to the Bitter Springs anomaly (~ 810 Ma), the use of d13C ratios are worldwide applicable for inter-basin correlations.

The late Mesoproterozoic–middle Neoproterozoic period (ca. 1300 Ma–800 Ma) heralded extraordinary climatic and biological changes related to the tectonic changes that resulted in the assembly (~ 1.0 Ga) and the break-up of Rodinia (880 Ma–850 Ma). In the Democratic Republic of Congo, these changes are recorded in the Mbuji-Mayi Supergroup which was deposited in the SE–NW trending siliciclastic-carbonate failed-rift Sankuru-Mbuji-Mayi-Lomami-Lovoy Basin. New LA-ICP–MS U–Pb laser ablation data on detrital zircon grains retrieved from the lower arenaceous-pelitic sequence (BI group) together with C and Sr isotopic data on carbonates from the upper dolomitic-pelitic sequence (BII group) and an 40Ar/39Ar age determination on a dolerite give a new depositional time frame between 1174 ± 22 Ma and ca. 800 Ma for the Mbuji-Mayi Supergroup. The upper age limit is based on the assumption that the transition between the BIIb and BIIc subgroups recorded the Bitter Springs anomaly. In terms of tectonic and paleoclimatic settings, the BII group was deposited in the eastern passive margin of the Congo Craton during warm periods interlaced with temporarily dry and wet seasons, suggesting greenhouse conditions during the fragmentation of Rodinia.

Ostracods from the Sourd d'Ave section have been collected in the Moulin Boreux and Fort Hulobiet Members (Fromelennes Fm., Givet Group) and in the Pont d'Avignon Member (Nismes Fm., Frasnes Group). Ostracods collected in the Fromelennes Fm. by Milhau (1983a) and in the Nismes Fm. by Casier (1987a) have been also reviewed. Forty-four ostracod species are identified in the Fromelennes Fm. and 25 in the Nismes Fm. They belong exclusively to the Eifelian Mega-Assemblage, and several assemblages indicative of restricted and shallow marine, sometimes agitated, environments are recognized in the Fromelennes Fm. The great rarity of ostracods in the upper part of this formation provides evidence for less favourable lagoonal conditions probably related to increasing aridity at the end of the Givetian. In the Frasnes Group, assemblages are exclusively open marine and indicative of increasing water depth. The majority of ostracod species recognized in the Givet Group are missing in the base of the Frasnes Group as a consequence of the Frasnes Event. A systematic list of ostracods identified in the Fromelennes Fm. at Sourd d'Ave is published as an annex. Systematic sampling has been carried out in order to establish the sedimentological evolution of the environments and to detail the Givetian-Frasnian (G/F) transition. This allowed recognition of 13 microfacies types that replicate the standard sequence of Preat & Mamet (1989) from open marine shallow subtidal to restricted supratidal near emersion. The Boreux Member and the Fort Hulobiet Member display restricted facies (Amphipora, spongiostromid and algal bafflestones and bindstones, loferites with desiccation lumps) with poorly fossiliferous beds interbedded with higher energy peloidal and sometimes oolitic grainstone facies. Laminite horizons, sometimes with small-sized lateral linked hemispheroid stromatolites are uncommon, and are associated with dolomicrites showing pseudomorphs of evaporite minerals. These evaporitic facies become common in the upper part of the Fort Hulobiet Member suggesting the palaeoclimate became more arid at the G/F transition. Metre-scale cyclicity is pervasive throughout the Givetian part of the section. The boundary between the Givet Group and the Frasnes Group is very distinctive in the field, and is characterized by a transition from restricted evaporative lagoonal facies to open marine interbedded marly shales and nodular limestones. The upper part of the Fort Hulobiet Member consists of interbedded biostromes (semi-restricted stromatoporoid boundstones) followed by Amphipora floatstones, then fossil-poor units and restricted supratidal laminites with well-developed fenestral fabrics. The Frasnian Pont d'Avignon Member contains a rich faunal assemblage (bryozoans brachiopods, molluscs, nautiloids, tentaculitids) suggesting an abrupt drowning from the marginal Givetian carbonate platform into a Frasnian distal ramp or deep basinal environment below or near storm wave base.

The Upper Diamictite Formation of the West Congo Supergroup is a diamictite-dominated succession variously interpreted as a continental tillite, glaciomarinite, and glacially-influenced or non-glacial debrite. This paper presents a detailed macroand microscale analysis of soft-sediment deformation structures in order (1) to resolve the long-standing debate on the genetic origin of the Upper Diamictite Formation, and (2) to constrain the paleoenvironmental conditions during the Marinoan global event. The predominance of ductile and brittle deformations and grain-to-grain compression, considered as evidence of high strain rates and local high stress conditions, indicate that the diamictites were deposited as mass flows. The presence of probable pelagic clays, lonestones, and the absence of direct ice-contact deposits point to a subaqueous gravity flow origin. These diamictites were deposited along the margin or at the foot of the basin slope. They were probably triggered by oversteepening and/or tectonic shocks in the Araçuaí-West Congo Orogen between 630 and 660 Ma. The Upper Diamictite Formation provides no support for the postulated global Marinoan glaciation at this time and underscores the importance of a local tectonic control on the sedimentation., SCOPUS: ar.j, info:eu-repo/semantics/published

The focus of this chapter is the West Congo Supergroup in the West Congo Belt (WCB), which extends along the western margin of the Congo Craton from Gabon in the north to northern Angola in the south, and the Lindi/Ubangi Supergroup of the Lindian and Fouroumbala – Bakouma Basins exposed on the northern margin of the craton. In both regions, up to two distinct diamictite horizons have been recognized, the younger of which is often associated with carbonate rocks. Geochronological constraints are generally rather poor, many of the deposits lack modern sedimentological analysis, and the glacial versus non-glacial genesis of the diamictites is a matter of debate in the literature. However, recent studies suggest a periglacial influence of diamictite deposition, particularly for the sequences in the WCB. The stratigraphy of the various basins is described, available geochemical and geochronological information collated, and recent work regarding the periglacial nature of the diamictites discussed. Finally, an updated chronostratigraphic correlation between the basins is presented. However, much more work is required, particularly in the Neoproterozoic basins on the northern margin of the Congo Craton, and more accurate geochronological constraints are required before the Neoproterozoic palaeogeography and depositional environments of the western and northern Congo Craton can be fully understood.

Resume Quarante-quatre especes d’ostracodes, la majorite en nomenclature ouverte, sont reconnues de l’Eifelien a la base du Famennien dans les coupes du Djebel Mech Irdane, de Bou Tchrafine et d’El Atrous, dans le Tafilalt (Maroc). Une espece nouvelle, Tubulibairdia tafilaltensis nov. sp., est instituee. Les associations d’ostracodes appartiennent principalement au Mega-Assemblage de l’Eifel et indiquent generalement des environnements marins relativement profonds et mal oxygenes sous la zone d’action des vagues de tempetes. Dans l’extreme base du Famennien de la coupe de Bou Tchrafine, les ostracodes appartiennent au Mega-Assemblage a Myodocopida qui indique une oxygenation beaucoup plus faible. Ils confirment que les conditions hypoxiques liees a l’horizon « Kellwasser superieur », se poursuivent dans l’extreme base du Famennien, en Afrique du Nord. Vingt-quatre especes d’ostracodes sont reconnues dans le stratotype de la limite Eifelien/Givetien au Djebel Mech Irdane. Le changement le plus important de la faune dans cette coupe est lie a un niveau de marne silteuse grisâtre situe 10 cm sous la limite Eifelien/Givetien, mais il n’est pas significatif d’une extinction a ce niveau. Trois zones d’ostracodes sont reconnues dans le Frasnien et dans l’extreme base du Famennien du Tafilalt.

Resume Etude de la repartition des Algues et microproblematiques observes dans une sequence regressive de roches carbonatees dont le milieu de depot va du marin ouvert jusqu’au lagunaire. La microflore algaire d’abord reduite a quelques Chlorophytes, augmente rapidement en nombre et diversite (Udoteacees, Dasycladales) dans les flancs des bioconstructions, pour pulluler dans les facies restreints (Pseudo-udoteacees). Une estimation de la bathymetrie se fonde sur les contraintes paleoecologiques des Algues reconnues et l’evolution de la zone euphotique et conduit a un ordre maximum d’une vingtaine de metres de la tranche d’eau. Les trois sequences regressives identifiees etant chacune constituees d’une soixantaine de metres de carbonates recifaux, perirecifaux et lagunaires, l’influence de l’eustatisme nous semble faible, celle de la subsidence, en revanche, majeure.

Until recently Cummingsella was only known by two rare and puzzling species, erected on very limited material. The genus was therefore considered as representing a very uncommon and puzzling form that played no role in carbonate sedimentation. Recent studies of Viséan carbonates in the American Midcontinent have disclosed a prolific Cummingsella flora that indicates the proximity of an algal bank. Cummingsella can therefore be considered locally abundant, cosmopolitan (Europe, Australia, North America), ranging in age from the Tournaisian to the Namurian (Serpukhovian).

The iron (Fe) isotopic composition of 17 Jurassic limestones from the Rosso Ammonitico of Verona (Italy) have been analyzed by Multiple-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS). Such analysis allowed for the recognition of a clear iron isotopic fractionation (mean -0.8 per thousand, ranging between -1.52 to -0.06 per thousand) on a millimeter-centimeter scale between the red and grey facies of the studied formation. After gentle acid leaching, measurements of the Fe isotopic compositions gave delta(56)Fe values that were systematically lower in the red facies residues (median: -0.84 per thousand, range: -1.46 to +0.26 per thousand) compared to the grey facies residues (median: -0.08 per thousand, range: -0.34 to +0.23 per thousand). In addition, the red facies residues were characterized by a lighter delta(56)Fe signal relative to their corresponding leachates. These Fe isotopic fractionations could be a sensitive fingerprint of a biotic process; systematic isotopic differences between the red and grey facies residues, which consist of hematite and X-ray amorphous iron hydroxides, respectively, are hypothesized to have resulted from the oxidizing activity of iron bacteria and fungi in the red facies. The grey Fe isotopic data match the Fe isotopic signature of the terrestrial baseline established for igneous rocks and low-C(org) clastic sedimentary rocks. The Fe isotopic compositions of the grey laminations are consistent with the influx of detrital iron minerals and lack of microbial redox processes at the water-interface during deposition. Total Fe concentration measurements were performed by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) (confirmed by concentration estimations obtained by MC-ICP-MS analyses of microdrilled samples) on five samples, and resultant values range between 0.30% (mean) in the grey facies and 1.31% (mean) in the red facies. No correlation was observed between bulk Fe content and pigmentation or between bulk Fe content and Fe isotopic compositions. The rapid transformation of the original iron oxyhydroxides to hematite could have preserved the original isotopic composition if it had occurred at about the same temperature. This paper supports the use of Fe isotopes as sensitive tracers of biological activities recorded in old sedimentary sequences that contain microfossils of iron bacteria and fungi. However, a careful interpretation of the iron isotopic fractionation in terms of biotic versus abiotic processes requires supporting data or direct observations to characterize the biological, (geo)chemical, or physical context in relation to the geologic setting. This will become even more pertinent when Fe isotopic studies are expanded to the interplanetary realm.

In the Maritime Alps (north-west Italy - south-east France), the Middle Triassic-lowermost Cretaceous platform carbonates of the Provençal Domain locally show an intense dolomitization. Dolomitized bodies, irregularly shaped and variable in size from some metres to hundreds of metres, are associated with tabular bodies of dolomite-cemented breccias, cutting the bedding at a high angle, and networks of dolomite veins. Field and petrographic observations indicate that dolomitization was a polyphase process, in which episodes of hydrofracturing and host-rock dissolution, related to episodic expulsion of overpressured fluids through faults and fracture systems, were associated with phases of host-rock dolomitization and void cementation. Fluid inclusion analysis indicates that dolomitizing fluids were relatively hot (170 to 260°C). The case study represents an outstanding example of a fossil hydrothermal system, which significantly contributes to the knowledge of such dolomitization systems in continental margin settings. The unusually favourable stratigraphic framework allows precise constraint of the timing of dolomitization (earliest Cretaceous) and, consequently, direct evaluation of the burial setting of dolomitization which, for the upper part of the dolomitized succession, was very shallow or even close to the surface. The described large-scale hydrothermal system was probably related to deep-rooted faults, and provides indirect evidence of a significant earliest Cretaceous fault activity in this part of the Alpine Tethys European palaeomargin.

International audience; The common presence of oil seepages in dolostones is widespread in Cretaceous carbonate successions of the Kurdistan Region of Iraq. This integrated field, petrographic, chemical, stable C, O and Sr isotopes, and fluid inclusion study aims to link dolomitization to the origin and geochemical evolution of fluids and oil migration in the Upper Cretaceous Bekhme carbonates. Flux of hot basinal (hydrothermal) brines, which is suggested to have occurred during the Zagros Orogeny, resulted in dolomitization and cementation of vugs and fractures by coarse-crystalline saddle dolomite, equant calcite and anhydrite. The saddle dolomite and host dolostones have similar stable isotopic composition and formed prior to oil migration from hot (81–115 °C) basinal NaCl–MgCl2–H2O brines with salinities of 18–22 wt.% NaCl eq. The equant calcite cement, which surrounds and hence postdates saddle dolomite, has precipitated during oil migration from cooler (60–110 °C) NaCl–CaCl2–H2O brines (14–18 wt.% NaCl eq). The yellowish fluorescence color of oil inclusions in the equant calcite indicates that the oil had API gravity of 15–25° composition, which is lighter than present-day oil in the reservoirs (API of 10–17°). This difference in oil composition is attributed to oil degradation by the flux of meteoric water, which is evidenced by the low δ13C values (− 8.5‰ to − 3.9‰ VPDB) as well as by nil salinity and low temperature in fluid inclusions of late columnar calcite cement.This study demonstrates that linking fluid flux history and related diagenesis to the tectonic evolution of the basin provides important clues to the timing of oil migration, degradation and reservoir evolution.

A condensed Mid-Upper Devonian succession of the TaWlalt (Anti-Atlas) contains numerous red carbonate intervals. Bioclastic tempestites form a transition between an outer shelf and a hemipelagic setting. The red pigmenta- tion is not related to the abundance of detrital iron oxides, but to the presence of iron bacteria and fungi. They form bioconstructions remarkably similar to those previously described in the Jurassic Ammonitico Rosso from Italy and Spain. Thus, dysaerobic microenvironments are proposed and not oxic conditions for these Fe-poor sediments.

The Uppermost Jurassic-Lower Cretaceous limestone succession in the Maiella region of Italy has been investigated in a profile covering more than 400 m stratigraphical thickness at Fara San Martino. The succession mainly consists of peritidal limestones, intertidal and supratidal sequences being dominant, together with subtidal lagoonal facies. As a consequence, the microfossil assemblages are generally poorly developed, Microfossils occur within the subtidal lagoonal facies of this suite of restricted sediments. This feature led us to recognise five informal biostratigraphic intervals, which are facies related - instead of biozones, each of the intervals being characterized by specific micropaleontological associations. The occurrence and/or disappearance of some marker microfossils were used as references for separating these intervals. Based on these markers, the Uppermost Jurassic-Lower Cretaceous carbonate deposits from Maiella region may be correlated with similar deposits from the Apennines and from other regions in the perimediterranean area.

This paper discusses ostracods and their environmental setting close to the Givetian/Frasnian boundary in the Puech de la Suque Global Stratotype Section and Point. The ostracod fauna belongs exclusively to the Eifelian mega-assemblage and is largely dominated by Podocopina instars. Consequently, the majority of the 33 species recognised and illustrated is described in open nomenclature. The abundance of instars also indicates that most of beds are related to storm deposition. The composition of the fauna suggests a regression in the late Givetian followed by a transgression at the beginning of the Frasnian. Only one ostracod assemblage collected in the upper part of the section indicates a deeper setting below the storm wave base. Eight taxa disappeared in two steps near the Givetian / Frasnian boundary, probably in relation to the Frasnes Event. The sedimentological analysis confirms that the section is constituted principally of storm deposits and reveals in addition the presence of numerous reddish hardgrounds highlighting an important condensation of the sequence.

New carbon isotopic data from the Devonian of Ardennes (Belgium) and partly from the Holy Cross Mountains (Poland) highlight an abrupt and high-amplitude negative excursion in the punctata conodont Zone. Published information from Moravia and China suggests that this Middle Frasnian negative excursion, jointly with the preceding large-scale positive shift, should be used as a global chemostratigraphic marker. Causation scenarios for this negative ‘punctata Event’ are correlated neither with major biota turnover nor major sea-level changes, but may be related to: (1) the Alamo Impact Event, that led to (2) the massive dissociation of methane hydrates and (3) the rapid onset of global warming.

Different pathways can be proposed to explain the origin of the red pigmentation present in many Phanerozoic limestones. We investigate three possible models: (1) telogenetic alteration; (2) detrital input and (3) role of iron-bacteria. The third hypothesis is supported by our data for a number of Phanerozoic series. By this hypothesis not only can we highlight the importance of the microenvironments at the dysoxic–anoxic water sediment interfaces for the bacterial Fe-oxidation but also can conclude that the dred limestonesT have no particular paleogeographic meaning. The seven studied red successions are rather poor in iron (350 ppm in the Ammonitico Rosso) without large fluctuations. The pigmentation is due to the dispersion of submicrometric (hydro)oxides (now hematite) formed by bacterial mediation during early diagenesis in various microaerophilitic environments. Different ferruginous morphs, e.g., filaments, coccoids, microstromatolites, suggest the presence of iron-bacteria. The limiting factor is the oxygen content which was low in these very quiet and relatively deep environments. The presence of iron-bacteria is also confirmed by iron isotope analysis on the Ammonitico Rosso. Fe isotopic compositions have been analyzed on residues as well as on leachates of red and grey facies of the same interval. d 56 Fe values are systematically lower in the red residues (varying from 1.19x to 0.34x) compared to the grey ones (0.27x to 0.09x). This indicates a significant accumulation of lighter isotopes in the red parts suggesting that hematite have been formed via a biological pathway. D 2005 Elsevier B.V. All rights reserved.

Illite Kübler index (KI) and oxygen isotope (brachiopods and micrites) investigations have been performed on more than 300 Frasnian limestones sampled in one borehole and numerous outcrops in the Dinant Synclinorium (Belgium, northern France) of the northern Variscan front. The illite Kübler index and δ18O data of a 3-km-thick, tectonically repeated Frasnian series from the Focant borehole are compared with their surrounding surface correspondents and document in-situ reheating induced by Variscan tectonic loading, which post-dated sedimentary burial alteration. The boundary between these two thermal processes (sedimentary burial and tectonic loading) on the Focant profile corresponds to an important location where the heat induced by the tectonic loading was equivalent to that Frasnian strata suffered during maximum sedimentary burial. Mainly based on this knowledge and on a former conodont colour alteration index study, the thickness of the eroded thrust sheet in the Focant area is estimated to be around 3,000 m. Oxygen isotopic exchange in these Frasnian closed carbonate systems, occurring under highest-grade diagenesis and anchimetamorphism, records two events. Brachiopods present a quite different and more homogeneous pattern, due to their higher resistance to heat alteration. These thermal events caused both δ18O records to become increasingly lighter than the presumed original seawater signature. The comparison between KI and δ18O profiles indicates that illite KI analysis is more appropriate than δ18O in highlighting the temperature variations in the burial metamorphism at the periphery of orogenic belts.

Resume L’etude de 14 coupes de l’Ammonitico Rosso Veronese (Callovien a Tithonique) du plateau de Trente montre une tres grande variete de facies allant du pelagique a la plate-forme. Malgre cette grande diversite dans les bathymetries, les facies rouges s’observent a plusieurs niveaux. Les microfacies sont fort semblables a ceux observes dans d’autres calcaires rouges du Paleozoique et Mesozoique avec une abondance de bioconstructions hematitiques. Nous pensons donc que l’origine de la pigmentation de ces facies est semblable et l’attribuons a la presence de ferrobacteries. Deux differences notables sont toutefois observees : la presence ici de manganese en quantite non negligeable et l’existence de mattes bacteriennes et de Fungi imperfecti in situ preserves dans les matrices. Ces mattes representeraient un volume appreciable, voisin de 20 %. Leur excellent etat de preservation serait lie au faible taux de sedimentation, tant des facies pelagiques que des niveaux de hardgrounds.

New carbon and oxygen isotope data from carbonates spanning the Frasnian-Famennian (F-F) boundary in the Guilin area, South China, show a broad positive δ13C rise and fall, with sharp, short-lived negative δ13C events; this pattern is comparable to that in Europe and North America. The integration of the isotope stratigraphy with high-resolution sequence stratigraphy corroborates the onset of the positive δ13C excursion during a third-order sea-level fall in the latest Frasnian. This can best be explained through increased burial of Corg during the sea-level fall, brought about by increased organic productivity caused by increased continent-derived nutrient flux to the ocean due to enhanced weathering through the proliferation of land plants in the Devonian. This scenario would have resulted in anoxic and eutrophic conditions over epicontinental seas and blooms of cyanobacteria, creating a highly stressful and fragile ecosystem for oligotrophic normal-marine benthic organisms and leading to their massive decline. The global third-order sea-level fall near the end of the Frasnian may have led to gas hydrate dissociation (giving the negative δ13C events), and caused wild climatic fluctuations. The subsequent short-term events of sea-level rise, anoxia and eutrophication in the latest part of the F-F transition would have placed additional environmental stresses on the already weakened biota, leading to their further demise.