Frédéric Boulvain

Palaeozoic Reefs and Bioaccumulations: Climatic and Evolutionary Controls.

The geological record contains a fascinating diversity of reefs and shell accumulations. As my other biosedimentary structures, their facies characterization requires careful observation at outcrop and sample scale, and in thin-section to provide information about the global geometries, fabrics and textures respectively. This collection of papers encompasses the breadth of sedimentary geometries and facies displayed by Palaeozoic reefs, shell accumulations, and transitional composite deposits. The definition of reefs and shell concentrations has given rise to variations in nomenclature. The papers in this volume cover specific problems regarding the nomenclature and facies characterization of reefs, shell accumulations and transitional composite deposits. However, rather than attempt a complete revision of terms, the authors have touched on some of the important issues at this stage of development in the field: the main climatic, environmental and evolutionary factors that controlled the Palaeozoic development of shell accumulation and reefs.

Iron microbial communities in Belgian Frasnian carbonate mounds

SummaryThe Belgian Frasnian carbonate mounds occur in three stratigraphic levels in an overall backstepping succession. Petit-Mont and Arche Members form the famous red and grey “marble” exploited for ornamental stone since Roman times. The evolution and distribution of the facies in the mounds is thought to be associated with ecologic evolution and relative sea-level fluctuations. Iron oxides exist in five forms in the Frasnian mounds; four are undoubtedly endobiotic organized structures: (1) microstromatolites and associated forms (blisters, veils...), possibly organized in “endostromatolites”; (2) hematitic coccoids and (3) non dichotomic filaments. The filaments resemble iron bacteria of theSphaerotilus-Leptothrix “group”; (4) networks of dichotomic filaments ascribable to fungi; (5) a red ferruginous pigment dispersed in the calcareous matrix whose distribution is related to the mound facies type. The endobiotic forms developed during the edification of the mounds, before cementation by fibrous calcite. The microbial precipitation of iron took place as long as the developing mounds were bathed by water impoverished in oxygen.

Sequence stratigraphy of the Frasnian-Famennian transitional strata: a comparison between South China and southern Belgium

The sedimentological evolution of the Frasnian-Famennian transitional strata in South China and southern Belgium has been investigated. A similar trend in the deepening and shallowing of the sedimentation environment occurs in the two palaeogeographically distinct areas. The stratigraphic succession has been subdivided into depositional sequences. A general deepening occurs in the Palmatolepis rhenana conodont zone. During the most rapid rise in sea level, a transgressive systems tract formed. This transgressive systems tract is followed by a highstand systems tract in the late P. rhenana zone. A major sequence boundary within the latter zone is indicated by an unconformity in intraplatform and platform areas and by a conformity in the basinal area. The uppermost sediments within the late rhenana zone correspond to a lowstand systems tract. A rapid flooding took place near the base of the Palmatolepis linguiformis zone and dark shales and limestones formed. A second sequence boundary coincides with or is very close to the Frasnian-Famennian boundary (base P. triangularis zone). The global extinction event at the Frasnian-Famennian boundary coincides with an important eustatic fall in sea level. The proposed stratigraphic subdivision should allow worldwide correlations of shallow and deep water deposits.

Sedimentology, magnetic susceptibility and isotopes of a Middle Frasnian carbonate platform: Tailfer section, Belgium

SummaryThe Tailfer section (Belgium, northern border of the Dinant Synclinorium) exposes Middle Frasnian shallow-water limestones. This paper proposes a sedimentological analysis based on classical petrology, which leads to the identification of 19 fourth order sequences, forming three third order system tracts. This sedimentological analyses is complemented by carbon isotopic and magnetic susceptibility (MS) data (based on the relation between MS and lithogenic input). Each system tracts is characterised by a particular evolution of the isotope and MS curve:--Biostromal Unit: the biostromes are built by lamellar and branching stromatoporoids and massive metazoans. MS values are weak, in relation with the important distance from landmasses and lesser amounts of lithogenic contribution. The carbon isotope values are close to Frasnian seawater values.--Lagoonal Unit: it corresponds to a lagoonal facies succession, from inter- to supratidal zones, rich inAmphipora, paleosiphonocladalesUmbella, and pellets, alternating with paleosoils. MS values are high, related to landmass proximity and high lithogenic input. The carbon isotopic curve shows strongly negative values, close to −7%., resulting probably from a continental influence.--Lagoonal and biostromal Unit: it consists of a lagoonal succession with some biostromal interruptions. MS values are lower than in the lagoonal unit and the isotopic values are higher (close to 0%.), related to an increase in distality.

Sedimentology and magnetic susceptibility of the Upper Eifelian-Lower Givetian (Middle Devonian) in SW Belgium: insights into carbonate platform initiation

Abstract The major part of the Hanonet Formation is deposited on a mixed siliciclastic—carbonate detrital ramp, whereas the top is dominated by carbonate-rimmed shelf-related sedimentation. The transition corresponds roughly to the Eifelian—Givetian boundary. This work is based on two stratigraphic sections located in the southern part of the Dinant Synclinorium. Petrographic study leads to the definition of 11 microfacies, which demonstrate important sedimentological differences existing between the sections. A curve showing microfacies evolution is interpreted in terms of changing bathymetry. An environmental model depicts the lateral transition from a multiclinal carbonate ramp (to the east) to a fore-reef setting (to the west). Magnetic susceptibility was used to establish accurate stratigraphic correlations between the two sections. It also leads to an appreciation of the relative importance of eustatic sea-level change and local sedimentation rate. The combined interpretation of the microfacies curves and the magnetic susceptibility provides a new view of the sedimentary dynamics of the studied sections and, in a more general way, a better understanding of the processes responsible for magnetic susceptibility variations in carbonate rocks.

Magnetic susceptibility and its relationship with paleoenvironments, diagenesis and remagnetization: examples from the Devonian carbonates of Belgium

To better understand the origin of the initial magnetic susceptibility (χin) signal in carbonate sequences, a rock magnetic investigation that includes analysis of acquisition curves of the isothermal remanent magnetization (IRM) and hysteresis parameters, was undertaken on Devonian carbonates from the Villers and Tailfer sections, Belgium. Both sections are divided into a lower unit, dominated by biostromal and external ramp facies (biostromal unit) and an upper unit, only consisting of lagoonal facies (lagoonal unit). The variations in χin signal are mainly driven by magnetite variation, mostly pseudo-single-domain (PSD) magnetite. Clay minerals, pyrite, hematite and obviously calcite and dolomite are also present but their contribution to the χin pattern is not significant. There is a correlation between detrital proxies (Zr, Rb, Al2O3, TiO2) and χin for the Tailfer biostromal unit and the entire Villers section. The pervasive presence of fine-grained magnetite is interpreted as related to remagnetization. In absence of external fluids, the iron released during the smectite to illite transition remains in situ. In those situations χin may reflect an inherited primary synsedimentary signal. In the lagoonal unit of the Tailfer section, remagnetization appears to have obscured the original detrital information prompting the need for an evaluation of the composition of the susceptibility signal for individual case studies.

Frasnian carbonate mounds from Belgium: sedimentology and palaeoceanography

Abstract The facies architecture, sedimentary dynamics and palaeogeographic evolution were reconstructed for a number of middle-late Frasnian carbonate mounds from the south side of the Dinant Synclinorium (Belgium). Nine facies were recognized in the buildups, each characterized by a specific range of textures and assemblage of organisms: spiculitic wackestone with stromatactis (facies Pm1), which becomes progressively enriched in crinoids and corals (Pm2); grey or pinkish limestone with stromatactis, corals and stromatoporoids (A3–L3, Pm3); grey limestone with corals, peloids and dasycladales (A4–L4, Pm4); grey, microbial limestone (A5–L5, Pm5); grey limestone with dendroid stromatoporoids (A6–L6); grey, laminar fenestral limestone, (A7–L7); and grey, bioturbated limestone (A8–L8). Sedimentological evidence suggests that facies Pm1 and Pm2 correspond to iron bacteria-sponge-dominated communities, developing in a quiet aphotic and hypoxic environment. A3–L3 developed between storm and fair-weather wave base, in an oligophotic environment. Facies A5–L5 developed close to fair-weather wave base. Facies A6–L6 and the fenestral limestone A7–L7 correspond to an environment with slightly restricted water circulation. Facies A8–L8 developed at subtidal depths in a quiet, lagoonal environment. The main differences between the middle and late Frasnian mounds concern facies architecture, and are a consequence of different palaeoceanographic settings. The large flattened middle Frasnian Arche and Lion buildups show limited vertical differentiation, large-scale progradation features, extensive exportation of material towards off-reef environment and development of inner lagoonal facies. They grew offshore from a well-developed carbonate platform with a healthy carbonate factory. Middle Frasnian sea-level fluctuations were relatively mild, and sedimentation was able to keep up with sea-level rise. At the opposite extreme, during the late Frasnian, severe eustatic rises, together with rising oceanic hypoxic conditions, were responsible for frequent collapses of the carbonate factory, drowning of the middle Frasnian carbonate platform, and development of buildups with relatively limited lateral extension, high vertical facies differentiation, low potential for material exportation and high content in microaerophilic iron bacteria.

Iron Oxidation and Deposition in the Biofilm Covering Montacuta ferruginosa (Mollusca, Bivalvia)

The shell of the bivalve Montacuta ferruginosa is covered with a rust-colored biofilm. This biofilm includes filamentous bacteria and protozoa encrusted with a mineral, rich in ferric ion and phosphate. The aim of this research was to study two possible microbial iron precipitation pathways in the biofilm, namely, microbial iron oxidation and microbial degradation of organic Fe(III) complexes. The iron-oxidizing activity was assayed spectrophotometrically by monitoring the formation of the dye Wurster blue in biofilm extracts. Iron-oxidizing activity was effectively detected in extracts obtained by oxalic acid treatment of biofilm fragments. Extracts obtained without oxalic acid treatment, heated extracts, or extracts supplemented with HgCl 2 did not show any activity. This suggests that an iron-oxidizing factor (IOF), possibly an enzyme, coprecipitated with the mineral. Additional information gathered by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, gel-filtration chromatography, and UV spectrophotometry indicate that the IOF would be a small peptide or glycopeptide (1,350 Da). Microbial degradation of organic Fe(III) complexes was assayed with biofilm fragments incubated in a medium containing ferric citrate. Analysis of the supernatants after various intervals revealed that the complex was degraded by living microorganisms much faster than in the heat-killed negative controls. We conclude that ferric iron precipitation in the biofilm may proceed by way of microbial Fe(II) oxidation as well as microbial degradation of organic Fe(III) complexes.The shell of the bivalve Montacuta ferruginosa is covered with a rust-colored biofilm. This biofilm includes filamentous bacteria and protozoa encrusted with a mineral, rich in ferric ion and phosphate. The aim of this research was to study two possible microbial iron precipitation pathways in the biofilm, namely, microbial iron oxidation and microbial degradation of organic Fe(III) complexes. The iron-oxidizing activity was assayed spectrophotometrically by monitoring the formation of the dye Wurster blue in biofilm extracts. Iron-oxidizing activity was effectively detected in extracts obtained by oxalic acid treatment of biofilm fragments. Extracts obtained without oxalic acid treatment, heated extracts, or extracts supplemented with H(g)Cl2 did not show any activity. This suggests that an iron-oxidizing factor (IOF), possibly an enzyme, coprecipitated with the mineral. Additional information gathered by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, gel-filtration chromatography, and UV spectrophotometry indicate that the IOF would be a small peptide or glycopeptide (1,350 Da). Microbial degradation of organic Fe(III) complexes was assayed with biofilm fragments incubated in a medium containing ferric citrate. Analysis of the supernatants after various intervals revealed that the complex was degraded by living microorganisms much faster than in the heat-killed negative controls. We conclude that ferric iron precipitation in the biofilm may proceed by way of microbial Fe(II) oxidation as well as microbial degradation of organic Fe(III) complexes.

Diagenetic and palaeogeographic significance of clay, carbonate and other sedimentary components in the middle Devonian limestones of western Ardenne, France

Abstract Givetian deposits accumulated in a more than 300-m-thick succession on a carbonate platform developed on the passive margin of the so-called Rheic ocean. These rocks crop out in the Ardenne massif at Glageon in a structural context marked by small fault-controlled basins subjected to moderate overburden, tectonic and metamorphic constraints. Data on the clay mineral distribution, associated with detailed information on limestone facies/microfacies, as well as data on the depositional profile and sequence stratigraphic evolution of these rocks indicate strong carbonate diagenesis and moderate clay diagenesis, the latter being favoured by the early occurrence of the former, which prevented further fluid-rock interaction. The clay assemblage distribution, which roughly parallels the palaeomorphologic and palaeobathymetric shape of the Givetian continental margin, is organized in five successive zones indicating large-scale sequence stratigraphic evolution and the control of regional sea-level fluctuations. The clay and other sedimentary components provide additional information on the warm and variably humid climate, northern, relative to southern, terrigenous sources, the open sea relative to restricted depositional environments, the temporary tectonic rejuvenation of the continental hinterland and the chemical conditions allowing early diagenetic modifications.

The astronomical calibration of the Givetian (Middle Devonian) timescale (Dinant Synclinorium, Belgium)

Abstract Recent advances in radiometric dating result in significant improvements in the geological timescale and provide better insight into the timing of various processes and evolutions within the Earths system. However, no radiometric ages are contained within the Givetian. Consequently, the absolute ages of the Givetian Stage boundaries, as well as the stages duration, remain poorly constrained. As an alternative, the analysis of sedimentary cycles allows for the estimation of the duration of this stage. We examined the high-resolution magnetic susceptibility signals of four Givetian outcrops in the Givet area for a possible astronomical imprint, to fully understand the rates of evolutionary and environmental change. All four sections are firmly correlated and wavelet analyses of the magnetic susceptibility signals reveal the imprint of astronomical eccentricity forcing. The highly stable 405 kyr cycles constrain the duration of the Givetian Stage at 4.35±0.45 Myr, which is in good agreement with the International Chronostratigraphic Chart (5.0 Myr). The studied sections also exhibit an imprint of obliquity, suggesting a climatic teleconnection between low and high latitudes. The corresponding microfacies curves demonstrate similar astronomical imprint, and thereby indicate that the observed 105 year-scale cyclicity is the result of climatic and environmental change.