Xavier Devleeschouwer

The Frasnian/Famennian boundary interval in the South Polish–Moravian shelf basins: integrated event-stratigraphical approach

Abstract The Late Devonian mass extinction event near the Frasnian/Famennian (F/F) boundary has been analysed using conodont biostratigraphy and biofacies, sedimentology, magnetic susceptibility and geochemistry in reference sections of the South Polish–Moravian shelf (Holy Cross Mountains, Cracow and Brno areas). High-resolution biostratigraphic study revealed difficulties in the precise recognition of this ‘natural’ stage boundary, but confirmed the occurrence of a major (third-order) sequence boundary in the F/F transition in an active synsedimentary tectonic setting, marked by erosional discontinuities, hardgrounds and brecciation or omission surfaces. Conclusive evidence of an extraterrestrial impact has not been found. Among Earth-bound factors, the main devastating role in the shelf habitats is ascribed to fluctuating anoxia and/or nutrient dynamics in a disturbed greenhouse climatic setting. The long-term facies changes were determined by a conspicuous break in carbonate production, accompanied by replacement of mature stromatoporoid–coral reefs by pioneer shelly-crinoid banks, microbial mounds and localised oolitic bars. The key F/F passage interval was marked by intermittent but generally accelerated periplatform ooze/debris input and severe storm events, as well as by probably highly fluctuating oxygenation and biological overproduction, best manifested in radiolarian–silicisponge and cephalopod acmes. Eutrophication phenomena, at least partly stimulated by various hydrothermal and volcanic processes, were one of the major biogeochemical processes during this climax of the Late Devonian biotic crisis. However, significant oceanographic perturbations were also linked with the onset of the transgressive–hypoxic Upper Kellwasser Event, still within the latest Frasnian linguiformis Zone, i.e., well before the F/F conodont collapse. Implied variations in the redox state of seawater through the Kellwasser timespan, probably in regionally highly variable temporal scales, are in agreement with recent palaeoecological and biogeochemical inferences, in particular suggesting recovering oxygenation just prior to the F/F boundary in the other Laurussian intrashelf basins. All the data support a long-time, multicausal Earth-bound crisis instead a worldwide cosmic catastrophe.

Chapter 8Productivity and bottom water redox conditions at the Frasnian-Famennian boundary on both sides of the Eovariscan Belt: constraints from trace-element geochemistry

Abtract The Late Devonian Frasnian-Famennian (F-F) mass extinction event coincides in many places with the depositionof C org -rich “Kellwasser” facies. Four F-F boundary sections representative of platform and basin environments from widely separated locations (Morocco, Germany, and France) were analysed for inorganic geochemistry, especially trace elements (redox and productivity proxies), in order to describe paleodepositional environments for the Kellwasser horizons. Ni/Co, V/Cr, U/Th, and V/(V+Ni) ratios, as well as redox trace metal concentrations indicate that oxygen-depleted conditions existed during the times of Kellwasser facies deposition. In platform settings, dysoxic conditions seem to be limited to the Late Frasnian. In basinal settings, oxygen depletion was stronger and persisted into the Early Famennian. Enrichments of Ba, Cu, Ni, that are limited to the Late Frasnian, show that surface productivity was relatively high and organic matter could accumulate, especially in the deeper environments. The stratigraphical distribution of several geochemical markers are linked with two positive excursions of the δ 13 C carb signal that result from enhanced organic matter burial. Reducing conditions likely resulted from high productivity of Late Devonian marine ecosystems. Intense nutrient supply resulted probably from the biogeochemical recycling of nutrients, and/or runoff from emerged lands. Coupled with other factors, such as rapid sea-level fluctuations and climatic changes, oxygen-depleted conditions and eutrophication would have modified Late Devonian environments and could be possible factors in the F-F mass mortality.

Microfacies, sequence stratigraphy and clay mineralogy of a condensed deep-water section around the Frasnian/Famennian boundary (Steinbruch Schmidt, Germany)

Abstract A multidisciplinary analysis (microfacies, sequential stratigraphy and clay mineralogy) was made on Frasnian/Famennian (F/F) boundary strata of the Steinbruch Schmidt section in Western Germany. Three major microfacies are recognised. Their succession records a shallowing-upward evolution from deep, quiet and poorly oxygenated environments, below the storm wave base, to environments influenced by current activities close to the storm wave base. The Kellwasser Horizons correspond to the deepest microfacies. The shallowest microfacies correspond to fine-grained calcareous tempestites or turbidites coming from a distant shelf of northwest Germany. The sequential pattern through the F/F boundary shows the succession of seven systems tracts. Two sequence boundaries are located just above the Lower Kellwasser Horizon and at the F/F boundary itself. These are underlined by hardgrounds suggesting time gaps. The Kellwasser Horizons correspond to sea-level highstands and the overlying beds record a transition from lowstand to transgressive systems tracts. Illite and kaolinite are the dominant clay minerals associated with mixed layers and traces of chlorite. Illite abundance is maximal during Kellwasser Horizons. Illite and kaolinite were probably inherited from a highly weathered source area although part of the illite is diagenetic. Kaolinite is the second most abundant clay mineral and is particularly well represented (up to 50%) between the Kellwasser Horizons. An unusual clay assemblage of illite and mixed layers is associated with a bentonite layer. Kaolinite increases during times when thin tempestites or turbiditic microbioclastic layers come from a distant shelf during sea-level falls. The kaolinite percentage reaches its maximum at the top of the lowstand systems tract. The high percentage of kaolinite suggests a hot–wet climate and could be related to global warming.

Magnetization carriers of grey to red deep-water limestones in the GSSP of the Givetian–Frasnian boundary (Puech de la Suque, France): signals influenced by moderate diagenetic overprinting

Abstract Limestones at the Puech de la Suque Global Boundary Stratotype Section and Point (GSSP) of the Givetian–Frasnian boundary show a drastic change towards much higher magnetic susceptibility values in the Givetian rocks. Different rock magnetic parameters indicate that ferromagnetic minerals are the main controlling factor. The ferromagnetic fraction is composed of low- (magnetite-type) and high-coercivity (hematite and goethite) phases. Confirmed by the spectral reflectance, high coercivity minerals are fluctuating along the section with a higher abundance in the basal Frasnian. These phases may be of secondary origin and produced during burial stage. The magnetite-type phase contains two different grain-size populations. The identified that Stable Single-domain/Superparamagnetic (SSD/SP) particles are of diagenetic origin and their amount decreases slightly upwards. A second group of magnetite grains correspond to coarse-grained particles identified using the squareness v. coercive force plot. It is tentatively suggested here that these particles present throughout the section are of primary origin. On the contrary, the paramagnetic minerals underwent a clear diagenetic overprinting and may represent secondary minerals. The presence of a primary ferromagnetic carrier allowed the use of spectral analyses, leading to the detection of spectral peaks at 1.25 and 4 cycles/m, which can be interpreted as the result of 405-kyr and 100-kyr eccentricity forcing, respectively.

Magnetic susceptibility application : A window onto ancient environments and climatic variations: Foreword

Abstract Magnetic susceptibility (MS) is a powerful tool, which is being applied increasingly on sedimentary rocks to constrain stratigraphic correlations, or as a palaeo-environmental or palaeo-climatic tool. The origin of the magnetic minerals responsible for the variations in MS can be linked to various phenomena such as detrital inputs, pedogenesis, bacterial precipitation or diagenesis. Therefore, it is critical to improve our knowledge of the origin of the MS signal in order to apply it for correlations or as a proxy. Here, we present a synthesis of the techniques that can be applied to get a better understanding of the origin of the MS signal, through comparison with other palaeo-environmental proxies, through magnetic measurements or through dissolution and direct observation of the extracted minerals. We also propose an overview of the different techniques applied in order to use MS as a correlation tool, and we show various examples of successful applications of MS as a recorder of change in past sea-level and climate. We also present the main results and activities of the IGCP-580 project ‘Application of magnetic susceptibility as a palaeo-climatic proxy on Palaeozoic sedimentary rocks and characterization of the magnetic signal’.

Insights into a million-year-scale Rhenohercynian carbonate platform evolution through a multi-disciplinary approach: example of a GIvetian carbonate record from Belgium

In this paper we formulate answers to three important questions related to Givetian carbonate records and their use for reconstructing million-year-scale past palaeoenvironmental changes. First, we provide detailed illustrations of the fascinating diversity that shaped a significant shallow reefal platform during early to late Givetian time in the Rhenohercynian Ocean; secondly we improve the sedimentological model of the extensive Givetian carbonate platform in the Dinant Basin; and thirdly we evaluate the application of magnetic susceptibility as a tool for long-term trend correlations and palaeoenvironmental reconstructions. These goals are reached by making a sedimentological, geophysical and geochemical study of the La Thure section. Through the early–late Givetian interval we discerned 18 microfacies ranging from a homoclinal ramp to a discontinuously rimmed shelf and then a drowning shelf. The comparison of these sedimentological results with those published for the south of the Dinant Syncline allowed us to provide an up to date model of the vertical and lateral environmental development of one of the largest Givetian carbonate platforms in Europe. This comparison also increased the knowledge on the distribution of facies belts in the Dinant Basin and allowed us to highlight the Taghanic Event. Palaeoredox proxies reveal a substantial change in the oxygenation level, from oxygen-depleted to more oxic conditions, between middle and late Givetian time. We demonstrated the relationship between variation in magnetic susceptibility values and proxies for siliciclastic input (such as Si, Al). The La Thure section is considered a key section for the understanding of internal shelf settings bordering Laurussia9s southeastern margin.

Sedimentary development and magnetic susceptibility evolution of the Frasnian in Western Belgium (Dinant Synclinorium, La Thure section)

Abstract Recent opening of the La Thure quarry in Western Belgium allowed the collection of new data from a poorly outcropping area of the Belgian Frasnian platform. The studied section covers an interval extending from the falsiovalis to hassi s.l. Zones. Sedimentological analyses allowed the reconstruction of depositional settings in the northwestern part of the Dinant Synclinorium after the demise of the extended Givetian carbonate platform. Two depositional models are distinguished: (a) siliciclastic drowned platform during the Early Frasnian; and (b) a fore-reef depositional setting belonging to a rimmed shelf during the Middle Frasnian. Moreover, interpreted depositional settings in the northwestern part of the Dinant Synclinorium allowed to constraint the direction of the main facies belts for the Belgian Frasnian platform. Combination of MS and geochemistry demonstrates the inherent-parallel link existing between variation in MS values and proxy for terrestrial input (such as Si and Al). This observation means that, despite the remagnetization occurring within the Belgian Frasnian lithologies, the main trends in the MS signal from the La Thure section still reflect some syn-sedimentary conditions. The increase in MS and clastic input proxies recorded in distal fore-reef deposit within the punctata Zone are likely to be considered as enhanced by rapid and strong shallowing events recognized in the ‘puncata Event’ interval.