Les Monts de Baileux section: detailed sedimentology and magnetic susceptibility of Hanonet, Trois-Fontaines and Terres d'Haurs Formations (Eifelian/Givetian boundary and Lower Givetian, SW Belgium)

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Abstract

ABSTRACT. This work details for the first time the sedimentology of Les Monts de Baileux section. This section, located in a quarry between Chimay and Couvin, exposes a remarkable succession of strata from the top of the Jemelle Formation to the base of the Mont d'Haurs Formation. It therefore offers the opportunity to investigate the entire Hanonet, Trois-Fontaines and Terres d'Haurs formations, biostratigraphically from Polygnathus ensensis to P. timorensis conodont zones. This large lithostratigraphic thickness of clayley and argillaceous, and also pure limestones encompasses the Eifelian/Givetian boundary in the lower part of Hanonet Formation, allowing a better understanding of the transition between the Eifelian which corresponds to a mixed siliciclastic-carbonate detrital ramp and the lower Givetian dominated by a carbonate rimmed shelf-related sedimentation.

Petrographic study leads to the definition of 21 microfacies integrated into two palaeogeographical models. The first model (13 microfacies) is proposed for the Jemelle, Hanonet and Trois-Fontaines formations, i.e. P. ensensis Zone and almost entire P. hemiansatus Zone. In this platform model, the fore-reef environment is characterized by a high influence of storm events and carbonate input coming from proximal settings. The reef is mainly composed of an accumulation of stromatoporoids, crinoids, tabulate corals and rugose corals with a peloidal matrix. The back-reef area is dominated by agitated environments and calmer lagoons. Locally, less agitated conditions allow growth of massive and laminar organisms. The second model (6 microfacies) concerns the Terres d'Haurs Formation with the end of P. hemianstus Zone and significant part of the P. timorensis Zone. This ramp model is divided into a mid-ramp characterized by open-marine sedimentation interrupted by storm-related events and an inner ramp composed of ooidal shoals, back-shoal sedimentation and storm related deposits. Two other microfacies are fragmentarily defined for the Mont d'Haurs Formation, within the main P. varcus / P. rhenanus intervals. This unit was affected by strong dolomitization processes, where scarcity of well preserved, primary sedimentary fabrics is not favourable for designing of a microfacies-based model at all.

The last part of the work concerns stratigraphic variations of magnetic susceptibility (MS). Values of mass MS of rocks were plotted and juxtaposed with semi-quantitative variation curves of microfacies. According to the prevailing magnitudes of MS, the relationships with two controlling parameters are evaluated: terrigenous influence (using the thin-section data on detrital quartz contents as proxies) and wave agitation (based on microfacies interpretation). The transgressive-regressive evolution of microfacies characteristics are compared with the juxtaposed trends in decreasing-increasing MS magnitudes. Approximately two thirds of this section suggests a good matching of the trends on generalized lithological and magnetic data. It is explained by common presence of clayey/silty impurities of slightly to moderately varying compositions which are greatly evidenced by means of thin-section studies and represent a principal and abundant paramagnetic component. The overall MS magnitudes actually show decreasing trends together with vigorous, eustatically driven sealevel rises. However, the remaining third of intervals in this section shows the rather complex than simple, equivocally or negatively arranged sequence/lithologic and MS stratigraphic trends.

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A propos de : Cédric MABILLE

A propos de : Frédéric BOULVAIN