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Ostracods, rock facies and magnetic susceptibility of the Trois-Fontaines and Terres d’Haurs Formations (Early Givetian) in the Rancennes quarry at the Mont d’Haurs (Givet, France)
Casier, J.-G.; Cambier, G.; Devleeschouwer, X.; Petitclerc, E.; Préat, A. (2010). Ostracods, rock facies and magnetic susceptibility of the Trois-Fontaines and Terres d’Haurs Formations (Early Givetian) in the Rancennes quarry at the Mont d’Haurs (Givet, France). Bull. Kon. Belg. Inst. Natuurwet. Aardwet. = Bull. - Inst. r. sci. nat. Belg., Sci. Terre 80: 85-114
In: Bulletin van het Koninklijk Belgisch Instituut voor Natuurwetenschappen. Aardwetenschappen = Bulletin de l'Institut Royal des Sciences Naturelles de Belgique. Sciences de la Terre. KBIN: Brussel. ISSN 0374-6291, more
Peer reviewed article  

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Keywords
Author keywords
    Ostracods, Sedimentology, Palaeoecology, Magnetic Susceptibility, Givetian, Dinant Synclinorium, Ardennes, France

Authors  Top 
  • Casier, J.-G., more
  • Cambier, G., more
  • Devleeschouwer, X., more

Abstract
    About 1,200 ostracods were extracted from 64 samples collected in the upper part of the Trois-Fontaines Formation (Fm) and in the base of the Terres d’Haurs Fm in the Rancennes quarry located along the western rampart of an historic entrenched military camp at the Mont d’Haurs (southern part of the Dinant Synclinorium, Ardennes Department, France). The ostracod richness and diversity are quite variable, and monospecificity prevails in numerous samples. Forty-nine ostracod species are recognised. In the Trois-Fontaines Fm, environments were lagoonal or semi-restricted, and the level containing numerous Leperditicopid ostracods (Herrmannina) indicative of (brackish?) lagoonal environments is 40 m thick. In the Terres d’Haurs Fm the environment was semi-restricted or more frequently shallow marine but the energy of the environment was apparently never very high. The level rich in Leperditicopida (Herrmannina) in the Trois-Fontaines Fm corresponds remarkably to the highest magnetic susceptibility (MS) value. The Rancennes microfacies point to a tidal flat system with various subenvironments such as restricted intertidal, supratidal and channel deposits. The system was bordered by subtidal open marine deposits where former reefal constructions have been destroyed. Frequent oscillations in this low-gradient shallow platform led to the exposure and modification of marginal ponds, floodplain environments or palustrine areas. No evidence of evaporitic environments or sabkha were encountered. The sedimentary system records the evolution of a shallow restricted carbonate platform (Trois-Fontaines Fm) to a carbonate ramp setting (Terres d’Haurs Fm). The evolution of the platfom to a ramp could be related to the cessation of the active role of a reefal barrier possibly as a response to synsedimentary tectonism and block faulting.The magnetic susceptibility curve established for the Rancennes quarry highlights 26 short-term magnetic evolutions that can be grouped into 10 major long-term magnetic sequences characterized by decreasing, increasing or stable magnetic susceptibility fluctuations. Magnetic susceptibility values range between 3.75 x 10-9 and 2.98 x 10-7 m³/kg. There is a general good correspondence between the microfacies and magnetic susceptibility curves, which are clearly mimetic at the smaller scale (i.e., 5th-order parasequences). The magnetic susceptibility curve could thus be interpreted as sea-level oscillations. A part of the magnetic minerals carrying the MS signal must have a detrital origin. Magnetization and coercivity ratios deduced from hysteresis loops indicate the presence of detrital coarse-grained multi-domain magnetite and authigenic mixtures of fine-grained superparamagnetic and single-domain magnetite. The MS signal of the Rancennes quarry seems to be controlled by the ferrimagnetic fraction (magnetite) with minor paramagnetic contribution (clay minerals and pyrite).The Rancennes quarry completes the stratotype of the Terres d’Haurs Fm because the section exposes the boundary with the Trois-Fontaines Fm unlike the previously proposed stratotype located on the south-eastern flank of the Mont d’Haurs entrenched camp.

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