Ostracods and lithofacies close to the Eifelian-Givetian boundary (Devonian) at Aisemont (Namur Synclinorium, Belgium)

The paper presents the sedimentological analysis and the ostracod study of the upper part of the Rivière Fm. and of the stratotype of the Névremont Fm. in the Aisemont section, the reference section for the Eifelian-Givetian boundary (Devonian) in the southern border of the Namur Synclinorium, Belgium.Thirteen major microfacies types are defined. Their stratigraphic succession records the transition from a mixed siliciclastic-carbonate ramp (upper Eifelian) to a carbonate platform (lower Givetian). About 6,350 ostracods were extracted. Forty species (19 in open nomenclature) are recognized in the Rivière Fm. and 48 species (27 in open nomenclature) in the Névremont Fm. They belong exclusively to the Eifelian Mega-Assemblage and their distribution is controlled mainly by water energy and salinity variations.The sedimentological analysis and the study of ostracods are in good agreement with recent conodont study of GOUWY & BULTYNCK (2003). Our work confirms the position of the Rivière and Névremont formations boundary and highlights a gap of the major part of the Hanonet and Trois-Fontaines formations of the southern Dinant Synclinorium. This hiatus is probably associated with the lowstand systems tract at the Eifelian-Givetian boundary interval in northern France and southern Belgium (PRÉAT, 2004).The presence of Quassilites fromelennensis and Jenningsina heddebauti, indicates that a great part of the transitional zone of GOUWY & BULTYNCK (2003) is of Givetian age

Evolution sédimentaire et Ostracodes de la base du Givetien à Resteigne (bord sud du Bassin de Dinant, Belgique)

The facies patterns of lower Givetian rocks (Trois-Fontaines, Terres d'Haurs and Mont d'Haurs Formations) at Resteigne quarry (southern border of Dinant Basin, Belgium) reveal a transition from mixed siliciclastic carbonate ramp to gently sloping carbonate platform without any true reefal barrier. Depositional conditions on the ramp were open marine, well circulated, well lit and very near or within wave base. The ramp evolved through a very shallow-subtidal to supratidal, restricted to semi-restricted, quiet stable platform during early Givetian times. A sedimentological model for this succession enables recognition of 12 major microfacies, ranging from open marine, below wave action zone, to reefal complexes, lagoons and supratidal mats. These microfacies define three principal facies belts. Four ecozones belonging to the "Eifeler ökotyp" are recognized in the Givetian of Resteigne quarry. Ecozone 0 is characterized by Leperditicopida in the middle and upper parts of the Trois-Fontaines Formation and indicates a restricted lagoonal environment; three ostracod sub-assemblages can be recorded in term of salinity. Ecozone I is characterized by a few species of Platycopina, Palaeocopina or Eridostraca, each present in large numbers; it indicates an open lagoonal environment in the Terre d'Haurs Formation and, exceptionally, in the middle and upper parts of the Trois-Fontaines Formation. Ecozone II, dominated by large, thick shelled Podocopina, indicates water turbulence at the base of the Trois-Fontaines Formation and in the Mont d'Haurs Formation. Ecozone III is rich in species of Podocopina, Platycopina, Metacopina, Palaeocopina and Eridostraca; it indicates a shallow open marine environment in the Mont d'Haurs Formation and, exceptionally, in the Terres d'Haurs Formation. Water movement and salinity are the principal ecological factors controlling the composition of the basal Givetian ostracod fauna at Resteigne. Their detailed analysis gives more information about the elementary regressive rhythms that produce cyclic patterns on the platform. A new species is erected: Evlanella lessensis n. sp

Sédimentologie et Ostracodes de la Limite Eifelien-Givetien à Resteigne (Bord Sud Du Bassin de Dinant, Belgique)

The global cyclic succession of the microfacies from the Upper Eifelian to Lower Givetian times shows evidence of shallowing from deeper settings, unaffected by storm-reworking, into «proximal» above storm wave base settings, near a shoreline coast. This overall regressive progradational megasequence indicates that deposition could have occurred on a ramp setting without any kind of continuous rims and-or typical lagoonal facies behind them. This general upward-coarsening progradational evolution corresponds probably to a phase of reduced sea-level rise or stillstand on the top of which the ramp setting has evolved into a carbonate progradational accretionnary platform. Forty-one ostracod species are reported and twenty-four described from the Hanonet Formation and from the base of the Trois-Fontaines Formation of the Resteigne quarry. Sixteen are known mainly from Germany and Poland and five of them are closely related to well established species. Sixteen species are placed in open nomenclature, three of which are new, and eight are comparable with previously described data. Aparchites resteignensis n.sp. is described in detail. The ostracod fauna indicates a shallow marine environment and an increase of water agitation near the Eifelian/Givetian boundary

Ostracodes et sédimentologie du sommet de l'Eifelien et la Base du Givetien, à Couvin (bord sud du Bassin de Dinant, Belgique)

About seventy ostracod species are present in the upper part of the Hanonet Formation (Upper Eifelian) and in the base of the "Trois-Fontaines" Formation (Lower Givetian) of the "La Couvinoise" quarry at Couvin (southern border of Dinant Basin, Belgium). The ostracods belong to the "Eifeler Okotyp" and four associations are recognized which characterize successively a quiet open marine environment on the outer ramp (ecozone III), a shallow marine environment with increasing of water turbulence on the mid- and inner ramps (ecozone II), an open lagoonal environment on the upper part of the inner ramp (ecozone I) and a restricted lagoonal environment on the platform (ecozone 0). This sequence is the same as in the Resteigne quarry (50 km E of Couvin) studied by CASIER & PREAT (1990 & 1991) and is closely related to environmental variations. The principal ecological factors controlling the composition of the ostracod fauna are water movement in the Hanonet Formation (Eifelian), and water movement and salinity in the Trois- Fontaines Formation (Givetian). Two new species are erected: Condracypris? covinumensis n. sp. et Microcheilinella? hanonetensis n. sp