OSTRACODES, ROCK FACIES AND MAGNETIC SUSCEPTIBILITY OF THE TROIS-FONTAINES / TERRES D'HAURS TRANSITION IN THE TYPE LOCALITY FOR THE GIVETIAN

The quarry of Rancennes at the Mont d'Haurs, close to Givet, exposes 60 m of gray pluri-decimetric limestone beds belonging to the Trois-Fontaines Fm and Terres d'Haurs Fm (Lower Givetian). 1,200 ostracodes were extracted by the hot acetolysis method from 64 samples collected in the quarry, and 49 species were recognized. The richness and diversity of ostracodes are very variable and the monospecificity prevails in numerous samples. Ostracodes appertain to the Eifelian Mega-Assemblage, and more precisely to several assemblages indicative of shallow marine, semi-restricted and lagoonal environments. These last are in great part characterized by Leperditicopida belonging to the genus Herrmannina. The thickness of the level rich in Leperditicopida which reach about 40 m in the Trois Fontaines Fm exposed in the Rancennes quarry, is recognizable on hundreds kilometers. The sedimentological analysis recorded 8 microfacies ranging from open-reefal to restricted supratidal environments. The evolution of the lithological curve allows the recognition of numerous regressive intervals in the Trois-Fontaines Fm ending with an important transgressive episode straddling the Trois-Fontaines / Terres d'Haurs boundary. The magnetic susceptibility (MS) acquired with a Kappabridge MFK1-A, ranges between 0.1x10-7 and 3.0x10-7 m³/kg. The MS curve is subdivided in several successive episodes, relatively well correlated with the sedimentological curve.

The Rancennes quarry completes advantageously the stratotype of the Terres d'Haurs Fm located on the southeastern flank of the entrenched camp of the Mont d'Haurs where the Trois-Fontaines / Terres d'Haurs boundary is not visible.