Résumé
La coupe classique dans les calcaires fossilifères de Tercis débute dans du Campanian et monte jusque dans la base du Danien. L'affleurement actuel est en grande partie un front de carrière continu où la limite Campanien-Maastrichien se situe dans un faciès homogène. Afin de tenter de qualifier cette section pour localiser le point stratotype global de limite d'étages (notion clé moderne pour définir les limites d'unités chronostratigraphiques), le front de carrière a été marqué mètre par mètre sur une épaisseur de 165 m. Une étude préliminaire a permis une subdivision de la coupe en huit unités fondées sur des critères lithologiques : présence/absence de glauconie et de silex notamment. Des fossiles et des sédiments sélectionnés ont été collectés pour réaliser une étude de stratigraphie intégrée bio-magnéto- et chimiostratigraphique. L'état de l'étude lithostratigraphique, y compris la présence des fossiles, est résumé dans ce travail.
Abstract
Campanian and Maastrichtian limestones are known for long from the Tercis and nearby localities (Landes, France). Fossils from there are preserved in collections and pictured in many monographs. The area is ideally located between the boreal and tethyan realms; similar macrofossils are found from Madagascar to Denmark, from Caucasus to North America. The modern quarry, abandoned this year, needed description before modern integrated study. With easy access and good outcropping, the main section comprises about 165 m of continuous exposure with further possible collection points above up to the Danian and some way below within sub-vertical layers. The limestones are generally homogeneous but contain flint-bearing units and diagnostic glaucony content the combination of which allows subdivision into 8 lithologic units (fig. 2). The abundance of glaucony has been evuluated from the number of grains (50-100 µm in diameter) per cm² in about 100 samples. The main section is paint marked every meter and most observations may be compared on up to 3 different levels of the section. Inoceramids are abundant (Dhondt, 1993). Echinoids are widespread and diversified. Ammonites are present, with Baculites locally abundant; many are poorly preserved. Foraminifera are rare to present; the calcareous nannofossils are common in more clayey layers as well as the dynokysts. Diversified other fossil groups are also present. J.M. Hancock et al. (1993) have shown the biostratigraphic potential of the quarry. Following this preliminary study, a series of additional works: biostratigraphy, magneto- and chimiostratigraphy. The section will be considered with special care for locating the Global Stratotype Point for the Campanian-Maastrichtian boundary under the aegis of the IUGS Commission on Stratigraphy; a working Group has been formally established for this purpose this year.
Dernière mise à jour le 29.07.2015