MIDDLE DEVONIAN CONODONT BIOSTRATIGRAPHY AND STABLE OXYGEN ISOTOPES AROUND THE EIFELIAN-GIVETIAN BOUNDARY OF THE COL DES TRIBES SECTION (MONTAGNE NOIRE, FRANCE)

Conodont biostratigraphy around the E-G boundary in the Montagne Noire region is not yet known in detail. In the local reference section of the Mont Peyroux area, limestone sedimentation is interrupted by the deposition of basinal shales and cherts resulting in no conodont data from the australis Zone into the Lower hermanni Zone. In the Cabrières area, the Middle Devonian is represented by a continuous limestone succession that has yielded a complete succession of conodont zones from the kockelianus Zone up to the base of the Frasnian, but the position of the E-G boundary in this section is uncertain as the marker taxon P. hemiansatus to date has not been found in samples below the Middle varcus Zone.

To address these problems, a new section, Col des Tribes, situated in the Mont Peyroux area and showing a continuous limestone succession for the Middle Devonian, has been sampled for conodont biostratigraphy. The conodonts retrieved allow the establishment of the conodont biozonation from the costatus Zone up to the norrisi Zone with the E-G boundary recognized based on the first appearance of P. hemiansatus. Additional new samples, taken in the Pic de Vissou Quarry in the Cabrières area, indicate the E-G boundary boundary falls within unit I of House (1995).

To estimate temperature trends across the E-G boundary, we are measuring δ_¹⁸O values of well preserved conodont apatite from single-taxon samples in both sections. A previous study of the Pic de Vissou Quarry, showed increasing δ_¹⁸O values around the boundary but was based on multiple-genera conodont samples (Joachimski, 2009). This lack of taxonomic resolution could be important as a shift from a neritic to a pelagic environment occurs in the region during the Middle Devonian. Timing differs slightly between the two examined sections, but the shift results in abundance changes among conodont genera. If there are consistent differences in δ_¹⁸O among taxa, measured δ_¹⁸O trends could be in part an artifact of changing composition of the assemblages analyzed. By constructing δ_¹⁸O trends based on single-taxon samples in both sections we should be able to separate temporal, facies, and taxonomic δ_¹⁸O trends.