MIDDLE DEVONIAN CONODONT BIOSTRATIGRAPHY AND STABLE OXYGEN ISOTOPE ANALYSIS IN THE MONT PEYROUX AREA (SOUTHERN MONTAGNE NOIRE , FRANCE)

Considerable uncertainty exists concerning the Middle Devonian conodont biostratigraphy of the Southern Montagne Noire. Carbonate sedimentation interrupted by shales in the Col du Puech de la Suque section (Mont Peyroux area) and unconformities in the Japhet section (Cabrières Klippen) preclude the establishment of a detailed conodont biozonation. In the Pic de Vissou area, on the contrary, conodonts recovered from a continuous Middle Devonian carbonate succession without intervening shales span the costatus Zone (lower Eifelian) to the hermanni Zone (Upper Givetian), but the position of the Eifelian-Givetian boundary is uncertain as the marker taxon P. hemiansatus was not found in samples below the Middle varcus Zone (Walliser, 1990; Aboussalam, 2003).

To address these problems and obtain parallel paleoceanographic data, conodonts from a new section, Col de Tribes, were studied and their oxygen isotopic values were measured. The section is comprised of limestone deposited on top of a Lower Devonian mud mound. The conodonts retrieved indicate deposition from the costatus Zone to the norrisi Zone. The position of Eifelian-Givetian boundary is recognized based on the first appearance of P. hemiansatus at its expected level within the conodont succession. Isotopic values for P. l. linguiformis vary around 20.5‰_VSMOW throughout the section suggesting no significant temperature trends across the interval studied. However, there are negative excursions of between 0.5‰ and 1‰ in the eiflius-ensensis zones and the semialternans Zone that could correlate with the Kaçak (late Eifelian) and Taghanic (late Middle Givetian) events, respectively. The Col de Tribes section lacks intervals of anoxic shale or black limestone often associated with these events, but the isotopic shifts indicate these may have been times of warming and/or freshening of local seawater.