OSTRACODS AND ROCK FACIES OF THE EIFELIAN/GIVETIAN AND GIVETIAN/FRASNIAN BOUNDARY STRATOTYPES

The E/G GSSP has been recently placed in a section at Djebel Mech Irdane in the Tafilalt of Morocco (WALLISER et al., 1995). The section studied on 4 m is composed of silty mudstones and wackestones, occasionally of packstones, with abundant cricoconarids. Ten cm below the E/G boundary the series are interrupted by 30 cm of black shales containing shocked quartz, Cr, As, V and Co anomalies, a large negative carbon isotop shift and microspherules indicative of a bolide impact (ELLWOOD et al., 2003). Twenty-four ostracod species are recognized in the stratotype. They belong to the Eifelian Mega-Assemblage and are indicative of poorly oxygenated deep marine environments below the storm wave base. The most important change in the ostracod fauna is observed in the black shales.

The G/F GSSP coincident by definition with the Middle/Upper Devonian GSSP has been placed at Puech de la Suque in the Montagne Noire, France (KLAPPER et al., 1987). The section studied on 6 m is constituted of thin bedded fine grained homogeneous limestones. The ostracod fauna composed of 33 species belong also to the Eifelian Mega-Assemblage and is largely dominated by instars of Podocopina. The abundance of instars and of separated valves indicates that most of beds are related to storm deposition. The composition of the fauna suggests a regressive evolution in the upper part of the Givetian, followed by a transgressive evolution in the base of the Frasnian. Eight taxa disappeared in two steps close to the G/F boundary, probably in relation with the Frasnes Event. The sedimentological analysis confirms that the section is constituted principally of storm deposits and reveals in addition the presence of numerous hardgrounds. The choice of the section for a GSSP is consequently questionable. The problem is the same for all the GSSP fixed in the Montagne Noire area. For example, the F/F boundary stratotype at Coumiac is highly condensed and contains numerous hardgrounds, and the GSSP has been fixed surprisingly at the top of a hardground! The coupled study of ostracods and sedimentology indicated also that the La Serre section is far from being an ideal stratotype for the D/C boundary since most of the fossils are reworked.