Accuracy and reliability of conodont zones: the Polygnathus asymmetricus "Zone" and the Givetian-Frasnain boundary

Limestones from the top of the Bouia Formation, Tafilalt, southeastern Morocco, that traverse the Givetian-Frasnian boundary (base of the Lower P. asymmetricus Zone) were sampled several times for conodonts. In the Bou Tchrafine section near Erfoud a styliolinid coquina bed 6 cm thick was sampled five times, in each case from the whole bed at the same place. The bed immediately below belongs to the Klapperina disparilis Zone and the overlying one to the Middle P. asymmetricus Zone. The conodont fauna from the styliolinid bed varies from one sample to another and has been attributed as follows: once to the upper part of the Lower P. asymmetricus Zone, once to the lower part? of the Lower P. asymmetricus Zone and three times to an interval covering the top of the K. disparilis Zone?, the entire Lowermost and the lower part? of the Lower P. asymmetricus Zone. There is no relationship between the stratigraphic accuracy obtained and the size of the sample. In two nearby outcrops, 300 m and 600 m to the southeast, the microcoquina is thicker (15 cm and 51 cm) and composite; each bed was sampled once or twice and the zonal attribution shows the same variation as in the first outcrop. The varying results from the consecutive samplings are explained not only by the depositional environment (a condensed limestone sequence) but also by the unsatisfactory definition of conodont zones between the K. disparilis Zone and the Middle P. asymmetricus Zone. It would be appropriate to revise that part of the standard conodont zonation on the basis of non-condensed sequences.