3.1. INTRODUCTION
The "Système du Calcaire de Frasne" was introduced by d'Omalius d'Halloy (1868) and the name "Frasnien" was used for the first time by Gosselet (1879) corresponding to the base of the "Calcaire et schistes de Frasne" and to the lower part of the Upper Devonian (Gosselet, 1880). At a meeting of the Subcommission on Devonian Stratigraphy (SDS) at Binghampton, N.Y. in 1981 the name Frasnian was formally retained for the lower stage of the Upper Devonian. The base of the Frasnian has been defined in the Global Stratotype Section and Point at the Col du Puech de la Suque in the Montagne Noire (France) on the basis of the earliest occurrence of the conodont species Ancyrodella rotundiloba (Klapper et al, 1987). This boundary almost coincides with the original boundary and it is accurately documented in a section at Nismes, near Frasnes, 1.5 m above the base of the Nismes Fm and the base of the Frasnes Group (Bultynck et al, 1988). This section was adopted by the SDS (Prague 1986) as an auxiliary boundary stratotype for neritic facies.
Frasnian rocks of the Ardenne are exposed in the Dinant and Namur Synclinoria and in the Vesdre Nappe. However, five more specific areas can be recognized on the basis of differences in the lithostratigraphic subdivisions: the southern flank of the Dinant Synclinorium; the Philippeville Anticlinorium; the northern flank of the Dinant Synclinorium, the southern flank of the Namur Synclinorium and the Vesdre Nappe; the eastern part of the northern flank of the Namur Synclinorium and the central part of the last mentioned area.
The names of some of the lithostratigraphic units used herein were already introduced in the 19th Century, e.g. "Schistes de Matagne, Schistes de Barvaux, Assise de Bovesse, Assise de Rhisnes...". The first detailed stratigraphic subdivision of the Frasnian, based on both lithological and paleontological criteria, was published by Maillieux & Demanet (1929), e.g. F2a, F2b,... These subdivisions have been widely used until the early seventies of the past century. From this period on, following the recommendations of the first edition of the International Stratigraphic Guide, formal lithostratigraphic units have been introduced. Tsien (1974) proposed a lithostratigraphic scheme for the Frasnian of the Ardenne, however showing still the deficiencies of a first attempt. A complete set of Frasnian lithostratigraphic units covering the Ardenne area and resulting from meetings of the Belgian Subcommission on Devonian Stratigraphy was published by Boulvain et al (1999). The present synthesis is based on the latter publication.
A generalized lithostratigraphic cross-section of Upper Givetian to Lower Famennian formations across the southern and northern border of the Dinant Synclinorium is given in figure 6.
