4.1. INTRODUCTION
The base of the Famennian stage is defined at Coumiac, Montagne Noire, France (Klapper et al., 1993). The limit with the overlying Tournaisian stage is defined at La Serre, Montagne Noire, France (Paproth et al., 1991). Numerical ages for the Famennian span the interval from 370 ±5 Ma to 355 ±5 Ma. Racki (1999) reviews the various hypotheses for the Late Devonian mass extinction near the Frasnian-Famennian boundary, but the prime causation of this event remains uncertain.
In Belgium, the Famennian lithostratigraphic units are outcropping in the Namur and Dinant Synclinoria, the Vesdre Nappe and the Theux Window. The pionneer works concerning the Famennian stage are those of Mourlon (1875-1886). Bellière (1954) made an up-dated synthesis in the "Prodrome d'une description géologique de la Belgique". In 1965, Bouckaert & Ziegler gave a conodont stratigraphy of the Famennian stage but the first complete biostratigraphical scale in the type localities in the Dinant Synclinorium was published by Bouckaert et al. (1968). Additional sedimentological and micropaleontological data may be found in Thorez et al. (1977) and Dreesen & Thorez (1980, 1994). A revision of the late Famennian miospore zonation scheme in eastern Belgium was presented by Maziane et al. (1999). An assessment of the old Frasnian/Famennian boundary at Senzeille is due to Bultynck & Martin (1995).
A schematic frame of the Famennian lithostratigraphic units is given in figure 7. As a general trend, the Famennian stage corresponds to a regressive megasequence with oscillating movements due to short terms transgressive pulses. Two main groups, which are mainly composed of siliciclastic rocks, may be distinguished, namely the Famenne and the Condroz Grps. Depositional environments comprise juxtaposed continental, restricted marine and shallow marine environments, including alluvial, lagoonal, evaporitic, tidal, sand barrier and fore-barrier settings. A new transgressive system overlay the Condroz Grp. It consists of interbedded siliciclastic and carbonate rocks of the Etroeungt = Comblain-au-Pont Fm, eventually with biostromal units (Dolhain Fm).
All Belgian and French geologists recognize the necessity to subdivide the Famennian stage into four substages : Lower, Middle, Upper and Uppermost (= Strunian) Famennian. However, an international acceptance has not been reached on this proposal (see Ziegler & Sandberg, 1997; Streel et al., 1998). A comparison between the lithostratigraphic units and the chronostratigraphic subdivisions based on biozones is given in table 1.
