1.1. INTRODUCTION
The Lower Devonian is divided in three stages: Lochkovian, Pragian and Emsian. The Gedinnian and Siegenian stages of the ancient terminology correspond more or less respectively to the Lochkovian and Pragian stages (see Godefroid et al., 1994). Gedinnian and Siegenian were replaced by Lochkovian and Pragian according to the decisions of the IUGS (Bassett, 1985).
The base of the Lochkovian stage is defined at Klonk, Czech Republic (Martinsson (ed.), 1977), the base of the Pragian stage is defined at Velka Chuchle, Prague, Czech Republic (Chlupáč & Oliver, 1989), and the base of the Emsian stage is defined in the Zinzilban Gorge, Uzbekistan (Yolkin et al., 1997). Numerical ages of the Lower Devonian span the interval from 410 ±8/5 Ma to 390 ±5 Ma.
After the main phases of the caledonian orogeny, the Old Red Continent merged in Europe. A new transgression on this continent began at the Latest Pridolian - Early Lochkovian during which the Ardenno-Rhenish geosyncline was mainly fed by arenaceous sediments starting with basal conglomerates. Late Caledonian movements during the “Bolandian phase” was responsible for the emersion and the erosion of part of the Emsian rocks.
The Lower Devonian lithostratigraphic units of Belgium are exposed in the Ardenne Nappe, i.e. in the Dinant and Neufchâteau Synclinoria, the Ardenne Anticlinorium and the Vesdre Nappe (figure 1).
The basic contributions to their study are those of Dumont (1848), Asselberghs (1946, 1954) and Godefroid et al. (1994). The present synthesis is essentially adapted from Godefroid et al. (1994) who have updated the ancient terminology and descriptions to the concept of lithological formations in the Vesdre Nappe, the Theux Window and the Dinant Synclinorium. However, Godefroid et al (1994) did not review the terminology for the Ardenne Anticlinorium and the Neufchâteau Synclinorium. Revision of the lithostratigraphic units of these areas is in progress (Godefroid, 1985, Godefroid & Cravatte, 1999, Bultynck et al., 2000) and new geological mapping started.
Correlations with the Lower Devonian Global Stratotype sections and points are mainly based on miospore biozonation (Steemans, 1989; Streel et al., 2000). Brachiopods are important for correlations in the Ardenne area and with the Rhenish facies.
The 3D geographical extension of the Lower Devonian formations is shown in figure 2. To clarify their lateral variations at the northern and eastern sides of the Dinant Synclinorium and in the Vesdre Nappe, a SW-NE more detailed section is given in figure 3.
