Mullion structures in the Ardennes and Eifel occur in a lower Devonian, shallow marine siliciclastic sequence, S and SE of the Stavelot-Venn Massif (Fig. 2), roughly between Dedenborn (Eifel) and SW of Bastogne (Ardennes). Mullion structures in the area are actually quite common. The most spectacular outcrops are well known from many publications.

Figure 2. Geological map of the Ardennes and Northern Eifel

Geological map of the Ardennes and Northern Eifel, showing the area where mullion structures occur (light brown) and the trace of the profile shown in Figure 3. Slightly modified after Fielitz and Mansy (1999). (Click for enlargement)

The rocks here were buried to 8 to 12 km depth in a passive continental margin in late Devonian times. During this burial phase the rocks underwent very low grade metamorphism (from 350¾C in the NE, up to 450-500¾C and 200 MPa in the SW (Lielitz and Mansy, 1999; Kramm et al., 1985). This phase was accompanied by the development of close-to-lithostatic fluid pressure conditions (Sintubin et al., 2000; Hilgers et al., 2000).

In the late Devonian and Carboniferous, this sequence was inverted and thrust in a NW-direction to form a foreland fold- and thrust belt, which is now part of the Rhenohercynian zone of the Mid-European Variscides (Fig. 3). The important differences in sediment thickness and the involvement of basement massifs in the thrust sequence are the main factors influencing the typical outcrop pattern of the region (Adams and Vandenberghe, 1999; Fielitz, 1992; Hollman and von Winterfeld, 1992; Hollmann and Walter, 1995; Hance et al., 1999).

Figure 3. Figure modified after Hollmann and Winterfeld (1999)

(a) Schematic profile along the line shown in Figure 2, containing Cambro-Ordovician to Carboniferous strata. Horizon shown in pink is the base of Devonian, and blue is the middle Siegenian. (b) Palinspastically reconstructed equivalent of (a), shown at the same scale, with the lower Devonian at the final stages of burial, before the onset of Variscan shortening. Figures (a) and (b) are modified after Hollmann and Winterfeld (1999). (Click for enlargement)