Data collected in this study come from about 150 outcrops containing mullion structures in lower Devonian shallow marine sands and shales (Fig. 5). In addition we report data from a smaller number of outcrops in the Bastogne area (Spaeth, 1986; Sippli, 1981).

Most of our observations are consistent with earlier descriptions from the literature. We supplement these by a number of observations not yet reported. The observations are illustrated in a series of figures. Extensive descriptions are for example (Bruhl, 1969; Pilger and Schmidt, 1957a; Jongmans and Cosgrove, 1994; Sintubin et al., 2000; Corin, 1933; Mukhopadhyay, 1972; Rondeel and Voermans, 1975; Tromme, 1997)

• Mullions are visible as highly cylindrical cuspate-lobate structures of exposed psammite-pelite interfaces (Fig. 6). The cusps always point towards the psammitic layer. The higher the lithological contrast, the better defined the mullion.

• Mullions are always associated with quartz-rich veins in the psammite layers, terminating close to cusps. This association is very strong, over 99% of all observed cases contain this association. In psammitic layers the quartz veins are better developed. Sometimes these intra-mullion veins cross the psammite-pelite interface and terminate inside the pelite layer.

In layers without veins no mullions are observed, but in the same outcrop veins can be present without mullions. (Fig. 7).

Mullions are found on both limbs and in the hinge zones of regional folds. In mullions, cleavage and bedding intersect at angles more than 40 degrees. Mullions do not occur on fold limbs where the cleavage is sub-parallel to bedding. In these uncommon cases the psammite layers show the "normal, extensional" boudinage with cleavage that follows the curvature of bedding in the boudin necks.

When the psammite layer's boundary is well defined and bounded by pelite on both top and bottom, mullions are formed with cuspate-lobate folds on both upper and lower interface. The lobes point away from the psammite layer, with the intra-mullion veins connecting the cusps. In layers with graded bedding mullions are present at one side of the layer only, with cusps associated with veins (Fig. 8).

Cleavage is well developed in the slate layers, and is refracted across layer boundaries. Near the cusps between mullions, cleavage forms well-developed fans convergent into the cusps (Fig. 8c) (Mukhopadhyay, 1972).

Composite mullions with two wavelengths are sometimes found, usually the cusps between the larger first order lobes are connected to thicker quartz veins (Fig. 8a, Fig. 7a).

Mullion axes are oriented close to but usually not parallel to the delta lineation (Fig. 4e). Well-defined mullions with their axis at more than 40 degrees to delta lineation have not been documented. Delta lineation is rarely exactly parallel to mullion axis. Brühl (1969) quotes, based on 50 data, the angle between mullion axis and delta lineation to have values usually between 0 and 20 degrees. To further illustrate this point, outcrop data were grouped into subsets with similar structures. Figure 14 shows stereoplots of bedding, cleavage, mullion axis and delta lineation for two of these groups. It can be seen that in some outcrop groups the delta lineation is sub-parallel to the mullion axis, and in others the orientations are clearly different. In a compilation plot (Fig. 15) of all mullion axes and delta lineations, this distinction is not so clear due to the regional trends in orientations. For all outcrops we calculated the angle alpha between delta lineation and mullion axis. Figure 16 is a histogram of these data. There is a clear maximum at around 20 degrees, with no reliable measurement over 30 degrees. Frequency also decreases towards low values of alpha, but less rapidly than for high angles. This is consistent with the observed correlation between azimuth of mullion axis and delta lineation (Fig. 16). In rare cases mullions are folded across small-scale folds with the fold axis at an angle to the (folded) mullion axis (Bruhl, 1969), consistent with the above observation.

In cross section, mullions show layering which is straight in the middle of the psammite layer, with the curvature increasing towards the outside. Bedding planes in the adjacent pelite layer follow the cuspate-lobate shape until they are about one mullion-amplitude away from the layer contact (Fig. 7e). In thin sections of the psammite layers in mullions, the amount of solution-precipitation deformation is highest in the centre of the layer (Fig. 10) (Spaeth, 1986).

Layer thickness correlates with the width of mullions (Jongmans and Cosgrove, 1994; Rondeel and Voermans, 1975). The aspect ratio H/W usually between 1 and 3 (Fig. 13). Several authors have noted that this is too slender in comparison with “normal” boudins formed by layer-parallel extension.

Mullions have orthorhombic symmetry when cleavage is normal to bedding, and are monoclinic on fold limbs (Fig. 4). In this case, symmetry is in agreement with the corresponding fold limb.

Mullions are highly cylindrical, with scatter in mullion axes typically less than 15 degrees in one large outcrop. Observations on cylindricity over more than 10 metres are much less frequent, as such outcrops are rare. In more detail, cusps between mullions sometimes end along strike, so that two mullions join into one (Fig. 7d, Fig. 13e).

Measured along the layering, veins comprise up to about 10% of bed length. This ratio gradually decreases towards the vein’s tip.

Veins between mullions are normally spindle- or lens shaped. They are generally oriented at high angle to bedding and in profile view are sub-parallel to the weakly developed fracture cleavage in the psammite layers.

Vein microstructure is variable: in some cases fibres parallel to bedding are found. In other cases, the quartz has a blocky microstructure (Fig. 10).

Figure 12. Microstructure of a deformed intra-mullion quartz vein showing evidence for crystal plasticity and incipient recrystallization. (Click for enlargement)

In thin sections, micro-scale veins parallel to the intramullion veins are occasionally found, with a stretched crystal microstructure indicating transgranular fracturing in a strong rock (Hilgers et al., 2000) (Fig. 9, Fig. 11).

Not unfrequently the intra-mullion veins have a pinch-and-swell structure (stretched in a direction sub-normal to bedding); these structures have the usual aspect ratios and are quite different from the Mullions (Bruhl, 1969; Jongmans and Cosgrove, 1994).

In thin sections of deformed veins the vein quartz shows evidence for plastic deformation, with subgrain formation and incipient recrystallisation (Fig. 12).

Fluids in intramullion veins have variable composition but can be H2O, N2 and/or CO2 – rich as has been documented in extensive studies (Fielitz and Mansy, 1999; Sintubin et al., 2000; Schroyen and Muchez, 2000; Darimont et al., 1988).