Fig. 15. Map View of Outcrop 55 (Field Trip Stop 5), Showing Bedding

15 Fig. 15. Map view of outcrop 55 (field trip stop 5), showing bedding, cleavage, fold hinge lines, axial surfaces, cleavage/bedding intersection lineation plunges, younging sense, and zones of pre-cleavage sediment deformation (adapted after Debacker et al., 2006). Note the occurrence of zones of sub-horizontal to moderately plunging folds (folds II03 to II12 and II27, II28), in between zones of steeply plunging folds. Details of insets are shown in Fig. 17. 16 II10, II11, II12) E- to NE-plunging folds, some of which In the southern outcrop part the folds (II13 to II26) have appear to occur as isolated lenses in the mudstone matrix open to gentle interlimb angles, steeply N-dipping axial (II11). Of all these folds, only fold II12 has a large axial surfaces and steeply W-plunging fold hinge lines and cleavage transection angle and a relatively large angle younging sense is towards the W (Fig. 15). However, between its axial surface and the cleavage. However, despite these differences, the relationship with cleavage as suggested by the variation of the cleavage/bedding is quite similar to that in the northernmost part of the intersection lineation, folds II7 and II8 also have an outcrop (e.g. folds II01, II02). The cleavage/bedding anomalous cleavage/fold relationship (Fig. 17). Within intersection lineation is sub-parallel to the fold hinge lines this zone of NW-SE-trending bedding, younging sense and cleavage is sub-parallel to only slightly oblique to the changes from NE-younging in the NW, to SW-younging fold axial surfaces, shows a well-developed divergent in the SE. This polarity reversal appears to occur in the cleavage fanning symmetrical about the fold hinges, and vicinity of fold II12. shows contrasting refraction patterns on opposite fold limbs. Similarly, the individual folds show a small axial cleavage transection angle (< 020°), of which the sense varies in between the different folds, but the overall axial cleavage transection (004° anticlockwise) as well as the angular difference between cleavage and the mean fold axial surface (004° clockwise) can be neglected. In the southernmost outcrop part, two gently S-plunging folds occur (II27 and II28), with a moderately E-dipping axial surface. These folds are cross-cut at high angles by cleavage, and hence have a pre-cleavage origin (Fig. 17). Fig. 16. Lower-hemisphere equal area projection showing Although, because of the predominantly finer lithology, bedding, cleavage and fracture data from field trip stop 5 much less well developed, the same two quartz vein (outcrop 55). Although data from suspected slump folds have sets are recognised as in stop 4. Also here, a similar not been incorporated, the cleavage/bedding intersection relationship exists between these two vein sets and lineation still shows a lot of variation. For more detailed stereographic projection analyses of the data of this outcrop, the steeply plunging folds, whereas these vein sets do the reader is referred to Debacker et al. (2006). not show any geometrical relationship with the gently plunging folds (folds II04-II12, II27, II28). Fig. 17. Details of outcrop 55 (modified after Debackeret al., 2006; see also Fig. 15). A) Plan view of the pre-cleavage deformation zone in the northern outcrop part, showing bedding, cleavage, fold hinge lines, axial surfaces, sense of younging and the plunge of the cleavage/bedding intersection lineation. From W to E, the trend of the bedding relative to that of the cleavage changes from clockwise to anticlockwise, thus demonstrating the pre-cleavage nature of the change in bedding orientation. B) Pre-cleavage folds II27 and II28, and their relationship with cleavage (southernmost outcrop part). 17 Fig. 18. Conceptual block diagrams of outcrops 51 and 55, with the pre-cleavage deformation zones marked in grey (not to scale; modified after Debacker et al., 2006). In the hinge of the large type B folds the pre-cleavage nature of the sub-horizontal to gently plunging folds is obvious due to the large angle between bedding and cleavage, whereas, because of the small angle between cleavage and the slump fold axial surfaces, this is not the case within the NW-SE-trending type 1 limbs of the type B folds. Within these limbs, it is mainly (if not only) the strong variation of the cleavage/bedding intersection lineation orientation across the folds that gives away the pre-cleavage fold origin. In both outcrops, zone 1 refers to NE-ward younging beds and zone 2 to SW-ward younging beds (sense of younging shown by arrows). Note that the relative stratigraphic position of the beds of outcrop 55 with respect to those of outcrop 51 is unknown. Also note the change in overall bedding dip across the polarity reversal zones (between zones 1 and 2), which is attributed to pre-cleavage deformation (slumping) and may have a profound effect on the (local) plunge of the type B folds. Remarks and interpretation cut by cleavage (e.g. II06). Similarly, cleavage folds itself around, and cross-cuts isolated sandstone lenses Also in this outcrop, the combination of the marked and disrupted and truncated sandstone beds, without the divergent cleavage fanning symmetrical about the fold truncation surfaces and detachments affecting cleavage hinge, the parallelism between the cleavage/bedding (Fig. 17). All these features point to a pre-cleavage intersection lineation and the fold hinge lines and deformation event. In addition, as concerns the majority the geometrical relationship with the two vein sets, of the moderately to gently E-plunging folds (folds II04, suggests that the steeply plunging folds are small-scale II05, II07, II08), the same reasoning as in stop 4 can be type B folds. The southward decrease in “z-shaped” applied. Although cleavage is almost axial planar to the asymmetry of these folds (compare folds II01, II02 and folds, shows a divergent cleavage fanning symmetrical II03 with folds II13 to II26 on Fig. 15), together with about the fold hinges and shows contrasting senses of an anticlockwise change in cleavage trend towards the cleavage refraction on opposite fold limbs, the strong south, suggests a position at the transition between a variation in cleavage/bedding intersection lineation type 1 limb and a hinge zone of a large-scale type B orientation across these folds, in combination with the fold. occurrence within or in the direct vicinity of a pre- In the southern part of the outcrop, cleavage cross-cuts cleavage sediment deformation zone, suggests a pre- the gently S-plunging folds (II27 and II28) at very high cleavage fold origin. angles, indicating a pre-cleavage origin. At several The younging sense changes from E-younging to places breccias occur that are cross-cut by the cleavage, W-younging in the pre-cleavage deformation zone in as well as local changes in bedding trend that are cross- the central part of the outcrop, somewhere around fold 18 II12 (Fig. 15). As the structural style in the northern of the Jodoigne Formation and the Blanmont Formation. part of this outcrop, is, except for the sense of younging However, this hypothesis was rejected by Raynaud and the fold plunge, almost identical to that in the (1952) after a magnetic survey. Also this gap likely southern part, also here the change in younging sense coincides with the presence of a black-slate-dominated should have a pre-cleavage origin. Hence, the change sequence, especially considering the particularly flat in polarity is either directly related to fold II12, having topography, the largely pelitic nature of the adjacent a pre-cleavage origin, or changes across an unobserved units, and the centimetric to decimetric, rhythmic pre-cleavage detachment. alternation of grey sandstone, siltstone and black slate, very similar to the Jodoigne unit, observed in borehole Also in this outcrop, the structural, lithological and 118W285 (Fig. 4). sedimentological observations allow attributing the moderately to gently plunging folds, the sudden changes Considering the lithological and sedimentological in younging sense and the pre-cleavage sediment characteristics of the Jodoigne Formation (see Herbosch disruption zones to slumping (cf. Debacker et al., 2001). et al., 2008) only two of the Cambrian formations As in stop 4, the atypical, near-coincidence of cleavage known from the Brabant Massif bear some resemblance and slump fold axial surfaces, observed in the central to to (parts of) this formation. northern part of the present outcrop, can be attributed to the extremely small angle between cleavage and overall The quartzites and quartzitic sandstones of the Orbais bedding. Again, this is due to a position within the (type unit, and especially the quartzites of the Maka unit, 1) limb of a type B fold, possibly combined with the may be difficult to distinguish from the quartzites of influence of post-buckle flattening. By contrast, the large the Blanmont Formation (cf. Verniers et al., 2001). This angle between cleavage and slump fold axial surfaces resemblance may explain why Rutot & Malaise (first observed in folds II27 and II28, is due to a position in geological map, 1893), Fourmarier (1921) and de la the hinge region of a large type B fold (Fig. 18). Vallée-Poussin (1931) placed the limit of the Jodoigne Formation to the east of the Maka unit. However, an important difference between the Jodoigne Formation 3. Arguments for a revised stratigraphic position of and the Blanmont Formation can be found in the the Jodoigne Formation intercalated fine-grained parts. As pointed out above and observed in field trip stop 3, the fine-grained parts of Below, a shortened, updated version is given of the the Jodoigne Formation consist of black, pyrite-bearing arguments used by Herbosch et al. (2008) for revising slate, whereas the fine-grained parts of the Blanmont the stratigraphic position of the Jodoigne Formation.

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