The famous "boudins" from the Ardenne and Eifel regions (Belgium, Germany) occur as regularly-spaced segments of Lower Devonian (meta-)sandstones separated by lens-shaped quartz veins. The whole is embedded in thick siltstone or shale horizons. Structural evidence throughout the Ardenne-Eifel region reveals a two-stage deformation sequence composed of a layer-parallel extension followed by a layer-parallel shortening. The latter results from the development of the Rhenohercynian fold-and-thrust belt during the Variscan orogeny (Carboniferous). During that period, the boudins inherited from the layer-parallel extension were strongly reworked and acquired their current extreme convex geometry, in parallel to a reduction of the aspect ratio (width/height) to ~0.5. We consider therefore that these structures should be named as ‘narrow reworked boudins’. The prefix ‘narrow’ indicates that before the reworking period the boudin aspect ratio was already small (~ 1.0). In this memoir, evidences of unquestionable narrow boudins are illustrated. Their formation results from the opening of a joint set at a fracture saturation stage, in which any additional layer-parallel extension is accommodated by the opening of the existing joints instead of the development of new joints. We present a new model including all these aspects for the formation and reworking of boudins as those from the Ardenne-Eifel region.

The Lower Devonian formations from the Ardenne-Eifel region include not only thick siltstone or shale horizons with isolated sandstone layers that were boudinaged and then reworked, but also sandstone-dominant to massive sandstone units, where fine materials represent only cm- to mm-interlayers. Their deformation patterns differ from those richer in fine materials. During the period of layer-parallel extension, both sandstone-dominant and massive sandstone successions were affected by the growth of complex arrays of single- and multi-layered quartz veins. Single-layered veins correspond to pure open-mode fractures, while multi-layered veins frequently represent a hybrid-shear fracturation mode. During the layer-parallel shortening period, small-scale folds were developed in the sandstone-dominant units in close association with the occurrence of multi-layered quartz veins. By contrast, the quartz veins in massive sandstone units seem to have no influence during the layer-parallel shortening period. In this case, the shortening was accommodated by common structures, such as large-scale folds and reverse faulting (ramp).