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).
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RBINS Staff Publications
Subtidal natural hard substrates (SNHS) promote occupancy by rich benthic communities that provide irreplaceable and fundamental ecosystem functions, representing a global priority target for nature conservation and recognised in most European environmental legislation. However, scientifically validated methodologies for their quantitative spatial demarcation, including information on species occupancy and fine-scale environmental drivers (e.g., the effect of stone size on colonisation) are rare. This is, however, crucial information for sound ecological management. In this investigation, high-resolution (1 m) multibeam echosounder (MBES) depth and backscatter data and derivates, underwater imagery (UI) by video drop-frame, and grab sediment samples, all acquired within 32 km2 of seafloor in offshore Belgian waters, were integrated to produce a random forest (RF) spatial model, predicting the continuous distribution of the seafloor areal cover/m2 of the stones’ grain sizes promoting colonisation by sessile epilithic organisms. A semi-automated UI acquisition, processing, and analytical workflow was set up to quantitatively study the colonisation proportion of different grain sizes, identifying the colonisation potential to begin at stones with grain sizes Ø ≥ 2 cm. This parameter (i.e., % areal cover of stones Ø ≥ 2 cm/m2) was selected as the response variable for spatial predictive modelling. The model output is presented along with a protocol of error and uncertainty estimation. RF is confirmed as an accurate, versatile, and transferable mapping methodology, applicable to area-wide mapping of SNHS. UI is confirmed as an essential aid to acoustic seafloor classification, providing spatially representative numerical observations needed to carry out quantitative seafloor modelling of ecologically relevant parameters. This contribution sheds innovative insights into the ecologically relevant delineation of subtidal natural reef habitat, exploiting state-of-the-art underwater remote sensing and acoustic seafloor classification approaches.
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RBINS Staff Publications 2021
