The terms 'boudin' and 'boudinage' have been coined in 1908 at Bastogne to describe the geometry of particular structures in Lower Devonian metasedimentary series, resembling an array of sausages lying side by side. Recent research shows that these structures are the expression of a polyphase deformation history in brittle-ductile deformation conditions in the middle crust. Hydraulic fracturing caused the formation of the vein arrays, while layer- parallel shortening is responsible for the cuspate-lobate morphology of both the lower and upper interface of the psammite layers. Taking into account the currently acknowledged geological nomenclature, particularly with respect to the terms boudin and mullion, the resulting structures should be described as mullions, or more particularly as double-sided mullions. Notwithstanding the fact that the kinematics giving rise to the particular structures in the Ardenne-Eifel area are not related to the process of boudinage, the Belgian geological community should strive for the creation of a heritage site at Bastogne, still the locality where the terms 'boudin' and 'boudinage' have been used for the first time.
Geomechanical models form an essential basis part our quantitative understanding of tectonic processes. In these models, a long-standing problem involves the quantification of the constitutive equations that describe the rheology of the middle crust (7-12 km). A combination of indirect methods has yielded first order descriptions of the rheology of these rocks, but much is still unknown and controversial. Constraints on rock rheology are needed from careful field and observational studies and mechanical modelling. Here we present a new method to quantify the rheology of fine-grained siliciclastic rocks, which are common in the middle crust at around 400 degrees C and deform by solution-precipitation processes. We use a combined structural and numerical analysis of the mullion structures developed in these rocks, to quantify the rheological parameters during flow at geologic strain rates. The method is based on a parameter estimation scheme developed in structural mechanics. Results consistently converge towards a set of rheological properties which are in agreement with observed microstructures and indicate that fine grained siliciclastic rocks in the middle crust have a Newtonian viscous rheology, approximately ten times weaker than wet quartz. Because siliciclastic rocks control the rheology of the middle crust in many sedimentary basins, our results provide new parameters for geodynamic modelling.
During excavation works for the construction of the TGV track along the motorway Brussels-Liège (E40) near the interchange Tienen-Hoegaarden-Jodoigne a number of interesting, but only temporary outcrops were created. These outcrops offered not only the occasion to learn more about the stratigraphy, paleogeography and paleobotany of the Paleogene, but also revealed some particular deformation features. The Outgaarden Section, SE of the interchange, showed chaotically deformed sands at the base of the Upper Paleocene Tienen Formation. The highly viscous deformation features originated in a high-energy depositional environment. Dewatering, mass movements and strong currents caused the syndepositional, probably gravitationally-induced sediment deformation. The Goudberg Section, NW of the interchange, exposed the Overlaar Petrified Forest, of which part of the silicified tree-stumps were recovered to constitute the main feature of the planned GEOPARK HOEGAARDEN. In this section tectonically-induced faulting could be demonstrated. The fault shows a clear normal displacement. A sinistral strike-slip displacement is moreover supposed. Faulting occurred prior to the deposition of the Pleistocene loam cover but definitively postdates the deposition of the Ypresian clays. It is proposed that this faulting event in the Tertiary sediments may be caused by a reactivation of underlying master faults in the Lower Paleozoic basement of the Brabant Massif.
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