Variation of Visean strata across the midi thrust, Belgium

Abstract

The Viséan strata of southern Belgium, between Namur, Aachen, Comblain-au-Pont and Dinant, record the deposition of shallow water carbonates along the southern margin of the Brabant Massif. Three areas of Dinantian outcrop: the Namur, Dinant and Vesdre regions were separated by folding and faulting associated with the Hercynian orogeny, particularly the Midi Thrust. The Viséan series comprises three stages of which the Livian has been the basis for this study. The Viséan sediments comprise numerous cycles 2-20m thick which are interpreted as being dominated by shallowing upwards sedimentary sequences. Petrographic study of the nine carbonate lithofacies has revealed three microfacies associations which are broadly described as bioclastic (microfacies association 1), microbial-algal (microfacies association 2) and micritic-peloidal (microfacies association 3). These microfacies associations occur in the same order in each cycle, although their component microfacies may not. Although there are signs of subaerial exposure at the top of cycles, well developed subaerial exposure horizons are rare. Calcrete formation is limited to zone or subzone boundaries and is well developed close to the Brabant Massif. The palaeo environment is interpreted as a warm, shallow water, carbonate platform, which dipped gently southward, from tidal flats in the north. Early diagenesis of these sediments was facies specific and commonly occurred under marine phreatic conditions. Early meteoric diagenesis is only patchily developed. Later effects of burial diagenesis are common to all sediments. Although close to areas of Hercynian earth movements, there are no signs of synsedimentary tectonics in the Livian sediments. Later movement was restricted to open folding, faulting and thrusting. Bentonite layers are the only evidence of local volcanism. They increase in thickness and abundance towards the east and this is presumed to be the direction of their source area. Only slight lateral variation in sedimentation suggests that there was minimal separation between the Namur region and Vesdre-North Dinant region at the time of deposition. The sedimentological observations presented here, provide limiting conditions for palaeogeographical reconstruction and tectonic controls. Crustal shortening and decollement by northward thrusting was the main form of earth movement. The effect of movement along the Midi Thrust was to juxtapose predominantly submarine sediments from below fair weather wave base (Dinant region) and nearshore peritidal deposits (Namur-Vesdre region). Since the original width of the platform is unknown, the amount of displacement along the thrust is uncertain. A minimum of about 20 km displacement along the thrust is believed to have affected the Dinantian sediments.