Palaeozoic clay mineral sedimentation and diagenesis in the Dinant and Avesnes Basins (Belgium, France): relationships with Variscan tectonism

Clay mineral investigations have been performed on more than 500 limestones and shales sampled in Lower Devonian (Emsian) to Lower Carboniferous (Namurian) outcrops in the Dinant and Avesnes Basins (Ardenne Massif, NW Europe). Clay mineral data have been placed in the palaeoenvironmental and structural histories documented by previous lithological, stratigraphical, palaeontological, diagenetic and tectonic contexts. The clay associations are dominated by illite and chlorite derived partly from the erosion of land masses surrounding the marine domain. The geothermal gradient estimated from correlation with conodont colour alteration index ranges between 40 and 70 degreesC/km. A diachronous northwards migration of the diagenesis/metamorphism interface links to uplift caused by Late Carboniferous compressional folding and overthrusting. Associated day minerals include smectite, locally preserved from diagenetic changes mainly by early pore closure, that reflect lagoonal or quiet offshore marine conditions. Smectite and subordinate kaolinite abundances decrease upwards during the Devonian in three successive intervals suggesting alternations of sub-arid to drier climates. The local occurrence of corrensite (ordered chlorite-smectite mixed-layer) is attributed to the moderate diagenetic transformation of pre-existing smectite (C) 2000 Elsevier Science B.V. All rights reserved.