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 (Namu rian) outcrops in the Dinant and Avesnes Basins (Ardenne Massif, NW Europe) . Clay mineral data have been placed in the palaeoenvironmental and structu ral histories documented by previous lithological, stratigraphical, palaeon tological, diagenetic and tectonic contexts. The clay associations are domi nated by illite and chlorite derived partly from the erosion of land masses surrounding the marine domain. The geothermal gradient estimated from corr elation with conodont colour alteration index ranges between 40 and 70 degr eesC/km. A diachronous northwards migration of the diagenesis/metamorphism interface links to uplift caused by Late Carboniferous compressional foldin g and overthrusting. Associated day minerals include smectite, locally pres erved from diagenetic changes mainly by early pore closure, that reflect la goonal or quiet offshore marine conditions. Smectite and subordinate kaolin ite abundances decrease upwards during the Devonian in three successive int ervals suggesting alternations of sub-arid to drier climates. The local occ urrence of corrensite (ordered chlorite-smectite mixed-layer) is attributed to the moderate diagenetic transformation of pre-existing smectite (C) 200 0 Elsevier Science B.V. All rights reserved.