Coalification in the northern Wildhorn nappe and adjacent units, western Switzerland. Implications for tectonic burial histories

Abstract

Vitrinite reflectance of fourteen Eocene coal samples from the northern Wildhorn nappe in the Kander valley have been analyzed and compared to nine agewise similar coals from adjacent tectonic units. The degree of coalification decreases regularly from semi-anthracite in the Kander valley to lignite in the plateau molasse. The degree of coalification in the Tertiary coals appears to be solely dependent on their tectonic history and indicates and estimated 6 km of burial by higher (Pennine) nappes for the Kander valley. The rank at this locality (low-volatile bituminous to semi-anthracite) is significantly higher than in the Border chain at Beatenberg (high-volatile A bituminous). The latter locality therefore was never buried deeper than about 4 km. A further decrease in rank is found in the subalpine flysch and molasse (high-volatile B bituminous) which in turn is significantly more mature than lignites from the folded plateau molasse at Linden. Two samples of Dogger coal from the Pennine “Préalpes médianes” are medium- to low-volatile bituminous that is intermediate between the Kander valley and Beatenberg. Their rank, however, was achieved before their final emplacement north of the Wildhorn nappe. By comparing calculated TTI (Time-Temperature-Indices)-values for time-depth-burial curves with measured vitrinite reflectances, maximum burial temperatures were estimated for the various tectonic units. For the Wildhorn nappe, 160–180°C were obtained in the Kander valley and 100–130°C in the Border chain as a result of tectonic burial due to the thrusting by the Pennine Prealps. Sedimentary burial led to temperatures of 100–120°C in the subalpine molasse below the Border chain prior to its thrusting.