Evidence of aerial volcanic activity during the Valanginian along the northern Tethys margin.
Résumé
Stratigraphic measurement and sampling on three sections (Vergol, La Charce, and Montclus) through Valanginian deposits from the Vocontian Basin (southeastern France) reveals the occurrence of centimetre thick ochre-coloured layers, which can be correlated from one section to another. At least twelve of these are identified in sediments dated from the Pertransiens to Furcillata ammonite Zones. These horizons appear similar to previously described Oxfordian and Aptian bentonites, also from the Vocontian Basin. Clay-mineralogical and geochemical data are similar in the Valanginian ochre horizons and their enclosing marls except in one of these that shows a clay fraction mainly composed of smectite and which exhibits an enrichment in trace elements (Zr, Ba, Th, Y, Hf, U, Pb, Nb, Ta). This horizon occurs in sediments attributed to the Campylotoxus Zone and is interpreted as a bentonite while the other ochre horizons were derived from the meteoric weathering of pyrite, which probably developed in oxygendepleted environments. Volcanic ash was presumed to be transported westward by trade winds from volcanic centres located along the northern active margin of the Tethys.