Environmental and climatic controls of the clay mineralogy of Albian deposits in the Paris and Vocontian basins (France)

Abstract

High-resolution clay mineral analyses were performed on lower and middle Albian deposits from the Paris and Vocontian basins in order to specify the weathering conditions that prevailed at that time. The clay mineral assemblages are composed of small proportions of chlorite and vermiculitic clays associated with abundant illite, R0 type illite-smectite mixed-layers (smectite) and kaolinite. Clay minerals originated from the physical alteration and chemical weathering of rocks and soils outcropping on the Variscan massifs bordering the studied areas. In the Paris Basin, the covariation of illite and kaolinite suggests the reworking of these latter minerals from palaeoalterations and saprolites coupled with an intense runoff on the London-Brabant Massif under a warm-temperate climate.In comparison with the Paris Basin, the lower Albian series deposited in the Vocontian Basin are enriched in smectite because of the preferential deposition of this mineral in deep offshore environments. In this basin, illite and kaolinite did not show any covariation suggesting that kaolinite is rather the chemical weathering product of rocks exposed in the French Massif Central. The stratigraphic distribution of clay minerals in both studied areas is consistent with a cooler and dryer climate around the Aptian/Albian transition. An enhanced precipitation rate is suggested by an increasing proportion of kaolinite from the Douvilleiceras mammillatum Zone upward. This humid interval ended in the Anahoplites intermedius Zone contemporaneous with the smectite-rich sedimentation in the Paris Basin revealing semi-arid conditions consistent with palynological data. This semi-arid interval encompassing the Dimorphoplites niobe and D. biplicatus zones (and possibly the Dipoloceras cristatum Zone) is interrupted at the base of the Mortoniceras pricei Zone by a return to a more humid climate.