EVIDENCE OF EARLY CARBONATE CRUST FORMATI ON ON TRIASSIC BASALT IN THE MIDDLE ATLAS (MOROCCO)

The first stages of the basalt calcitization were studied along a weat hering profile located near Elhajeb city, within the Moroccan Middle A tlas. This study allowed us to distinguish between smectitic weatherin g of basalt and its calcitization. The weathering front is up to 35 m deep. It is characterized by the transformation of primary minerals to smectites with a strong depletion of Ca. Calcitization is developed u ntil 1.5 m deep and the lower limit of the calcitization front is mark ed by an increase of the Ca contents. LO to 15 % of this calcium is in herited from basaltic material and is in situ incorporated in secondar y products. The 85 to 90 % remained are of external origin to the prof ile and attributed to either Quaternary carbonate (travertine) surroun ding the profile or to Liassic carbonates outcropping above Elhajeb ba sin.