Geochemistry, mineralogy and micropaleontoly of a pedogenic calcrete profile (Slimene, NE Tunisia)

摘要

A calcrete profile developed on the top of a calcareous consolidated dune in a coastal area of NE Tunisia (semi-arid climate) was studied focussing on: (i) geochemistry of rare earth elements (REE) and other trace and major elements during the processes associated with calcrete formation, and (ii) mineralogy of the different horizons of the profile. The profile shows a vertical sequence: surface horizon, sub-surface horizon, lithoclast-rich horizon, laminar-structured level, and the old dune. Chemical and mineralogical analyses were performed by neutron activation analysis and X-ray diffraction (whole samples and clay fraction). The calcrete is mainly composed of quartz, calcite and K-feldspar. Phyllosilicates were also found in the other levels. Hematite was only detected in surface levels. The clay minerals present are kaolinite, illite and chlorite. Smectite also occurs in the matrix of the nodular horizon. The microfauna identified is the one commonly found in dunes close to the sea (terrestrial environment). Co, U, Br and heavy REE enrichment occur in the evaporite carbonate rich levels, while the light REE, Hf, Zr, Cr, Th, Cs, Ta, Ga, Rb, and K appear to be retained in the upper levels.