Abiod Formation in Boukhil section (Jebel M'rhila, Central Tunisia): Facies, geochemistry, mineralogy and deposition sequences

Abstract

The field study and the petrographic observations of the limestone samples particularly from the Boukehil section allowthe identificationof four carbonate facies: (1) calcisphere-rich mudstone-wackestone, (2) Conglomeratic limestone, (3) bioclastic wackestone-packstone, and (4) siliceous dolostones. According to the vertical change of the deposition facies and the occurrence of unconformities, it was possible to divide the AbiodFormation in this area into three third-order depositional sequences bounded either by subaerial exposure surfaces or by submarine erosional surfaces. These sequences include only transgressive and highstand systems tracts. The SEM observation of the limestone shows three types of micrite: bio-detrital, biological and chemical. Despite poor occurrence in our samples, the existence of some biological structures similar to bacterial fabrics highlights the role of bacteria in the carbonatogenesis. The XRD mineralogy and the calcimetry analyses show that the limestone of the Abiod Formation is very rich in calcium carbonate. The CaCO3 concentrations vary with the systems tracts and the depositional sequences controlled by the sea-level fluctuation. For instance, higher concentrations of CaCO3are recorded in the transgressive intervals of the eustatic cycles (with some samples yielding 100 wt% of CaCO3), whereas through highstand intervals, the CaCO3 concentration falls to 83 wt %. The atomic absorption geochemical analysis of limestones shows that the major elements concentrations change with systems tracts and thus with sea level fluctuation. For example the highest concentrations of Fe in the carbonates are observed in the marine highstand and regressive intervals, and those of Mg and Ca observed in the transgressive intervals.