Isotope geochemistry, petrology and depositional environments of the diatomite-dominated Tripoli Formation (Lower Messinian), Sicily



ABSTRACT The Tripoli Formation (Lower Messinian) in Sicily includes diatomites irregularly alternating with marl and carbonate beds and lies, stratigraphically, between the Tortonian pelagic marls and the evaporitic Calcare di base. The relationships between mineralogy, textural features and oxygen-carbon isotopic compositions of carbonate components point to a wide variability of depositional conditions and suggest that Tripoli sedimentation occurred in small basins characterized by periodic and marked restriction from the open sea.

The isotopic values of calcite and dolomite in the diatomites suggest an evolution from normal marine towards more restricted environments. Evaporating conditions are also indicated by the occurrence of anhydrite, length-slow chalcedonic quartz and moulds of gypsum. In a more advanced stage, the precipitation of heavy δ180 dolomite in the interstitial pores of fossil-poor diatomites denotes an environment with highly evaporated water. Mixing of meteoric and marine waters, on the other hand, might have favoured the precipitation of a dolomite characterized by relatively low δ180 and δ13C values.

The deposition of marl and carbonate beds alternating with or overlying the diatomites took place in an environment with highly evaporated marine waters on the basis of δl18O values of dolomite (up to + 9.10‰) and aragonite (up to + 5.83‰), occurrence of evaporitic minerals and lack of fossils. The presence at these levels of calcite with extremely negative δ13C values (down to - 38.40‰), filling gypsum moulds, suggests activity of sulphate-reducing bacteria. Some aragonitic marls, however, bear evidence of deposition in relatively normal marine conditions.