MINERALOGY AND GEOCHEMISTRY OF MESOZOIC BLACK SHALES AND INTERBEDDED CARBONATES, SOUTHEASTERN SICILY - EVALUATION OF DIAGENETIC PROCESSES

Upper Triassic/Lower Jurassic organic-rich shales and interbedded carb onates (Rhaetian --> Sinemurian) are widespread in the subsurface of s outheastern Sicily where important oil fields have been found hosted i n Triassic reservoirs. Core samples from wells drilled offshore and on shore were studied from petrographic and geochemical viewpoints. In th e Hettangian/Sinemurian shale-carbonate sequences, which accumulated i n a rapidly subsiding basin, the micritic aragonitic mud is still larg ely preserved. Mixed-layer I/S has remained randomly interstratified t o a depth > 4000 m. Diagenetic carbonates are non-stoichiometric finel y crystalline, pore-filling dolomite and/or calcite. The carbonate com ponent exhibits a high Sr content and fair amounts of Fe and Mn. Carbo n and oxygen isotopic values suggest a subsurface interstitial formati on for the digenetic carbonates in an essentially closed system. Based on all accumulated data it is suggested that anoxic marine waters wer e retained in the sediment pores for a long time after deposition, thu s enhancing the preservation of significant amounts of the original or ganic matter. In contrast, Rhaetian tidal-flat deposits hosting black shales display a clay component characterized by ordered illite-rich I /S and a carbonate mineralogy dominated by low-Mg calcite in the upper most beds and by near-stoichiometric dolomite in the lowermost ones. P etrographic, chemical and isotopic data indicate early cementation in an oxidizing phreatic environment and lower down in the sequence perva sive dolomitization in a sabkha-type environment.