Diagenesis of Microbial Carbonates: A Case History from the Norian of the Italian Peninsula

Laiena, Fabio ¹; Gasparrini, Marta ²; Hamon, Youri ²; Iannace, Alessandro ²; Nader, Fadi H.³
(1) Earth Sciences Department, Università di Napoli "Federico II", Naples, Italy. (2) IFP Energies nouvelles, Rueil-Malmaison, France. (3) Ministry of Water and Energy Lebanon, Beirut, Lebanon.

A diagenetic study was accomplished on Norian dolomites from Southern Italy along a carbonate platform-to-slope transect. The succession, formed in a shallow water carbonate platform dissected by stressed intra-platform troughs, bear widespread microbial mounds. Despite the complex diagenesis and burial history, this study aims to characterise timing and conditions of early to late cementation including dolomitization and silification.

The studied outcrops comprise different facies: a) peritidal, cyclic back-margin with dasycladales and bivalves grainstones, planar stromatolites with fenestral porosity, mega tepees and cemented neptunian dikes; b) margin to upper slope build-ups showing thrombolitic, stromatolitic and oncoidal structures, surrounding serpulids and calcareous sponges; c) middle to lower slope dark and laminated dolomudstone with graded bedding and gravity-driven deposits.

Early calcite and aragonite cements pre-dated the main dolomitisation, which is pervasive and mimic in all facies and comprise different replacive and void-filling phases. Two dolomite populations were geochemically distinguished: one includes replacive and early cements of inner-margin facies (positive δ18O), whereas the other comprises replacive and void-filling dolomites of build-ups and slope facies and late cements of inner-margin facies (lower δ18O). Similar geochemical trends are recorded in Dolomia Principale of Apennines, Alps and Hungary. An early diagenetic evaporative to reflux process better matches the dolomites of inner margin facies, whereas different scenarios can justify the geochemical signature of the other facies:1) an early process by more diluted brines, coeval with the inner margin dolomitisation, the difference in salinity being related to the different settings (restricted back-margin versus more open margin-to-slope); 2) geochemical evolution of the reflux dolomitizing fluid at shallow depth with decreasing 18O values away from the source of the brine; 3) a later shallow burial process from fluids with geochemical signature of Rhaetian seawater.

Silica occurs both as early low temperature chalcedony incrustations and as late burial mega quartz including fabric selective replacive and void-filling phases, which plumb the remaining porosity. The occurrence of chalcedony mainly in the microbial facies suggest a genesis related to the peculiar restricted and eutrophic deposition environments of the microbial buildups.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.