Dolomitization in basinal limestones of the Lagonegro Units: from early diagenesis to tectonically-driven fluid flow

The Triassic pelagic carbonates of the Lagonegro Basin Units from the Southern Apennines fold and thrust belt (Italy) are often replaced by dolomites, which in some cases show fabric and petrographic characteristics typical of late burial saddle dolomites displaying zebra-like structures (Iannace et al., in press).The aim of this thesis was to carry out a systematic regional study on Lagonegro dolomites, both on outcrops and on subsurface samples (cuttings from oil wells), in order to define dolomites extent, timing relative to Southern Apennines tectonic history, and fluids origin, by means of a combination of field work, and petrographic and geochemical investigations. The Lagonegro Basin succession, together with the Apennine Platform successions, is now imbricated in a pile of nappes of the Southern Apennines chain. This latter is a NE-directed fold and thrust belt, formed by progressive collision between Afro-Adriatic and Eurasian plates during the Neogene (Mazzoli and Helman, 1994, and references therein), with the Apulian promontory representing the orogenic foreland. The classical restorations of the pre-orogenic (Triassic to Paleogene) palaeogeography of the Southern Apennines show that the African (Apulian) passive margin was characterized by the Meso-Cenozoic pelagic Lagonegro Basin, located between two coeval carbonate platforms, Apennine and Apulian platforms (Pescatore and Tramutoli, 1980; Mostardini and Merlini, 1986). The evolution of the Lagonegro Basin was punctuated by two massive dolomitization events, which generated two distinct dolomite types displaying different petrographic and geochemical features. A fine crystalline and fabric preservative dolomite crops out in the northern area (Molise, Campagna and San Fele-Mount Pierno), whereas a coarse crystalline and fabric destructive dolomite, showing zebra-like and hydrofracturing structures is present in the southern area (from Pignola to the high Val d’Agri). Geochemical data (enrichment in 18O in respect with typical Upper Triassic seawater values, high Sr and low Mn and Fe content, Sr isotopes consistent with Upper Triassic seawater) allow to interpret dolomites of the northern area as the result of an early replacement caused by cold, marine and oxidizing fluid, in a shallow burial. It is proposed that dolomitizing fluids were expelled through a reflux from the Norian Apenninic carbonate platform toward the proximal Lagonegro Basin, or alternatively they were put into motion through a Kohout convection of seawater, heated thanks to a geothermal anomaly.Dolomites located in the southern area, display a depletion in 18O compared to Upper Triassic seawater values and show lower Sr, higher Mn and Fe contents, and higher Sr isotopes ratios in respect with early dolomites of the northern area. It is suggested that the southern area was affected, during Neogene time, by a late burial dolomitization, which was the result of a large scale “squeegee” fluid flow during Southern Apennines fold and thrust belt formation. The geochemistry of saddle dolomite samples indicates an origin by warm fluids (105-125 °C). Integration with available thermal data (Corrado et al., 2005; Mazzoli et al., 2008) into the regional deformation history, by assuming a fluid in thermal equilibrium with the host rocks, suggests that the fluid flow took place after maximum burial, in early stages of exhumation, between 5 and 4 Ma and under 3-4 km of burial. The dolomitizing fluids of southern area dolomites had salinities in the range of slightly modified to normal marine seawater (1.9-6.4 eq. wt % NaCl with a mean of 3.7) and oxygen isotopic composition typical of saline formation waters. Two possible sources for such fluids can be envisaged. They could be formation waters, diluted with fresh waters delivered during the smectite-illite transformation at depth, squeezed out from the surrounding Triassic and Cretaceous fine-clastic formations or Miocene marine pore-waters expelled from the mélange zone that separates the allochthonous wedge from the buried Apulian platform. The fluid flow event was a major phenomenon affecting the fold and thrust belt and it could be expected that the dolomite bodies which are reported in the Apulian subsurface (Murgia et al., 2004) might have the same origin of Lagonegro dolomites from the southern area. As the carbonates of the Apulia platform host major oil fields of continental Europe, the fluid flow that took place along with Southern Apennines belt formation could be of economic significance, providing secondary porosity development.