Carbonate Diagenesis Related to Methane-rich Fluid Migration on Continental Margins : Example of the Dolomite Concretions in the Neogene Marls of South-East Spain.
Résumé
The Tortonian, Messinian and lower Pliocene marly sediments in S.E. Spain basins (Fortuna, Murcia, Lorca, Aledo, Huercal Overa, Vera) contain abundant authigenic dolomite concretions. The concretions are essentially composed of pure dolomite with a composition from stoichiometry to iron-rich. The oxygen and carbon isotopic compositions of these dolomites exhibit wide ranges (-7.9 < δ18O‰ VPDB < +6.3; -35.5 < δ13C‰ VPDB < +9.8) indicating that carbonate precipitation occurred within the marly sediments due to circulation of fluids of various composition. The low δ18O values indicate that relatively warm fluids were migrating through faults system whereas the high δ18O values characterize either restricted evaporitic conditions or 18O-rich water issued from clay minerals dehydration or gas hydrates dissociation. The low δ13C values are indicative of anaerobic oxidation of methane whereas the high δ13C values are related to methanogenesis, both processes promoting the increase of alkalinity in pore solutions and thus leading to carbonate precipitation. High methane concentrations in the migrating fluids are thought to have created locally gas hydrate reservoirs that were dissociated after the abrupt sea level fall during the Messinian salinity crisis. Finally, the methane-rich fluid seepage was always active during the Pliocene in the Vera basin, which was the only one in SE Spain reflooded by the Pliocene marine waters.The authigenic dolomite concretions are testifying of intense methane-rich fluid migration through faulting and fracturing of the sedimentary deposits that was active during a long period of at least 7 Myrs, where tectonic uplift and major paleoenvironmental changes occurred in the Mediterranean sea.