Mud volcanism and authigenic carbonates related to methane-rich fluids migration in the late Neogene marls of S.E. Spain

Abstract:

Methane-rich fluids that are generated at depth in organic-rich deposits migrate within the sediments to the seafloor where they are expelled to form mud volcanoes or pockmarks. Moreover, these migrating fluids are involved in diagenetic processes as authigenic carbonate formation and they may participate to gas hydrate formation. These features are well-known in the present-day continental margins but their fossil records are relatively scarce. The outcropping Tortonian and Messinian marls in S.E. Spain basins (Lorca, Fortuna, Columbares, Huercal Overa) contain abundant authigenic dolomite nodules. The oxygen and carbon isotopic compositions of these dolomites exhibit wide ranges (-1.4 < δ¹⁸O < +5.6 ; -25.6 < δ¹³C < +9.3) indicating that carbonate precipitation occurred within the marly sediments due to circulation of fluids modified by gas hydrates formation/dissociation, where anaerobic oxidation of methane and methanogenesis were active. In the Huercal Overa basin, there is a well-preserved mud volcano intruding vertically the Messinian marls. These two features, methane derived authigenic dolomites and mud volcanism, are testifying of the intense methane-rich fluid migration in the marly deposits of the western Mediterranean basins during the late Neogene, which was the time of major paleoenvironmental changes in the Mediterranean sea climaxing during the Messinian salinity crisis.