Brecciated structures and fluid-flow conduits as indicators of methane-derived carbonates (northern Apennines, Italy).

The recognition of fossil carbonates as products of microbial chemosynthesis associated with fluid and hydrocarbon venting at the sea floor (chemoherms) has been linked to two main indicators (Sakai et al., 1992; Terzi et al., 1994; Kelly et al., 1995; Corselli and Basso 1996): the marked depletion in carbon isotope composition of the authigenic carbonates and the low diversity/high abundance of benthic chemosymbiotic communities (densely packed mussels and clam bivalves, gastropods, tube worms). Nevertheless, not all methane-derived carbonates enclose remains of large chemosymbiotic taxa, making the recognition of unfossiliferous methanogenetic limestones in the field problematic. Recently, new studies (Cavagna et al., 1998; Conti and Fontana, 1998) have shown that other diagnostic criteria, such as geometric, sedimentologic and compositional characteristics, can be useful in the identification of fossil seep environments and associated carbonates. In particular, brecciated structures and fluid-flow conduits, which frequently occur in methane-related carbonates, are of outstanding importance in connecting fluid ventings, slumping of pelitic sediments and tectonics. Indeed, these structures could permit the reinterpretation of many chaotic deposits as due to diapiric processes.Carbonate deposits bearing fossil chemosynthetic assemblages are concentrated in middle-late Miocene of the northern Apennines and are found in various domains and basin types (Vai and Ricci Lucchi, 1994; Conti and Fontana, 1998): from internal tectonic zones (Epiligurian basins) to external ones (foredeep). Enclosing sediments are mudstones, muddy sandstones and marlstones; commonly they have low permeability but are severely fractured. They are present in syntectonic deposits, related to compression and thrusting of the Apennine chain. In the foredeep, chemoherms are moderately reworked and concentrated in slumped pelitic horizons, suggesting a close relationship between sediment instability and hydrocarbon-fluid venting. Field and petrographic examination of methane-derived carbonates permits different lithofacies to be recognized: light blue to dark grey micritic limestones, biomicritic marly limestones, light brown to grey calcareous marls, fine to coarse calcarenites, calcarenitic limestones and various types of calcareous breccias and fractured limestones.