In the Miocene of the northern Apennines, methane-derived limestones from various geologic settings represent important examples of fossil chemoherms and provide a basis for more accurate interpretations of seepage systems. The systematic study of fossil chemoherms has allowed the definition of new field and compositional criteria for the recognition of these deposits, which were previously based almost exclusively on negative carbon isotope composition and peculiar chemosynthetic communities; this has constrained the relations between seepage and sedimentary and tectonic instability processes. We suggest that the peculiar brecciated structures associated with fossil chemoherms are related to diapirism of overpressured pelitic sediments due to fluid venting. Fluidization of sediments increases the instability of mudstones in which chemoherms are contained, thus favoring gravity slumping processes and the reworking of many chemoherms.

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