PLIOCENE CHEMOHERM FORMATION WITHIN DEEP-SEA REDUCING ENVIRONMENTS, ITALIAN NORTHERN APENNINES

Well known occurrences of ancient hydrocarbon seeps in the Mediterranean basin are the “Calcare a Lucina” limestones (CAL) punctuating the Miocene turbiditic wedges of the Apennine chain. Post-Messinian imprints of cold submarine defluidization are scarcer in relation of the attenuation of post-collisional tectonic activity backing the building of this mountain chain. Conspicuous examples of Pliocene chemoherms are those associated with organic-rich deep-sea sediments within the Argille Azzurre Fm of the northern Italian Apennines. Decimetric to metric carbonate lenses occur along the Stirone and Lamone river valleys intimately associated with organic-rich laminites and hemipelagites whose age is constrained between 3.1 and 2.6 Ma. A chemosynthetic food chain is documented by specialized macrobenthic assemblages of infaunal bivalves, such as large Solemya and lucinids symbiotic with autotrophs capable to exploit sulfur. HsS-rich fluids easily generated within the anaerobic-dysareobic sediments were a major source fueling these communities. Eventually, these bivalves were incorporated within authigenic carbonate concretions and limestones whose d13C signature (- 24.5 permil PDB) suggests a contribution by hydrocarbons. The primary connection of such Pliocene chemoherms and allied specialized biota with reducing environments, in turn forced by paleoceanographic factors, appears obvious. However, hydrocarbon fluid seepage at the Stirone River site was potentially influenced by fluid expulsion related to a Pliocene reactivation of the thrust-related Salsomaggiore anticline.