HYDROCARBON-DERIVED LIMESTONES INTERBEDDED WITH LOWER MESSINIAN EUXINIC SHALES (NORTHERN APENNINES, ITALY)

A 10.3 m section of laminated, grey-dark, organic-rich pelites (euxinic shales) of Lower Messinian age in the Vena del Gesso basin (Monticino quarry at Brisighella, Northern Apennines of Italy) are cyclically interbedded with up to eleven layers of marls and limestones. Massive gypsum beds of the Gessoso-solfifera Formation, considered equivalent to the Lower Evaporites of Sicily and Spain, cap the marine Messinian section. The limestones consist of highly indurated light yellowish to grey beds, about 30 cm in thickness. Interbedded marls are fossiliferous and show a remarkable faunal change from bottom to top of section. Grey marls contain a diverse benthic fauna dominated by bivalves (e.g., Nuculana, Propeamussium, Cardiomya), gastropods (e.g., Aporrhais, Nassarius), decapods, echinoids and foraminifera. The abundant pelagic component includes foraminifera (e.g. Orbulina) and thecosomatous pteropods, together with necktic remains (fish otoliths). The uppermost dark euxinic shales are characterized by quasi-monospecific assemblages of chemosynthetic lucinid clams (?Myrtea). d¹⁸O values of the limestones change upsection by about 10.5 per mil (i.e., from -4 per mil PDB to +6.5 per mil PDB ) and indicate a shallowing-up of the basin under increasing evaporitic and stratified conditions. The association of abundant pyrite with fine micritic limestone that yields highly negative d¹³C values (down to -35 per mil PDB) suggests that the carbon in the limestones is derived from hydrocarbons through coupled BSR/AMO (bacterial sulfate reduction/anaerobic methane oxidation) processes.

The exact source of the carbon is open to debate. The possibility that the interbedded limestone layers contain imprints of gas hydrate dissociation events caused by rapid tectonic uplift that destabilized the deepwater gas hydrates is intriguing but cannot be fully answered at present. Ongoing studies of gas escape features, micro-brecciation, microbial textures and biomarkers in the limestone beds are aiming to resolve remaining ambiguities.