Fluid flux and migration conditioning Miocene-seep carbonate precipitation in the northern Apennines.

In the Miocene satellite and foredeep basins of the northern Apennines, numerous outcrops of carbonate bodies share specific palaeoecological, sedimentological, compositional and isotopic features with present-day seep-carbonates.These carbonates occur in large turbiditic bodies (Mt. Cervarola and Marnoso-arenacea Formations) and in slope hemipelagites (Vicchio and Verghereto Marls, and Ghioli di letto mudstones). Apennine hydrocarbon seep-carbonates form isolated, irregularly-shaped masses of limestones with distinctive fabrics, specialized fossil fauna (chemosymbiotic clams and mussels) and exibit typical 13C-depleted signatures indicative of methane influence during precipitation. Several characteristics allow two type of seep-carbonates to be distinguished in the field (type 1 and 2). The type 1 is composed of a horizontal repetition of decametric to heptometric carbonate bodies, lenses and pinnacles. The basal portions of these huge bodies are strongly brecciated, made up of intraformational and rarely extraformational polygenic breccias. The type 2 is made of numerous and small marly-calcareous lenses, irregular column-like bodies aligned along bedding strikes, or horizontally and vertically scattered and not related to a precise stratigraphic level. Many of them consist of several lenticular units vertically stacked and separated by thin pelitic levels, thus suggesting periodic growth. Carbon isotope measurements show that carbonates from type 1 and 2 are typically depleted in 13C but the amount of depletion seem to differ between them. Therefore type 1 seep-carbonates appear significantly depleted in δ13C (δ13C ranging from -30‰ to -55‰ relative to the PDB standard) while type 2 seep-carbonates are only moderately depleted (δ13C varying from -10‰ to -23‰ relative to the PDB standard). Concerning oxygen isotopic measurements type 1 seep-carbonates seem to be enriched in δ18O (δ18O ranging from 1‰ to 6‰ relative to the PDB standard) whereas type 2 seep-carbonates show a more scattered range (δ18O varying from -3.00‰ to 2‰ relative to the PDB standard). Our presentation will report the result of detailed field observation of the two types of seep-carbonates coupled with petrographic, mineralogical and geochemical studies. In particular we discuss the isotope geochemistry, the mineralogy, the geometry in context with the precipitation and recrystallisation processes of the carbonates, the origin of carbon rich fluids and with different mechanisms of seep-carbonate formation.