Miocene seep-carbonates as indicators of style and intensity of fluid migration.

Ancient hydrocarbon Miocene seep-carbonates of Miocene age have been reported from marine sedimentary successions of the northern Apennines. They are recognized by their peculiar palaeoecological, sedimentological, compositional and isotopic features as products of the microbial oxidation of methane-rich fluids and represent an excellent on-land example of carbonate bodies interpreted as the remains of ancient cold seeps. These seep-carbonates occur from internal tectonic zones (Piedmont Terziary basins, epi-Ligurian and minor basins) to external zones of the foredeep. In the Miocene foredeep, they occur crop out in large turbiditic bodies (Mt. Cervarola and Marnoso-arenacea Formations) and in slope hemipelagites (Vicchio and Verghereto Marls, and Ghioli di letto mudstones). Dominant rock types are calcilutitic/marly limestones, calcareous marls and calcarenites. Enclosing sediments are hemipelagic/turbiditic mudstones, muddy sandstones and marlstones. In the Apenninic chain, the abundance and the extent of the outcrops provide a rare opportunity to study the geometry, facies distribution and internal structures of fossil methane-derived carbonates.On the basis of morphological and stratigraphic features two main types of seep-carbonates were 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. They have a thickness of 5 - 30 m and an extension that ranges from 10 m to 100 m. The basal portions of these huge bodies are strongly brecciated, made up of intraformational polygenic breccias and rarely extraformational. The Sasso Streghe (Modena Apennines) and Monte Petra (Romagna Apennines) carbonate outcrops are excellent examples of this type of seep-carbonates. The type 2 is made of numerous marly-calcareous lenses, irregular column-like bodies with a dimension ranging from some decimetres to 3 – 4 m and a thickness of 20- 30 cm to 3 m. Carbonate bodies are aligned along bedding strikes, or horizontally and vertically scattered and not related to a precise stratigraphic level. The Vicchio outcrops (Tuscan Apennines) are representative of this second type of carbonates. Mineralogical analyses of type 1 carbonate samples indicate that dolomite and ankerite represent the most dominant phases while Low-Mg calcite represents type 2 dominant carbonate phase. The carbon and oxygen isotopic compositions of the carbonates display very large ranges, from -10‰ to -55‰ Vienna PeeDee Belemnite (V-PDB) and from -3‰ to 6‰ V-PDB, respectively. Seep-carbonates type 1 appear significantly depleted in δ13C (ranging from -30‰ to -55‰ V-PDB) while seep-carbonates type 2 are only moderately depleted (δ13C varying from -10‰ to -23‰ V-PDB). Petrographic observations show complex facies relationships, as indicative of different stages in seep-carbonates growth.Our presentation will report the result of a detailed field observation of the two types of seep-carbonates coupled with geochemical, petrographic and mineralogical studies. In particular we discuss distinctive characters, geometry, isotope geochemistry and mineralogy, in context relationship with precipitation and recrystallisation processes of the carbonates, the origin of carbon rich fluids, and with different mechanisms of seep-carbonate formation.