Seep-carbonates and fluid expulsion processes in the Miocene of the northern Apennines.

In the Miocene satellite and foredeep basins of the northern Apennines numerous carbonate bodies were identified by specific palaeoecological, sedimentological, compositional and isotopic features as products of the microbial oxidation of methane-rich fluids (Conti and Fontana, 1999a, 2002 and references herein).These methane-derived 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). Dominant rock types are calcilutitic limestones, marly limestones, calcareous marls and calcarenites. Carbonates exhibit typical 13C-depleted signatures indicative of methane oxidation On the basis of morphological and stratigraphic features two main types of seep-carbonates were distinguished in the field (Type 1 and 2 Conti and Fontana,1999a).The first type (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 lateral contacts with marls and arenitic mudstones (host sediments) vary from sharp to transitional. The basal portions of these huge bodies are typically strongly brecciated, made up of intraformational polygenic breccias and rarely extraformational. The Sasso Streghe (Modena Apennines) and Monte Petra (Romagna Apennines) carbonate outrops are excellent examples of this type of methane-derived carbonates.The second type (Type 2) consist of numerous marly-calcareous lenses, irregular column-like bodies with a dimension that ranges from some decimetres to 3 – 4 m and a thickness of 20- 30 cm to 3 m. Carbonate Type 2 bodies can be 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. The Vicchio outcrops is clearly representative of this second type of carbonates.Carbon isotope measurements show that carbonates from Type 1 and 2 are tipically depleted in 13C but the amount of depletion seem to differ between them. Therefore seep-carbonates Type 1 appear significantly depleted in δ13C (up to …) while seep-carbonates Type 2 are only moderately depleted (…). Our presentation will draw attention to the distinctive characters of the two types of chemoherms, In particular we discuss geometry, isotope geochemistry and brecciation, in context with the composition and origin of carbon rich fluids, the flux discharge and rate, and with the mechanisms of expulsion (diffused or focused)