Fluid expulsion and typology of seep-carbonates in the northern Apennines.

Miocene seep-carbonates have been reported from marine sedimentary successions of the northern Apennines in the form of huge isolated carbonate bodies (type 1) and numerous horizontally and vertically scattered marly-calcareous lenses (type 2) (Conti and Fontana, 2005 and references herein). 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 example of carbonate bodies interpreted as the remains of ancient cold seeps. 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 seep-carbonates. The main parameters which control the composition and development of a carbonate body are the methane pressure in interstitial sedimentary fluids, the flux discharge and rate in the venting zone, and the evolutionary path of the rising fluids (Roberts, 2001). Mineralogical, petrographic analyses, carbon and oxygen isotopic compositions of type 1 and 2 carbonate samples indicate that different mechanisms of hydrocarbon fluid expulsion and variations in the upward methane flux control carbonate types and mineralogy.Mineralogical analyses of type 1 carbonate samples indicate that dolomite and ankerite represent the dominant phases, while low-Mg calcite is the type 2 dominant carbonate phase. The carbon and oxygen isotopic compositions of the carbonates display very large ranges, from -10‰ to -55‰ V-PDB, and from -3‰ to 6‰ V-PDB, respectively. Seep-carbonate type 1 appear significantly depleted in δ13C (ranging from -30‰ to -55‰ V-PDB) while seep-carbonate 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.Type 1 seep-carbonates could be related to constant and discrete fluid seepage conditions while type 2 could be explained by variations in the upward methane flux (increasing, decreasing flow rates). The intraformational and rarely extraformational polygenic breccias which are often observed in the basal portions of type 1 seep-carbonates could indicate phases of violent venting of gaseous-rich fluids. The explosive escaping of carbon-rich fluids interrupted the conditions of constant fluid seepage and caused brecciation of the carbonate body. 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.