A contribution to the reconstruction of Miocene seepage from authigenic carbonates of the northern Apennines (Italy)

Abstract

Authigenic carbonates from outcrops of the northern Apennines consist of small and irregular lenses and exhibit numerous features indicative for cold-seep settings. Detailed petrographic, mineralogical and geochemical studies from two Miocene deposits are presented. The first carbonate outcrop named Fosso Riconi is located in the foredeep basin of the Apenninic chain whereas the second deposit represents a satellite basin called Sarsetta. The stable isotope data from specific carbonate minerals show a wide range of values well known from other paleoseeps of the Apennine Mountains. From all carbonates analyzed during this study the majority of seep carbonates are formed by low-Mg calcite and ankerite. Those minerals have δ13C values between -7 and -23‰ V-PDB suggesting variable amounts of carbonate derived from oxidized methane, from seawater (DIC), and from sedimentary organic matter. Dolomite samples have the lowest δ13C values (-30.8 to -39.0‰ V-PDB) indicating methane as the main carbon source.The results from this study indicate an evolutionary formation of the seeps and development of the authigenic carbonates, influenced by the activity of chemosynthetic organisms from which large lucinid clams are preserved. Bioirrigation of those clams controlled the sediment-water exchange, and is here considered as an explanation for the anomalous Mg content of the calcite. We hypothesize that seep-carbonates are formed during periods of active methane-rich seepage, whereas during periods of slow seepage carbonate formation was reduced. Despite different geological settings, the two examined deposits of Sarsetta and Fosso Riconi show similar features, suggesting that the pattern of fluid circulation at both seep sites played a major role in their carbonate formation

Biogeochemistry and timing of methane-derived carbonate formation at Leirdjupet fault complex, SW Barents sea

“…iso-and anteiso-C 15 fatty acids (Table 2), is consistent with assimilation of methane-derived carbon. Intense microbial activity left typical petrographic features in the carbonate such as peloidal microfacies (Figure 3A) similar to other modern (Feng et al, 2008) and fossil methane-derived carbonates (Conti et al, 2010) and spherules possibly representing fossil sulfate-driven AOM aggregates (Himmler et al, 2022). In sample CarC-05, the vein filling displays three to five generations of cements separated by dissolution surfaces.…”

An enigmatic kilometer-scale concentration of small mytilids (Late Miocene, Guadalquivir Basin, S Spain)

“…Bivalves have been common cold-seep inhabitants since the Early Cretaceous onward, but they become particularly dominant during the Cenozoic (Campbell and Bottjer, 1995;Little and Vrijenhoek, 2003;Conti et al, 2010). Fossils in these communities usually cluster in the vicinity of fluid-seepage conduits although the lateral extension of fossil concentrations varies from tens to hundreds of meters.…”

Evidence of climatic control on hydrocarbon seepage in the Miocene of the northern Apennines: The case study of the Vicchio Marls

“…2 and 3). Previous studies have connected the origin of these carbonates to hydrocarbon-seepage, based on the carbon and oxygen isotopic signatures (Conti et al, 2010b), the fossil assemblage and the similarities with other seep-carbonates in the northern Apennines (Taviani, 1994;Cavagna et al, 1999;Taviani, 2001). Geochemical analyses reported in Conti et al (2010b) show d 13 C values ranging from À39 to À4.7& V-PDB and d 18 O values from À2 to 4& V-PDB.…”

“…4) ranges from 50 cm to 7e8 m, and the lateral extension from 1 to 3 m. Carbonate lithologies include marly limestones and weakly marly limestones, dark to light gray in color. The carbonate content ranges from 75 to 93 wt.% CaCO 3 ; the dominant carbonate phases are represented by calcite (40e95 wt.%), ankerite (0e15 wt.%) and dolomite (0e37 wt.%) associated with illite-muscovite (0e13 wt.%), chlorite (0e6 wt.%), quartz (0e6 wt.%), and albite (0e4 wt.%) (Conti et al, 2010b).…”

“…In contrast, other seep-carbonates located in the Apenninic foredeep, such as those observed in the Vicchio Marls examined in this paper, seem not to be related to tectonic structures. The Vicchio Marls in the Tuscan Apennines contain excellent exposures of a Miocene seepsystem (more than 80 carbonate bodies; Conti et al, 2010b) enclosed in marly sediments. A detailed stratigraphic and biostratigraphic study of these seep-carbonates and enclosing marls allows us to check the correlation between the onset of the seep-carbonate precipitation and the glacio-eustatic sea-level fall during the middle Miocene climate cooling.…”