Sluggish and steady focussed flows through fine-grained sediments: The methane-derived cylindrical concretions of the Tertiary Piedmont Basin (NW Italy)

Abstract

a b s t r a c tIn the Cenozoic succession of the Tertiary Piedmont Basin (NW Italy) a wide array of methane-derived carbonate rocks including both seafloor and sub-seafloor products have been reported in the last decades. The most intriguing features are cylindrical concretions, more than 1 m long and around 10 cm across, that were found within fine-grained sediments of Messinian age in their original stratigraphic position, crossing the bedding planes at high angles. These concretions result from cementation of the host muddy sediments by intergranular dolomite microcrystals, and lack any evidence of exposure at the sea floor. Their C and O stable isotope signature suggests that dolomite precipitation was induced by anaerobic oxidation of methane, possibly sourced by gas hydrate destabilization. Such cylindrical morphologies are interpreted as the result of carbonate precipitation and cementation of the sediments around fluid conduits and record a relatively long period of steady, sluggish flux of methane-rich fluids that opened their way rising up through fine-grained unconsolidated sediments. They formed immediately below the sea floor, in a narrow horizon bracketed between the sulphateemethane interface and the oxic/anoxic interface. Cylindrical concretions represent only the highest part of pipes through which methane-rich fluids flowed up. The deeper part of the conduits, conversely, left no trace in the stratigraphic record because of the lack of chemical/microbiological conditions favourable for carbonate precipitation.

Geocronologia e geoquímica isotópica do Grupo Passa Dois (Permiano) na borda leste da Bacia do Paraná (Estados de Santa Catarina e São Paulo, Brasil)

“…The pioneering work of Jenkins (1930) points to a Paleogene oil migration from the older Moreno Shale to the Kreyenhagen Based on stratigraphic relationships and stable isotope compositions of authigenic carbonates and clastic dikes, Minisini & Schwartz (2007) demonstrated the contemporaneity between these structures and conclude that the dikes controlled the distribution of the seeps. Cavagna et al (2015) and others, reported methanederived carbonate rocks, including both seafloor and sub-seafloor products in the Cenozoic succession of the Piedmont Basin of NW Italy. These authors argue that cylindrical concretions formed just below the seafloor are the result of carbonate precipitation around fluid conduits and record a long period of stable and slow methane seepage.…”

“…However, several studies on Phanerozoic seep deposits revealed methane-derived carbonates with a wide range of 13 C values, varying between -30 to +24‰ (see Kauffmann et al 1996, Martens et al 1999, Stakes et al 1999, Greinert et al 2001, Peckmann et al 2002, Reitner et al 2005, Wirsig et al 2012, Cavagna et al 2015, Lash et al 2015. Thus, a plausible methane source in the Irati Formation is compatible with the modestly 13 C-depleted carbonate cements from the Serra Alta concretions and clastic dikes.…”

“…In the sulfate-methane transition zone (SMTZ) of anoxic marine settings, the anaerobic oxidation of methane occurs in a thin interval characterized by methaneand sulfate-depleted pore waters (Campbell 2006). Furthermore, AOM coupled with bacterial sulfate reduction can stimulate sulfur isotopic enrichments in different solid phases including pyrite (Lash & Blood 2014, Cavagna et al 2015, Lash 2015, Lin et al 2016 (Longstaffe 1987. Several processes can explain negative δ 18 O values for early diagenetic carbonates, such as: (i) meteoric water input, (ii) oxidation of organic matter in the sulfate-reduction zone, (iii) chemical interaction between watersediments and volcanogenic or juvenile materials, (iv) ascension of hot fluids, and (v) increasing temperature and recrystallization at deep burial (Mozley & Burns, 1993).…”

“…Different processes can explain pathways for upward fluid migration at hydrocarbon seep systems, including: (i) sand injections (Cosgrove, 2001, Schwartz et al 2003, Duranti & Mazzini 2005, Hurst et al 2011), (ii) faults (Moore et al 1990, Ogawa et al 1996, (iii) seepage structures (Cavagna et al 2015, De Boever et al 2011, Zwicker et al 2015, (iv) hydraulic fracturing (Mazzini et al 2003a(Mazzini et al , 2003b) and (v) submarine erosion (Campbell et al 2002).…”

“…Several studies have demonstrated that the relationships between clastic dikes, tubular concretions and mineralized conduits as the potential plumbing system for methane seep sites (Mazzini et al 2003, Duranti & Mazzini, 2005, Minisini & Schwartz, 2007, Blumenberg et al 2015, Cavagna et al 2015, De Boever et al 2015, Wirsig et al 2015, Zwicker et al 2015. The pioneering work of Jenkins (1930) points to a Paleogene oil migration from the older Moreno Shale to the Kreyenhagen Based on stratigraphic relationships and stable isotope compositions of authigenic carbonates and clastic dikes, Minisini & Schwartz (2007) demonstrated the contemporaneity between these structures and conclude that the dikes controlled the distribution of the seeps.…”