Abstract
Abstract. The mechanisms that govern the vertical growth of seep carbonates were deciphered by studying the sedimentary architecture of a 15 m thick, 8 m wide column of limestone encased in deep-water marl in the middle Callovian interval of the Terres Noires Formation in the SE France Basin. The limestone body, also called “pseudobioherm”, records intense bioturbation, with predominant traces of the Thalassinoides/Spongeliomorpha suite, excavated by decapod crustaceans. Bioturbation was organized in four tiers. The uppermost tier, tier 1, corresponds to shallow homogenization of rather soft sediment. Tier 2 corresponds to pervasive burrows dominated by large Thalassinoides that were later passively filled by pellets. Both homogenized micrite and burrow-filling pellets are depleted in 13C in the range from −5 ‰ to −10 ‰. Tier 3 is characterized by small Thalassinoides that have walls locally bored by Trypanites; the latter represent tier 4. The diagenetic cements filling the tier-3 Thalassinoides are arranged in two phases. The first cement generation constitutes a continuous rim that coats the burrow wall and has consistent δ13C values of approximately −8 ‰ to −12 ‰, indicative of bicarbonate originating from the anaerobic oxidation of methane. In contrast, the second cement generation is dominated by saddle dolomite precipitated at temperatures >80 ∘C, at a time when the pseudobioherm was deeply buried. The fact that the tubes remained open until deep burial means that vertical fluid communication was possible over the whole vertical extent of the pseudobioherm up to the seafloor during its active development. Therefore, vertical growth was fostered by this open burrow network, providing a high density of localized conduits through the zone of carbonate precipitation, in particular across the sulfate–methane transition zone. Burrows prevented self-sealing from blocking upward methane migration and laterally deflecting fluid flow. One key aspect is the geometric complexity of the burrows with numerous subhorizontal segments that could trap sediment shed from above and, hence, prevent their passive fill.
Highlights
Seep carbonates are produced by the anaerobic oxidation of methane (AOM) or other heavier hydrocarbons coupled with seawater sulfate reduction (Boetius et al, 2000; Orcutt et al, 2010; Zwicker et al, 2018)
Blouet et al.: What makes seep carbonates ignore self-sealing and grow vertically bons, the depth of the reaction front, the so-called sulfate– methane transition zone (SMTZ), typically lies a few centimeters below the seafloor (Regnier et al, 2011), where a strong redox gradient favors the settlement of chemosymbiotic macrofauna (Kiel, 2010)
Oceanographic observations demonstrated that hydrocarbon seepage at the seafloor is a ubiquitous phenomenon in the world’s oceans (Judd and Hovland, 2007), and seep carbonates preserve this elusive phenomenon in the rock record
Summary
Seep carbonates are produced by the anaerobic oxidation of methane (AOM) or other heavier hydrocarbons coupled with seawater sulfate reduction (Boetius et al, 2000; Orcutt et al, 2010; Zwicker et al, 2018). Seep carbonates typically appear as concretionary bodies exhibiting a wide diversity in shape and size ranging from isolated nodules a few centimeters in diameter (e.g., Haas et al, 2010) to massive moundshaped structures tens of meters in diameter (e.g., Kauffman et al, 1996). They are commonly associated with other fluid expulsion features such as pockmarks (Ho et al, 2018a, b). The morphology and vertical variation of amplitude anomalies are useful to qualitatively reconstruct the history of fluid leakage intensity (Ho et al, 2012)
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