What makes seep carbonates ignore self-sealing and grow vertically? The role of burrowing decapod crustaceans

Abstract. We investigated the mechanisms that govern vertical growth of seep carbonates by studying the sedimentary architecture of a 15-m-thick, 8-m-diameter column of limestone encased in a deep-water marl succession in the Middle Callovian interval of the Terres Noires Formation in the SE France basin. The limestone body, a.k.a. pseudobioherm is characterized by intense bioturbation, with predominant burrows of the Thalassinoides/Spongeliomorpha suite, excavated by decapod crustaceans. Bioturbation is organized in three tiers. The upper tier corresponds to shallow homogenization of soupy sediment and the second one to pervasive burrowing dominated by Thalassinoides passively filled by pellets. Both homogenized micrite and burrow-filling pellets are depleted in ¹³C in the range −5 to −10 ‰. The deepest tier in contrast is filled by diagenetic cements arranged in two phases. The first cement phase makes a continuous rim coating the burrow wall, in which carbon isotope data show consistent ¹³C depletion near −8 ‰ to −12 ‰, indicating precipitation by anaerobic oxidation of methane in the sulfate-methane transition zone. In contrast, the second cement phase is dominated by saddle-dolomite indicating precipitation at a temperature > 80 °C, largely post-dating the burial of the pseudobioherm. The late final blocking of the burrows means that vertical fluid communication was possible over the whole thickness of the pseudobioherm up to the seabed during its active growth. Vertical growth is related to the presence of this open burrow network, providing a high density of localized bypass points across the intra-sediment calcite precipitation zone in the sulfate-methane transition zone and preventing self-sealing from blocking upward methane migration and laterally deflecting fluid flow. One key characteristic that prevented passive fill of the burrows is their geometric complexity with numerous subhorizontal segments that could trap sediment shed from shallower bioturbation tiers.