Methane seeps on an Early Jurassic dysoxic seafloor

摘要: Abstract The rhythmically bedded limestone–marl–shale succession of the Blue Lias Formation (Lias Group, Early Jurassic age) Kilve in Somerset (SW England) preserves a suite large conical concretions that formed around methane seeps. These are 1–2 m high, and elliptical plan (axes 2–4 m), with an outer limestone shell forming flanks cone. cone flank is composed micritic carbonate (20–30 cm thick), which locally includes sheets pods intraclasts bioclasts. cycle-forming beds host strata dark grey micrite carbon-isotope values (δ13C = 0.6 to 0.8‰) consistent carbon sourced from mixture seawater by sulphate reduction, oxygen-isotope (δ18O = − 6‰) suggesting some degree later diagenesis. pale forms sides mounds three-dimensional ammonites intraclasts, thus cemented close sediment–water interface prior compaction. mound-forming markedly isotopically light respect carbon, but not oxygen (δ13C = − 24.3 − 26.4; δ18O = − 2 − 3.5‰). isotope signature indicates cements were probably derived sources included biogenic methane. within suggest syn-depositional physical brecciation mixing had occurred, rich-fluids overlying surface waters likely also have occurred. relatively heavy may be indicative anaerobic oxidation mound-bearing interval benthos-poor comprises predominantly laminated black shales, characteristic poor bottom water oxygenation. largest however, capped fossiliferous breccias. Thus either benthic islands elevated biota into oxic zone or, alternatively, they supported chemotrophic community. Although cold seep deposits been documented previously still comparatively rare. This example one oldest Europe, unusual amongst described ancient seeps preserving relief extended above seafloor.