Outcrop analogues of pockmarks and associated methane-seep carbonates: A case study from the Lower Cretaceous (Albian) of the Basque-Cantabrian Basin, western Pyrenees

Abstract

Late Albian deep-water sediments of the Black Flysch Group in the Basque-Cantabrian Basin (western Pyrenees) preserve a fossil pockmark field including methane seep carbonates and associated macrofauna. The geometry of the pockmarks is reconstructed from repeated lens-shaped turbidite deposits with centrally located carbonate bodies. Early diagenetic carbonate phases such as clotted micrite and yellow calcite with δ13C values as low as −41.6‰, and hydrocarbon biomarkers (e.g. 2,6,10,15,19-pentamethylicosane) with strong depletions in 13C indicate that the carbonates precipitated due to anaerobic oxidation of methane. The pockmarks probably formed due to subsidence induced by dewatering and degassing of the gas-charged seabed perhaps enhanced by the weight of the carbonate bodies. The macrofauna resembles that of other late Mesozoic deep-water methane-seeps world-wide, and is dominated by large lucinid and Caspiconcha bivalves, and hokkaidoconchid gastropods. During late diagenesis the carbonate δ18O values were reset to a narrow range of −12 to −10‰, the remaining pore spaces and fissures were filled with pyrobitumen, and additional carbonate phases precipitated, potentially due to thermochemical sulfate reduction processes.