Characteristics and geological significance of tubular carbonate concretions in the Lower Shaximiao Formation of Yunyang, Chongqing, China

Methane-derived carbonates provide strong evidence supporting the emission of unstable gas-hydrates from the seafloor. However, existing studies have shown that methane-derived carbonates are mainly formed in underwater marine environments, and there are few reports regarding their formation in terrestrial basins. A large number of nodular and tubular concretions were developed in the Middle Jurassic Shaximiao Formation in Yunyang, Chongqing. This paper focuses on the two types of calcareous concretions, and their detailed macroscopic, microscopic and geochemical characteristics are described. Our results indicate that the observed nodular concretions were the product of continuous evaporation under arid-semiarid conditions. The morphologies, microscopic features and carbon and oxygen isotope compositions of the tubular concretions are similar to those of tubular methane-derived carbonates. Therefore, we believe that the tubular concretions in the study area are related to methane emissions and that they acted as channels for the migration of methane-rich fluids and muddy sediments. Based on the background of sediment liquefaction in the study area, we suggest that these tubular concretions might be the most direct geological records of mud volcanic activity.