Characteristics and significance of the cold-vent carbonate rocks from the Quse Formation in the Shuanghu area, Qiangtang Basin, northern Xizang

The carbonate concretions occur in large numbers in the Quse Formation, Shuanghu area, Qiangtang Basin, northern Xizang, and display hummocky, ellipsoidal, lenticular, stratoid, vein, dendritic, clotted, vesicular and seepage textures and structures. These nodules consist dominantly of micritic carbonate minerals with minor amount of clay minerals, quartz, framboidal and subhedral pyrite with an average grain size of about 5.0 μm,in which the sun-flower structures may be observed. The organic fossils such as bivalves, ammonites, worm-like or dendritic fossils and ultra-microbial fossils are found to be arranged with a higher density. The carbon isotopes in the analyzed samples are noticeably negative excursion while the sulfur isotopes in the analyzed samples exhibit a markedly distinct positive excursion. The results of research outlined above are in good agreement with the mechanisms for the formation of the cold-vent carbonate rocks produced by the seepage and release of the modern submarine natural gas. It is inferred from this reason that the carbonate concretions in the study area may be originated from the seepage and eruption of ancient natural gas. The presence of numerous submarine natural gas vents implies that there is a direct correlation between the oceanic anoxic events and the eruption of the submarine natural gas hydrates in the Shuanghu area, Qiangtang Basin during the Early Jurassic.