Late Early Jurassic Posidonia shale facies in Qiangtang Basin and its chronostratigraphic significance

Among the Mesozoic global oceanic anoxic events, the Posidonienschiefer (Posidonia shale) facies in southern Germany represents the earliest one, which is well-known all over the world. Up till now, the Posidonia shale facies has been found at 4 localities in the Qiangtang Basin, including drill holes and outcrops. In Qiangtang Gas Hydrate Scientific Well QK-5, a more than 150m black shale sediment sequence was discovered, which yielded the Early Jurassic Cleviceras elegans-Bositra buchii assemblage. The locality of Posidonia shale facies in Well QK-5 is the northernmost one, on the northern side of the “middle uplift zone” of the Qiangtang Basin, while the other three are spread in areas from Shuanghu to Sewa, on the southern side of the “uplift zone” of the basin. Actually, there is few evidence to indicate that the basin had ever been separated by a uplift during Early Jurassic. In addition, the Bailongbinghe Formation (Sowa Formation) in northeast Qiangtang, formerly thought of belonging to the Late Jurassic age, is proved to range from Early to Middle Jurassic by the Toarcian and Early Callovian ammonite successions. The discovery of Posidonia shale facies in the Qiangtang Basin is helpful to further understanding the Mesozoic sedimentary and evolution history of the basin. Considerable stratigraphic thickness and distribution area of the carbon-rich Posidonia shales in the basin would provide a very optimistic outlook for shale gas exploration.