Discovery of bituminous gravels in the Lower Cambrian in Qingchuan, northern Sichuan, China and its geological significance

Bituminous gravels are found in Lower Cambrian clastic deposits at Baijia Township, Qingchuan County, northern Sichuan Province. The black bituminous gravels are mostly subrounded or subangular, with a smooth surface by no luster, and have a maximum diameter of 5 cm, and there is distinct boundary between them and their cements. It is inferred that these gravels may have come from the denuded Dengying Formation. The deposition of bitumen, carbonate rocks and shale as gravels from the Lower Cambrian implies that a palaeo-petroleum-reservoir had existed in/near the Qingchuan area, northern Sichuan, before the Early Cambrian Qiongzhusian and that hydrocarbon source rocks may occur in the Precambrian in northern Sichuan.