The discovery of the Upper Jurassic to Lower Cretaceous marine strata on the southwestern margin of the Qiangtang block

On the basis of 1:50 000 regional geological survey, a succession of the Upper Jurassic to Lower Cretaceous marine strata has been recently recognized in the formerly Upper Triassic Riganpeico Group in the Jihang area, southwestern Qiangtang block, Qinghai-Xizang Plateau. The present paper gives a preliminary study of sedimentary environments and facies in the light of rock associations and palaeontological features in the marine strata. A LA-ICP-MS zircon U-Pb age of 118 ±2.1 Ma for the basalts in the marine strata and fossils data for the corals, bivalves, brachiopods and gastropods collected from the developed bioclastic limestones show that the marine strata were formed during the Late Jurassic to Early Cretaceous. The discovery of the Upper Jurassic to Lower Cretaceous marine strata has reflected that during the Late Jurassic to Early Cretaceous, the sea water still occupied the southern Qiangtang block, where the marine deltas were once developed in local parts.