羌塘地块西南缘上侏罗统-下白垩统海相地层的发现

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

关键词:  羌塘  鸡夯  雪山组  上侏罗统-下白垩统

Key words:  Qiangtang  Jihang  Xueshan Formation  Upper Jurassic to Lower Cretaceous

摘要:

通过1:5万区域地质调查,在青藏高原羌塘地块西南缘鸡夯地区原划上三叠统日干配错群中新识别出一套上侏罗统-下白垩统地层。本文根据该套地层的岩石组合以及古生物面貌特征,初步探讨了该套地层的沉积环境和沉积相特征,对其中发育的玄武岩夹层采用锆石U-Pb (LA-ICP-MS)同位素测年方法,获得其年龄为118.3±2.1Ma。在发育的生物碎屑灰岩夹层中采集了珊瑚、双壳类、腕足、腹足类化石,化石资料显示该套地层形成于晚侏罗世-早白垩世。这是首次在南羌塘地块发现该时期海相地层,这一发现证明南羌塘地块在晚侏罗世-早白垩世时期海水并未完全退出,而是局部发育海相三角洲。

Abstract:

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.