宜兴组的建立及有关问题讨论

ESTABLISHMENT OF YIXIN FORMATION AND DISCUSSION ON SOME RELATED PROBLEMS

本文介绍了宜兴组(P_2y)的典型剖面。以《国际地层指南》等为理论依据,从宜兴组的岩性、区域分布及时代等方面讨论了建立宜兴组的必要性和充分性。并且还讨论了前人所认为的江阴古岛实际上可能是一个浅滩。

There are two kinds of sedimentary rocks in the lower Yangtze stratigraphic region in the late interval of the Late Permian Epoch.One is called Changxin formation composed of shallow sea carbonate rock containing chert and palaeofulina biota,the other one is under the name of Dalong Formation composed shallow sea siliceous clastic rock containing pseudotirolites biota. But there exists a suit of different strata in Jiulishan,Yixin city which is composed of both the limestone and siliceous rock with a rate of 1:2.Acturally,it is the transitional sedimentary strata between Changxin and Dalong Formations.The content of limestone decreases from Changxin to Dalong Formations.According to the ‘international stratigraphic guide’,the author considers. that it is necessary to establish a new formation on tish transitional strata and named it Yixin Formation.Regionally,the Yixin Formation appears belt-like and NE trending,paralled to the Jiangying-Lishan shallow barrier shoal and located southeast to it,220km long and 6km wide, mostly 20km wide.It is conformablely overlying Longtan Formation and underlying the tran- sitional earliest Triassic Qinglong Formation. As to the Jiangying-Lishan shallow barrier shoal,it was considered as a relative uplift pre- viously which had been denuded and divided the region into two sedimentary facies:southeast side limestone facies(Changxin Formation)and northwest clastic facies(Dalong Formation).Accor- ding to the continuity of sedimentary facies,denudation of Longtan Formation and the orien- tation of terrigenous clastic grain,it may not exist,but a underwater shallow barrier shoal.