论两种二叠-三叠系界线

<正> 近年来许多地质学家围绕着二叠-三叠系界线的划分问题做了大量的工作,关于二叠-三叠系之间界线定义,尚未取得一致意见。目前议论较多的主要是两条界线:1.Otoceras层之底——传统界线

Geological age and depositional feature of the upper Xiushan Formation and Xiaoxiyu Formation of west Hunan

A lot of work have been done on the researches of the Permian-Triassic boundary during recent years. But there is so far no international agreement on the definition of the Permlan-Triassic boundary. There are mainly two views on this problem; placing the boundary at the base of the Otoceras bed——the traditional boundary and putting it at the base of the Ophiceras bed. The former is considered as the boundary of eventstratigraphy and the latter the boundary of biostratigraphy. A detailed research has revealed that some geological events such as crustal uplift, regional regression, volcanic explosion, reversal of magnetic poles, and the Ir anomalies have happened during the late Late Permian time in South China. The geological events produced the crises in the ecosystems and made a mass extinction of faunas. Both the biotic extinction and the geological events formed a event-stratigraphy boundary——traditional boundary at the base of the Otoceras beds. The occurrence of geological events not only caused biotic extinction but also provided some niches for some faunas, made the groups which had potentialities to capture the new niches, newborn and radiantly developing. Thus, a biostratigraphical boundary was formed at the base of the Ophiceras bed.