西秦岭中、晚二叠世生物群更替事件

西秦岭地区中、晚二叠世的生物,在其发展过程中经历了一次动物群的更替事件,表现为中二叠世茅口晚期迭山组底部开始的竺蜓类化石的突然锐减或消失及腕足动物的大量繁盛。茅口中期的热让沟组上部,有Afghanella及丰富的Parafusulina,Chusenella等竺蜓类化石。但上覆的迭山组底部开始,仅有少量Parafusulina,Chusenella及Schwagerina等断续出现。而同时交替出现了以Urushtenia,N eoplicatifera为代表的小型腕足动物群及小型单体四射珊瑚。这一动物群的更替事件受到当时沉积环境变迁和区域构造活动的制约。迭山组底部的沉积环境由开阔台地的浅水环境转入半局限台地较深水滞流环境,导致了竺蜓类的消失和小型腕足动物的兴起。这种沉积相的变化与动物群的更替在层位上是完全吻合的。研究区处于西秦岭晚海西裂陷带两侧的碳酸盐台地或台地边缘,其生物群的更替和沉积相的变化必然受到区域上裂陷活动的影响。根据本区生物地层的研究,可以更为确切地认为,本区二叠纪裂陷活动的时间始于中二叠世茅口期Afghanella schencki时的后期(或N eoschwagerina时的后期) ;约终于晚二叠世长兴期Palaeofusulina时之前。

Fauna Replacement Event of the Middle and Late Permian in the West Qinling Mountains

The middle and late Permian faunas from the west Qinling Mountains have suffered a replacement event during the developing history. It behaves that the fusulinids occurring from the bottom of the late Maokouan Dieshan Formation of the middle Permian were sharply reduced or disappeared, and the brachiopods were prosperously developed. The fusulinids are well developed in the middle Maokouan Reranggou Formation, such as Afghanella and plenty of Parafusulina, Chusenella. But from the very beginning of the superjacent Dieshan Formation, only a small amount of Parafusulina, Chusenella and Schwagerina appeared intermittently, and meanwhile a small brachiopods fauna, represented by Urushtenia and Neoplicatifera, and small solitary tetracorals alternately occurred. This fauna replacement event was under the control of the variations of sedimentary environments and regional tectonic activities. The sedimentary environments of the Dieshan Formation was changed from the early shallow water of open platform into stagnated water of semi-restricted platform, and thus resulted in disappearance of fusulinids and flourish of small brachiopods. The change of sedimentary facies and the replacement of faunas are well inosculated in stratigraphic horizon. The study area is located on the west margin of the carbonate platform or the carbonate platform margin of the late Hercynian rift zone in the west Qinling Mountains, so both of the changes of sedimentary facies and the replacement of faunas must be influenced by regional rift activities. Based on biostratigraphic study in this area, it is definitely known that the Permian rift activity of this region started at the late chron of the middle Permian Maokouan Afghanella schencki (or the late chron of Neoschwagerina), and finished before of the late Permian Changxingian Palaeofusulina.