云南保山上三叠统诺利阶南梳坝组和大水塘组的牙形石及沉积环境特征

CHARACTERISTICS OF CONODONTS AND SEDIMENTARY ENVIRONMENT OF UPPER NORIAN NANSHUBA AND DASHUITANG FORMATIONS IN BAOSHAN, YUNNAN PROVINCE

云南保山地块晚三叠世诺利期(Norian)海相沉积发育。通过野外观察、实测保山大堡子剖面,对剖面南梳坝组中部以及大水塘组顶部灰岩进行了牙形石分析,经鉴定有1属3种,分别为Mockina bidentata,Mockina englandi和Mockina aff.englandi,时代为晚诺利期Sevatian1亚期。岩石显微薄片分析显示,南梳坝组中部灰岩和大水塘组顶部灰岩中含大量被方解石完全或部分交代的放射虫,陆源碎屑岩中见磨圆程度不同的碎屑。结合滑塌沉积构造,推测南梳坝组沉积时期大堡子地区为混积陆棚环境,发育了与岛弧(微板块)有关的特殊陆棚沉积模式。

Norian marine deposits are developed in Baoshan block, Yunnan Province. So far, few studies on palaeochronology have been carried out. In this study, the Norian sections at Dabaozi and Hongyan in Baoshan are measured and sampled. Conodont fossils were obtained from the limestone in middle Nanshuba Formation and the top part of Dashuitang Formation, These conodonts identified as Mockina bidentata, Mockina englandi and Mockina aff. englandi, are with the age of Sevatian 1 Subage, late Norian. Based on the analysis of thin sections, it is found that the limestones of Nanshuba and Dashuitang formations contain a large number of radiolarians, and debris with different degrees of roundness in clastic rocks can also be seen. In view of the slump structures seen in outcrops and the microscopic features of sedimentary rocks, it is deduced that Nanshuba Formation was deposited in a mixed shelf environment at Dabaozi area, controlled by the tectonic setting of an island arc(or microplate) in the Norian Tethys Ocean.