湘西托口地区南华系沉积地层学研究及其地质意义

A SEDIMENTOLOGICAL AND STRATIGRAPHICAL STUDY OF THE NANHUA SYSTEM IN THE TUOKOU AREA,WESTERN HUNAN AND THEIR GEOLOGICAL SIGNIFICANCE

一般认为，华南南华系是指板溪群之上、陡山沱组之下的一套地层，其时限大致为720～635Ma。湘西托口地区的南华系原来被划分为江口组和南沱组，由于江口组的停用以及高精度年代学资料的缺乏，加上古沉积环境恢复研究程度较低，严重制约了新一轮南华系区域地层对比与岩相古地理重建。通过对该地区南华系及相关地层的野外地质调查、区域地层对比和碎屑锆石U—Pb同位素测年等研究，认为该区南华系可以被重新划分为滨岸冰融相长安组、障壁岛相富禄组、滨岸冰融相古城组、潮坪相大塘坡组和冰海大陆架相洪江组；其中，长安组和古城组的沉积时代分别晚于732Ma和705Ma。结合相关资料研究初步认为，富禄组应同大塘坡组一样属于间冰期沉积；华南南华纪可能至少发生过三次冰川事件，即长安冰期（〈732Ma～？）、古城冰期（〈705～670Ma）和南沱冰期（651～635Ma）．

It is generally accepted that the Nanhua System in South China represents a suite of strata that lie above the Banxi Group and below the Doushantuo Formation, the depositional age of which is probably between 720~635 Ma. The Nanhua System in the Tuokou area of western Hunan Province was previously divided into the Jiangkou Formation and the Nantuo Formation. However, because Jiangkou Formation has been discontinued as a stratigraphic term and there are very few radiometric ages from the Nanhua Sys- tem in the study area, the regional stratigraphic correlation and paleogeographic reconstruction of Nanhua System in western Hunan Province have been a challenging task. To overcome these difficulties, we car- ried out a detailed field outcrop investigation, regional stratigraphic correlation, and U-Pb radiometric dat- ing of detrital zircons of the Nanhua System and related strata in the Tuokou area. The results show that the Nanhua System in the Tuokou area should be divided into the Changan Formation （shoreface glacier melting facies）, Fulu Formation （barrier island facies）, Gueheng Formation （shoreface glacier melting fa- cies）, Datangpo Formation （tidal flat facies）, and Hongjiang Formation （glacial marine and continental shelf facies）. The depositional age of the Changan Formation and Gucheng Formation is not older than 732 Ma and 705 Ma, respectively. Combined with other data, the FUlU and Datangpo formations are both in- terpreted as interglacial deposits. There may have been three glacial periods in South China during the Nanhuan Period, including the Changan （〈732 Ma）,Gucheng （〈705~670 Ma）, and Nantuo glacial ep- ochs （651~635 Ma）.