湘黔桂邻区板溪期与南华冰期之间的沉积转换及其地层学涵义

Depositional Transformation from Banxi Period to Nanhua Glacial Period in Southeast Margin of Yangtze Block and its Implications to Stratigraphic Correlation

前人一直认为湘黔桂邻区的板溪群及其相当层位的下江群、丹洲群与南华冰期的沉积序列是连续或部分连续的,没有明显的沉积—构造转换界面。本次调查的锦屏隆里剖面、黎平肇兴剖面、从江黎家坡剖面、三江泗里口剖面、罗城黄金友洞剖面等均显示,板溪期与南华冰期的沉积转换非常明显,主要表现是界面上下的地层岩性、岩石结构构造和沉积相类型等迥然不同。其区域表现是:板溪沉积晚期为海退进积序列,从陆块内部到陆块边缘,地层缺失逐渐减少;江口冰期沉积从陆块边缘到陆块内部,显示出明显的渐次海侵上超特征,越靠近边缘早期沉积序列越完整。我们认为这就是前人提出的"雪峰运动"的沉积表现。因此,在扬子东南缘的湘黔桂邻区一带南华系地层序列是相对完整或近似完整的,应是层型剖面和点(GSSP)选择的最有利地区。目前首要问题是要进一步明晰"南华系"含义,而深入研究和探讨南华大冰期沉积前之板溪群及其相当层位地层的构造属性及其与冰期沉积的转换过程则是建立"南华系"的必要前提。

The depositional-structural transformation and the regional correlation of relevant important strata units from Banxi period to Nanhua glacial period in southeast margin of Yangtze block have been a major focus of debate for a long time. Predecessors had been regarded that they wear conformity without obvious boundary surface between the Danzhou Group and Nanhua glacial deposition in North Guangxi and East Guizhou. However, the authors survey of some im- portant sections in Guizhou-Guangxi neighboring region, such as Jinping Longli section, Liping Zhaoxing section, Congjiang lijiapo section, Sanjiang Silikou section and Luocheng Huangjin section, indicate that the transforming sur- face from Banxi period into Nanhua glacial period was clearly and sharply, for the upper part and down part of the sur- face were different completely in rock component, contexture, construction, and depositional facies types. The regional change of the transformation token on that the stratigraphic missing of the Banxi Group and its equiv- alences with a regressive progradational sequences become more and more obvious from the margin to the interior of Yangtze Block, and in reverse, the sedimentary sequences of the Nanhua glacial period with a transgressing process gradually become more and more integrated from the interior to the margin. We think that this was the depositional re- cords of Xuefeng Movement brought forward by predecessor. Therefore, it was more probability and feasibility in the Hunan-Guizhou-Guangxi neighboring region of the southeast margin than the Gorges region in the interior of Yan- gtze Block to find a relatively integrated sedimentary sequence of Nanhua glacial period, and so the region should be the most favorable area to select the Global Standard Section and Point （GSSP） for Nanhua System. At the same time, the available geo-chronological data suggest that the top boundary of Banxi period should be towards 720Ma, which is close to the onset age of Sturtian Glacial stage, and should not be limited at 780Ma subjec- tively. The most important is that the depositional sequence of Banxi period was a tact sedimentary cycle standing for the filling of mid-Neoproterozoic rift basin, and also there was not observable and identifiable hiatus in the sequence. so this sequence should not be dismembered or fragmented, otherwise we will be going against the principle of Interna- tional Commission on Stratigraphy （ICS） for the classification of Precambrian strata. Therefore, at present, we con- sider that 720Ma is reasonable for the top boundary age of Banxi Group. And now, the first thing is to get more distinct and clear concept and implication of Nanhua System. So it is necessary for its foundation to research and discuss deeply the tectonic settings of the Banxi Group and its equivalences and the depositional transformation from Banxi period to the Nanhua glacial period.