江南过渡带奥陶系层序地层分析

Characteristics of sequence stratigraphy for Ordovician strata around Jiangnan transitional zone

文章根据露头层序地层原理和方法,考虑层序关键界面、体系域特征、层序区域追踪对比性,对江南过渡带周边(石台—黟县一带)江南地层分区、江南过渡地层分区及下扬子地层分区奥陶系进行了露头层序地层研究。研究表明:江南过渡带周边奥陶系可识别出2个完整二级层序(SS1、SS2)与1个二级层序低水位体系域(SS3-LST)和11个三级层序(OSQ1～OSQ11),平均每个三级层序约为4.05 Ma,反映了研究区奥陶纪具有11次相对较大的海平面升降旋回;三级层序发育海侵体系域(transgressive system tract,TST)和高水位体系域(highstand system tract,HST),有时缺失低水位体系域(lowstand system tract,LST),局部凝缩段(condensed section,CS)难以识别;二级层序能进行较好的区域对比,三级层序不能完全对比;研究区北西高南东低,海水向北西方向持续海侵;北西侧灰质成分较高,水体相对较浅,而南东侧泥质成分较高,水体较深,在中间的江南过渡带,主体为斜坡相沉积,局部发育重力流沉积,其成因可能与江南断裂为同沉积断裂有较为密切的关系。

Based on the principle and method of outcrop sequence stratigraphy, considering the key surfaces of sequence, characteristics of systems tract and comparison of sequence region tracking, the outcrop sequence around Jiangnan transitional zone(from Shitai to Yixian area) during the Ordovician including the Lower Yangtze stratigraphic region, Jiangnan transitional stratigraphic region and Jiangnan stratigraphic region is researched. The results show that during the Ordovician the Jiangnan transitional zone can be divided into two complete second-order sequences(SS1,SS2), one lowstand system tract(SS3-LST), eleven third-order sequences(OSQ1-OSQ11), and the average age of each third-order sequence is 4.05 Ma. It is indicated that the studied area has eleven relatively large eustatic cycles during the Ordovician;the third-order sequences developed transgressive system tract(TST) and highstand system tract(HST) with local lowstand system tract(LST), partial condensed section(CS) is difficult to identify;the second-order sequences can be correlated, while the third-order sequences can not be completely correlated. Through the comparison research of five profiles, it is shown that the studied area inclined from NW to SE, and the continuous regressive happened to south-east;on the NW side, Lower Yangtze stratigraphic region is rich in lime and its water body is shallow;on the SE side, Jiangnan stratigraphic region is rich in argillaceous components and its water body is relatively deep;the middle part is Jiangnan transitional zone, the main part of it is the clinothem slope facies,and the gravity flow sediments are found in some areas. The origin of the complex sedimentation is related closely to Jiangnan fault zone with the synsedimentary movements.