中-上扬子地区寒武系层序地层格架

Cambrian sequence stratigraphic framework in the Middle-Upper Yangtze area

以典型露头剖面实测分析为基础,结合岩石薄片、钻测井、全岩碳氧同位素和地震剖面的综合分析,将中—上扬子地区寒武系海相地层划分为6个区域上可对比的三级层序,建立了年代地层、岩石地层与层序地层之间的关系。层序CSQ1相当于下寒武统下部,由牛蹄塘组(或筇竹寺组)构成;层序CSQ2相当于下寒武统中部,由杷榔组(或沧浪铺组)构成;层序CSQ3相当于下寒武统上部,由清虚洞组(或龙王庙组)构成;层序CSQ4相当于中寒武统下部高台组;层序CSQ5相当于中寒武统上部—上寒武统下部,由孔王溪组和耗子沱群(或洗象池群)下部构成;层序CSQ6相当于上寒武统中—上部,由耗子沱群(或洗象池群)中—上部构成,6个三级层序纵向上叠置构成1个完整的寒武系二级层序。识别出2种层序界面类型,即相对海平面下降形成的Ⅱ型层序界面和暴露有关的Ⅰ型层序界面,区域上以Ⅱ型层序界面发育为主,仅层序CSQ1的底界面属典型的Ⅰ型层序界面。通过对比反映全球海平面相对变化的碳酸盐岩全岩碳同位素旋回和反映沉积古水深相对变化的沉积旋回,分析揭示,在古气候和沉积充填速率影响不大的背景下,中—上扬子地区寒武系三级层序CSQ1—层序CSQ4和层序CSQ6属于全球海平面层序,其可容空间的增加主要取决于全球海平面的上升,而层序CSQ5属于主要受控于扬子区西缘构造活动影响的构造层序。层序CSQ1发育于震旦纪末期桐湾运动灯影组剥蚀夷平面之上的碎屑陆棚环境,层序CSQ2—层序CSQ3演变为混积陆棚—碳酸盐岩缓坡环境;中—晚寒武世,受中—上扬子区碳酸盐岩台地边缘加积-进积作用的逐渐加强以及全球海平面上升的影响,陆源物质输入减少,层序CSQ4—层序CSQ6演变为典型的局限镶边台地环境。

Based on analyzing the measured data of typical outcrop sections,in combination with the comprehensive analysis of thin sections of rocks,drilling and logging,whole-rock carbon and oxygen isotopes,and seismic profiles,this paper divides the Cambrian marine strata in the Middle-Upper Yangtze region into six third-order sequences for comparison and establishes the relationship between chronostratigraphy,lithostratigraphy and sequence stratigraphy.The sequence CSQ1 is equivalent to the lower part of the Lower Cambrian and composed of Niutitang Formation(or Qiongzhusi Formation);CSQ2 is equivalent to the middle part of the Lower Cambrian and composed of Palang Formation(or Canglangpu Formation);CSQ3is equivalent to the upper part of the Lower Cambrian and composed of Qingxudong Formation(or Longwangmiao Formation);CSQ4 is equivalent to the Gaotai Formation of the lower part of Middle Cambrian;CSQ5is equivalent to the upper part of Middle Cambrian and lower part of Upper Cambrian and composed of the Kongwangxi Formation and the lower part of Haozituo Group(or Xixiangchi Group);CSQ6 is equivalent to the middle-upper part of the Upper Cambrian and composed of the middle-upper part of Haozituo Group(or Xixiangchi Group).The six third-order sequences are stacked vertically to form a complete Cambrian second-order sequence.Two types of sequence boundary have been identified,namely the type Ⅱ sequence boundary formed by the relative fall of sea level and the type Ⅰ sequence boundary correlated with the exposure.The type Ⅱ sequence boundary is mainly developed in the region,and only the bottom boundary of the sequence CSQ1 is a typical type Ⅰ sequence boundary.By comparing the whole-rock carbon isotope cycles of carbonate rocks reflecting the relative changes of global sea level and the sedimentary cycles reflecting the relative changes of sedimentary ancient water depth,the analysis reveals that under the background that the paleoclimate and sedimentary filling rate have little influence,the Cambrian third-order sequences CSQ1-CSQ4 and CSQ6in the middle-upper Yangtze region belong to the global sea level sequence,of which the increase in accommodation space mainly depends on the rise of global sea level,while the sequence CSQ5is a tectonic sequence mainly controlled by tectonic activities in the western margin of the Yangtze region.The sequence CSQ1 developed in the clastic shelf environment above the denuded planation surface of Dengying Formation during the Tongwan Movement at the end of the Sinian,and the sequence CSQ2-CSQ3 evolved into a mixed shelf-carbonate gentle slope environment.During the middle-late Cambrian,influenced by the gradual strengthening of the aggradation and progradation of the carbonate platform margins in the middle-upper Yangtze region and also the rise of global sea level,the input of terrigenous materials decreased,and the sequence CSQ4-sequence CSQ6 evolved into a typical enclosed rimmed platform environment.