塔里木盆地上泥盆统—下石炭统滨岸-混积陆棚三级层序发育特征

Development characteristics of the third-order sequence of Upper Devonian–Lower Carboniferous shore-mixed shelf in Tarim Basin, NW China

基于塔里木盆地晚泥盆世—二叠纪形成的原型沉积盆地地层沉积相研究结果,按照层序地层学理论对原型沉积盆地进行了五级层序划分。研究认为原型沉积盆地下部上泥盆统—下石炭统滨岸-混积陆棚是典型的三级层序并建立了层序发育模型;研究发现三级层序实际是海侵体系域-高位体系域旋回组合层序,Ⅰ型层序(无低位体系域)是三级层序的典型发育模式,识别标志是从海向陆存在巨大的泥岩+灰岩楔形体。五级准层序是基本层序,为单旋回层序;混积陆棚区较易识别的薄层灰岩等时沉积体是最大或次级海泛面的等时沉积物;混积陆棚区属于内源和外源沉积物均衡沉积剖面,没有凝缩段,外源碎屑沉积物在海域内倾斜大前积沉积现象在混积陆棚区减弱或消失;三级层序底部古地貌为阶地-坡折带,这种古地貌特征既是造成东河砂岩段穿时的主要原因,也是造成东河砂岩段与其上含砾砂岩段构成海侵-高位多个四级旋回层序的主要原因。

Based on the study of stratigraphic sedimentary facies of the prototype sedimentary basin formed in the Late Devonian to Permian Period in the Tarim Basin,the prototype sedimentary basin are divided into five order sequences according to the theory of sequence stratigraphy.It is considered that the Upper Devonian-Lower Carboniferous shore-mixing shelf in the lower part of the prototype sedimentary basin is a typical third-order sequence and a sequence development model is established.The third-order sequence is actually a transgression systems tract and highstand systems tract cycle assemblage,and the type I sequence (no lowstand systems tract) is a typical developmental mode of the third order sequence,and the identification mark is the existence of huge mudstone and limestone wedges from sea to land.The fifth-order parasequence is the basic sequence,which is a single cycle sequence.The thin layer limestone isochronous sediments,which are easy to be identified in the mixed shelf areas,are the isochronous sediments of the largest or secondary marine-flooding surface.The mixed shelf area belongs to the equilibrium sedimentary profile of internal and external sediments,and there is no condensed section.The inclined deposition phenomenon of extraneous source sediments in the sea area weakens or disappears in the mixed shelf area.The paleogeomorphology at the bottom of the third-order sequence is a terrace-slope break zone.This paleogeomorphological feature is not only the main reason for the sedimentary diachronism of Donghe sandstone,but also the main reason for the formation of the transgression-highstand fourth-order cyclic sequences of the Donghe sandstone section and its upper gravel-bearing sandstone section.