鄂尔多斯盆地奥陶系马家沟组层序地层格架研究——兼论陆表海沉积作用和早期成岩作用对相对海平面变化的响应

在论述马家沟组三级层序地层成因的基础上,重新厘定了鄂尔多斯盆地马家沟组层序地层格架。马家沟组马一段至马五段划分为4个三级层序,除了Sq1为Ⅰ型层序之外,Sq2、Sq3、Sq4都是Ⅱ型层序。在陆架内Sq1和Sq4由TST和HST构成;Sq2、Sq3由TST、HST和ISLST构成。TST以正常盐度浅海中形成的生物灰岩和泥晶灰岩为特征,HST以准同生期近地表的略为咸化海水成因的泥晶—细粉晶白云岩为特征,陆架内低水位体系域(ISLST)由陆架内盐湖形成的巨厚的蒸发岩系构成。ISLST和HST在空间分布上和成因上是密切相关的,ISLST置于Ⅱ型层序的顶部或者近顶部比较合理。传统的层序地层学将层序界面置于高水位体系域与低水位体系域之间,在此建议将层序界面置于低水位体系域与海侵体系域之间的海侵面。编制了层序3的海侵体系域、高水位体系域、低水位体系域全盆地层序古地理略图。

SEQUENCE STRATIGRAPHY OF THE MAJIAGOU FORMATION,ORDOS BASIN:SEDIMENTATION AND EARLY DIAGENESIS RELATED TO EUSTATIC SEA-LEVEL CHANGES

On the basis of third-order sequence stratigraphic study,this paper reestablishes a sequence stratigrphy framework of the Ordovician Majiagou Formation.The five members of the Majiagou Formation(Ma1—Ma5)can be divided into four third-order sequences.The Sq1 sequence is a typeⅠ sequence,and the other three sequences(Sq2,Sq3,Sq4)are all typeⅡ sequence.On the continental shelf,Sq1 and Sq4 each consists of a TST and a HST,whereas Sq2 and Sq3 each consists of TST,HST and ISLAT.The transgressive systems tract(TST)is characterized by bioclastic limestone and micritic limestone deposited in epeiric sea normal salinity.The highstand systems tract(HST)is characterized by micritic or microspraritic dolostone that were formed penecontemporaneously in subbrackish sea water and in sub-surface environments.The inner shelf lowstand systems tract(ISLST)is characterized by thick evaporates that were deposited in saline environments in the continental shelf.ISLST and HST are closely associated indicating that they are genetically related.It is feasible to place ISLST at the top or near the top of typeⅡ sequences.Traditionally,sequence boundaries are placed at the surface between the highstand systems tract and lowstand systems tract.We suggest that sequence boundaries can be placed at the transgressive surface between lowstand systems tract and transgressive systems tract.We identifed the TST,HST and ISLST of Sq3 and compiled an Ordovician paleogeographic map of the Ordos Basin.