蒸发背景沉积序列精细刻画及沉积学解译:以鄂尔多斯盆地中部中奥陶统马五_(6)亚段为例

Datailed characterization and interpretation of sedimentary sequences under evaporitic environments: a case from the Ma5_(6) submember of Middle Ordovician in central Ordos Basin

云膏共生组合是受化学沉积分异作用控制而形成的岩石组合,对其沉积序列精细刻画,有利于揭示诸多蒸发环境下的沉积学信息。依据鄂尔多斯盆地中部奥陶系马家沟组五段6(简称"马五6亚段")亚段钻井、岩心资料,开展了岩石宏观和微观观察、典型沉积序列刻画等工作。研究结果表明:(1)鄂尔多斯盆地中部马五6亚段普遍发育碳酸盐岩与蒸发岩共生组合,主要由10种岩石类型构成;(2)主要发育5种沉积序列,下云上膏的岩性组合和序列顶部多发育暴露面等特征表明单个序列具有向上变浅、变咸的演化特征,是蒸发台地叠合海水渐次补给作用的结果;(3)在局限—蒸发台地环境下,主要发育潟湖/滩间海、颗粒滩、灰泥丘、台坪4种沉积亚相及11种沉积微相,与经典的蒸发潮坪序列在岩性组合与沉积构造方面存在明显的差异;(4)马五6亚段沉积期发育2次海平面升降,沉积环境可分别对应于云(膏)质潟湖—灰泥丘/颗粒滩—膏质潟湖—蒸干膏质潟湖和灰泥丘/颗粒滩—膏质潟湖—蒸干膏质潟湖—台坪。该研究结果可为鄂尔多斯盆地马五6亚段沉积环境与海平面变化研究提供系统的岩石学证据。

Assemblages of co-occurred dolomite and gypsum are controlled by chemical sedimentary differentiation,and precise characterization of sedimentary sequences is conducive to revealing sedimentary information in evaporatic environments.Based on drilling and coring data of the Ma56 submember of the Ordovician Majiagou Formation in the central Ordos Basin,we studied characterization of typical sedimentary sequences based on macro-and microscopic observation of rocks.The study shows that:(1)paragenetic assemblage of carbonate and evaporite rocks is widely developed in the Ma56 submember of the Ordovician Majiagou Formation in the central Ordos Basin,including 10 subtypes of facies;(2)five kinds of sedimentary sequences were summarized,characterized by the lithological assemblage of the lower dolomite and upper gypsum facies and exposed surfaces at the top of each sequence,which shows shallowing and salinizing upward due to gradual replenishment of superposed seawater from evaporative platforms;(3)in the restricted evaporation platform environment,there are mainly four sedimentary subfacies and11 sedimentary microfacies:lagoon/inter-shoal marine,shoal,lime mud mound and platform flat.Furthermore,the sedimentary sequences in the study area and those in typical evaporative tidal flat are distinguished from each other in terms of lithologic association and sedimentary structures;(4)there were two cycles of sea-level rises and falls recorded in the Ma56 submember,and the sedimentary environments could be corresponding to dolomite lagoon,lime mud mound and carbonate shoal,gypsum lagoon,evaporite lagoon,and tidal flat.The above results can provide systematic petrological evidence for the study of sedimentary environments and sea-level change of the Ma56 submember in Ordos Basin.