鄂尔多斯盆地中奥陶统马五6亚段高频旋回层序研究:论海平面升降与横山凸起沉积分异效应*

高频旋回分析有利于揭示浅水碳酸盐岩台地海平面升降、沉积物响应、古地理单元演化及对盆地内沉积分异影响等诸多沉积学信息。依据鄂尔多斯盆地横山地区丰富的钻井取心资料,通过岩石类型、典型五级旋回结构等分析,对中奥陶统马家沟组马五₆亚段开展了高频旋回研究。研究表明: (1)基于研究区内马五₆亚段碳酸盐岩与蒸发岩共生组合的9种岩石类型,识别出5种典型T-R单旋回,其中旋回Ⅰ、Ⅱ、Ⅲ为云质潟湖—颗粒滩—灰泥丘沉积,Ⅳ、Ⅴ为膏盐质潟湖沉积,并以5种单旋回为基本单元搭建骨架井五级高频旋回结构剖面;(2)依据五级高频旋回结构剖面建立的Fischer图解,认为马五₆亚段处于三级旋回晚期,内部发育“2个”四级旋回,具有初次海退—快速海侵—再次海退的相对海平面变化趋势;(3)结合五级旋回厚度统计,认为第1套四级旋回沉积期处于隆拗格局发育阶段,该时期横山凸起作为水下高地,对东西两侧海水封隔明显;至第2套四级旋回沉积阶段,填平补齐作用导致横山凸起影响减弱,海水循环得到改善,凸起东西两侧盐度明显下降。以马五₆亚段为例的高频旋回研究,在横山地区马五₆亚段地层学、沉积学研究中取得了较好的效果,以期在鄂尔多斯盆地马家沟组未来研究中得到进一步推广和实践。

High-frequency cyclic sequence of the Ma56 submember of Middle Ordovician Majiagou Formation in Ordos Basin:indication for sea-level change and sedimentary differentiation effect of Hengshan uplift

Studies in high-frequency cyclic sequence is helpful to reveal much sedimentary information,such as sea-level changes,deposition response and palaeogeographic evolution during depositional stage of shallow carbonate platform,and their impacts on the sedimentary differentiation in the basin. Based on the rich drilling core data,the lithology characteristics and typical fifth-order cyclic sequence of the Ma5₆ submember of Middle Ordovician Majiagou Formation are studied. The results indicate that(1)5 types(Ⅰ~Ⅴ)of typical T-R single cycle have been identified based on the 9 rock types of symbiotic combination of carbonates and evaporites. The cycles of Ⅰ,Ⅱ and Ⅲ are characterized by dolomitic sediments of lagoon-shoal-mud mound,while cycles of Ⅳ and Ⅴ are characterized as gypsum-salt sediments of lagoon. The fifth-order high-frequency cyclic structural sections have been established according to the 5 types of single cycle;(2)Fischer plot established according to the fifth-order high-frequency cyclic sections indicates that the Ma5₆ submember developed in the late stage of the third-order cycle,which comprises 2 fourth-order cycles representing the trend of initial regression-rapid transgression-another regression;(3)the thickness statistics of fifth-order cycles shows that the development of uplift and depression corresponds to the period of the first fourth-order cycle. The Hengshan uplift was the depositional highland,which isolated the seawater between the west and east sides. During the second fourth-order cycle period,filling and leveling up weakened the impact of Hengshan uplift,promoting the seawater circulation and the salinity at the east and west sides of the uplift decreased. The study on the high-frequency cycles of the Ma5₆ submember has been well applied in the analysis of stratigraphy and sedimentology of the Ma5₆ submember in the Hengshan area,which can be further applied in the Majiagou Formation in the Ordos Basin.