Abstract: The characteristics of lithology, mineralogy and sedimentary geochemistry of dolomite in the fifth member of Majiagou Formation in the middle Ordos Basin were analyzed. The genesis of dolomite in this area was discussed, and the formation mode of dolomite in this area was established. The deposition environment of the fifth member of Majiagou Formation is the intertidal-subtidal zone of continental sea, and the shape of dolomite stone shows lentoid with cross-bed and climbing properties. The trace element analysis indicates that the contents of manganese, sodium and strontium are low. Meanwhile, the isotope of δ¹³C is around null, and the values of δ¹⁸O display negative distribution. All these evidences indicate that the dolomite is resulted from dolomitization due to mixture of sea water with fresh water from the penesyngenetic period to the early diagenetic stage in the environment of intertidal-subtidal zone of continental sea.

Key words: Ordos Basin, Majiagou Formation, dolomite genesis, geochemistry characteristics, dolomitization mode

摘要： 对鄂尔多斯盆地中部气田马五段白云岩的岩石学、矿物学和沉积地球化学特征进行了研究,讨论了白云岩成因,并建立了本地区白云岩的形成模式。区内马五期沉积环境为陆表海潮下—潮间坪碳酸盐沉积,白云岩产状呈透镜体状,具有穿层、爬坡特征,岩性以微晶、粉晶白云岩为主;微量元素表现为低锰、低锶、低钠;碳、氧同位素特征为:δ13C值在零值附近,而δ18O值偏负。该地区的白云岩成因为潮间—潮下环境下的准同生期—成岩早期海水—淡水混合白云岩化作用形成的结果。

关键词: 鄂尔多斯盆地, 马家沟组, 白云岩成因, 地球化学特征, 白云岩化作用模式