Genesis of dolomite in the fifth member of Majiagou Formation in the middle Ordos Basin

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 δ~(13)C is around null,and the values of δ~(18)O 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 intertidalsubtidal zone of continental sea.