Characteristics and implications of carbon and oxygen isotopes in dolomites of the Ordovician Zhuozishan Formation in northern Tianhuan Depression, Ordos Basin

The origin and geological significance of dolomites in the Ordovician Zhuozishan Formation in the northern Tianhuan Depression of the western Ordos Basin were interpreted based on spatial distribution and the characteristics of carbon and oxygen isotopes of different types of dolomites. There are three main types of dolomites in the study area, including powder-microcrystalline, residual sand-fine crystalline and fine-medium crystalline ones. The δ¹³C values of powder-microcrystalline, residual sand-fine crystalline and fine-medium crystalline dolomites are 0.78‰, 0.53‰, and 0.47‰, while the δ¹⁸O values are -5.0‰, -6.5‰ and -6.3‰, respectively. These values belong to normal marine carbonates, indicating that the dolomite fluid might come from ancient seawater. Carbon and oxygen isotope evolution and diagenetic effects showed that the powder-microcrystalline dolomites were formed during the quasi-synchronic stage, and the residual sand-fine crystalline and fine-medium crystalline dolomites were formed during the shallow burial stage.