Diagenetic Facies and Its Geochemical Characteristics of Dolomite: A case study of Ma5₅-Ma5₁ Sub-Members of Majiagou Formation in central-eastern Ordos Basin

Key words: 
• central-eastern Ordos Basin /
• Ma5₅-Ma5₁ sub-members /
• dolomite /
• diagenetic facies /
• geochemical characteristics

Abstract: It is important to study on the diagenetic facies of dolomite for the restoration of its diagenetic environment. The cores and thin sections as well as the whole rock major and trace element geochemistry, O, C and Sr isotopic compositions were analyzed to study the diagenetic facies types and lithofacies of the dolomite as well as the geochemistry of different diagenetic facies of Ma5₅-Ma5₁ sub-members of Majiagou Formation in the central-eastern Ordos Basin, respectively. The results show that the diagenetic facies of dolomite of Ma5₅-Ma5₁ sub-members in central-eastern Ordos Basin mainly include the gypsum and salt dissolution, active reflow seepage dolomitization and implicit reflux seepage dolomitization diagenetic facies in a near-surface diagenetic tract, and weathering karstific breccia diagenetic facies in an epidiagenetic tract. The former dolomite samples display lower δ¹⁸O and δ¹³C values than those of the Ordovician seawater, higher Mn and lower Fe contents, medium-strong cathodoluminescence, higher ⁸⁷Sr/⁸⁶Sr ratios than the coeval seawater as well as their high Al and Ti contents may be caused by the effects of fresh water or mixed water, while the latter dolomite samples may be affected by the epigenetic coeval seawater shows relatively high Mn and significant high Fe contents, weak or non-cathodoluminescence, obvious higher ⁸⁷Sr/⁸⁶Sr values than those of the coeval seawater, significant high Al and Ti contents. The results implicate that the δ¹⁸O, δ¹³C, ⁸⁷Sr/⁸⁶Sr, major and trace elements are the effective indicators for identifying the diagenetic facies of dolomite.