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The carbonates in the Middle Ordovician Ma submember of the Majiagou Formation in the northern Ordos Basin are partially to completely dolomitized. Two types of replacive dolomite are distinguished: (1) type 1 dolomite, which is primarily characterized by microcrystalline (<30 µm), euhedral to subhedral dolomite crystals, and is generally laminated and associated with gypsum-bearing microcrystalline dolomite, and (2) type 2 dolomite, which is composed primarily of finely crystalline (30-100 μm), regular crystal plane, euhedral to subhedral dolomite. The type 2 dolomite crystals are truncated by stylolites, indicating that the type 2 dolomite most likely predated or developed simultaneously with the formation of the stylolites. Stratigraphic, petrographic, and geochemical data indicate that the type 1 dolomite formed from near-surface, low-temperature, and slightly evaporated seawater and that the dolomitizing fluids may have been driven by density differences and elevation-related hydraulic head. The absence of massive depositional evaporites in the dolomitized intervals indicates that dolomitization was driven by the reflux of slightly evaporated seawater. The δO values (−7.5 to −6.1 ‰) of type 1 dolomite are slightly lower than those of seawater-derived dolomite, suggesting that the dolomite may be related to the recrystallization of dolomite at higher temperatures during burial. The type 2 dolomite has lower δO values (−8.5 to −6.7 ‰) and Sr concentration and slightly higher Na, Fe, and Mn concentrations and Sr/Sr ratios (0.709188-0.709485) than type 1 dolomite, suggesting that the type 2 dolomite precipitated from modified seawater and dolomitic fluids in pore water and that it developed at slightly higher temperatures as a result of shallow burial. [ABSTRACT FROM AUTHOR]