Dolomitization mechanism for thick bedded dolostones of the Ordovician Majiagou Formation (Member 5) in the eastern Ordos Basin, China

Abstract

The genesis of thick bedded dolostones is considered closely related to the “Dolomite Problem”. This study investigates the mineralogical, sedimentological, and geochemical characteristics of thick bedded dolostones of the Ordovician Majiagou Formation (Member 5) in the eastern Ordos Basin, China. These dolostones consist of stacked meter to submeter scale thin bedded cycles, characterized by three key facies: cryptocrystalline dolostone with spherical gypsum molds, microcrystalline dolostone with nonplanar crystal boundaries and lath shaped gypsum pseudomorphs, and fine crystalline dolostone with planar boundaries lacking gypsum. The vertical distribution of these facies correlates with sea level cyclicity and geochemical variations in dolomite stoichiometry, indicating that dolomitization was controlled by high frequency sea level oscillations. The preservation of horizontal gypsum pseudomorphs, together with diagnostic Th/U ratios and bell-shaped REE patterns, supports a low energy, restricted cratonic setting. We apply a syndepositional model wherein evaporative concentration in a warm, shallow greenhouse sea led to calcite and gypsum precipitation, elevating the Mg/Ca ratio and triggering dolomitization. The resulting dense brine percolated downward, replacing unconsolidated calcareous precursors to form the observed dolostone types. This case demonstrates that high frequency sea level oscillations and their resultant thin bedded cycle stacking can constitute an efficient formation mechanism for thick bedded dolostones in cratonic basins.