Multi-phase paleokarst reconstruction over hundreds of millions of years: A case study of the Majiagou Formation, Ordos Basin

Karstification over hundreds of millions of years has profoundly influenced the evolution of carbonate reservoir globally. However, the timing of karst stages and their impact on pore preservation remain poorly constrained. The Ordovician Majiagou Formation in the Ordos Basin, China, which was subjected to subaerial weathering for approximately 150 million years during the Late Caledonian-Hercynian orogenies, serves as an ideal analog for investigating long-lived karst systems. The upper submembers (Ma5₁-Ma5₇) of the Majiagou Formation were analyzed using core examination, major and trace elements measurements, δ¹³C and δ¹⁸O analysis, and calcite U-Pb dating to clarify the impact of surface exposure on the original deposition and the age of calcite cementation in diagenetic phase. The results reveal cyclic “collapse-accommodation” breccia patterns in the upper Ma5 submembers (Ma5₁-Ma5₄), where alternating high/low-mud breccias destroyed early dissolution pores, supported by geochemical indicators like elevated Mn/Sr ratios (>2) and strong terrigenous element (Al/Th/Zr)-REE correlations (r² > 0.95) reflecting intense meteoric water input. Tight limestones in Ma5₅ acted as a barrier, evidenced by lower Mn/Sr ratios (0.86-1.30) and weaker correlations in deeper submembers (Ma5₅-Ma5₇). U-Pb dating of calcite cements (252.7 ± 6.3 Ma to 308.6 ± 3.9 Ma) constrains post-uplift cementation to the Late Carboniferous-Permian, lasting > 44 million years with horizontal fluid flow. These findings demonstrate that protracted karstification, dominated by collapse cycles and terrigenous inputs, is detrimental to pore preservation. Critically, U-Pb dating provides the essential absolute chronological framework that deciphers the specific sequence and spatio-temporal linkages of these complex, multi-phase events.