Features of Karst Paleogeomorphology and Sedimentary Environment of Ordovician Majiagou Formation in Eastern Ordos Basin

The formation and evolution of carbonate weathering crust-type reservoirs are closely linked to paleokarst processes. This study aims to restore and analyze the karst paleogeomorphology and sedimentary environment of the Ma2 and Ma3 sections in the eastern Ordos Basin, using the compensation thickness impression method. The Ma3 section is characterized by widespread distribution and considerable thickness, with clear inheritance among the sublayers and minimal variation in thickness across the section. In contrast, the Ma2 section exhibits a similar thickness for each sublayer, with a notable ring-like pattern of increasing thickness toward the center of the basin. The Ma3 section primarily consists of evaporite platform facies under a regression system, with intraplatform lagoons and evaporite tidal flats being the dominant subfacies. In contrast, the Ma2 section is mainly composed of open platform facies under a marine transgression system, with the primary development of intraplatform depressions and tidal flat subfacies. The top of the Ma4 formation is selected as the regional isochronous surface for the restoration of paleogeomorphology. The paleo geomorphological framework of the Ma2 and Ma3 sections is located within the first-level geomorphological unit of the carbonate platform. Within this carbonate platform, 3 s-level geomorphological units have developed: the interior platform area, the eroded paleo-land area, and the platform margin area. These units represent key components of the region's paleo geomorphological structure and provide a comprehensive understanding of the sedimentary evolution and karst development in the Eastern Ordos Basin.