Control of paleogeographic pattern on sedimentary differentiation of evaporite-carbonate symbiotic system: A case study of the sixth sub-member of Ordovician Majiagou Formation M5 Member in central-eastern Ordos Basin, NW China

The relationship between paleogeographic pattern and sedimentary differentiation of evaporite-carbonate symbiotic system is examined based on logging, core and thin section data, by taking the sixth sub-member of fifth member of Ordovician Majiagou Formation (M56) in the central-eastern Ordos Basin as an example. (1) Seven sub-geomorphic units (Taolimiao west low, Taolimiao underwater high, Taolimiao east low, Hengshan high, East salt low, North slope and Southwest slope) developed in the study area. (2) The “three lows” from west to east developed dolomitic restricted lagoon, evaporite evaporative lagoon and salt evaporative lagoon sedimentary facies respectively, the “two highs” developed high-energy grain beach and microbial mound, and the north and south slopes developed dolomitic flats around land. (3) The paleogeographic pattern caused natural differentiation of replenishment seawater from the northwest Qilian sea, leading to the eccentric sedimentary differentiation of dolomite, evaporite and salt rock symbiotic system from west to east, which is different from the classic “bull's eye” and “tear drop” distribution patterns. (4) As the Middle Qilian block subducted and collided into the North China Plate, the far-end compression stress transferred, giving rise to the alternate highland and lowland in near north to south direction during the sedimentary period of M56 sub-member. (5) Taolimiao underwater high and Hengshan high developed favorable zones of microbial mounds and grain shoals in south to north strike in M56 sub-member, making them favorable exploration areas with great exploration potential in the future.