Distribution and depositional model of microbial carbonates in the Ordovician middle assemblage, Ordos Basin, NW China

Based on outcrop profiles, drilling cores, cast thin sections etc., the types, microfacies combinations and distribution pattern of microbial carbonates in the Ordovician middle assemblage of the mid-eastern Ordos Basin have been systematically analyzed. The middle assemblage of Ordovician in the mid-eastern Ordos Basin has microbial carbonates formed by the calcification of cyanobacteria, including microbial biostromes and microbial mounds made of stromatolites, thrombolites, and on-colites. The distribution of the carbonates shows obvious stratum-control and regional characteristics. The microbial biostromes 2-3 m thick each are controlled by sequence cycles and sedimentary facies changes, and were mainly formed in the tidal flat environment during the depositional stages of the Ma5(6) and Ma5(5) sub-members. The microbial biostrome in the Ma5(5) sub-member occurring near the carbonate-evaporite transition interface in the early stage of the transgression is distributed mainly in the Mizhi subsag in the eastern part of the basin; the microbial biostrome in the Ma5(6) sub-member turns up near the carbonate-evoporite transition zone in ring shape in the east of the central uplift. The ancient landform had noticeable control on the distribution of microbial mounds. The microbial mounds or mound-shoal complexes developing mainly during the depositional stages of Ma5(7)_Ma5(10) sub-members are about 15-25 m thick in single layer and distributed largely in the Wushenqi-Jingbian paleouplift. The development model of the microbial carbonate rocks shows that the carbonate-evaporite lithologic transition zone and the Wushenqi-Jingbian paleouplift are favorable exploration zones of microbial carbonates in the Ordovician middle assemblages.

Automated summary

Analyzing outcrop profiles, drilling cores, cast thin sections, etc., this study systematically analyzed the types, microfacies combinations, and distribution patterns of microbial carbonates in the middle assemblage of the mid-eastern Ordos Basin during the Ordovician period. The distribution of microbial carbonates is influenced by sequence cycles, sedimentary facies changes, and ancient landforms, and they are mainly found in the tidal flat environment during specific depositional stages in different sub-members of the Ordovician middle assemblage.