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

Abstract

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 oncolites. 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 Ma56 and Ma55 sub-members. The microbial biostrome in the Ma55 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 Ma56 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 Ma57_Ma510 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.