Abstract:

The deep-water environment in lakes of the middle and late Paleogene in the Liaoxi sag, Bohai Bay Basin, formed a special group of paleontological fossils. In this study, we characterized the microfossil assemblages through investigating the paleontological drilling data in this study area. Five fossil assemblages of pollen algae and five mesomorphic fossil assemblages were established. In the western and northwestern Shahejie Formation, distinct near trapezoidal mesomorph fossils Candona sagmaformis, C. postabscissa, and membrane tube algae fossils Hystrichosphaeridium tubiferidium tubiferi and Membranilarnacia originally formed in deep-water were observed. Moreover, mesomorph fossils Tianjinia fulgida, Bohaicythere sp, Parabohainia elliptica, and others formed in deep-water were widely observed in Dongying Formation. These fossil signatures indicate that the range of sedimentary center in deep water expanded gradually from Shahejie Formation to Dongying Formation, and it connected with the sedimentary centers of the Liaozhong depression in the end. Meanwhile, comparison study of a large number of paleontological fossils and their rock lithology, we concluded that the Liaoxi sag and the low convex zone of Liaoxi sag had a large range of sedimentary breaks when deposited in the second section of Shahejie Formation. Also, the upper subsection of the third section of Shahejie Formation is widely developed. This study provides a strong evidence for the division and comparison of the Paleogene strata in this region, and meanwhile this study enriched the relevant studies on sedimentary environment.

Key words: Liaoxi sag, biological assemblage, stratigraphic division, sedimentary environment