Under the promotion of exploration and development for shale oil and gas, marine shale has become a hotspot of fine-grained sedimentary studies in China. Paleo-environment reconstruction has always been an important aim, especially for black shale. Based on a large number of samples collected from Wufeng and Longmaxi Formations of two field sections in western Hubei and eastern Chongqing, this study compares the vertical variations in total organic carbon content, quartz/clay mineral value, and trace element compositions of these two profiles to reflect the paleo-environment changes from the latest Ordovician to the earliest Silurian. The results show that during the later Ordovician period, when Wufeng Formation deposited, the study area experienced a water depth change from relative deep facies to shallow marine, and paleo-redox changed from oxygen rich to anoxic, and even euxinic, followed by the content of oxygen increase. When the Guanyinqiao Bed deposited, the basin suddenly became shallower, and the deposition environment changed to be oxygen enriched for high water energy, circulating oxygen enriched. In the earliest stage Silurian, when the Longmaxi Formation formed, and the paleo-water also suffered several changes of sea level, and the paleo-environment also changed from anoxic to oxic. These changes can correspond to the third-forth order sequences well.

The Permian–Triassic mass extinction has been considered the largest bio-crisis of the Phanerozoic, with more than 90% of marine species extinct. Previous studies showed that ostracods suffered various extinction patterns in different localities and were relatively enriched in the lowermost Triassic shallow marine microbialites. Multiple hypotheses have been put forward to explore the reasons for this phenomenon. Abundant ostracod fossils were collected from the microbialite-bearing Youping section in the Nanpanjiang Basin. 45 species in 22 genera from Wujiaping Formation increased dramatically to 104 species in 33 genera from the microbialites of basal Luolou Formation. However, Ostracods from the Youping section suffered severe extinction during the second phase of the Permian–Triassic crisis, i.e., the earliest Triassic mass extinction (ETME), rather than the first phase, i.e., the latest Permian mass extinction (LPEM). In addition, the Sørenson coefficient has been used to examine the similarity of faunal associations among different sections of the Permian–Triassic transitional beds. There was no significant differences for ostracods between microbialites and non-microbialites sections based on similarity analysis. Thus, we proposed that “Shallow marine refuge” hypothesis could explain the high diversity and high abundance of ostracods of the Permian–Triassic transitional beds. Besides, ostracods showed remarkable geographical differentiation at both regional and global scales during the Permian–Triassic transitional beds and were presumably controlled by geographical isolation.

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