Sedimentary environment and evolution of Late Carboniferan-Middle Permian in southeastern Ordos Basin

By conducting analysis and testing of trace elements in mudstone samples from the Upper Paleozoic strata in the southeastern Ordos Basin, and integrating various methods such as sedimentary geological characteristics and geophysical logging, this study qualitatively, semi-quantitatively, and quantitatively analyzes the sedimentary water body environment and paleoclimate features of the Late Carboniferous Benxi Formation to the Middle Permian Xiashihezi Formation He 8 Member. Furthermore, it explores the evolutionary characteristics of the sedimentary environment based on these findings. The results indicate that the Upper Paleozoic strata are primarily characterized by sedimentary structures reflecting traction flow conditions. During sandstone deposition periods, water dynamics were notably strong. The Sr/Ba value suggests that the study area is situated within a mixed water-freshwater environment. Qualitative and semi-quantitative assessments based on rock composition, typical self-formed minerals, the values of V/Cr, Ni/Co, V/(V+Ni), δU, as well as natural gamma ray spectral logging data collectively indicate that an oxygen-rich shallow water environment predominates in the study area. Furthermore, quantitative judgment using La-Co law indicates that paleo-water depths in this region generally did not exceed 25 meters but exhibited frequent fluctuations. The Sr/Cu value points to a warm and humid ancient climate prevailing in this area. From Benxi Formation to Xiashihezi Formation He 8 Member, the salinity of water bodies gradually decreased while their depth steadily diminished. Additionally, the paleoclimate transitioned from humid to dry, revealing a sedimentary evolution process from nearshore basin to inland basin for Upper Paleozoic strata in southeastern Ordos Basin.