Sedimentary response and restoration of paleoshoreline of Taiyuan-Shanxi Formations in North China basin

The Permian Taiyuan-Shanxi Formations in the North China Basin are a series of terrestrial-marine strata whose depositional evolution exhibits unique particularities. Current studies involving the paleoenvironment of the Taiyuan and Shanxi Formations are mainly supported by sedimentary facies or geochemical analysis, yet studies on the sedimentary and geochemical responses of the paleoshoreline remain inadequate, which leads to the inconsistency of the concluded paleoshoreline distribution. Based on outcrop data, well data and major and trace element analysis, sedimentary and geochemical features of the paleoshoreline in both formations are identified. Combining sedimentary facies and geochemical analysis, this study aims to restore the distribution of the paleoshoreline in the Taiyuan and Shanxi Formations as well as its migrations. The outcrop and well data show that the paleoshoreline of the Taiyuan and Shanxi Formations features 3 types of sedimentary responses: (1) distributary channel deposits of the delta plain + argillaceous deposits of the delta front; (2) peat mire of the delta plain + argillaceous deposits of the delta front; and (3) distributary channel deposits of the delta plain + tidal flat-lagoon deposits. Based on major and trace element results, Sr/Ba, m = 100 × (MgO/Al2O3), Mn/Fe, f = (Rb/K)/(Zr/Al), CIA, Sr/Cu, V/(V + Ni), and V/Cr are selected as proxies for paleosalinity, paleowater depth, paleoclimate and paleo-redox conditions. The results show that transitional-brackish water was dominant during the transgression period of the Taiyuan Formation with mild changes in paleowater depth, while fresh water primarily covered the North China Basin during regression during the Shanxi Period, which was a time when paleo-water depth exhibited evident fluctuation in the northern region. Generally, the paleoclimate during the Taiyuan-Shanxi Period was warm and moist to hot and humid, and the paleo-redox conditions were anoxic to reductive. Under the influence of transgression during the Taiyuan Period, the boundary of the freshwater zone closely matches the maximum range of marine deposition, indicating a generally steady transgression background; additionally, the paleoshoreline was distributed in the vicinity of Yulin - Shijiazhuang - Baoding. During the regression event of the Shanxi Period, the paleoshoreline migrated southeast to the south of ZK03-2 and southeast of Yu88, extending southward along Sanmenxia - Luoyang - Zhengzhou - Yuzhou, and the freshwater zone extended southward and covered a considerable range of marine deposition, indicating a strong flux of terrestrial freshwater with periodic transgression. The analysis and conclusions in this paper would hopefully provide insights and a reference for the paleoenvironmental evolution and shoreline changes in the Taiyuan - Shanxi Formations of the North China Basin.