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Sedimentary Model of Coal and Shale in the Paleogene Lijiaya Formation of the Huangxian Basin: Insight from Petrological and Geochemical Characteristics of Coal and Shale

Paleogene Huangxian Basin which lies in Eastern China contains deposits of many types of coal and shale associations. The sedimentary models of coal and shale associations are very important to explore the coal and shales. However, whether the transgression affected the lithofacies and geochemical characteristics of coal and shale in the Paleogene Huangxian Basin in eastern China has not been reported in detail, which is important to construct the sedimentary models. In this study, petrological and geochemical methods are used to comprehensively analyze the mineral composition, contents of major and trace elements, and distribution characteristics of geochemical parameters of coal and shale in Huangxian Basin. The results show that coal and shale have different macerals. The study of typical geochemical parameters (Mn/Ti, K₂O/Na₂O, Sr/Ba, and Th/U) shows that the no. 1 coal seam (CB1) and no. 4 coal seam (CB4) are least affected by seawater, while the no. 1-3 coal seam (CB1-3) is affected by seawater to some extent. Shale 1-3 (S1-3), shale 2 (S2), and shale 4 (S4) are affected by seawater, while shale 1 (S1) is little affected by seawater. In order to discuss the sedimentary model of coal and shale, the tectonic background, provenance, paleoclimate, and fault movement of Huangxian basin are analyzed. The results show that the basin is located at the continental island arc and the active continental margin. Transgression and tectonic movement affect the input of terrigenous debris. The paleoclimate during coal formation was warmer and wetter, while shale was cooler and drier. During basin subsidence, CB4–S4 and CB1–S2–S1 combinations were basically formed under the influence of transgression driven by paleoclimate change. The combination of S1-3 and CB1-3 was formed in a stable freshwater lacustrine environment, followed by transgression.