Geochemical Characteristics of Lower Cretaceous Chengzihe Formation Mudstones in the Jixi Basin, China: Implications for Provenance, Paleoenvironment and Tectonic Setting

Abstract

The exploration of coal measure gas is currently in its early stage, and it is necessary to study the paleosedimentary background of coal-bearing mudstone/shale to provide a foundation for research such as coal measure gas reservoir formation. In this work, based on the total organic carbon and major and trace elements, the provenance, paleoenvironment, and tectonic setting are further revealed to provide the necessary theoretical basis for the subsequent exploration of coal measure gas in the Jixi Basin, China. The diagrams of major elements (TiO₂ vs Al₂O₃ and TiO₂ vs Zr), trace elements and REE (Th/Sc vs Zr/Sc and La/Th vs Hf), and the distribution of igneous rock in the source area (the Lesser Xing’an Range-Zhangguangcai Range) suggest that the provenances of the Chengzihe Formation mudstones are dominated by the Early Jurassic adamellites (after 187 Ma). Furthermore, the paleosalinity proxies (Sr/Ba, N-value) and paleoredox proxy (C_org/P) indicate that the Chengzihe Formation mudstones were deposited in an oxic-anoxic brackish bottom water which has low-energy, moderate productivity reflected by low Zr/Rb and medium Ba_bio. In addition, the chemical index of alteration values and the Al₂O₃–CaO* + Na₂O–K₂O (A-CN-K) ternary diagram indicate that the source rocks have experienced low-moderate chemical weathering. For the tectonic setting, the binary diagrams (K₂O/Na₂O-SiO₂ and SiO₂/Al₂O₃–K₂O/Na₂O) and the ternary diagrams (La–Th–Sc, Th–Co–Zr/10, and Th–Sc–Zr/10) demonstrate that the tectonic setting of the source area is mainly an active continental margin, followed by a continental island arc. The results of this study can provide the theoretical basis for future geochemical characterization of terrestrial mudstones/shales and exploration of terrestrial coal measure gas.