Paleoenvironment of marine-continental transitional shales in the lower Permian Shanxi formation, southeastern Ordos Basin, China

Abstract

The paleoenvironment of shales can be reconstructed to some extent using the combinations or concentrations of elements that correlate strongly with environmental conditions. In this study, we analyzed rare earth elements (REEs), major elements, and trace elements in the marine-continental transitional shales (transitional shales for short) of the Shan 23 submember of the Shanxi Formation in the southeastern Ordos Basin. The purpose is to deduce the paleoenvironmental conditions of the shales, encompassing paleoredox, paleoclimate, paleoproductivity, and paleo-provenance. The Shan 23 submember comprises four sections, namely Shan 23-1, Shan 23-2, Shan 23-3, and Shan 23-4. The Ba/Al, P/Al, and Cu/Al ratios, along with biogenic barium (Babio), indicate that the paleoproductivity of the submember peaked during the Shan 23-1 deposition and exhibited a downtrend upward in other sections. Trends in the Uau and the Ni/Co, V/Cr, U/Th, and V/Sc ratios suggest that suboxic conditions prevailed during the Shan 23-1 deposition, with the oxidation level gradually increasing from Shan 23-1 to Shan 23-4. C-value and the Sr/Cu vs. Ga/Rb cross-plot indicate a warm and arid paleoclimate during the Shan 23-1 deposition, which transitioned to cooler, drier conditions during the deposition of other sections. Indicators sensitive to paleoclimate, such as the K/Rb and Th/U ratios, along with the ICV, PIA, and Chemical Index of Alteration (CIA), highlight elevated weathering from Shan 23-2 to Shan 23-4, with Shan 23-1 exhibiting the weakest weathering during its deposition. As suggested by the REE data, the Zr/Sc vs. Th/Sr cross-plot, provenance discriminant functions, and the cross-plots of Hf vs. La/Th, Th vs. Hf–Co, and ∑REE vs. La/Yb, the sedimentary provenance for the transitional shales of the Shan 23 submember is of multiple origins, with significant contributions from the Upper Continental Crust (UCC). Discriminant diagrams, including those of Th–Co–Zr/10, Th–Sc–Zr/10, La–Th–Sc, and K2O/Na2O vs. SiO2, suggest that the transitional shales of the Shan 23 submember were primarily deposited under tectonic settings such as continental island arcs (CIAs) and passive continental margins (PCMs).