Geochemical characteristics and depositional environments of mudstones from the Triassic Zhifang Formation in the Tongchuan Area, southern Ordos Basin, China

Abstract

Significant sedimentary and tectonic information is preserved in the geochemical characteristics of the Middle Triassic Zhifang Formation in the southern Ordos Basin, North China. In this paper, elemental geochemical analysis is performed for 12 mudstone and siltstone samples from the Zhifang Formation of the Liangzhai River–Liulinchuan section of Tongchuan City, Shaanxi Province, with a major element analysis performed via X‐ray fluorescence spectrometry, trace element and rare earth element (REE) analyses performed via inductively coupled plasma mass spectrometry (ICP‐MS). The palaeoenvironment and tectonic backgrounds of the study section are also studied by analyzing the elements and elemental ratios sensitive to palaeoenvironmental conditions. The geochemical proxies, including the concentrations of U, and ratios of Fe2O3/FeO, U/Th, V/(V + Ni), V/Cr, and Ni/Co indicate that the depositional environment of the Zhifang Formation was likely an oxic–dysoxic condition without strong lake‐water stratification. The oxic–dysoxic condition is interpreted as one of the key controlling factors for the poor development of source rocks in the Zhifang Formation, although this formation is dominated by black fine‐grained sediments. Because of the Qinling orogenic belt, the geochemical characteristics reflect a tectonic background of the provenance area that was an active continental margin related to the continental island arc.