Characteristics of the Tectonic Background and Sedimentary Paleoenvironment of Coal-Bearing Strata from the Yan’an Formation in the Ordos Basin: Evidence from the Geochemistry of Major and Trace Elements

Abstract

The coal-bearing strata of the Jurassic Yan’an Formation in the Ordos Basin, China, contain various energy sources, including coal, oil, and gas. However, due to the complex geological conditions of the coexistence of multiple energy sources, it is difficult to explore coal, oil, and gas. Geochemical features record important geological information about rock formations at that time. Therefore, it is of great significance to study the characteristics of major and trace elements and the paleoenvironment of the coal-bearing strata of the Jurassic Yan’an Formation in the Huangling area for the subsequent smooth mining of coal mines and other energy sources. An X-ray fluorescence spectrometer and inductively coupled plasma mass spectrometer were used to determine the major and trace elements of the Yan’an Formation, respectively, and the coal-bearing strata of the Yan’an Formation were comprehensively analyzed from the vertical and different lithology aspects: the paleoweathering characteristics, provenance environment, sediment source, and tectonic background. At the same time, the paleosedimentary environment characteristics of coal-bearing strata were recovered through a series of environmental discrimination indicators, including Sr/Cu, Sr/ppm, Sr/Ba, Th/U, U/Th, V/Cr, V/(V + Ni), V/Sc, and V/Ni. The results show that the sedimentary mode of the Yan’an Formation in the Ordos Basin is mainly fine-grained deposition, and tight sand-mud interbeds and mudstone layers are developed in fine-grained sandstone deposits. The sedimentary rocks are dominated by granite (obvious negative Eu anomaly), mixed with some basement volcanic rocks and ancient sedimentary rocks, and the parent rocks have many geneses (high SiO2, low MgO, and low CaO). The degree of weathering is generally stable but has not experienced complete weathering. Among them, the lower clastic particles have undergone moderate weathering from the source area to the sedimentary basin, and the sandstone is slightly higher than the mudstone. The paleoclimate of the Yan’an Formation was warm and humid in the early stage and tended to be dry and hot. The overall paleowater depth was less than 10 m, and the local area showed a shallow lake subfacies. It experienced a transition from a saline lake facies to a freshwater environment. It is typical brackish–brackish water with a partially oxidized environment (i.e., a sub-rich oxygen environment).