Control of coal-bearing claystone composition by sea level and redox conditions: An example from the Upper Paleozoic of the Datong Basin, North China

In order to investigate the genesis of different types of claystones and facies-related variations in clay mineralogy, a mineralogical and geochemical investigation of the coal-bearing Upper Carboniferous Taiyuan Formation of North China was undertaken in the Wangping and Xiaoyu mines of the SE Datong Basin. This formation contains both kaolinitic and mixed-layer illite/smectite (I/S) claystones having distinctly different compositions. The former is composed of kaolinite with minor illite, quartz, calcite, and boehmite, whereas the latter consists mainly of R1 and R3 mixed-layer I/S clay and kaolinite with minor illite, siderite, quartz, dolomite, and anatase. The two types of claystone formed from the same parent materials, as shown by Al2O3/TiO2 ratios and rare earth element (REE) patterns. Paleosalinity indices (B/Ga and Sr/Ba) and redox proxies (Mo and U enrichment factors and Corg/P ratios) indicate that the kaolinitic claystones formed under freshwater, oxic-anoxic conditions, whereas the I/S claystones formed under brackish, oxic-suboxic conditions. Chemical index of alteration (CIA) values show that the kaolinitic claystones experienced more intense leaching than the I/S claystones, even though both formed in a warm and humid climate, and this result is consistent with late Carboniferous-early Permian global climate and weathering intensity. The kaolinite developed through recrystallization and alteration of terrigenous clastics and an aluminosilicate colloidal solution in a reducing, freshwater peaty swamp environment. The formation of smectite was linked to marine transgressions that altered the salinity and oxygen content of sedimentary porewaters, with subsequent deep-burial conversion of smectite to mixed-layer I/S. The frequent changes in salinity and redox conditions that caused mineralogical variations in Taiyuan Formation claystones were driven by glacio-eustatic fluctuations of the Late Paleozoic Ice Age (LPIA), which led to episodic marine incursions into the Datong Basin.