Sedimentary provenance and paleogeographic environment of a Mississippian coal-bearing unit in South China: Constraints from detrital zircon U-Pb ages and sedimentologic and geochemical evidence

Abstract

The Mississippian Xiangbai Formation in South China is one of the oldest coal-bearing sedimentary units in China. However, the sedimentary provenance, paleogeography, and coal formation patterns of the coal-bearing strata are not well constrained. We address this key issue by using a combination of detrital zircon U-Pb data, geochemical analyses of mudstone and shale, drilling data, and geologic field observations. With the exception of the early Paleozoic orogenic events (ca. 440–420 Ma) in the eastern part of the study area, the samples from the Xiangbai Formation exhibit similar detrital zircon U-Pb age distribution with major age peaks at ca. 980–960 Ma and several subordinate age peaks at ca. 800–780 and 600–500 Ma. These detrital zircon grains mainly originated from recycled sedimentary units. Field and drilling data reveal that the Xiangbai Formation was formed in a tidal flat environment with water depth gradually increasing from the bottom to the top, indicating an overall trend of transgression. The Mississippian Xiangbai Formation was deposited in an interglacial period. The Lower Xiangbai Formation was deposited in a relatively cold paleoclimate, gradually transitioning upwards to a warm and humid climate, creating favorable conditions for coal formation. The alternation of warming and cooling climates triggered high-frequency eustatic fluctuations, which led to multiple sedimentary cycles and resulted in thin, unstable coal seams within the Xiangbai Formation. Mississippian sedimentary paleogeography, palaeoclimate, and sedimentary provenance together dominate the coal-bearing clastic sediments in southwestern South China.