In this paper, we study the sedimentary facies and geochemistry of the Middle Devonian (Givetian) Haikou Formation of the Qujing Basin, eastern Yunnan, China, to understand the depositional environments of some of the oldest coal-bearing strata in the world. The succession shows an palaeoenvironmental evolution, from base to top, from tidal flat to deltaic deposits, progressing to a barrier island-lagoon system, and ultimately transitioning to shoreline deposits. Geochemical analysis of mudstone samples suggests that deposition occurred mostly under brackish to fully marine conditions, although the coal seam developed during a freshwater lagoonal episode, associated with a short-lived regression, in middle part of the Haikou Formation. We propose that a humid climatic interval occurred during the initial stage of regression, leading to a significant increase in terrestrial runoff, which facilitated freshening of the coastal lagoon and promoted plant growth; these factors contributed to coal seam formation.
 

Depositional Environment,coal accumulation,sea-level change,Middle Jurassic,paleosalinity