Deposition of coal and oil shale in NE China: The Eocene Huadian Basin compared to the coeval Fushun Basin

Abstract

The Huadian and Fushun basins, located along the DunMi fault zone (NE China), are filled with non-marine coal- and oil shale-bearing sediments of Eocene age. Despite similar tectonic setting, the habitat of coal and oil shale differs significantly. During this study the depositional environment of organic-rich sediments in the Huadian Basin and factors controlling differences between both basins were investigated. Early in the history of the Huadian Basin, thin coaly and bituminous mudstones accumulated in shallow lake environments (Pyrite Member). Water depth increased during deposition of the Oil Shale Member, but did not exceed a few tens of meters preventing stable water column stratification. Moreover, deposition of fan delta sediments interrupted accumulation of fine-grained rocks. Nevertheless, algal blooms and oxygen-depleted conditions resulted in accumulation of 13 thin (<7 m) oil shale layers. Oil shale quality varies between different layers due to variable portions of terrestrial organic matter. Ash-rich coal layers mined in the Huadian Basin developed during the regressive late stage of basin evolution (Carbonaceous Shale Member). Biomarkers indicate a change from an angiosperm-to a gymnosperm-dominated vegetation. In the Fushun Basin, a single 120-m-thick coal seam formed during early basin subsidence. Subsequently, high subsidence rates established deep lacustrine conditions with photic zone anoxia and a water depth probably exceeding 150 m. Stable, strictly anoxic conditions allowed accumulation of a 300-m-thick oil shale layer. The main factor controlling differences between the Huadian and Fushun basins is tectonic subsidence.High subsidence rates in the Fushun Basin favored deposition of thick coal during the transgressive stage and of oil shale in a deep lake. In contrast minor subsidence in the Huadian Basin resulted in a lake with moderate water depth and deposition of several oil shale layers with varying quality. In this basin economic coal developed only during the regressive stage.