Mineralogy and geochemistry of fine-grained clastic rocks in the Eocene Huadian Basin (NE China): Implications for sediment provenance, paleoclimate and depositional environment

Abstract

The Huadian Basin is a small fault-controlled basin in northeast China. It is filled by the Eocene Huadian Formation comprising thick lacustrine oil shale-and coal-bearing sediments. Oil shale, mudstone and carbonaceous shale samples have been collected to determine their mineralogical and geochemical (major, trace and rare earth elements) characteristics. These data are used to evaluate sediment provenance as well as paleoclimate and depositional environment. The fine-grained sediments in the Huadian Formation are derived from felsic volcanic rocks and granites, mixed with minor amounts of mafic and sedimentary rocks. Geo-chemical proxies confirm sediment recycling in the source region. Clay mineralogy and indices of chemical alteration suggest that a subtropical warm and humid climate prevailed during deposition of the fine-grained sediments. The data also suggest climatic changes during deposition of the Huadian Formation, from a stable warm and humid climate causing intermediate chemical weathering (Pyrite Member), to a seasonal dry-wet climate (Oil Shale Member), to a stable warmer and more humid climate causing strong chemical weathering (Carbonaceous Shale Member). Based on inorganic proxies, the fine-grained sediments in the Huadian Formation have been deposited in an anoxic fresh-water environment. Only the sediments of the Oil Shale Member reflect fluctuating freshwater and brackish conditions. The Eocene climatic change controlled lake level variations and water chemistry. A brackish and strictly anoxic environment together with a warm and humid climate was beneficial for the formation of high quality oil shale, whereas fresh-water conditions and warm and more humid climate favored peat accumulation.