Mixed sedimentation in saline lacustrine basins during initial Eocene thermal maximum period:A case study on Kongdian Formation in Dongying Sag,Bohai Bay Basin
Mixed sedimentation,a special sedimentary phenomenon,has always been a hot topic in sedimentology domain.This article documents a systematic study on the mixed sedimentation in saline lacustrine basins during the initial Eocene thermal maximum (IETM) period and the formation of Dongying Sag,Bohai Bay Basin based on analyses of core,thin section,isotopic and element data.The results reveal that there are two kinds of mixed sedimentation in the Kongdian Formation of Dongying Sag:mixed sedimentary sequences and structure-mixed deposits.The former mainly includes deposits of continental siliciclatic rocks interbedded with carbonates and that interbedded with sulfates.The latter mainly include deposits of structurally mixed continental siliciclastic with carbonates and that with sulfates.Various mixed siliciclastic-carbonate facies zones are identified in different stages of the deposition of the Kongdianzu Formation.Mixed sedimentation in the formation occurred in shore-shallow lake mud flats and sand bars during the early stage,and then in mud flats,sand bars and high-salinity deep water settings during the interim stage due to the thermal maximum.Later on,with a continuous salinization of the lake basin,sedimentation in the place went on in such settings as mud flat,sand bar,high-salinity deep water as well as bioclastic shoal and etc.It was dominated by facies-margin mixing,with accidentally discrete or autochthonous mixing.Controlling factors include sediment input,water hydrochemistry,hydrodynamic force,and fluctuation of water-level.The study confirms that,during the initial Eocene thermal maximum period,the lake basin in the area had gone through an intense salinization process,which occurred simultaneously with clastic input into the basin and formed typical mixed siliciclastic-carbonate deposits in the saline lacustrine basin.Dissolution of carbonate and sulfate during burial process of the mixed sediments and fracturing contributed to the formation of hydrocarbon-bearing reservoirs.The original chemical constituents in the mixed deposits could be used to restore the paleo-environment of the lake.