Sedimentary Characteristics And Depositional Model Of Hyperpycnal Flow In Lacustrine Rift Basin

The sedimentary periods of the Eocene Shahejie Formation in Dongying depression of Bohai Bay Basin and the Cretaceous Xiguayuan Formationin Luanping Basin were both in the rifting stage dominated by semi-deep to deep lake settings with large-scale lacustrine gravity flow deposits.According to the sediment supply and trigger mechanism,two kinds of gravity flow deposition,which were triggered by slumping from intrabasinal sediments and flooding from extrabasinal sediments,are easy to develop in lacustrine rift basin besides that triggered by volcanic activity.For a long time,the slumping gravity flow deposition has been widely used to interpret lacustrine gravity flow,however the flooding gravity flow / hyperpycnal flow deposition is still poor understood.Based on the comprehensive utilization of core data,well log data,well logging data,3D seismic data and laboratory data,this paper has carried out the research on the sedimentary characteristics and process of the hyperpycnal flow of Shahejie Formation in Minfeng Sag,Dongying depression and Xiguayuan Formation in Luanping Basin,defined the lithofacies type,lithofacies combination characteristics,deposition process,fluid genesis,and influencing factors of the hyperpycnal flow,summarized development rules and distribution patterns of hyperpycnal flow in different tectonic environments.The research results show that twenty-three basic lithofacies of hyperpycnal flow have been identified in different structural zones of the study area.Among them,the gentle slope zone is mainly composed of sandstone and muddy siltstone,which is characterized by an inverse-positive composite rhythm;the steep slope zone and the long-axis fan delta are coarse-grained,mostly conglomerate and gravel sandstone,and the vertical See block structure and inverse-positive complex rhythm.Based on the lithofacies type and lithofacies combination characteristics,the sedimentary processes(fluid types)such as gravelly bedload,sandy bedload,gravelly suspended load,sandy suspended load,and muddy plume suspension settlement can be explained.Sediment transport characteristics of coexistence of bed load and suspension load.The hyperpycnal flow usually coexists with the slumping gravity flow in study area,but the hyperpycnal flow is widely distributed and extends further.Two types of flooding deposition,sudden hyperpycnal flow and normal hyperpycnal flow,are identified.As the results of these two hyperpycnal flow,fan delta,nearshore submarine fan,sublacustrine fan and gravity current channel are identified,and the main channel,braided channel and overbank deposits,inter-channel,debris flow tongue body,sand sheet,turbidite thin-bed are also identified in studied area.Based on discussions on facies composition and their depositional architecture,the sedimentary model of hyperpycnal flow in lacustrine rift basin is put forward in this study.