Depositional Processes And Models Of Slump-derived And Flood-induced Gravity Flows In Lacustrine Basins

During the rifting phase from the Upper Sub-member of the 4^th Member to the Middle Sub-member of the 3^rd Member of the Eocene Shahejie Formation,the Dongying Depression is mainly dominated by semi-deep to deep lacustrine environments and gravity flows significantly developed.According to the triggering mechanism,gravitly flows can be subdivided into two broad types,including the slump-derived type and the flood-induced type.The former is resulted from the re-transport of intrabasinal older sediments while the latter is induced by extrabasinal floods.Theories associated with slump-derived gravity flows have been widely applied to the research of lacustrine gravity flows over the past few decades,while less attention has been paid to the importance of flood-induced gravity since recent decades.The difference in depositional process,deposits and depositional model between these two different gravity flow types is currently unclear,which contribute one of the hotspots in the area of gravity flow researches,thus hold great scientific significance.The slump-derived gravity flow deposits which developed in the Middle Sub-member of the 3^rd Member of the Eocene Shahejie Formation in front of the Dongying Delta and the flood-induced gravity flow deposits which developed in the Upper Sub-member of the 4^th Member and the Lower Sub-member of the 3^rd Member of the Eocene Shahejie Formation in the Well Yan 18 block,have been studied through the integration of core data,well logging data,3D seismic data and laboratory data.The depositional processes and depositional responses have been analysed and definited,the development and distribution patterns have been summarized,and the depositional models have been finally proposed for the slump-derived and flood-induced gravity flows,respectively.It is shown that,nine base types of resedimented lithofacies can be recognized in slump-derived gravity flow deposits,which indicate mass transport and flow transport processes.Seven lithofacies associations are identified,indicate the transformation of three types of transport medium with different rheologies,which can be subdivided into nine specific gravity flow types.Quantitative lithofacies analysis suggest that the slump-derived gravity flow depositional system is dominated by debris flows,while turbidity currents are less important.The slump-derived gravity flow depositional system can be subdivided into massive transport system and flow transport system.The former contains two depositional elements,including slide and slump,while the latter contains debris flow channel,debris flow fan and turbidite sheet.Additionally,a deep-water depositional model for slump-derived gravity flow systems is proposed,which is composed of four main types of process-response patterns.Nineteen base types of lithofacies and twelve types of lithofacies associations are identified,which jointly indicated two types of transport medium with different rheologies.Eleven types of specific gravity flows can be further suggested,but exclude mass transport deposition.An integrated gravelly flood-induced gravity flow contains five components,including gravelly bedload,gravelly suspended load,sandy bedload,sandy suspended load and lofting plume.Two types of flood-induced gravity flows are identified and defined as sudden flood-induced gravity flows triggered by fault activities and normal flood-induced gravity flows,which form nearshore subaqueous fans and lake-floor fans,respectively.Furthemore,three sub-facies including inner-fan,middle-fan,outer-fan and some main depositional elements such as main channel,overflow deposits,braided channel,space between braided channel,debrite tongue,sheet-like sands,thin turbidites,and distal muds are identified and classified.Accordingly,a depositional model is proposed to depict flood-induced gravity flow systems,which emphasising the control by episodic fault activities in the processes-responses of flood-induced gravity flows.This study may have some reference meaning in exploring applicable gravity flow theories in rift basins.