“Source-to-Sink” System And Sandbody-Controlling Model Of The Liushagang Formation In The Weixinan Sag,Beibuwan Basin

The Liushagang Formation of the Weixinan sag is investigated comprehensively under the guidance of the method of “source-to-sink” system in this study.Constrained by sequence stratigraphic framework,seismic information,well logging and cores are used to reveal the characteristics of source rocks,transport channels and slope break belts of the convergence zones for different geological times.“Source-channel-sink” assemblages of “source-to-sink”systems are established,which are followed by characterization of types and sizes of sedimentary sandbodies at the terminal of “source-to-sink” system.Thereby,the “sourcechannel-sink” assemblages of various “source-to-sink” systems in the same geological period are compared to expound their influence on sedimentation.Moreover,the evolution of the“source-channel-sink” assemblages of the same “source-to-sink” system are analyzed to explicate its influence on sedimentation.The ultimate aim of this study is to summarize the controlling factors of the sandbody sedimentation in the “source-to-sink” systems,and establish sandbody sedimentary patterns within the evolution of the “source-to-sink” systems.The “source-to-sink” systems of the Liushagang Formation in the Weixinan Sag are divided by the boundary of source regions and tectonic zones,and the “source-channel-sink”assemblages of the “source-to-sink” systems are described subsequently.In the sedimentary period of Liushagang Formation,there are five “source-to-sink” systems in the Weixinan sag,namely 1)the Yuegui uplift-west area of north steep slope “source-to-sink” system,2)the Yuegui uplift-east area of north steep slope “source-to-sink” system,3)the Yuegui uplift-west principal axis area “source-to-sink” system,4)the Qixi uplift-south slope belt “source-to-sink”system,and 5)the Weixinan low bulge-north slope belt “source-to-sink” system.Under the control of tectonic activity and climate transformation,the vertical evolution parent rock properties,transport channels,slope belt and accommodation the sag constitute the spatiotemporal configuration relation of each “source-to-sink” system.Results suggest that: 1)a sequence of fan delta,nearshore subaqueous fan/hyperpycnal sublacustrine fan and fan delta is developed in the Yuegui uplift-west area of north steep slope“source-to-sink” system;2)a sequence of fan delta,hyperpycnal sublacustrine fan and delta is developed in the Yuegui uplift-east area of north steep slope “source-to-sink” system;3)a gradually transitional sequence from braided river delta to meander river delta with the development of sublacustrine fan driven by slide gravity flow in the delta front is developed in the Yuegui uplift-west principal axis area “source-to-sink” system;4)a sequence of fan delta,braided river delta-beach bar and meander river delta-sublacustrine fan driven by slide gravity flow is developed in the Qixi uplift-south slope belt “source-to-sink” system;5)a sequence of fan delta,fan delta-sublacustrine fan driven by slide gravity flow or hyperpycnal flow and meander river delta is developed in the Weixinan low bulge-north slope belt “source-to-sink”system.Elements including rock types of source regions,shape and size of transport channels,transport distance and morphtectonic patterns are regarded as the main controlling factors to determine the differences of sandbody sedimentation in these two “source-to-sink” systems during the sedimentary period of the first member.Rocks of source regions on the periphery of the Weixinan Sag are dominated by metasedimentary rock and magmatic rock.These metasedimentary rocks are apt to be subject to physical weathering under the semi-arid to semihumid climate,resulting in the formation of large-scale glutenite body at the terminal of“source-to-sink” system.Incised valley is characterized by “V” shape,the transporting capacity of which is positively correlated with its scale.And scale of the sandbody at the system terminal is also proportional to the scale of incised valley.Moreover,the increase of transport distance can provide more time for the sediments sorting,and fine-grained sediments are deposited in the lake basin as a result.Intense activities of the Weixinan Fault and No.1 Fault lead to the geomorphic mutation and the formation of small-scale sag between these two faults,obstructing the transportation of segments toward the lake basin.Coarse-grained fan delta or nearshore subaqueous fan are commonly developed at the steep slope belt.Multiple fault slope breaks are favourable for the long-distance transportation of sediments along the fault zone toward the sag centre,resulting in the formation of sublacustrine fan in the sag centre.Besides,sedimentation is characterized by fast deltaic progradation toward the centre due to the abundant source supply.As a result,slope angle of the delta front is too large to keep balance and fluxoturbidite is commonly developed in the delta front.By comparing the vertical evolution of “sourcechannel-sink” assemblages for the same “source-to-sink” system and their sedimentary responses,the patterns of sandbody depositing controlled by “source-to-sink” system are established and the relevant controlling factors are concluded as well.