| There are diversified tidal and river joint-dominated sedimentary systems under the control of multiple geological factors developed during the Pinghu Formation in the Western Slope Belt of Xihu Sag,East China Sea Shelf Basin.Based on high-precision3 D seismic data covering the study area,drilling core and zircon dating data,by analyzing the controlling factors of basinal architecture,sequence stratigraphic framework,provenance intensity difference,and relative sea-level changes in the Western Slope Belt,this research aimed to reveal its sedimentary feedback mechanism,and defines the sedimentary characteristics,development model and distribution characteristics of tidal and river joint-dominated sedimentary systems.Then to provide reliable guidance for the prediction of favorable reservoirs in the Western Slope Belt,and provide an example and reference for the study of tidal and river joint-dominated sedimentary systems in the sea to land transition zone at the continental margin.Major research results and understandings are as follows:(1)According to the fault and geomorphic characteristics of the Western Slope Belt,this research divided the sub-zones in the basin during the period of Pinghu Formation,and defined the basinal architecture of different sub-zones.Due to differences of faults and geomorphic characteristics in the period of Pinghu Formation,the Western Slope Belt can be divided into Tiantai Slope Area(steep-slope),Tiantai-Pingnan Interaction Area(fault and steep-slope),Pingbei Slope Area(paleo-uplift and fault terrace),and Hangzhou Slope Area(gentle-slope)from south to north.From west to east,it can be divided into Uplift-Denudation Zone,Stress Regulation Zone and Low-lying Stability Zone according to the differences of faults and landforms.(2)The sequence stratigraphic framework under the sea-land transitional sedimentary background of the fault-depression transition period of Pinghu Formation in the Western Slope Belt is defined.The Pinghu Formation Western Slope Belt is divided into three third-order sequences(E2p SQⅠ,E2 p SQⅡ,and E2 p SQⅢ)from bottom to top,representing three large-scale transgressions under the background of overall regression.The sequence interior presents episodic changes,and each third-order sequence interior can be further divided into three system tracts(LST,TST,HST).Due to the differences in the internal structure of the basin,different sequence configurations are developed from south to north.In the Tiantai Slope Area,it developed a steep-slope sequence configuration,the Tiantai-Pingnan Interaction Area developed a fault-slope sequence configuration,in the Pingbei Slope Area,it is characterized by transformed slope and paleo-uplift-horst and graben type,while in the Hangzhou Slope Area,it formed multi-level slope sequence configuration style.(3)Provenance analysis showed that the Western Slope Belt is supplied by the Haijiao Uplift and Yushandong Uplift.At the same time,since the E2 p SQⅡ,the Hangzhou Slope Area and the Pingbei Slope Area were partially affected by the provenance of the middle Yangtze River catchment outside the basin.The Tiantai Slope Area was supplied by the granite basement of Yushandong Uplift,and three large-scale subparallel river systems were developed with weak provenance intensity;The Tiantai-Pingnan Interaction Area was located in the transition area between Yushandong Uplift and Haijiao Uplift that only two river systems with a certain scale are developed,and the provenance intensity is the lowest.Because the Pingbei Slope Area is directly adjacent to the Haijiao Uplift and affected by the provenance of the middle Yangtze River catchment transported from the Tiantai Slope Area,there are four large-scale dendritic river systems developed with strong provenance supply;The Tiantai Slope Area was directly adjacent to the provenance on the north side of Haijiao Uplift,and it was also the main influence area of provenance in the middle Yangtze River,then three sets of large dendritic and comb river systems are developed,and the provenance intensity is the largest.(4)This research established a comprehensive small-medium-large scale systematic discrimination method of sedimentary facies under the background of tidal and river joint-dominated sedimentary background and divided the major tidal controlled area and major river-controlled area.In addition to the typical facies indicators of tidal and fluvial bedding characteristics and lithologic assemblages,the zircon crystal shape analysis in the main tidal controlled area indicates that the grains are mainly distributed in the 6-10 cycle zone and the dentation degree of noise reduction logging curve ΔGr>10,the average dentation degree of an interval ΔGr2>7.In terms of seismic facies,the main tidal controlled area with tidal action as the background is usually dominated by the typical isolated ridge seismic reflects,beaded ridge and continuous ridge seismic reflects;The zircon crystal shape analysis in the main river controlled area indicates that the grains are mainly distributed in the 1-5 cycle zone and the dentation degree of the noise reduction logging curve ΔGr<10,the average dentation degree of an interval ΔGr2<7,and characterized by continuous sheet reflection seismic reflects.(5)The main sedimentary facies types and plane distribution of the Western Slope Belt were defined.In the E2 p SQⅠ period,except some parts of the Hangzhou Slope Area and the Pingbei Slope Area were mainly controlled by rivers and river-controlled deltas were developed at the outlet of the main river systems,other areas were mainly tidal controlled areas and tidal deltas and tidal flat deposits are developed.By the time of E2 p SQⅡ,the scope of river action was significantly weakened that a small near-source area on the north side of Hangzhou Slope Area and Pingbei Slope Area was dominated by small-scale tidal deltas.Most areas in the study area were the main tidal controlled areas,and the scale of deltas becomes smaller on the whole.During the E2 p SQⅢ period,the main river-controlled area significantly increased and was dominated by large-scale river deltas.Part of the Tiantai Slope Area and Tiantai-Pingnan Interactive Area were still affected by tide and large-scale tidal delta and small-scale tidal flat sediments were developed.(6)This research established the sedimentary model under different control factors of tidal and river joint-dominated background in the Western Slope Belt.Affected by the difference of provenance supply,relative sea-level changes and basin architecture,the Western Slope Belt developed different tidal and river joint-dominated sedimentary models.The Tiantai Slope Area was mainly dominated by tidal controlled delta under the background of "medium-weak provenance and steep-slope" background,while the Tiantai-Pingnan Interactive Area was dominated by tidal delta under the background of "weak provenance and fault-slope" background.The Pingbei Slope Area developed typical tidal controlled and river-controlled delta sedimentary mode under the background of "medium to strong provenance,paleo-uplift and multistage fault terrace",and the Hangzhou Slope Area was dominated by "strong provenance with gentle slope" river-controlled delta sedimentary mode. |
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