Evolution Of Sedimentary Environment And 3D Modeling Of Geological Structure In Hangzhou Bay Since The Pleistocene

Global environmental change is increasingly an issue of great concern to the international society that numerous scholars have raised considerable attentions on the global climatic and environmental evolution history since the Pleistocene.In recent years,through the Quaternary sedimentary structure and spatial distribution characteristics to study the geological evolution and paleogeographic environmental evolution since the Pleistocene is a hot research focus,of which scientific issues such as the definition of stratigraphic sequence,sea level change,paleoenvironmental evolution and transgression have provoked ardent discussions.The analysis of sedimentary stratigraphic sequence since the Pleistocene in Hangzhou Bay area is of great significance for studying the development history of sedimentary processes in Hangzhou Bay and paleogeographic environmental evolution,and exploits the distinctive role of geographic location in the south wing of Yangtze River Delta for the research on the alluvial plain formation and paleogeographic environmental evolution in Yangtze River Delta of China.Hangzhou Bay is located in the northeast of Zhejiang Province,China.The north bank of Hangzhou Bay is the south rim of Yangtze River Delta which developed from a deep trough along the river bank.The south bank is Ningshao Plain of broad riparian beach land.In recent years,the research on the paleoclimate and eustatic sea level change since last glacial period in Hangzhou Bay has accomplished a phenomenal achievement,laying a foundation for further study in sedimentary characteristics,stratigraphic sequence,and paleoenvironmental evolution since the Quaternary,especially from the Holocene.However,due to technological limitations such as drilling difficulty,drilling depth and sample analysis,there are few researches on sedimentary environmental evolution since the Pleistocene,also exist some pendent problems to be solved.3D visualization of geological structures can provide a factual and intuitive display of underground geological phenomena and structures,which has become the key and core technology that promote the construction of“transparent city,smart city”and the services of“geology+”,attracting attentions of many scholars.The modeling of 3D geological structure has been applied to various geological fields,and accordingly the technical methods have been gradually improved.However,there are some critical issues such as accurate description and refined expression of geological structure and strata properties,and the practical application in service resource management of 3D geological structure models have to be settled urgently.As the pilot region of Yangtze River development strategies of Zhejiang Province,Hangzhou Bay area is at preliminary stage of the modeling and application of 3D geological structure.In allusion to issues and heat research focus mentioned above,this paper did systematic analyses of high-resolution granularity,palynology,optically stimulated luminescence?OSL?,AMS¹⁴C dating and paleomagnetism through core sampling in the Quaternary geological boreholes,obtaining abundant data since the Pleistocene in the study area.It carried out a specific and comprehensive research on sedimentary unit division,sedimentary types,sedimentary facies,paleogeography,stratigraphic age,and stratigraphic framework since the Pleistocene.By establishing stratigraphic sequence and geological section,combined with historical data collection and collocation,screening,database construction and comprehensive analysis,a 3D geological structure model of Jiaxing City located on the north bank of Hangzhou Bay has been preliminarily constructed.By sampling from thirteen Quaternary boreholes in both banks of Hangzhou Bay,99 OSL ages,74 AMS¹⁴C ages and 5,178 paleomagnetic sample data have been obtained.For the BZK02,BZK04,and BZK05 boreholes on the north bank of Hangzhou Bay,lithostratigraphy,biostratigraphy,climatostratigraphy,magnetic strata,chronostratigraphy,granularity analysis,and comprehensive logging analysis methods were used to establish Quaternary multiple stratigraphic division and comparison map that includes sedimentary facies,paleoclimate,and eustatic sea level change and stratigraphic classification information.The Quaternary stratigraphic framework on both banks of Hangzhou Bay was established by compiling eight high-precision Quaternary geological sections.The paleoclimatic evolution since the Pleistocene in the study area was divided into eight climatic warm-cold cycles to perform more accurate comparison on the strata in the climatic cycles according to the isochronism principle of base level cycle,and the paleogeographic evolution map of the 20 periods since Pleistocene in the study area was compiled that reflected the evolution law of sedimentary facies and paleogeographic environment in the study area.Jiaxing City located in the north bank of Hangzhou Bay was selected for 3D geological structure modeling and analysis.A 3D original database of geological boreholes was established by selecting representative boreholes on the north bank of Hangzhou Bay.By using the 3D software tools in MapGIS K9,the analyses of 3D geological model construction,3D model visualization,3D model cutting and analysis of 3D model grating profile display based on surficial geological map and geological-geophysical-drilling joint deductive profile were carried out.A total of 116representative Quaternary boreholes,more than 1,100 engineering geological boreholes and 445 hydrogeological holes were selected to construct 3D Quaternary geological structure model,3D engineering geological structure model and 3D hydrogeological structure model of Jiaxing City on the north bank of Hangzhou Bay respectively.Based on the 3D geological structure model,the effective management and 3D visualization of geological borehole data by data system were preliminarily achieved.This paper has preliminarily explored the on-demand real-time evaluation and 3D visualization dynamic display of the suitability for the utilization of underground space in any region and any depth range,and also studied the practical application of 3D geological structure models in the service resource management and ground-underground 3D visualization management.The main innovations of this paper are:?1?It studied the sedimentary stratigraphic structure,sedimentary facies sequence and paleogeographic evolution since the Pleistocene in Hangzhou Bay,analyzed the coupling relationship between sedimentary sequence and paleoenvironmental evolution of Hangzhou Bay,determined eight warm-cold paleoclimatic cycles,and compiled 20 paleogeographic maps that restored the evolution of lithofacies paleogeography in Hangzhou Bay.?2?By using 3D tools of MapGIS K9,it carried out the 3D geological modeling study and 3D visualization analysis of the study area,and constructed the 3D Quaternary geological structure model,3D engineering geological structure model,and3D hydrogeological structure model in Jiaxing City of the north bank of Hangzhou Bay.?3?It preliminarily explored the practical application of real-time on-demand evaluation and dynamic 3D visualization display of suitability for the utilization of underground space in any region and any depth range based on the modeling of 3D geological structure,providing supports for the promotion of“transparent city,smart city”construction and“geology+”services in Hangzhou Bay.