The Late Holocene Stratum And Evolution In The Luanhe River Delta

The Luanhe River originates from the foothills of Bayantuer in Fengning County in Hebei Province,and empties into the Bohai Sea through the Yanshan Mountains.It had undergone many diversions during the historical period,forming alluvial fan-delta in different scales with apex in Tingliuhe,Qianan and Luanxian.The alluvial fan-delta of the Luanhe River is adjacent to Yanshan in the north and the Bohai Sea in the south.It is structurally located in the transitional zone between the Huanghua Depression and the Yanshan Uplift Belt.By analyzing the sedimentary process and development pattern of the luanhe river delta in the Late Holocene,we can understand the response process of the Luanhe River Delta under future global changes better.In order to understand the sedimentary evolution process in the Luanhe River Delta in the Late Holocene better,eight boreholes were analyzed in chronology,sedimentology,micropaleontology and grain size.27 samples for micropaleontology and 52 samples for grain size analysis were used to establish sedimentary facies,7AMS ¹⁴C ages used to establish era framework.And based on the characteristics of the remote sensing image of the study area from 1984 to 2016,the development process and the pattern of the luanhe river delta in the late Holocene were recovered.Before 2.7 cal.ka B.P.,the lacustrine delta front,including the riverbed facies and the underwater channel facies,had developed in the study area.During 2.7?1.6cal.ka B.P.,with the sea level rising,the coastline had reached the present-day Luanhe River Delta.The western area of the Jianggezhuang Forest Farm had developed the lower delta facies of the lacustrine delta and deltaic plain facies,including the lacustrine facies,marsh facies,the lakeside facies,branch channel facies and the crevasse splay,respectively.And in the east it had formed the transgressive barrier-lagoon geomorphic sedimentary system.Since 1.0 cal.ka B.P.,the study area had generally formed lands,the western areas were mainly the interfluvial lowland and the crevasse splay,and most of the eastern areas were branch channel facies.From the 17th century to the 19th century,coastal dunes had formed around the area of Feicuidao-Jianggezhuang Forest Farm,and had caused the rivers such as Erluanhe River and Laomigou to turn from the east to the south.The Luanhe River rushed apart the coastal dunes of Feicuidao-Jianggezhuang Forest Farm and formed the current wavy delta in 1915 A.D.The study area has experienced three-phase geomorphic sedimentary combinations.The Late Holocene combination included the western lacustrine delta and the eastern transgressive barrier-lagoon geomorphic sedimentary system.The combination from the 17th century to the 19th century included the delta plain and the transgressive barrier-lagoon geomorphic sedimentary system of Feicuidao-Qilihai.The combination after 1915 A.D.included the delta plain,the transgressive barrier-lagoon geomorphic sedimentary system of Feicuidao-Qilihai and the wavy delta.