Study Of Sedimentary Sequence And Development Model Of Coastal Dunes In Changli County, Hebei Province

The coast of Chanli County, Hebei Province is one of the largest and most typical coastal sand dune fields in China. This paper struggles to show the genesis, developing process and model of the coastal dunes in this area. For this purpose, we performed our study by analyzing images from Ground-penetrating radar (GPR), optically-stimulated luminescence (OSL) ages, grain-size characteristics and data about evolution of sedimentary environment in the Holocene in study area. The primary conclusions of the research have followed:(1) The images from GPR show the coastal dunes developed based on the beach barrier outside the Qilihai Lagoon since 2000 aBP. There is a strong reflector inside the dunes. Cross-lamination shaped by the wave action is found, below the strong surface. Both in landward and seaward slope, the cross-lamination develops where it dips landwards and seawards respectively. On the windward slope, we can also find some reactivation or erosion surfaces which are influenced by the secondary changes of wind. Trough cross-bedding that is caused by the seasonal variation of wind direction is developed on the top of the transverse dune. On the landward slope, the structure is mainly composed of high angle cross bedding. The dip direction of these bedding is SWW and the dip angle ranges from 30°to 32°.(2) The processes of barchan dune are more complex. The structure of the barchan dune demonstrates the transition from a dome dune to a typical barchan dune and eroded. Within the barchan dune, some extensive reactivation surface and erosion surface are apparent.(3) Particle size characteristics in the study area show that, the size composition and the frequency distribution curve of the costal dunes are similar with beach sand. We know sand of dune comes from beach sand. The dune sand is finer and well sorted slightly compared with beach sand. That is because that sand was transported by wind in short distance. The sand of the barchan dune is finest and the beach sand is the coarsest. There is tendency that the sandy sediment become finer and better sorted except the place on the wind slope of barchan dune because of the erosion event.(4) The OSL ages of the 7 samples range from 184±16 a to 13±3 a.220? is a fitting preheat temperature for the young samples. At this temperature, the recycling ratio and recuperation are within acceptable limits. The ages are decreasing from foot slope to top of the traverse dune which indicates the dune moved landward and upward. The ages of coastal barchan are close which indicates erosion event on the windward slop.(5) By analyzing the information of evolution of sedimentary environment in the Holocene in study area, the beach barrier formed 2000 aBP, when the sea level declined. And then the Qilihai Lagoon formed and decreased continuously in the past 2000 a. The high large dunes are the result of steady accretion under the action of wind for a long time and they moved landward.