Study Of The Sedimentary Structure,Grain Size Characteristics And Sedimentary Environment Of Coastal Dunes In Zhangpu County,Fujian Province

The southeast coast of Fujian Province is one of the main distribution areas of coastal dunes in China. In this paper, we took the coastal dunes on Gulei Peninsula, Liuao Peninsula and Chihu town, ZhangPu County of southeastern Fujian Province as our research objects. Based on outcrop observation in the field, we used ground penetrating radar (GPR) to detect the coastal dunes and preliminarily summarized their sedimentary structures. Meanwhile, granular sampling was implemented on the typical coastal dune profiles. We analyzed the granularity characteristics of coastal dunes by size distribution, frequency curves, cumulative probability curves and particle size parameters. On such basis, the sedimentary structure and granularity characteristics of coastal dunes in the study area were comprehensively analyzed. By comparing the results of previous research, we summarized their sedimentary facies and sedimentary environments. Several preliminary understandings are achieved:(1) The results of ground penetrating radar (GPR) detection indicate that Chihu (CH) Profile mainly consists of discontinuous horizontal or sub-horizontal beddings, showing the sedimentary structure characteristics of low and undulating coastal transverse ridges or coastal sand sheets. In addition, there is an obvious trough interface in the radar profile which is perpendicular to the local prevailing wind (NE), the width of which is 15 m or more, indicating sand erosion in the process of sand deposition. The ground waves are in a mess, which may have been interferenced by the roots of vegetation or human activities. The profile parallel to the prevailing northeast (NE) wind in Longmei (LM) Coastal dune is mainly made of downwind small-angle tilt beddings, while the profile perpendicular to the prevailing wind are obviously composed of continuous horizontal beddings, together with some wedge-shaped cross-beddings, showing the sedimentary structure characteristics of parabolic dunes. At the same time, it reflects that the prevailing wind direction in the study area is NE, while the SW wind is occasional and weaker. Gulei (GL) Coastal dune mainly consists of moderate continuous or discontinuous, horizontal or sub-horizontal beddings, obviously dipping to the SW direction in parallel to the prevailing northeasterly direction by landward, showing the sedimentary structure characteristics of parabolic dunes and transverse ridges. Through the interpretation of GPR images of the three coastal dunes in the study area, we summarized five kinds of radar facies. We found that there are some differences from different types or different parts of the same type of sand dunes in radar facies.(2) Particle size analysis of the coastal dunes in the study area shows that, compared with the coastal dunes in temperate China and other parts of the southern China, the size composition of the coastal dunes in Zhangpu County, Fujian Province is noticeably coarser, poorly sorted, mainly negative skewness and medium kurtosis. Along the direction of prevailing northeast (NE) wind of the coastal zone, the particle size become coarser gradually, namely CH<LM<GL. There is a finer trend of sand particle size, from bottom to top of the three coastal dunes profiles. Combined with the frequency curves, cumulative probability curves and C-M diagrams, it suggests that the lower parts of most dunes are possibly wave action and storm surge deposited, transport momentum of which is strong, and the upper parts of the dunes are wind dominated, with weaker transport momentum.(3) The sedimentary environments revealed by the sedimentary structure and granularity characteristics of coastal dunes indicate that:Chihu (CH) Profile belongs to coastal sand sheets or gentle coastal transverse ridges, and beach sand is its main source; Longmei (LM) Coastal dune and Gulei(GL) coastal dune are respectively parabolic dune and transverse ridge located on the tombolo of Liuao Peninsula and Gulei Peninsula, and coastal plants play an important role in the development of coastal dunes. There are two main sources of LM Coastal dune and GL Coastal dune:the first is tombolo sand deposition near the coastal dunes, particle size of which is coarse, and it mainly distributes in the lower parts of the dune; the second is beach sand, deposited landward in the process of NE wind, mingled with typhoon or storm surge, and it mainly distributed in the upper sand dunes.(4) In terms of grain size analysis methods, as the grain size of coastal dunes in the study area is coarse, we adopted both the sieving method and laser diffraction method for the contrast analysis of grain size. The results show that, due to the different principles of the two methods, there are some differences between the obtained results, but the overall trend of grain size distribution is the same. The cumulative probability curves show that the results of sieving method and laser diffraction method coincide best in medium particle. In the tests of sediment grain size, sieving method is more suitable for measuring coarse sediments, while laser diffraction method is more applicable to the sediments of fine particle size.