基于GPR图像的河北昌黎海岸横向沙脊北段沉积构造及其成因

The sedimentary structure and genesis of the northern transverse dune on the coast of Changli county, Hebei province from ground penetrating radar

运用探地雷达对河北昌黎海岸带横向沙脊的沉积构造进行探测,通过对探地雷达探测影像进行处理与解译,获得海岸沙丘约20 m深度范围内的海岸横向沙脊沉积构造图像,包括沉积层理的倾向、倾角与平面展布信息等。研究发现,昌黎海岸横向沙脊北段的沉积构造主要由高角度交错层理和波状交错层理组成,两者之间界面应为古海滩沿岸沙坝的顶面。界面以上发育稳定的向SWW倾斜的高角度交错层理,倾角约为30°~32°,为向岸风作用形成的前积层理;横向沙脊的丘顶可见槽状交错层理,可能是沙丘脊顶部沉积作用对风向季节性变化的响应。界面以下波状交错层理发育,为波浪作用形成的海滩沉积构造。结合前人对探测区地层调查以及全新世晚期以来海岸线变迁研究成果综合分析,认为海岸横向沙脊是大约2000 a BP以来,以澙湖海滩沿岸沙坝为基础经长期风力吹扬作用持续增长而发育的风成沙丘。

The sedimentary structure of dune systems provides available information about the development of dunes. Along the coast of Changli County, Hebei Province, China, a high large transverse coastal dune has been formed. This place is a typical coastal sand dune field in China, where the highest point of the dune reaches up to 40 m above sea level. The transverse dune runs NNE-SSW and the dominant wind direction is NE or NNE in the study area. In order to gain more detailed data of the sedimentary structure and to understand its formation better,we performed a study to detect the inner structure of the transverse dune using groundpenetrating radar（GPR） and to analyze the grain- size characteristics. GPR is a non- invasive technique that allows the study of the structure of dune systems when outcrops are limited. The images of sedimentary structure at a depth of 0- 20 m obtained from the GPR reveal dip direction, dip angle and distribution of the bedding. There is a strong reflector at about 6 m depth inside the dune. Above the reflector, high-angle cross bedding is generally found; the dip direction of the bedding is SWW and the dip angle ranges from 30° to 32°. The high- angle cross bedding formed by onshore wind is a kind of foreset beds. We can also find some reactivation or erosion surfaces on the windward slope which are influenced by the secondary changes of wind. On the top of the transverse dune, trough cross- bedding caused by the seasonal variation of wind direction is developed. Below the surface, cross- lamination shaped by the wave action is found. The cross- lamination develops both in landward and seaward slopes, where it dips landwards and seawards respectively. The results of grain- size analysis indicate that the sediment is well-stored medium sand. But the grain size at the base of the dune is coarser than that at the top of the dune. The sediment at the base deposited under the action of wave, while the sediment at the top deposited under the action of wind. Therefore, based on the characteristics of sedimentary structure and grain size, we conclude that the strong reflector is the interface of an old beach barrier and aeolian dune. Integrating previous studies on the changes of the coastline since late Holocene in the study area, around 2000 years ago suggest that the beach barrier controlled by wave action exposed above sea water and the Qilihai Lagoon formed on the landward side. The transverse dune developed based on the beach barrier outside the Qilihai Lagoon since 2000 a BP and it was eroded by wave, at the beginning of the formation of the aeolian dune. The high large transverse dune is the result of steady accretion under the action of wind for a long time.