基于GPR图像的福建海坛岛北部海岸冲积扇沉积构造及其成因的初步研究

福建海坛岛北部海岸发育一座覆盖淡灰黄色风成砂层、相对高度达30 m的大沙山，外貌很像风成大沙丘，但同时又具有冲积扇形态。其成因和沉积环境有待考证，而探明大沙山内部沉积构造是判别其成因的主要途径之一。在野外考察的基础上，运用探地雷达（GPR）对大沙山内部沉积构造进行探测，获得约20 m深度范围内的沉积构造图像数据。结果表明:大沙山中上部发育由棱角状砾石或粗砂细砂组成的沉积旋回和沟槽沉积构造，显示出冲积扇上部的沉积构造组合特点；中下部层面中交替呈现弧形沉积构造，大沙山南侧发育有向南倾斜28°～30°的前积层理。结合大沙山西侧冲沟剖面沉积构造、大沙山表面的棱角状砾石特征以及现代风沙地貌分布规律，初步认为大沙山是山前多期沟谷洪流携带的碎屑物沉积形成的冲积扇残余堆积体，沉积间隙受海岸带风沙活动影响。

Unravelling the Sedimentary Structure of Coastal Alluvial Fan and its Origin in the Northern Haitan Island,Fujian, Using Ground Penetrating Radar

Alluvial fan could provide valuable information in recording local climate change, reflecting neotectonic movement and rebuilding paleogeographic environment. Compared with inland areas, coastal zone as the interface of three layers of atmosphere, ocean and land has distinct differences in depositional environment, resulting in unique material sources and dynamic condition. Thus, alluvial fan in coastal zone may have special geological landscape process and sedimentary structure characteristics. There is a great sandy hill which covered by grayish yellow eolian sand layer in the northern Junshan along the coast of Haitan island, Fujian Province. It seems to be a big aeolian dunes, but in fact alluvial fan shape. In this paper, we used the ground penetrating radar (GPR) to detect the internal sedimentary structure of the great sandy hill, and tried to find out its origin. Detection results show that in the middle-upper part of the great sandy hill, there developed sedimentary cycle composed of angular gravel or coarse to fine sand and groove depositional structure, which presents the depositional characteristics of traditional alluvial fan; in the middle-lower part of the great sandy hill, there developed curved depositional structure presented alternately in the layer. Based on the results, we also found that detecting depth of the alluvial fan record the wide range of three to five times of piedmont ancient flood sedimentary history. Combining the geological investigation in the great sandy hill, we assumed that the great sandy hill is the remnant of alluvial fan, which contains the characteristics of aeolian sediments along the coastal zone. The research is benefit for further understanding of the evolution of paleogeographic environment in the coastal zone of South China, and enriching the research of dynamic geomorphological processes on coastal zone and the sedimental structures of alluvial fan.