香港海湾沙坝发育过程中的风沙作用

海湾沙坝是香港海岸一种主要的沉积地貌，以大屿山岛贝澳湾的沙坝最为典型。关于贝澳湾古（内）沙坝的成因曾有过较多的争论，但各种成因假说都缺乏有力的证据支持。本文分析了湾内沉积物特征和沉积过程与海平面变化的关系，揭示了风沙在海湾沙坝发育过程中所起的作用。古（内）沙坝的沉积成分随着内沙坝向上加积贝壳含量急剧减少，其上部沉积成分已明显不同于外沙坝和现代海滩，而与其前后的干泻湖上部相似，显示了沉积过程中沉积物来源的变化。内沙坝的沉积物粒径向上变细，分选向上变好，而粒径最小处不在沙坝的最上部，则是在贝壳含量开始向上减少的高度，表明沉积动力随着沉积来源的变化而改变。电镜扫描也显示内沙坝上部的部分颗粒不是来自海滩，具有磨圆差、缺乏水下撞击痕迹以及上翻解理与贝壳状断口共存的特征。内外沙坝下部海滩岩的高度、位置与年代还表明内外沙坝间的沉积高度与海平面变化无关。研究表明：香港海湾沙坝发育过程向上加积的动力由原来的海动力变为风动力；随岸线向海推进内沙坝的沉积物来源由来自海滩物质变为同时来自海滩物质及干泻湖上层物质，只有风才能堆积这些物质；内外沙坝间的沉积高度差只是风积高度不同的结果，与海平面变化无关；海湾内风沙作用并不显著。

AEOLLAN ACTION IN THE DEVELOPMENT OF BAY BARRIERS OF HONG KONG

One of the major depositional coastal features in Hong Kong is hey barriers (bay bars).The typical one is found in Put O bay of Lantun Island, Hong Kong. Previous studies have provideddifferent hypotheses for the origin of the old (inner) bay bar at Pui O. However, lack of directevidence was seen in those studies. This study shows the aeolian action in the development of the baybarriers in Hong Kong by analysis of the sediment characteristics in the bay and relationships betweenthe deposition process of the bay bars and sea level changes at Pui O.The percentage contents of shells in the deposits.of the inner bar decreased upwards sharply so thatthe deposits in the upper part of the inner bar apparently differed from those of the present beach butwere similar to those in the upper layers of the dried lagoon, indicating that the sediments from the seadecreased and ones from other sources were added. Grain size became finer upwards and the sortingdegree improved upwards in the inner bar. However, the finest sands were not located in the top layersof the inner bar but in the layers where the shells begsn to decrease, showing that the deposition agentchanged in association with the change of the sediment source for the inner bar. Some of the grains inthe upper layers of the inner bar being characterized by irregulars outline and lack of subaqueoustexture with upturned plates and conchoidal fracture, denoting that they did not come from modernbeach.The evidence above shows the following regulations in the development of hay bars in Hong Kongcoasts: Firstly, the deposition agent alters from waves to wind bringing depositS up to the bay bornerin the later sarge of the bay barrier development. Secondly, the sediment source for the inner barchanges from the beach material to the aeolian deposit brought by wind from both the upper layers ofthe dried lagoon and modern beach. Thirdly, the height difference of the inner and outer bay barsresulted from the different degree of the aeolian deposition and had no relationship to sea level changesin the bay and finally, the aeolian actions in the hays were not very prevalent.