香港地区港湾沙坝的沉积物来源——以香港大屿山岛贝澳湾为例

The Sediment Sources for the Bay Barrier on the Hong Kong Coasts——A Case Study in Pui O Bay in Lantau Island, Hong Kong

香港港湾沙坝海滩的沉积物主要来源于河流带来的物质 ,其他来源是海湾内的残余沉积物和沿岸岩石的侵蚀带来的物质 ,沙坝的形成与第四纪冰后期海侵无关。由于香港地区河流短小 ,沉积物数量有限 ,故香港港湾砂坝坝体小 ,仅位于湾顶附近 ,向海推进年代较晚 。

The bay barrier in Pui O Bay on the south coast of Lantau Island, Hong Kong was selected as an example for the study of the sediment sources for bay barrier on the Hong Kong coasts. The studies of geomorphology and sediments of the selected bay barriers include the analysis of longshore variations of beach profiles and grain size and that of heavy minerals form the beach and the inland streams, which are compared to those in the surface deposits on the continental shelf off the Hong Kong coasts. Longest beach profiles and beach deposits with coarsest grain size, worst sorting and positive skewness are found near the outlet where the wave energy is smallest, indicating that the main source for the barrier beach is the materials from the inland streams. Although the heavy minerals of the beach and the inland streams are similar in mineral species assemblage, the fact that very stable minerls occurring in the beach are not found in the streams, that different ratios of contents of heavy minerals with similar stability and specific gravity are in the beach and streams and, that minerals of the same species in the present beach always have different surface textures indicates that there are other sources for the beach from the sea. However, the heavy mineral assemblage of the surface deposits on the continent shelf differs very much from that in the present beach. This suggests that the source must be the deposits from somewhere between the present beach and inner continent shelf, that is the remnant sediments in offshore zone within the bay. This is also supported by the previous study which indicated that the sand movement is restricted within the bays on the Hong Kong coasts due to low coastal energy. The fact that some very unstable minerals, which cannot be kept for a long time in deposits, are only contained in the present beach deposits shows that the final source for the beach is the materials eroded by waves from coastal headland rocks of the bay. The case study of Pui O Bay indicates that under the local geological and climatic conditions, the materials for bay barrier development in Hong Kong are mainly from the inland streams, which are limited resulting in the bay barriers on the Hong Kong coasts having developed in a small size. The bay barriers could not have been the result of the postglacial marine transgression in Hong Kong. The bay barriers in Hong Kong prograded very late in the Holocene period and are still located near the head of bays in the present day, which differs from those with the progradation reinforced by inner shelf sands, because of the limited source from inland streams, and also because of this, the barrier beaches are very sensitive to regular and irregular changes in coastal environments.