香港及其邻近海域海洋沉积物研究

THE MARINE SEDIMENTS OF HONG KONG AND ITS VICINITY

香港及其邻近海域的表层沉积物以粘土质粉砂为主,测得的全新世沉积物最大厚度为17.8m,全新世平均沉积速率为0.81mm/a;维多利亚港因后期的潮流作用改造,全新世沉积很薄。第四纪沉积层中有5次海进及5次海退过程,体现了第四纪以来香港及其邻近海域的环境演变过程。香港海域的沉积环境与珠江密切有关,珠江不仅塑造了香港的5层陆相地层,同时为现代香港水域的潮流通道奠定了基础。

This paper mainly deals with the marine sediments of sea area of Hong Kong and its vicinity. The Holocene deposits in this region are mainly consisted of clayey silt which exceeds 70% in the most samples, they reflect a low energy depositional environment. However, sand and gravels can be found in the eastern part of this region, because the marine dynamics dominated there, they also reflect a kind of depositional environment with high energy. The thickness of Holocene sediments varies from 1～17.8m, and is measured from sea bottom to the top of the first weathered crust while it is preserved, or to the first disconformity of the underlying terrestrial sediments, or to the top of consolidated marine deposits. The average rate of Holocene sedimentation ranged from a minimum of 0.35mm/a to 1.41mm/ a with an overall mean of 0.81mm/a. These values are lower than the modem sedimentation rate. The annual rates of sedimentation in Shenzhen Bay by comparison of navigation charts and ^(210)Pb dating are 8～15mm/a and are 3.5～5.6mm/ a by studying cores using ^(210)Pb and ^(137)Cs dating in sea area of Hong Kong and its vicinity, while the corresponding values in Dacan Bay are 4～21mm/a. The difference between the Holocene sedimentation rate and modem sedimentation rate is great because the average rate of sedimentation during Holocene is based on the entire record of Holocene, these data are representation of the longer term rates, it is relative lower than the actual present sedimentation of the sea area. The offshore Quaternary strata in the sea area of Hong Kong and its vicinity can be divided into two types: marine facies and terrestrial facies, they are different in geological, physical, sedimentological, mineralogical and chemical features. Terrestrial sediments provenance was mainly from ancient Pearl River. Five marine and terrestrial strata have been analyzed, respectively, and a number of general observations have been made: (1) The M1 (Holocene marine layer) unit decreases in thickness with increasing sea-bed depth. (2) The presence of the 5 terrestrial units appears to be restricted to sites where alluvial deposition existed during low sea-level stands of the glacial periods.(3) Weathered crests are frequently observed on top of the pre-Holocene marine units. This is confirmative of their subaerial origin during low sea-level stands of glacial periods.(4) There exist disconformities between different units, some of the units missed due to the erosion by wave and tidal current. The Pearl River plays an important role to the sedimentary environment of Hong Kong sea area. The sediments of Pearl River Estuary are sandy materials in its upper reach, and muddy in the lower part before entering Hong Kong area. The records of sporopollen in the core of No. 17940 demonstrated not only the variations of vegetation, temperature and humidity, but also the changes of paleo-monsoon. Comparing with the period of Late Pleistocene, the winter monsoon had weakened from the beginning of Holocene. Human impacts on the coastal water environment are becoming more and more often, as dredging submarine sand for construction materials. The land of Hong Kong is approximately 1 100km^2 with about 10% created by reclamation from the sea. With the construction of Hong Kong new airport and other increasing engineering, as well as the growth of population, the human impacts will be more intensive in the near future.