香港海域全新世地层及风暴潮的建造作用

ROLE OF STORMS IN HOLOCENE INNER SHELF SEDIMENTS OF HONG KONG

香港全新世地层中发现有6种沉积构造类型,包括:1)隐构造淤泥层(SM型)、2)砂质粒序层(GS型)、3)层理状沉积层(L型)、4)贝屑粒序层(GSh型)、5)无序贝屑层(USh型)和6)均一粉砂层(SS型)。其中,后5种类型与风暴潮有关。沉积物的粒度分布有7种类型。其中,双峰型粒度分布与风暴潮对海底沉积物的悬浮和再分配密切相关。贝壳的破碎度可指示沉积物受改造的程度。沉积构造受改造的程度主要取决于沉积速率和生物扰动。

Hong Kong, located on the north coast of the South China Sea, is under the influence of typhoons. Because of the potential damage during typhoons, the determination of the retum period of typhoons during the Holocene is particularly relevant to coastal engineering. The main objective of this study is to examine the role of storms in the Holocene inner shelf sediments of Hong Kong. Four boreholes, VB1, PV3, PV18 and DEW42, located in eastern, central and western waters of Hong Kong have been selected for the present study. Six types of sedimentary beds have been identified including:(1) structureless mud bed (SM); (2) graded sand bed(GS); (3) laminated bed(L); (4) graded shell bed(GSh); (5) ungraded shell bed(USh), and(6) structureless silt bed(SS). In addition to the sedimentary beds, shell pockets(ShP) and sand pockets(SP) may also be present. GS, L, GSh, USh, SP and ShP are indicative of the dynamic nature of the sedimentary environment Seven types of grain-size distributions are found in association with the six types of sedimentary beds. Grain-size type V associated with bed SM is thought to be the result of a low energy environment. Types Ⅱ, Ⅲ and Ⅳ, characterised by bimodal distribution are associated with GS, GSh, USh and L. The bimodal distribution is attributed to the result of the resuspension of SM. The fine silt fraction is explained by the presence of mud pellets deposited together with the fine sand and coarse silt fraction during storms. Type Ⅵ is associated with bed SS which is found in the eastern waters of Hong Kong, and is thought to be the result of sediment mixing including storms. The low degree of shell fragmentation in GSh indicates rapid accumulation during storms, while the high degree of shell fragmentation in the structureless beds, such as beds SM and SS, is explained by a combination of bioturbation and mechanical breakdown associated with sediment recycling. In the western and central waters of Hong Kong, graded sequences of centimetre to decimetre scale down to 3m below the sea bed are common. Their sedimentological characteristics are similar to modern storm sequences found in other inner shelves. The ungraded shell beds often present from 3 m below the sea bed possess similar bimodal grain-size distribution and sediment composition as the graded sequences. They are explained by insufficient reworking of the graded beds. The upward increase in frequency and thickness of storm beds is explained by the decrease in water depth of the sea bed and the higher sedimentation rate caused by the progradation of the Pearl River Delta. As yet, no firm evidence has been found to show an increase in frequency and magnitude of storms during the late Holocene. The well preserved beds are attributed to high rates of sedimentation and low rates of biomrbation. In the eastern waters, the poor preservation of storm beds in borehole DEW42 is explained by low rates of sedimentation, high rates of bioturbation, and sea bed disturbance through trawling.