长江水下三角洲向南延伸泥质带浅地层结构及沉积物特征初探——以朱家尖岛以东海域为例

A Preliminary Study of the High Resolution Sub-bottom Stratigraphic Structures and Sedimentary Features in the Mud Area along Southern Distal Yangtze Subaqueous Delta: An example from the area off eastern Zhujiajian island

文章对取自长江水下三角洲向南延伸带上舟山泥质区边缘的58个表层沉积物样品的粒度进行了分析,并且对舟山近岸泥质区进行了浅地层剖面探测。结果显示,该区沉积物以黏土质粉砂为主;含水率较高(平均为46%),干容重较低(平均为1.44 g/cm3);孔隙比大,高压缩性,抗剪强度低,因此其抗冲蚀能力较弱。区内地层剖面上部为全新世浅海相沉积,岛礁区及南部剖面可见少量下伏基岩,剖面I东部可见浅层气出露,其顶面埋藏深度约为12 m。全新世以来该区域沉积厚度为4~23 m,千年时间尺度沉积速率约0.57~3.29 m/ka,低于长江口门外水下三角洲泥质沉积中心,属弱淤积沉积环境。由于近年流域人类活动导致长江入海泥沙减少,南下沿岸流携带的泥沙可能减少,浙江沿岸面临泥质沉积速率下降或甚至侵蚀的威胁。

In this paper,based on 58 surface sediment samples collected in the mud deposited zone along the southern distal Yangtze subaqueous delta recently,the grain size of the surface sediment was analyzed and the sub-bottom stratigraphic structure was also measured. Results show that the clayey silt is the main type of the sediment in the study area. These sediments are characterized by high water content( 46%),high porosity,high compressibility and low shear strength which are potentially vulnerable to coast erosion. The upper part of the seismic profiles shows horizontal neritic strata. Underlying bedrocks are discovered in the area of reefs and the southern profiles. Sparse shallow gas is found in the eastern part of Profile I,with top imbedded depth about 12 m. The thickness of the Holocene sediment is 4 ~ 23 m and the sedimentation rate is estimated about 0.57 ~ 3.29 m / ka. Considering low sedimentation rate of the study area,recent accelerated human activity in the river basin,leading to decreased sediment discharge from Changjiang River,will cause a potential threat to the seabed stability of the Zhejiang coast.