冲绳海槽南部A23孔浊流沉积层的粒度特征

GRAIN-SIZE CHARACTERISTICS OF TURBIDITE SEDIMENTS IN CORE A23 FROM THE SOUTHERN OKINAWA TROUGH

对取自冲绳海槽南部的A23孔中浊积层的沉积层序特征进行了详细研究,结果表明,浊积层在平均粒径、分选系数、偏态、峰态等粒度参数、粒度组成、粒度分布上表现出显著的特征性。以A层(342～362cm)为例,浊积层的平均粒径在层内具有细-粗-细的变化,粒度分布呈以粗粒组分为主的双峰分布,正常沉积则为以细粒组分为主的单峰分布;在C M粒度图上,砂质层的投点规律性地平行于C=M线性排列,表现出典型的浊流沉积特征。A层浊积层在层序上与典型的鲍马层序存在一定的差异,可能反映了浊流层序的区域性特征。通过上述结果和其他资料,认为冲绳海槽南部A23孔中浊流沉积层的形成主要由地震引起,而小规模浊积层的发育可能是海底峡谷中的内波/内潮汐作用的结果。

Sequence characteristics of turbidite layers in Core A23 from the southern Okinawa Trough have been analyzed in detail. The grain-size analysis results show that the grain-size parameters, distribution, compositions of the turbidite layers are evidently distinguishable compared with those of normal sediments (non-turbidite). Taking Layer A in Core 23 as a typical example, we can understand the sequence characteristics and the sedimentation process of the turbidite layers. The grain-size distribution curve of Layer A sediments is characterized by “Bi-modal” type, whose main population is coarse grain (>32μm), while the normal sediments is “Uni-modal”. In the C-M diagram, the samples are plotted linearly parallel to the line of C=M, which is distinctly characteristic of turbidite sediments. The small difference in sequence compositions between Layer A and classical “Bauma Sequence” may reflect the regional differences among the turbidite sediments. According to the analysis results and other information, we suggest that the large-scale turbidite layers were mainly caused by earthquake, and the small-scale layers may be the results of regional internal wave and/or internal tides that occurred in the submarine canyons in the western margin of the Trough. The turbidite frequency varied greatly in the longer geological time scale, but the relationship between the turbidite sedimentation and the sea-level change is unclear because of the lack of the chronological data.