基于激光粒度测试法的闽粤沿海花斑粘土成因研究

近年来主流的激光粒度分析是一种集激光、光电以及计算机技术对颗粒的粒度进行测试的方法。其原理是：不同粒径的颗粒对入射激光会产生不同角度的散射光，通过测量不同角度散射光的强度，可计算样品的粒度分布。由于操作简便、测试快速、精度高，在沉积学中有着重要的应用。闽粤沿海第四纪盆地的上更新统与全新统之间，普遍发育一层“花斑粘土”，其多被认为是晚更新世末期的河、海相沉积，在末次盛冰期，顶部暴露地表风化而成。而初步研究显示，花斑粘土与下伏沉积在颜色、结构、成分等方面存在显著差异，两者相交截然，不具备典型的风化成因关系；花斑粘土具粉砂感，干燥后易随风飞扬，沉积特征与风成黄土具有较好的可比性。粒度分布是判断沉积物的沉积环境及成因的重要指标，为了探明花斑粘土的沉积属性及形成机制，选取珠江三角洲三个代表性第四纪钻孔，采用激光粒度方法，对花斑粘土的粒度特征进行研究。结果显示，花斑粘土以10～50 μm的粗粉砂为众数粒组，<5 μm的粘粒为次众粒组，分别对应黄土的“基本粒组”和“挟持粒组”，是风成沉积的典型粒组构成；各项粒度参数均符合风成沉积的范围；粒度参数散点集中，粒度指数展布范围统一，均与典型黄土一致，而与下伏沉积差异截然；判别结果显示，花斑粘土属风成沉积；粒度相分析表明，花斑粘土与典型黄土投点范围重合，而与其下伏沉积显著不同。最终提出，广布闽粤沿海第四纪地层中的花斑粘土与其下伏沉积层在沉积特征及成因上并无关联，花斑粘土并非原地风化的产物，而属末次盛冰期外来的风成堆积。该结论对今后闽粤沿海地区晚第四纪古气候环境的重建有着重要的科学意义。可见，基于光学散射原理的激光粒度方法为判断沉积物的沉积环境及成因提供了有效的科学证据。

Research on Origin of Mottled Clay in the Coastal Areas of Fujian and Guangdong Provinces Based on Laser Particle Technology

Laser particle size analysis integrated laser, photoelectric and computer technologies has recently become a mainstream method of grain size testing. As particles with different sizes produce scattered light at different angles to the incident laser, the particle size distribution of the sample can be calculated by measuring the intensity of scattered light at different angles. Because of simple operation, rapid test and high precision, it has an important application in sedimentology. A layer of “mottled clay” is widely developed between the Late Pleistocene and Holocene in Quaternary basins in the coastal areas of Guangdong and Fujian provinces. Current research attributes its origin to exposure weathering of Late Pleistocene marine/fluvial deposits during the last glacial maximum. However, our studies find that the mottled clay has no transition in color, structure and composition with its underlying layer and is therefore not formed from weathering. The mottled clay is silty, easily raised by the wind and similar to typical loess. In order to ascertain the character and origin of the mottled clay, three drill cores in the Pearl River delta area were chosen in this study with the method of laser grain size analysis. The results show that grain size composition is characterized by the modal grain size group of coarse silt(10~50 μm), followed by the group of clay grain (<5 μm), both of which are typical particle compositions of an aeolian deposit. All grain size parameters are in accordance with that of an aeolian deposit. Both particle size parameter scatter diagrams and index distribution range of the mottled clay are consistent with typical loess but different from the underlying deposits. The discriminant analysis exhibits an aeolian origin of the mottled clay. The phase analysis also shows that the mottled clay points coincide with the range of typical loess, but has no genetic correlation with its underlying deposits. It is concluded that the mottled clay is not a weathering product of its underlying sediments but an exotic aeolian deposit. This conclusion is of great scientific significance for reconstructing the paleoenvironment of the last glacial period in Fujian and Guangdong coastal areas in the future. The laser particle size method based on optical scattering provides effective scientific evidence for judging the sedimentary environment and origin of the sediments.