沉积物粒径趋势分析:原理与应用条件

Grain Size Trend Analysis:Principle and Applicability

长期以来,沉积学家尝试用粒度数据来识别沉积环境的类型或判定物质运动的方式,但只取得了部分成功。粒度参数还有一项可能的用途,即用其平面差异来获取物质输运信息。沉积学家将粒度参数的平面差异定义为"粒径趋势",并建立了以"粒径趋势分析"为基础的定性物质输运模型。粒径趋势分析的基本科学问题包括:如何提取粒径趋势信息;如何确定含有物质输运信息的粒径趋势类型;粒径趋势分析的应用条件是什么。粒径趋势信息可通过粒径趋势矢量的定义及其各向异性显著性的检验而获得,而经验证据也显示,含有物质输运信息的粒径趋势类型是存在的。初步研究表明,粒径趋势分析应满足以下条件:粒径趋势矢量具有显著性;底质采样深度应代表同一时间尺度;平面采样间距应符合地统计法的规则;避免使用位于采样网格边缘上的采样点的粒径趋势矢量;沉积物样品应属于同一个输运体系或研究区处于堆积状态。但是,由于不同来源的物质混合、源区沉积物特征及变化、不同水动力条件下的物质输运、悬沙沉降、溶解态—颗粒态物质转换、物质输运动力的侧向分布等因素与粒径趋势形成之间的关系还不够明确,因此这些条件还不是完备的,这个问题的解决依赖于粒径趋势形成的过程和机制的进一步研究。

For a long time sedimentlogists have used grain size parameters as a critierion to determine the type of sedimentary environments or to identify the mode of transport before accumulation. However, such efforts have been only partly successful. Another possible ussage of grain size data is to delineate sediment transport pathways on the basis of the spatial distribution patems of grain size parameters, which is defined as ＂grain size trends＂. Based upon analysis of grain size trends, qualitative models for sediment transport have been established. The basic scientific problems for grain size trend analysis include： the procedure to subtract the information on transport; the method to determine which types of the trends contain transport information; and the conditions under which the method can be applied. For the first two questions, it has been proposed that the patterns of the trends can be obtained by defining the ＂trend vector＂ and identifying its spatial anisotropy （the significance of which can be tested using statistical methods）, and empirical evidence has shown the existence of the trend types that contain transport information. With regard to the third question, preliminary studies indicate that the following conditions should be satisfied for the application of grain size trend analysis ： （ 1 ） the spatial anisotropy of the trend vectors is statistically significant; （2） the sampling depth matches the temporal scale of the transport phenomenon ; （3） the sampling interval is in agreement with the geostatistical principle ; （4） the edge effect is eliminated ; and （5） the samples are from the same transport system or the sedimentary environment is in an accretional status. However, these conditions are still incomplete, because of the uncertainties from the effects of material mixing, the characteristics and their changes of sediment in source areas, transport in different hydrodynamic settings, settling processes of suspended sediment, transformation between dissolved-particulate materials and lateral distribution patterns of sediment movement. A thorough solution to the problem depends upon future progress in the study of the processes and mechanisms for the formation of grain size trends.