物源分析的一个误区:砂粒在河流搬运过程中的变化*

长期以来,中国沉积学界存在一个认识误区,认为“碎屑物质在流水搬运过程中,其不稳定成分逐渐变少,粒度逐渐变小,圆度逐渐变好”,并把这一认识错误地运用到对砂岩的物源分析中。作者基于文献调研和现代河流砂研究成果,评述了关于砂粒的不稳定矿物含量、磨圆度、粒度这3个重要参数的研究进展,认为河流搬运过程中砂粒并没有向下游出现明显的不稳定矿物丢失,粒度和磨圆度也没有明显变化,这3个参数与搬运距离并无必然关系。因此,根据长石、岩屑、不稳定重矿物的多寡以及粒度大小、磨圆度好坏来推测砂岩物源区的距离远近是缺乏科学依据的。

A misunderstanding in provenance analysis: Sand changes of mineral,roundness, and size in flowing-water transportation

There is a big misunderstanding in the community of Chinese sedimentologists and was written into the textbooks since 1970s,that is,“during sediment transport in flowing water,unstable minerals will be eliminated,while the surviving minerals become rounded progressively,and minerals sizes get smaller”. This idea was wrongly applied in sandstone provenance analysis. This paper overviewed the literatures over one century on this topic,and introduced the research advances of modern river sand studies,especially on the physic controls of sand mineral,roundness,size during transportation. Numerous studies in big rivers sands in the world indicated that the unstable minerals such as olivine,pyroxene,amphibole,feldspars and lithic grains survive in thousands of kilometers transportation,while the sand size do not get smaller and sand roundness do not increase with distance downstream. Therefore,the idea to infer distance of provenance based on the amount of unstable minerals,size,and roundness of sandstones is not based on scientific investigation,and should be corrected in the textbooks.