南京周家山下蜀黄土石英颗粒特征及其物源意义

下蜀黄土的成因和物源一直是学术界研究的热点。对南京周家山下蜀黄土的石英粒度和石英表面微结构进行分析,结果显示:粉砂粒级石英颗粒（5~50 μm）占绝对优势;<20 μm组分平均含量为42.76%,<30 μm组分平均含量为62.98%;粒度分布曲线和累积曲线总体具有颗粒偏细,呈正偏态,分选较差,峰形尖锐,双峰曲线不对称的特征;粒级-标准偏差曲线呈“双峰”分布,两个明显的标准偏差峰值分别出现在7.962 1 μm和39.905 2 μm。石英颗粒表面形态主要以次棱角状为主;表面机械结构具有丰富的蝶形坑、曲脊、贝壳状断口,部分表面出现平行节理面、V形坑;不同粒级组分表面形态和机械结构特征存在差异。分析表明,南京周家山下蜀黄土属典型风成成因堆积物,是多源区物质高度混合搬运堆积的结果。

Characteristics of Quartz Grains of the Xiashu Loess in Zhoujiashan Nanjing and Its Provenance Significance

The genesis and source of the Xiashu Loess has always been a hotspot of academic research. We have analyzed the grain size of quartz and it's surface microstructive of the Xiashu Loess in Zhoujiashan Nanjing. The results show that silt fraction of quartz grains(5~50 μm) accounts for absolute advantage. The content of the corase silt fraction of quartz grains(10~50 μm) occupies 65.4%. The content of <30 μm component occupies 62.98%, while the content of <20 μm component occupies 42.76%. There is an opposite changes of the content in section between clay fraction and coarse silt fraction. The Xiashu Loess in Zhoujiashan Nanjing is similar to typical Northern Loess in China with the particle size distribution and cumulative curves. Overall, it presents the characteristics of very fine skewness, poorly sorting, leptokurtic and asymmetrical bi-modal curve in quartz grain size parameters. Grade-standard deviation curves present the distribution of "twin peak". Two obvious peaks in standard deviation is nearly 7.962 1 μm and 39.905 2 μm. The scope of the grain size fraction, corresponding to the two peaks, present to 0.399 1~15.886 6 μm and 15.886 6~112.468 3 μm, respectively. The surface shapes of quartz grains are mainly sub-angular. Surface mechanical structures not only have the characteristics of eolian features such as rich dish-shaped pits, curve ridge, parallel joint surface, textured structure and so on, but also slightly show the underwater environment characteristic as conchoidal breakage fractures, underwater ground surface and V-shaped pits. Meanwhile, it is different to the surface microstructive between 5~30 μm component and 30~50 μm component. Finally, our analysis has found that the Xiashu Loess in Zhoujiashan Nanjing belongs to the typical eolian-formation debris. It is probably a high degree of mixture with multiple sources. Our study has an important theoretical significance to understand the area dust transport mechanism.