浙江普陀山岛老红砂的成因与环境

Origin and Circumstance of Old Red Sand Sediment On Putuo Island

作者根据野外实地考察,描述了浙江普陀山岛老红砂的性状,通过样品的采集及室内分析,从老红砂的沉积结构、石英砂颗粒表面微结构特征、微量元素等揭示了老红砂属于古风砂沉积。通过与中国华南地区老红砂的比较分析,结合老红砂的年代测定数据以及普陀山岛的地理环境特征等,揭示了该地的古地理环境。

The old red sandy sediment on Putuo Island, Zhejiang Province was once recorded in the local chorography, but no systematical research has ever been done on such old red sandy sediment. However, since the old red sandy sediment on Putuo Island proved to be a very rare case found on the north subtropical coasts in China middle latitude area, this research turns out to be a very meaningful one, for it helps explain the origin of the old red sandy sediment as well as the ancient geographical environment in China＇s middle latitude zones. The field investigation conducted by the author reveals that the old red sandy sediment on Putuo Island is mainly consisted of terrigenous clastics, the sandy sediment has mainly medium-fine grain size with more silts and clays, better-medium sorting, sandy sediment in the dominant color of brown red or black brown-red. Mostly in its stripshaped forms, the majority of the old red sandy sediment is distributed on the land capes vertical to the coast extending toward the sea. Beneath the sediment is always a granite regolith with rough surface. Taking the Feisha＇ao Cape as the specimen, the authors embarked on a sample collection and a series of laboratory analysis, including granularity analysis, quartz electronic microscope scanning, XRD analysis, thermoluminescene dating test, etc. , and the paper elaborated the sedimentary characteristics, origin and evolvement law of the old red sandy sediment in the area. The research results show that, the old red sandy sediment has shown that its sediment character are similar to that of recent coastal wind-drift sands, and so may be clearly distinguishable from the beach sands. The surface textures include dish-shaped pits, snakelike ridges and up-curved cleavage thin section, etc. , reflecting the typical features of aeolian environment, as well as the crystal grouth showing features of surface soil weathering. Elements found in the sediment and their ratios also recognize itself as continental sediment, therefore, it could be concluded that the old red sandy sediment falls into the category of ancient effloresced sand sediment. As shown in a comparative analysis on the old red sandy sediment found in South China, the old red sandy sediment on Putuo Island is not as typically evolved as that in South China. Combined with the age-testing data of the old red sandy sediment and geographical circumstance characters of Putuo Island, it is proposed that the old red sandy sediment on Putuo Island is the effloresced sand sediments in the medium and late period of the late Pleistocene. Since the sediment was evolved in cold and dry climate with low sea level and slight oxidization, such sediment on Putuo Island is not as typical as that in South China. The sedimentary process of the red sandy sediment in this region is mainly characterized by the self-reddening in its post-sedimentary period. As a result of regional difference, the red pigment on Putuo Island, dominant by acus-shaped iron ore, is quite different from that in South China. The difference in color is reflected by a grey dark shade, dominant by brown red and black brown red.