Study Of Sedimentary Environment And Its Rapidcementation Causes Of Jiangbei Conglomerate

Jiangbei conglomerate which is called Conglomearte and sandstone of Jiangbei’epoch by Liu Xingshi and called Jiangbei’gravel by Li Chunyu is proposed by Ha Anmu. Conglomearte and sandstone of Jiangbei’epoch is the most scientific name.People used to call it Jiangbei conglomerate so this paper will continue call it Jiangbei conglomerate. But it’s the same meaning as Conglomearte and sandstone of Jiangbei’epoch. Jiangbei conglomerate is a kind of allothigenic supergene cemented rock that wide-spreads in lower terrace of Quaternary sediment along the river in Sichuan Basin and it’s adjacent area, and consists of conglomerate and sandstone.Hence,it is also named Jiangbei conglomerate and sandstone rock.Jiangbei conglomerate has it own characteristic different from other river sediment, namely it cemented to solid rock on earth’s surface and only spread on the lower terrace. Jiangbei conglomerate look like concrete on earth’s surface, many visitors take Jiangbei conglomerate for the concrete.However in order to correct people’s understanding researchers is strongly interested in the reason of its rapidly cemented on earth’s surface, they speculation for the cementing causes and environment of Jiangbei conglomerate through their own researches. Though predecessors has had a more detailed study to jiangbei conglomerate, there are still many dispute and unresolved issues, such as the causes of the rapid cementation on earth’s surface of Jiangbei. etc.The analysing of gravel group of Jiangbei conglomerate and dating shows that Jiangbei conglomerate of Xishi Fuzhong section has experience two sedimentary spinning back, fine to thick and thick to fine. Concretely speaking it can be divided into seven sedimentary spinning backs and each spinning back represents a flood period. Based on it there superposition more still more small scale of crude, fine changes, it is speculate that these changes of gravel is caused by frequently fluctuations of climate. According to the research chongqing and upstream area the precipitation had been decreased after a little increase between24.3KaB.P. and10KaB. P.. In particularly, the gravel degree have seven times from thick to fine,so the precipitation may has seven times from more to less in chongqing and upstream area between24.3KaB.P. and10KaB. P..It shows that Precipitation intensity changes greatly in this time. Among them in22.5KaB.P. sedimentary a sedimentary which long axis reach31cm, it shows that The water power was very strong. But the layer is thickness, so we speculated that the rainfall took a short time. Relatively wet period abundant precipitation, the water power is stronger, the big gravel can been take to the beach and sedimentary by the flood. Otherwise,relatively dry period less rainfall, the water power is weak, it can only bring small gravel and sedimentary,many fine sand was taken to sedimentary and in formed many sandstone layers.Therefore, we speculation that the change of gravel form fine to thick in Small scale is connected wit h frequent fluctuations between dry and wet.Previous research analysis shows that, jiangbei conglomerate formed in the period of24.3KaB.P.and10KaB.P.At this period the general trend of climate change in Chongqing area is from warm wet to dry cold. In this article, through the data of analysis of the Rb/Sr ratio of jiangbei conglomerate and magnetic susceptibility, we find that the Rb/Sr ratio of jiangbei conglomerate is greater than the Rb/Sr ratio of sandstone and the magnetism of conglomerate is also greater than the magnetism of sandstone. As is known to all in warm wet climate conditions, precipitation, river water volume and the erosion of jiangbei conglomerate are strong, in this climate conditions Sr is easy to leach away, thus Rb/Sr ratio is greater. High value magnetism is in corresponding with a warm wet climate, and low value represents a dry cold climate. So we conclusion that between24.3KaB.P. and10KaB.P. in Chongqing area, the trend of climate change is from warm wet to dry cold in general. But in this background there still exists a few small scale of climate change. In relatively warm wet climate condition, gravels accumulate and change into conglomerate; in relatively dry wet climate condition, it formed sandstone.