五峰期上扬子海古盐度分析

<正> 上扬子区五峰期地层发育含笔石黑色岩系,五峰期上扬子区的沉积环境一直是研究者关注的焦点(穆恩之1954;穆恩之等1981;俞剑华等1984;戎嘉余1984;陈旭等1987;戎嘉余、陈旭1987;汪啸风1989)。五峰期上扬子区的沉积环境因素是多方面的,就水体

Analysis of paleosalinity during the Wufeng age in Upper Yangtze Sea

The Wufengian strata in Hanyuan area of Sichuan Province are products deposited or, the western border of Upper Yangtze Sea, so the geochemical information contained in them is very sensitive to the changes in palaeosalinity and sea level. According to the test results of Al2O3, MgO, Cl-, Sr and Ba, the palaeosalinity of the sea is analyzed and, on the basis of this, some discussions about the relative changes in the sea level are given.1.Synthetic indexes of palaeosalinity. The synthetic indexes of palaeosalinity used by the authors include the ratio of magnesium to aluminium (m = 100×MgO/Al2O3), the ratio of strontium to barium (Sr/Ba) and the amount of chlorine (Cl-). The correlation coefficients among them vary from 0.84 to 0.93. This strongly positive correlation indicates that m, Sr/Ba and the amount of Cl- are dependable when they are used to deduce the relative changes in palaeosalinity of the Wufengian Upper Yangtze Sea.2.Characteristics Of palaeosalinity changes. According to the synthetic indexes of palaeosalinity, the following characteristics of palaeosaline changes could be recognized: the palaeosalinity of the Upper Yangtze Sea was obviously changed through the Wufengian age, experiencing two relative reduction periods and two relative increase periods. Although the relative changes in palaeosalinity are restricted to the scope of marine salinity, the lower everage value of m (37.75) in two reduction periods shows that the Wufengian Upper Yangtze Sea suffered strong desalinization. Two desalinization events occurred respectively in the early Wufeugian (the lower part of the Dicellograptus szechuanensis zone) and in tile late Wufengian (the Diplograptus bohemicus zone - the Glyptograptus persculptus zone).3.Changes of the sea level. The palaeosaline changes in the Wufengian Upper Yangtze Sea might have been related to global glaciation or strongly pouring of fresh water. Both the glaciation ang pouring of fresh water could create the same effects: the sea level fell ami the palaeosalinity increased when global glaciers were formed or fresh water was not enough; contrarily, the sea level went up and the palaeosalinity was reduced during melting of global glaciers or pouring of abundant fresh water. Therefore, based on the palaeosalinity changes in the Wufengian Upper Yangtze Sea, the authors could conclude that the sea undergone two transgressions and two regressions, and two transgressive summits happened resoectively in the early Wufengian when the lower part of the Dicellograptus szechuanensis zone was deposited, and in the late Wufengian during which the Diplograptus bohemicus zone as well as the Glyptograptus persculptus zone were formed. This conclusion is different from the previous view that the Wufengian Upper Yangtze Sea passed through a continuous regression.