Geochemical characteristics of the Lower Permian Liangshan Formation in Dushan area of Guizhou Province and their implications for the paleoenvironment
Corresponding relationship between geochemical characteristics and depositional environments was established through analyzing distribution of mudstone, content and indexes of the trace and rare earth elements from the Liangshan Formation, Lower Permian, Dushan area, southern Guzhou. The results show that the gross amount of REE (SigmaREE) varies from 127.23*10~(-6) to 450.07* 10~(-6) with an average of 271.09 * 10~(-6),which is obviously higher than the gross amount of American shale REE. And the ratio of LREE/HREE is usually between 7.66 and 15.67 with an average of 9.51, which indicates enrichment of LREE. Relevance between Ce anomalies and Eu anomalies, Dy_n/Sm_n ratios, and bulk REE show that diagenesis had some effects on REE of the Liangshan Formation during the depositional transformation period, and it seems that abnormal values of Ce anomalies are not a reliable redox indicator. An oxygen and oxygen-depleted seawater environment was judged through trace element ratio indexes, such as Ni/Co 0.20,U/Th0.2, V/Cr1.09, deltaU0.73, V/Sc7.43; in addition, abundant development of benthos can be a strong evidence. The authors argue that the seawater environment of sedimentary period during the Liangshan Formation was an evolution process with dynamic transformation, rather than a pure oxygenate, low oxygen or reducing environment. More specifically, the initial seawater environment represented oxygenate condition for biological reproduction, then gradually evolved into low oxygen or even reducing environment which hindered degradation of organic matter. The authors put forward two possible cases: (1) Deposition flux of high organic-matter under humid climate and rapid accumulation of shallow-water deposit used much oxygen of bottom water, which resulted in a transformation from oxygenate to low oxygen until the reducing environment;(2) The water interface overlapped or fluctuated near the oxygenate to low oxygen interface of the Liangshan Formation, and the oxygenate to low oxygen interface rose gradually with continuous transgressive process, then a low oxygen environment appeared with the death of early organisms, and oxygen exchange between deposits and water became less and finally transferred into a reducing environment.