Geochemistry Of Cretaceous-early Paleogene Detrital Rocks And Geological Significance In Simao Basin, Yunnan Province

Simao Basin is one of important salt-forming basins in China, where there are rich potassium and rock salt resources. Mengyejing potash deposits in Jiangcheng of Yunnan is an unique ancient solid potash deposits which is in the clastic sediments in China. The deposits is mainly consist of chloride-type salt sediments especially to saline rocks and sylvinite. Since the deposits were detected, a great deal geologic reconnaissance have been researched for the basin evolution、the form of tectonic fault zone、the genesis and material sources. However, hardly any researches aim to study the geochemical characteristics of clastic sediments from salt-bearing formation, overlying and underlying strata. Sedimentary stratum over 10 thousand meters in thickness dominated by sand shale in Mesozoic and Cenozoic period of Simao Basin, which provides an advantage for discussing provenance properties and tectonic evolution process from the point of geochemical characteristics of clastic rocks. The geochemistry of sandstones and mudstones a sedimentary basin tends to be influenced by provenances, and thus may provide important geological information about the sediments. Therefore, tectonic setting and provenance properties of sedimentary basin could be revealed by geochemical characteristics of clastic rocks.The Cretaceous-Early Paleogene strata in the Simao Basin consist of the Mangang Formation(K1m), the Pashahe Formation(K1p), the Mengyejing Formation(K2me) and the Denghei Formation(E1d) which are primarily composed of sandstones and mudstones. Based on the geochemical research into clastic rocks of which 19 from Jiangcheng Erguan Zhai-Heping Zhai-pashahe Road Course section and 16 from SHK4 drilling in Jiangcheng Mengyejing deposit. Three conclusions have been yield which are presented as follows: Firstly, in theses samples, δCe vs. δEu, δCe vs. ∑REE, CIA vs. Th/U、CIA vs. Th/Sc show no correlation which imply that the Cretaceous- Early Paleogene clastic rocks are mainly controlled by source region rather than chemical weathering and oxidation-reduction conditions. Secondly, the distribution pattern of rare earth element are basically the same, LREE/HREE ratios are high, Eu negative anomalies and combines with microscopic identification indicating are evident the same source region of the Cretaceous- Early Paleogene sediments with little mantle material adding. The rocks are mainly pro-granite felsic, and the parent rock debris come predominately from the upper crust which are in accordance with the features displayed by the La/Th-Hf、La/Sc-Co/Th、REE-La/Yb 、 Hf-Co-Th discrimination diagrams. Thirdly, after the deposition of Pashahe Formations, a sudden rise of Ca O content, Gd content, ICV,(Gd/Yb)N ratio demonstrating that the provenance area is in an rapid uplift and erosion stage.From the above, we can obtain such conclusions as follows by geochemical characteristics and lithofacies paleogeographical study of sedimentary rocks. The sediment provenance has been originated from the ancient continent and mountains the east and the west sides of Simao Basin since Cretaceous-Early Paleogene. However, the sediment provenance varies in different sedimentary evolution period. In Cretaceous-Early Paleogene strata, the original substance was originated from upper crust, and the provenance property is pro-granite type on the whole. The tectonic setting of source area of clastic rocks in Cretaceous-Early Paleogene strata belongs to the passive continental margin, and the result is consistent with the tectonic setting of the Simao Basin which located in the Sanjiang orogenic belt.