Santanghu Basin, Permian Sedimentary Facies And Depositional Environment

Santanghu Basin is a congruent & reconstruct innertief, which developed on the basis of rift-fault basin of Permian. Permian Series distributed widely in the baisin. Lower Permian Series was composed of acid, neutral, and basic volcanics, subvolcanic rocks, volcanoclastic rocks and few normal sedimentary rocks. From western to eastern, the volcanics transited from acid volcanics to neutral and basic volcanics. Middle Permian Series was found Tiaohu Fm. and Lucaogou Fm. inside the basin, while Jingjingzaigou Fm. and Wulabe Fm. were found in the margin around the basin. Lucaogou Fm. was mainly composed of thick dark mudstone, micritic limestone, argillaceous dolomite, which reflected a shallow to fairly deep-lake sedimentary formation. The distribution of Tiaohu Fm. was similar to the distribution of Lucaogou Fm., but the lithologic character varied completely. Tiaohu Fm. was made up of thick neutral and basic volcanics, volcanoclastic rocks and few normal sedimentary rocks. The sedimentary rocks of Tiaohu Fm. were mainly composed of terreverte or mauve mudstone, sandstone and glutenite. Upper Permian Series only distributed scattered in the margin of the basin, and it was not found inside the basin.From the spatiotemporal distribution of the volcanics of Kalagang frn. and Tiaohu fm., we can see that there are not only eruptive facies such as rhyolite but also effusion facies such as andesite, basalt and shallow intrusive rock inside the basin. Because the volcanics and sedimentary rocks alternated with one and another, the volcanic activity of these two periods had the characteristics of many a time, multipoint of erupting.Early Permian was initial stages of expanding of the lake basin. Inside the region of interest, there developed fan delta, braided delta and lake facies. From northern and western to southern and eastern, the sedimentary facies transited from fan delta, braided delta to lake facies. Tiaohu period in middle Permian was the first period of broad lake-basin. The lake covered almost the whole basin. The border facies did not appear at the south edge of Tiaohu seg. In the basin, the braided river facies and braided delta facies were only found at the foot of Chahaquan palaeohigh in the west-south edge of the basin, the meandering delta was found from Mazhong structural belt to Fangfangliang horseback, and the other area were found covered by the shallow and fairly deep lake facies. Flat bedding, wavy bedding and vermiglyph were often found in the shallow lake facies, and the log was usually straight or slight dentation. In the fairly deep lake facies there found abundant bivalve fossils, such as the standard fossil of Permian in north Xingjiang-Tarfaniasp., Ostracoda, and so on. In the deeplake facies of Niujuanhu and Mazhong structural belts, there were also discovered gravity-flow microfacies, which was mainly composed of turbidite in which the slump structure and convolute bedding were often found. At the beginning of Tiaohu period, the lake basin shrank, and the water became shallow. Braided river facies and braided delta facies were found on both side of the basin, and deep lake facies disappeared, and shallow lake facies was only found in the southern of Tiaohu seg and central part of Malang seg.Use varied kinds of methods to analyze the depositional environment of Permian. The conclusion showed that the palaeoclimate represented the characteristic of half-aridity to moistness then to half-aridity. In early Permian the water salinity of the lake was half-salt water, while in Lucaogou period the lake water changed to salt water and hypersaline water, when the time to Tiaohu period, the water salinity declined back to freshwater and half-salt water. In Lucaogou period some regions even appeared terrestrial playa lake since the ratio of Sr/Ba even reached 42.5. Besides the aridity climate, the reason caused the high Salinity of the lake water possibly was the frequent transgression in the early and middle Permian.