Research On Clay Mineral Characteristics And Its Environment Significance Of The Middle Permian In Susong Area, Anhui Province

The Middle Permian strata in Susong are well developed, which are mainlycomposed of carbonate, clastic rocks and siliceous rocks. Based on field geologicalsurvey, microfacies analysis, clay mineral separation and quantitative experiments,this paper analyzed characteristic of lithofacies, sedimentary facies, clay minerals ofthe Middle Permian, and discussed the sedimentary environment and paleoclimateevolution in Susong area.The main types of rocks developed in Middle Permian of the study area arecalcirudite, bioclast grainstone, bioclast packstone, bioclast wackestone, micriticlimestone, siltstone, shale and a few dolomite. The lithofacies analysis shows that: thebottom detrital stone member in Qixia Formation belongs to littoral swamp facies; thelower part of swine limestone member belongs to carbonate platform and the upperbelongs to middle part of carbonate slope; the lower silicon member was deposited inthe lower part of carbonate slope or deep basin; the lower part of middle limestonemember belongs to carbonate platform and the upper belongs to lower part ofcarbonate slope or deep basin; the upper silicon member was deposited in lower partof carbonate slope or deep basin; the lower part of top limestone member belongs tocarbonate platform and the upper belongs to lower part of carbonate slope; GufengFormation was deposited in a deep basin; Wuxue Formation was deposited incarbonate platform. In Middle Permian, the study area experienced three majortransgressions, in which, the lower silicon member, the upper of middle limestone, theupper silicon member and Gufeng Formation were formed.By the experiment of clay mineral separation, the clay minerals in carbonaterocks, clastic rocks and siliceous rocks were extracted. The main steps includecrushing samples, removing carbonate and organic matter and clay mineral extraction.Air drying oriented slices, ethylene glycol treatment oriented slices and heat treatmentoriented slices are made for each clay mineral sample. Then, by professional software(jade7.0), the types and relative contents of clay minerals were obtained.The analysis of clay mineral shows that illite (60.37%) is most abundant,kaolinite(28.48%) takes second place, while chlorite (3.72%) is the least.Illite/smectite has an average content of7.43%. The values of Kübler are greater than0.25Δθ°, which indicates the crystallinity of clay mineral is poor and the clay mineralhas a low metamorphic degree. Therefore, clay minerals can be used to restore the paleoclimate.Combining with the feature of sedimentary environment and clay mineralcomposition, illite is used as paleoclimate indicator factor. When the climate is coldand dry, the weathering degree of rocks is low, so illite content is high. In the contrarycase, the content of illite is low. Based on the content of illite, the Middle Permian canbe divided into five periods, four cycles of cold-warm stage and one cold stage. QixiaStage has three cycles of cold-warm stage, Gufeng Stage has one cycle of cold-warmstage and Wuxue Stage has one cold stage. Although Wuxue Stage has high content ofillite, the content of illite decreases, which indicates the climate has a tendency tobecome warm. On the whole period of Middle Permian, the content of illite at bothends is more than the content in the middle. From Qixia Stage to Gufeng Stage, theclimate changed from cold and dry to warm and wet gradually, in Wuxue Stage, theclimate became cold and dry suddenly. The trend of Middle Permian climateevolution is about the same as the result of Bo Chen et al.（2013）, who used δ18O inconodonts from South China to restore paleoclimate. But the timing of climate changeis later than the result in this paper. Volcanoes and hydrothermal activities may slowedthe drop of seawater temperature. Another possibility is that conodonts’ life habitschanged. As a result, the perched water layer of conodonts became shallower andhigher temperature is recorded.