Clay Mineral Assemblages in the Lower Carboniferous of Central Hunan, South China

Lower Carboniferous in central Hunan, South China includes Shaodong. Menggongao Liujialang, Shidengzi, Ceshui and Zimenqiao formations. Shaodong Formation and Ceshui Formation mainly consist of clastic rocks and formed in the offshore muddy shelf and shoreline barrier-lagoon envi-ronments respectively. The other four formations mainly consist of carbonate rocks intercatated with the calcareous mudstone and shale, in which Menggongao and Shidengzi formations mainly formed in the offshore shallow carbonate shelf, Liujiatang formation mainly formed in the deep shelf and shelf basic and Zimenqiao formation formed in the peritidal and offshore shallow carbonate shelf environments. Mudstone (including mudstone, silty mudstone, calcareous mudstone and calcareous shale) is one of the major rock types in each formation. X-ray diffraction and SEM analyses show that the clay minerals of Lower Carboniferous mudstones in central Hunan are kaolinites, illites. I / S mixed layer minerals and chlorites, in which kaolinite and illite are the two important minerals. Clay minerals are distributed regularly in the Lower Carboniferous section. The highest and lowest contents of kaolinite occur in Ceshui and Lijiatang formations respectively, and illite shows the reverse trend. I/ S mixed laver mineral is up to the highest content in Menggongao- Shidengzi and Zimenqiao formations. Chlorite is less common and with higher contents in Shaodong formation. Refering to the major lithofacies of each formation, the environmental distribution model can be outlined. From clastic barrier-lagoon to offshore muddy shelf, offshore shallow carbonate shelf, deep shelf and shelf basin, that is from the clastic shoreline to the shelf basin, kaolinites decreases suddenly and the highest content is in the clastic shorline, illites increases gradually and up to the highest content in the shelf basin, I / S mixed layer minerals and chlorite increase firstly and then decrease and their highest contents are in offshore shallow carbonate shelf and offshore muddy shelf respectively. The environmental distributions of clay minerals are mainly controlled by water acidity and salinity of the depositional environment. On the landward, the acid and fresh-brackish water favous the deposition of kaolinite such as in Ceshui formation, and on the basinward, the alkaline and marine water favors the deposition of illite such as in Liujiatang formation. This conclusion also supports the idea that the assemblages of clay mineral can act as the indicator of the paleosalinity. Ceshui formation is the major coal measure of Lower Carboniferous. The vertical distributions of clay minerals show that, from the bottom to the top, the major type of clay minerals changes from illite, kaolinite to illite, which suggests that the depositional environment of Ceshui formation experienced weak alkaline marine water, acid fresh-brackish water to alkaline marine water.