Geochemical characteristics and palaeoenvironmental significance of the paleo-weathered crust at the Carboniferous bottom in Wudang of Guiyang

A well-preserved paleo-weathered crust of carbonate rocks is found at the bottom of Carboniferous Xiangbai Formation at Xiaoguankou Section of Wudang, Guiyang. Based on field observation and indoor comprehensive research, it can be referred that the paleo-weathered crust is mainly composed of grey-green or brick-red ferrous kaolinite claystone and limonite-bearing silty claystone. Al₂O₃, TiO₂ and Fe₂O₃ as the major elements are rich in the paleo-weathered crust. SiO₂ is leached slightly, but K₂O, Na₂O, CaO and MgO are leached seriously. Trace elements, including Sc, V, Cr, Cu, Zn, Ga, Ba, Th and Pb, are relatively enriched, and Sr, Mn as the soluble elements are leached. The indexes, including Sr/Cu, A/NK, A/CNK, OI and CIA, all indicate that the paleo-weathered crust should be formed under the tropical to subtropical paleoclimate conditions of strong weathering, torridity and humidity. In addition, due to the stability of the paleo-weathered crust and the lithological characteristics of parent-rock, the authors inferred that ancient karst landform including karst depression and highland had been widely developed during the formation of paleo-weathered crust.