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The ancient weathered crust studied occurs on the razed surfaces of various physiographic periods atdifferent altitudes in the Sichuan red basin.A typical profile of the ancient weathered crust can be divided intothree weathered belts from the surface to the basement:the clastic clay belt(Belt Ⅰ),the elastic breccia-clasticclay belt(Belt Ⅱ),and the clastic breccia belt(Belt Ⅲ).These belts are different in clay mineral composition,reflecting varying degrees of chemical weathering.Wide variations are recognized in the abundance of chemical elements in the profile of th ancientweathered crust.In comparison with the underlying purple parent rocks,Fe_2O_3>Al_2O_3>TiO_2>SiO_2 arerelatively concentrated in the ancient weathered crust,while CaO>P_2O_5>MnO>Na_2O>MgO>K_2O arerelatively depleted due to leaching in order of Belt Ⅰ>Ⅱ>Ⅲ.The extent of chemical weathering in the ancient weathered crust in the Sichuan red basin is closelyrelated to the time of formation of the razed surface.For this reason,the contents of Al_2O_3 and Fe_2O_3 or theirconcentration values,and the leaching amounts of CaO,MgO,K_2O,Na_2O,and P_2O_5 are higher in the ancientweathered crust at high altitudes than those at low altitudes.
The ancient weathered crust studied occurs on the razed surfaces of various physiographic periods at different altitudes in the Sichuan red basin. A typical profile of the ancient weathered crust can be divided intothree weathered belts from the surface to the basement: the clastic clay belt ), the elastic breccia-clastic belt belt (Belt II), and the clastic breccia belt (Belt III) .These belts are different in clay mineral composition, reflecting varying degrees of chemical weathering.Wide variations are recognized in the abundance of chemical elements in the profile of th ancientweathered crust.In comparison with the underlying purple parent rocks, Fe_2O_3> Al_2O_3> TiO_2> SiO_2 arerelatively concentrated in the ancient weathered crust, while CaO> P_2O_5> MnO> Na_2O> MgO> K_2O arerelatively depleted due to leaching in order of Belt Ⅰ> Ⅱ> Ⅲ. The extent of chemical weathering in the ancient weathered crust in the Sichuan red basin is closely related to the time of formation of the razed surface .For this reason, the contents of Al_2O_3 and Fe_2O_3 or theirconcentration values, and the leaching amounts of CaO, MgO, K_2O, Na_2O, and P_2O_5 are higher in the ancientweathered crust at high altitudes than those at low altitudes.