Abstract：The spheroidal weathering of sedimentary pyroclastic rocks which deposited at the bottom of Emeishan basalt is intensive. The weathering extent is stronger evidently than that of the overlying basalt. The saprolited crust have spongy microtexture and are characteristic of high porosity，high intrinsic permeability，high specific water capacity，low specific yield and low density. Both of the sedimentary volcanic clasolite have a homothetic chemical composition. Also，the changes of chemical composition in the weathering process are nearly the same. The temperature fluctuation in vadose zone can form to tiny slot between different clasts. The condensated vapor water in the tiny slot dissolve the limestone clast firstly，and then a sponginess crust occur，whose water absorbability and storativity are stronger than other saprolized crusts. The liquid water which has been absorbed and stored in the sponginess crust can both ensures the dissolution of the pyroclasts in the crust itself and supports the spread of the weathering front. The weathering process of basic calcareous sendimentary pyroclast can be divided into two stages which can not be separated absolutely. In the first stage，limestone clasts are mainly dissolved and basaltic clasts are dissolved in second stage.
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2011, Vol. 30 
