Based on field observations and indoor researches, the authors hold that there exists an unconformity between Sinian Wanlong Formation and Badaojiang Formation in southern Jilin, as evidenced by three paleo_weathering crusts discovered there. In the past, however, the contact was believed to be a conformity contact. Chemical analysis shows that there is a remarkable change of chemical components from base rocks to the paleo_weathering crust. Unlike CaO and LOI, the total amount of SiO_2 Al_2O_3 of the weathering crusts has increased from 13.57% to 55.63%, suggesting the existence of an obvious formation process of paleosol. The variation of chemical index of alteration (CIA) and that of moving index of mass from the base rocks to the weathering crust show the same trend. CIA is up to 39.43%, whereas the moving index of mass of Ca is up to -0.97, implying that Ca component has been almost completely depleted. All the above analyses show that these weathering crusts belong to the second paleosol formation stage of carbonate rocks, i.e., they were developed by the accumulation of undissolved materials in base rocks and hence belong to the 2nd_order sequence surface.