Predecessors had carried out a lot of researches on the sedimentary—tectonic transformation of Proterozoic-Phanerozoic, but did not reach a common censensus. Through the field survey, the writers found that a layer of 10～20cm tuff interbed developed on the top of the siliceous rocks of the Sinian Laobao Formation around the Changping Bahuang, Tongren, which was located in the transitional phase of platform—basin of the south eastern margin of Upper Yangtze Block. Based on petrological study, the writers proved that the tuff interbed was a layer of crystal tuffs, and also measured the SHRIMP zircon UPb age of crystal tuffs, gaining the depositional age of 556±5Ma. Based on predecessors’ achievements of biostratigraphy and sedimentology, the writers believe that the interlayer between siliceous rocks and black shales on the top of the Laobao Formation represented an important depositional event during the sedimentary—tectonic transition of Proterozoic—Phanerozoic. The depositional event started at ca. 556Ma and was connected to the Rodinia Supercontinent eventual breakup and the formation of Iapetus Ocean at 550Ma, corresponding to these extensive volcanic events on the Yangtze Craton.