Lower Cambrian sequence-stratigraphic framework of southeasternGuizhou Province

Lower Cambrian strata are quite completely developed in southeast Guizhou,and the transitional facies characterized by intense facies change comprise an upward shallowing sedimentary succession,which constitutes a second-order sequence.This second-order sequence can be further divided into five third-order sequences.From northwest to southeast,with the change of sedimentary setting from slope to basin,Lower Cambrian strata gradually thin and eventually form a condensed succession which is difficult to divide in the third-order scale.Based on two kinds of natures of the third-order sequence,i.e.,the regularity of lithofacies succession in space and the synchronism of environmental change in time,and taking the two kinds of facies-transition surface and two kinds of diachronisms in stratigraphic records as the fundamental elements in building sequence-stratigraphic framework for the Lower Cambrian strata of the study area,the authors hold that the sequence-stratigraphic framework demonstrates the special third-order sedimentary succession patterns formed after the platform drowning event caused by the rapid transgression at the early stage of Cambrian.