Sequence stratigraphic division and framework of the Lower Cambrian in the Upper Yangtze region

The Lower in the Upper Yangtze region,especially in Guizhou and its adjacent areas,is completely developed and marked by a succession from black shales to carbonate rocks.The drowning event of the platform that occurred at the turn from the Sinian to the resulted in the formation of a series of black shales of anoxic basin facies represented by the Niutitang Formation in the study region.With shallowing of the sedimentary environment a sequence of carbonate rocks represented by the Qingxudong Formation formed at the top of the Lower Cambrian.Thus the Lower forms a second-order sequence,which may be subdivided into five third-order sequences,forming a regular cyclic succession of transgression-regression.The third-order sequences in the second-order sequence form a regular vertical stacking pattern.From the bottom to the top,the third-order sequences change from the succession of CS (condensed section)+HST (highstaud systems tract)to the succession of TST (transgressive systems tract)+CS +HST,and correspondingly the drowning-type sequence boundary changes to the pronounced exposed hiatus-type one,implying the similarity between the second-order sequence and the third-order sequence in respect to lithofacies organization.This temporal change produced some differences in different paleogeographic settings.From northwest to southeast, the sedimentary environment deepened and the Lower became thin and changed into a condensed succession that cannot be subdivided into third-order sequences.On the basis of the sequence stratigraphic divisions at typical sections with different paleogeographic environments and according to the elementary features of the third-order sequence,i.e.the regularity of sedimentary facies succession in space and the synchroneity of environmental changes in time,we may establish a sequence stratigraphic framework that can demonstrate types of facies change surface and two types of diachroneity in the stratigraphic record.This sequence stratigraphic framework shows that the Lower is a complex and regular stratigraphic succession, representing the growth process of the carbonate platform with the condensed succession as the base. Concomitantly with the rapid transgression in the Early Cambrian,the ecological space expanded,which formed the antecedent conditions of paleogeographic setting for the Cambrian explosion of life.In addition,the very complex,close relationship between the depositional events and the biological diversity events means many problems that remain to be further studied in the future.