From Sequence-Stratigraphic Division to the Establishment of Sequence-Stratigraphic Framework,the Middle to Upper Cambrian in the Upper-Yangtze Region

In Upper-Yangtze region,especially in Guizhou Province and its adjacent areas,the Middle to Upper Cambrian is well developed and is marked by a succession from limestone to dolomite.The progressive diachronism represented by great-thick Group indicates a progression of carbonate platform resulted from second-order regression.Thus,a complex and regular sequence-stratigraphic framework of Middle to Upper Cambrian was formed in Upper-Yangtze region.In shallow-water background,the Middle to Upper Cambrian is made up of both thin Douposi Formation with fossils and great-thick Loushanguan Group lacking fossils,in which there are some difficulties to discern third-order sequences.But regularity of of Middle to Upper Cambrian in deeper background make it easy to discern third-order sequence,and this regularity can be used to make up shortage for division of third-order sequences in shallow-water background.The Middle to Upper Cambrian in study area constitute a second-order sequence that can be further divided into seven third-order sequences,which forms a regularly transgressive-regressive cyclic succession.Along with these temporal changes,the palaeogeographic background of Middle to Late Cmbrian in study area become deep toward southeast,and Middle to Upper Cambrian marked by a great-thick dolomite in shallow setting is changed into a set of muddy shale with limestone in deep setting.According to elementary features of third-order sequence,i.e.the regularity of succession in space and synchronism of sedimentary-environment changes,the detailed division of third-order sequence at main logged section in different palaeogeographical background become basis to establish sequence-stratigraphic framework that can demonstrate two types of facies-changing surface and two types of diachronism in stratigraphic record.There is an important meaning for this sequence-stratigraphic framework,i.e.the sequence-stratagraphic study on basis of the regularity represented by temporal succession of sediments and spatial pattern of sedimentary-facies is a useful tool to get more regularities from complex stratigraphic records.