The Early Triassic Sequence Stratigraphic Framework in Southwestern Margin of Yangtze Platform

The early Triassic lithostratigraphy in southwestern margin of Yangtze Platform is divided into 4 depositional sequences by studying the structure and stacking pattern of stratigraphic bodies, the features of unconformity-bound. The depositional sequences are subdivided into lowstand system tract, transgressive system tract and highstand system tract. After researching the data about conodonts obtained, and correlating them with Triassic conodont biozones established by Haq et al., the boundary ages of sequence and system tract are determined initially (lower boundary age of sequence 1 is 251Ma; lower boundary age of sequence 2 is 245.2Ma; lower boundary age of sequence 3 is 243Ma; lower boundary age is 240.5Ma, upper boundary age is 239.4Ma in sequence 4). Thus early Triassic stratigraphic framework with higher resolution is established in this area. The characters of sequence-combination in framework are displayed clearly in carbonate platform margin.The platform margin retreated successively toward continent during highstand system tracts of sequence 1-2, showing combinatorial relation overlapped. While platform margin advanced orderly in highstand systems tract stage of sequence 3-4, showing combinatorial relation offlaped. The characters of sequence-combinations mentioned over were controlled by the tectonic subsidence of sedimentary basin and the change of up-down subcycle of 2 order sea level. The sequences showed retrograding combinatorial relation when the tectonic subsidence was accelerated and sea level rised; on the contrary, the sequence appeared prograding combinatorial relation when the tectonic subsidence was gentle and sea level declined. In addition, the change law of carbon-oxygen isotope researched in sequence 1-bottom sequence 3 in slop zone (Gaimao, Guiyang) is as follow: The change of carbon-oxygen isotope composition isnot large in transgressive system tracts-highstand system tracts. δ~ 13C is mainly positive, but it move obviously to negative direction in top highstand system tracts. The change of carbon-oxygen isotope composition is very big and negative number in lowstand system tracts. These geochemical characters provide basis for dividing stratigraphy.