The Middle—Late Triassic Sequence Stratigraphic Framework in Southwestern Guizhou and Adjacent Regions

The Middle—Late Triassic strata in southwesten Guizhou and adjacent regions can be divided into 5 depositional sequences based on the study of features, structures and stacking patterns of the stratigraphic boundaries. Besides, characteristics of all systems tracts in the depositional sequence were described. Then we investigated the obtained biostratigraphic data and correlated these data with Triassic biozones established by Haq et al. On this basis, boundary ages of sequences and systems tracts were determined preliminarily (the lower-boundary age of sequence 1 is 239.4Ma; that of sequence, 2, 238Ma; that of sequence 3, 233Ma; that of sequence 4, 227Ma; the upper-boundary age of sequence 4 is 223Ma). Thus, a high-resolution Middle—Late Triassic stratigraphic framework was established in this area. When the second-order sea level is falling slowly, the combination pattern of the sequence stratigraphy in the framework is affected largely by tectonic subsidence of the sedimentary basin. A retrograding combination pattern could be seen when the tectonic subsidence is accelerated (e.g. sequence 1 and sequence 3); contrarily, the sequence is characterized by prograding combination when tectonic subsidence is relatively gentle (e,g., sequence 2 and sequence 4).