Sequence-Stratigraphic Framework And Its Palaeogeographical Evolution Of The Lower Cambrian Series In Guizhou And Its Adjacent Areas

A large scale carbonate platform is drowned from Sinian to Cambrian in Guizhou and its adjacent areas, which leads to Niutitang Formation sedimented abroad. After that, in shallow marine, Mingxinsi Formation, Jindingshan Formation and the lower Qingxudong Formation are formed; simultaneously, in the deeper marine, Jiumenchong Formation, Bianmachong Formation and Palang Formation are produced. Qingxudong Formation standing in vivid contrast in these two areas. In the deepest marine, it is just called Zhalagou Formation because it is too thin to divide. Lithostratigraphic succession shows a carbonate platform`s development. The Lower Cambrian in the study area constitutes a second-order sequence that can be further subdivided into five third-order sequences, and forms a regularly cyclic succession of the transgression-regression. There is a regularly vertical stacking pattern for the third-order sequences in the second-order sequence. From the bottom to the top, the third-order sequences that are characerized by the succession of"the CS ( Condensed Section ) + HST (High-stand System Tract )"are changed into those marked by the succession of"the TST (Transgressive System Tract) + CS + HST", correspondingly, the drowning-type sequence boundary is changed into exposure-type one. Therefore, both the second-order and the third-order sequences have a similarity of sedimentary-facies fabric. A concomitant with these temporal changes is that the Lower Cambrian containing five third-order sequences with about 1, 000 meter thickness is changed into a condensed succession that third-order sequences can not be identified toward the southeast with the deepening of the sedimentary environment. Ultimately, a series of the panel diagrams of sequence-stratigraphic framework shows the temporal and spatial changes of sedimentary-facies succession of the third-order sequence, and a series of the corresponding palaeogeographic maps indicates the evolution of palaeogeographic setting that forms the third-order ssequences.