Abstract:

In the Upper-Yangtze region, especially in Guizhou Province and its adjacent areas, the Lower Cambrian is well developed and is marked by a succession from black shale to carbonate rocks. And this stratigraphic succession makes up a complex and regular sequence-stratigraphic framework. The Lower Cambrian in the study area constitutes a second-order sequence that can be further subdivided into five third-order sequences, and form a regularly cyclic succession of the transgression-regression. There is a regularly vertical stacking pattern for the third-order sequence in the second-order sequence. From the bottom to the top, third-order sequences that are characterized 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 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. The sequence-stratigraphic framework can demonstrate both the elementary features of the third-order sequence, i.e. the regularity of sedimentary-facies successions in space and the synchronism of sedimentary-environment changes in time, and two kinds of facies-changing surfaces as well as two kinds of diachrononism in the stratigraphic records. And the sequence-stratigraphic framework represents its complex forming-setting of palaeogeography. Third-order sequences with different facies-succession mean particular paleogeographic backgrounds; therefore, five third-order sequences of the Early Cambrian represent a complex paleogeographic variance in the study area. 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 paleogeographic maps indicates the evolution of paleogeographic setting that forms the third-order sequences.

Key words: the Upper-Yangtze region, the Lower Cambrian, sequence-stratigraphic framework, paleogeographic background