Stratigraphic sequences and sedimentary facies of Qigebulak Formation at Xianerbulak, Tarim Basin

Sinian is one of the most important stages of the development of passive continental margin basin in the Tarim Basin. Based on the measurement and analysis of profiles of the Qigebulak Formation (Z₂g) of the upper Sinian at Xianerbulak of Aksu, it has been concluded that the Ⅲ-order sequence boundary surfaces with hiatus and lithofacies transition and the Ⅱ-order surfaces with transgressive onlap and sequence unconformity interface develop at the bottom and on the top of the Qigebulak Formation, respectively. In the middle of the formation, deepening interruption surface and shallowing-upward Ⅳ-Ⅴ sequences develop. The Milankovitch cycle of carbonate rocks has been found, under either peritidal (becoming shallow, fine-grained and thin upwards) or subtidal (becoming shallow, coarse-grained and thick upwards) environments controlled by sea level changes. Stromatolites are widespread, mainly consisting of various microbial mats. The Qigebulak Formation can be divided into 3 lithologic sections from bottom to top, among which the lower section changes from supratidal to intertidal, the lower part of intertidal to intertidal, and intertidal to supratidal; the middle section is characterized by the frequent variation from the lower part of intertidal to subtidal and the stable environment of subtidal. Carbon isotopes are strongly positive at the bottom, stable in the middle and upper sections, and weakly positive at the top. Sea levels change frequently at the bottom, and stably decrease in the middle and upper sections.