Astronomical forcing in Lopingian coal-bearing cycles: a case study of Bijie area in northwestern Guizhou

Abstract

To systematically study the sedimentary cycles and their astronomical forcing in the Lopingian coal-bearing series in Bijie area, northwestern Guizhou, the GR logging data from four boreholes in this area were analyzed and tested by using time series analysis and correlation coefficient method for the cyclostratigraphy analysis and astronomical testing.The cyclostratigraphy analysis results showed that the Lopingian coal-bearing series recorded the 405 kyr long eccentricity cycle, 125 kyr and 95 kyr short eccentricity cycle, and 40.6 kyr and 33 kyr obliquity cycle, and some 17 sedimentary cycles forced by 405 kyr long eccentricity cycle were identified.Astronomical testing results showed that the possibility of Lopingian coal-bearing series forced by astronomical periods is more than 99 %.According to the identified sedimentary cycles, and combining previous sequence subdivision results, a higher resolution sequence stratigraphic framework were established for the Lopingian in the study area, in which three third-order sequences and 17 fourth-order sequences were subdivided.At the same time, the precise age constrained by the tuned time scale was applied to establishing the astronomical time scale.A comprehensive analysis of sequence stratigraphic framework and the astronomical time scale showed that the deposition duration of the Lopingian was between 6.47 and 7.16 Myr, and there was a diachroneity of 0.69 Myr in the bottom part.In addition, the deposition duration of third-order sequence CSⅠ, CSⅡ and CSⅢ were 2.34 Myr, 1.98 Myr and 2.84 Myr, respectively.Finally, sedimentation rate variation of each third-order sequence was analyzed.The results showed that the average sedimentation rate of the Lopingian was between 2.91~4.11 cm/kyr, the sedimentation rate of CSⅠ displayed a change trend of "slow to fast" or "fast to slow and back to fast", the sedimentation rate of CSⅡ displayed a relatively stable state, and CSⅢ showed a cyclic trend of "fast-slow-fast-slow-fast".The established sequence stratigraphic framework can provide reference for high precision stratigraphic correlation of the Lopingian in Bijie, and at the same time, the established astronomical time scale can provide a high-resolution chronological framework for the study of the evolution of paleoclimate, paleo-environment and major historical events in the Lopingian.

Keywords

• bijie
• lopingian
• cyclostratigraphy
• astronomical testing
• high-resolution sequence stratigraphic framework
• astronomical time scale