乐平统含煤岩系旋回地层的天文周期驱动:以黔西北毕节地区为例

为了系统地研究黔西北毕节地区乐平统含煤岩系中存在的沉积旋回及其天文周期驱动，本文采用时间序列分析法和相关系数法对该地区4个钻孔剖面的自然伽马(GR)测井数据进行旋回地层学分析与天文检验。旋回分析结果显示，研究区乐平统含煤岩系记录了405 kyr长偏心率、125 kyr和95 kyr短偏心率、40.6 kyr和33 kyr地轴斜率周期信息，并且可识别出17个由405 kyr长偏心率周期驱动形成的旋回；天文检验结果显示，乐平统含煤岩系受到天文周期驱动的可能性大于99 %。根据识别出的沉积旋回并结合前人在西南地区的层序划分结果，在研究区乐平统建立了高分辨率层序地层格架，划分出3个三级层序和17个四级层序，同时在经过调谐后得到的时间标尺上加以精确的年龄约束，建立了乐平统的天文年代标尺。对层序地层格架和年代标尺的综合分析表明，乐平统的沉积时限在6.47~7.16 Myr之间，并且底部存在0.69 Myr的穿时现象，3个三级层序的沉积时限分别是2.34 Myr、1.98 Myr、2.84 Myr。最后分析和总结了各三级层序的沉积速率变化规律，乐平统平均沉积速率在2.91~4.11 cm/kyr之间，CSⅠ的沉积速率整体呈现出“慢—快”或“快—慢—快”的变化趋势，CSⅡ沉积速率变化规律不明显，CSⅢ整体呈现出“快—慢—快—慢—快”的变化趋势。本研究所建立的层序地层格架可为毕节地区乐平统煤系高精度的地层对比提供参考，同时所建立的天文年代标尺可为研究乐平世古气候、古环境和重大历史事件的演化提供高分辨率的年代地层框架。

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

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.