藏南晚白垩世旋回地层学研究:以定日贡扎剖面为例

旋回地层学是提高地质年代标尺精度的一种重要方法。特提斯海相沉积是旋回地层学研究的良好对象,与西特提斯比较,东特提斯的旋回地层学研究程度较低,第四纪之前的研究精确度有待进一步的提高。西藏南部广泛出露海相白垩系,其中定日贡扎剖面出露的上白垩统尤为连续和完整。在对贡扎剖面野外观察及其层束厚度测量的基础上,通过快速傅立叶变换(FFT),分析该地层蕴含的地球轨道周期,得到了与理论值较为接近的结果:偏心率周期,4092 ka/周期(E1),1279 ka/周期(E2),93 ka/周期(E5);斜率周期,512 ka/周期(O1),409 ka/周期(O2),389 ka/周期(O2)。在功率谱分析过程中,利用层束和超层束组的关系进行了调谐的处理,并比较调谐前后结果,来确定调谐的重要性,证明该地层沉积速率的变化:地层由老至新,其沉积速率的变化趋势整体为由小变大,变化范围为086～253 cm/ka。进一步对比东西特提斯和其他地区研究结果,证明东西特提斯海相沉积在外形、成因和所蕴含的轨道周期的一致性,地球轨道周期变长与理论值变化的一致性。最后给出了下一步工作的建议。关键词：藏南; 定日; 晚白垩世; Coniacian阶; 旋回地层学

Cyclostratigraphic study of the Upper Cretaceous of Southern Tibet,China:A case study of Gongzha section

The Cyclostratigraphic study has become a principal tool for refinement of the geologic time scale.Compared to the Western Tethys,the Cyclostratigraphic study on the Eastern Tethys is relatively insufficient.The precision of the study for the pre-Quaternary still needs to be advanced in Eastern Tethys.Cretaceous marine sediments are cropped out widely in Southern Tibet,among them the late Cretaceous in the Gongzha section of Tingri Area is rather continuous and integrated.Based on the field observation and the measurement of the bundle thickness,we have analyzed the cycles of the orbital parameters with Fast Fourier Transport(FFT) in this paper.And the result well matches the theoretical value as that the cycles of the precession are 409.2 ka(E1),127.9 ka(E2),and 93 ka(E5),and the cycles of the eccentricity are 51.2 ka(O1),40.9 ka(O2),and 38.9 ka(O2).In the process of power spectral analyses,a tuning treat based on the relationship between bundle and superbundle set was conducted,and those results before and after the tuning were compared to confirm the significance of the tuning.Moreover,the variety of sedimentation rate was confirmed as well.Generally,the sedimentation rate is in the range of 0.86-2.53 cm/ka,which increased from the bottom to the top of the strata.In addition,the result of this study is compared with the published results of Cretaceous marine sediments in Western Tethys and those from other areas in different ages,which indicates that in Eastern and Western Tethys,the shape,genesis and the cycles of the orbital parameters of Cretaceous marine strata are similar.And the conclusion that the orbital cycles becoming longer are well consistent with the theoretical value.Finally,a suggestion for further research is given.