天津蓟县剖面前寒武系洪水庄组—铁岭组米兰科维奇旋回

显生宙沉积旋回会受到地球轨道参数偏心率、地轴斜率和岁差的周期性变化(米兰科维奇旋回)的控制,但目前对前寒武系的相关研究较为薄弱。天津蓟县剖面中元古界洪水庄组-铁岭组为一套潟湖-潮坪相沉积,主要呈砂岩-页岩互层的韵律性产出。为探索这种岩性韵律是否与轨道旋回有关,对其进行了高分辨率的岩性刻画,并以磁化率和伽马能谱数据作为古气候-古环境替代性指标,通过频谱分析等方法进行旋回地层学分析。结果表明各指标均记录了完整的米兰科维奇旋回。铁岭组一段下部由短偏心率、斜率和岁差控制的地层旋回厚度分别为1.2~1.5 m、0.4 m和0.17~0.19 m,其中0.1 m的半岁差周期也较明显;洪水庄组二段顶部由长偏心率、短偏心率、斜率和岁差控制的地层旋回厚度分别为1.1~1.8 m、0.34~0.54 m、0.14~0.16 m和0.08~0.09 m。以识别出的稳定长偏心率周期405 ka旋回对洪水庄组进行了天文年代校准,并估计出当时的短偏心率、斜率以及岁差周期分别为100 ka、22~24 ka和15 ka。在洪水庄组中记录的偏心率振幅变化周期为~2 Ma,比现今~2.4 Ma的周期略短。

Milankovitch Cycles of the Precambrian Hongshuizhuang-Tieling Formations at Jixian Section in Tianjin

Sedimentary cycles preserved in the Phanerozoic Eon can be associated with the changes in the ellipticity of Earth’s orbit, the tilt and the precession of Earth’s rotational axis, but evidence from the Precambrian sequences is still lacking. The Mesoproterozoic Hongshuizhuang-Tieling formations at the Jixian section in Tianjin were deposited in the lagoonal-tidal flat environment, and are dominated by rhythmic sand-shale interbeds. To explore whether the rhythmic sequences are related to the Milankovitch cycles, a high-resolution cyclostratigraphy study was conducted on the paleoclimate-paleoenvironment proxies of the lithology, magnetic susceptibility (MS), and gamma-ray (GR) features of the Hongshuizhuang-Tieling formations. Spectral analyses show that the wavelength ratios of the cycles in these stratigraphic units correspond to the hierarchies of the Mesoproterozoic Milankovitch cycles. The sedimentary cycles of 1.2-1.5 m, 0.4 m and 0.17-0.19 m can be associated with the short eccentricity, obliquity and precession, respectively, in the lower part of Member 1 of the Tieling Formation. The 0.1 m sedimentary cycle may be associated with semi-precession cycle in the Tieling Formation. The major sedimentary cycles of 1.1-1.8 m, 0.34-0.54 m, 0.14-0.16 m and 0.08-0.09 m are considered to be caused by long and short eccentricity, obliquity, and precession in the upper part of Member 2 of the Hongshuizhuang Formation, respectively. After calibrating to the 405 ka long eccentricity cycles, the spectra represent periods of the 100 ka short eccentricity, 22-24 ka obliquity and 15 ka precession, respectively. Moreover, the eccentricity amplitude modulation cycles show main periodicities of ~2.0 Ma, which is shorter than that at present (~2.4 Ma).