Constrained Magnetostratigraphic Dating of a Continental Middle Miocene Section in the Arid Central Asia
Constrained Magnetostratigraphic Dating of a Continental Middle Miocene Section in the Arid Central Asia
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DOI：
10.3389/feart.2018.00049
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发表时间：
2018-05
影响因子：
2.9
通讯作者：
V. Verestek;E. Appel;S. Voigt;Konstantin Frisch
中科院分区：
文献类型：
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作者：
V. Verestek;E. Appel;S. Voigt;Konstantin Frisch
哈萨克斯坦东南部伊犁盆地阿克陶山脉新近系序列是非常适合研究中亚在中新世气候演化中的作用的陆地档案。基于磁性地层学和旋回地层学和生物地层学的约束，我们提出了一种综合的方法来确定出露良好的巴斯陶组的年龄。阶梯退磁作用产生的特征剩磁方向与中亚中新世的预期一致。浅红色冲积滩沉积和灰色湖相沉积以磁铁矿和赤铁矿为主要磁性载体，表现出部分复杂的磁性行为，退磁行为多变，非偶极正反极方向。磁铁矿的溶解作用和新形成作用(包括显著的二次磁化作用)的沉积变异性和磁矿物学改变效应很好地解释了所观察到的磁性。反极性方向的平均值比预期的中中新世地球磁场平坦，这是支持原生源的倾角浅化的存在的标志。利用详细的岩石磁学分析，分析特征剩磁的性质，区分原生和次生剩磁方向，以获得可靠的磁性地层学结果。Bastau组的年龄为15.3 Ma~13.9 Ma，与已知的生物地层背景相对应，与中亚前陆盆地的典型沉积速率相吻合，与该剖面的地球化学指标光谱分析相吻合。由此得到的年龄模型为中亚地区新近纪气候动力学的古气候重建提供了一个可靠的框架。
The Neogene succession of the Aktau Mountains in the Ili Basin, southeast Kazakhstan, is a terrestrial archive well suited for researching the role of Central Asia in Miocene climate evolution. We present an integrated approach for dating the well-exposed Bastau Formation, based on magnetostratigraphy and constraints from cyclostratigraphy and biostratigraphy. Stepwise demagnetization yielded characteristic remanence directions that are consistent with those expected for the Miocene in Central Asia. The reddish-colored alluvial floodplain deposits and grey lacustrine deposits show partly complex magnetic behavior with magnetite and hematite as the main magnetic carriers, with variable demagnetization behavior and non-dipolar normal and reverse polarity directions. The observed magnetic properties are best explained by depositional variability and magneto-mineralogical alteration effects of both dissolution and neo-formation of magnetite, including significant secondary magnetization. The mean of reverse polarity directions is flatter than the expected Middle Miocene Earth magnetic field, which is an indicator for the existence of inclination shallowing that supports a primary origin. Detailed rock magnetic analyses are used to analyze the nature of the characteristic remanent magnetization and to discriminate primary and secondary remanence directions in order to obtain a reliable magnetostratigraphic result. The proposed age of 15.3 Ma to 13.9 Ma for the Bastau Formation corresponds to the known biostratigraphic setting, matches with typical sedimentation rates of foreland basins in Central Asia, and coincides with spectral analysis of geochemical proxies of that section. The resulting age model serves as a robust framework for paleoclimate reconstruction of Neogene climate dynamics in Central Asia.