Abstract
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.
Highlights
Central Asia’s climate dynamics during the Cenozoic, with a general trend of aridification (Guo et al, 2002; Tang et al, 2011; Caves et al, 2015), was mainly driven by global cooling and uplift of the Tibetan plateau (Zhisheng et al, 2001; Lu et al, 2010; Miao et al, 2012)
The extent of the Paratethys, together with its advance and retreat largely influenced the atmospheric moisture supply in Central Asia (Fluteau et al, 1999; Bosboom et al, 2011). To investigate these influencing factors, we study a Constrained Magnetostratigraphic Dating Central Asia well-exposed paleoclimatic archive, the Neogene sedimentary succession in the Aktau Mountains (SE Kazakhstan)
We present a detailed magnetostratigraphy of the Bastau Formation (Fm) as a fundamental basis for better understanding the Neogene climate dynamics in Central Asia
Summary
Central Asia’s climate dynamics during the Cenozoic, with a general trend of aridification (Guo et al, 2002; Tang et al, 2011; Caves et al, 2015), was mainly driven by global cooling and uplift of the Tibetan plateau (Zhisheng et al, 2001; Lu et al, 2010; Miao et al, 2012). Magnetic Minerals Temperature dependent results suggest the existence of at least two magnetic phases in all lithologies of the Bastau Fm. In all studied samples, a marked decrease around 550–580◦C is seen in high-temperature susceptibility runs, which clearly reveals the presence of magnetite (Figure 6). Strongly enhanced values appear in the cooling curves accompanied by a continuous increase during cooling (Figure 6B) We explain this observation with new formation of ultrafine superparamagnetic magnetite particles (with a range of different grain sizes) at high temperatures during heating, which transform from the fully unblocked state to the state of higher susceptibility during cooling (Bowles et al, 2009). The contributions are more variable, with 30–90% for the first component, indicating a relatively higher magnetic fraction than in the other two facies types.
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