Cenozoic evolution of geomorphic and sedimentary environments in the Tianshui-Qinan regions

Yuan, B.Y. [Institute of Geology and Geophysics, Chinese Academy of Sciences] Guo, Z.T. [Institute of Geology and Geophysics, Chinese Academy of Sciences] Hao, Q.Z. [Institute of Geology and Geophysics, Chinese Academy of Sciences] Peng, S.Z. [Institute of Geology and Geophysics, Chinese Academy of Sciences] Qiao, Y.S. [Institute of Geology and Geophysics, Chinese Academy of Sciences] Wu, H.B. [Institute of Geology and Geophysics, Chinese Academy of Sciences] Xiao, G.Q. [Institute of Geology and Geophysics, Chinese Academy of Sciences] Ge, J.Y. [Institute of Geology and Geophysics, Chinese Academy of Sciences] Sun, B. [Institute of Geology and Geophysics, Chinese Academy of Sciences] Zhou, X. [Institute of Geology and Geophysics, Chinese Academy of Sciences] Yin, Qiuzhen [UCL] Liang, M.Y. [Institute of Geology and Geophysics, Chinese Academy of Sciences] Qin, L. [Institute of Geology and Geophysics, Chinese Academy of Sciences] Liu, L. [Institute of Geology and Geophysics, Chinese Academy of Sciences] Yao, Z.Q. [Institute of Geology and Geophysics, Chinese Academy of Sciences] Liu, T.S. [Institute of Geology and Geophysics, Chinese Academy of Sciences] et al.[show all]

In the western Loess Plateau in northern China,the Miocene eolian sequences reported in the recent years from different sites have extended to about 22Ma ago. The eolian origin of these sequences are evidenced by (1)their wide distribution mantling the broad highlands; (2)the spatially correlative magneto- and litho-stratigraphy,magnetic susceptibility and grain-size records; (3)the presence of several hundreds of paleosols with the interbedded loess layers; (4)the fine silty textures with the maximum grain-size mostly <120μm throughout the ca.16Ma sequence; (5)the similarity of quartz grain morphology and geochemical properties to Quaternary loess;(6)the well-preserved land snails and lack of aquatic species throughout the sequences; and (7)the cyclical changes of various climate proxies along the sequences,similar to those in Quaternary loess-soil sequences in China. These Miocene eolian sequences,combined with the well-known loess-soil sequences of the last 2.6Ma and the eolian Red-Clay (8.0～2.6Ma),provide a nearly continuous terrestrial climate record since the Early Neogene.In this study,we address the evolution of geomorphic and sedimentary environments in the Tianshui-Qin'an regions where Miocene loess deposits widely spread. Earlier geological studies showed that the Tianshui and Qin'an regions are within two distinct tectonic units,which have different tectonic histories. During the Paleogene,both regions are dominated by pluvial-plain landscapes as evidenced by the widely spread pluvial sandy-gravel deposits. A tectonic event occurred near the Oligocene-Miocene boundary,leading to the formation of plateau and basin settings in these regions. Since the early Miocene,typical loess-soil sequences started to form on the plateaus while secondary loess (loess-like deposits) were formed in the basins.During Late Miocene times,the Tianshui region experienced an intense erosion leading to the formations of some small shallow lakes. Their deposits are characterized by interbedded grey-green silty-clays with coarser fluvial sandy or gravel layers. Some of them are affected by pedogenesis. These indicate that these small lakes, were frequently dried up along with the fluctuations of paleoclimates. Loess deposits in the Qin'an region have also experienced erosions,resulting in the formations of some shallow depressions in which water-reworked loess or shallow-water deposits were formed. They sometimes contained large mammalian fossils. On the contrary, typical loess-soil sequences continued to form in flat areas, that contained only micromammalian and landsnail fossils. This Late Miocene erosion period,probably characterized by several erosion events,was started ca.11Ma ago and lasted until 7Ma. It may correspond to the Late Miocene growth of the Himalayan-Tibetan complex.The basic patterns of the hilly landscape in these regions were formed in Late Pliocene or Early Pleistocene times due to another intense erosion. The severely fragmented topography of the regions since then has not been favorable for latter loess deposition,thus the Pleistocene loess is purely preserved in these regions. This Late Pliocene or Early Pleistocene erosion may be linked to another tectonic event or to the climate changes relative to the northern hemispheric cooling.