Cenozoic Sedimentary Evolution Of The Weihe Basin: Basin-Orogen Coupling And Eolian Sediments

The Weihe Basin, which is also known as the Weihe graben or Guanzhong Basin (Plain), a Cenozoic rift basin, is located in the North China block. The Weihe Basin and the Fenhe Basin together are called the Fenwei Basin, which is one part of the graben basin systems around the Ordos Basin among with the Hetao-Yinchuan graben basin. Its geographical and tectonic location both are very important, it’s located south of the Ordos block and north of the Qinling orogenic belt. The tectonic and sedimentary evolution of the Weihe Basin is closely related to the Ordos Basin in the north and the Qinling orogenic belt in the south. The Weihe Basin, within the Cenozoic sedimentary debris piled seven to eight kilometers, unconformably on top of Cambrian granite. Quartzite sits on top of the basement, the deepest basin-Xi’an Huxian depression deposition thickness of more than 7000 m, and the highest peak of the Qinling Mountains (altitude 3776 m) formed the over 10 000 meters of the elevation. The form of basin is the result of the northward collision of the Indian plate and the remote response of the uplift of the Tibetan Plateau during the Cenozoic era. Therefore, to study the rift and sedimentation process of the Weihe basin can be highly valuable for understanding many major scientific issues, such as:1) the Cenozoic uplift and erosion history of the Qinling orogenic belt and 2) the dynamics of the transformation process of the Ordos Basin and the regional environmental evolution and environmental effects caused by the uplift of the Tibetan Plateau.The Weihe Basin has been studied by a lot of exploratory researchers during for over 100 years, however, restricted to the limitations of the technology in previous times and the relatively simple methods, they often recognize the Weihe Basin as an isolated single content and almost never study the Weihe Basin as part of the larger Orodos Plateau-Qinling Mountains system with the idea of "Basin-Orogen Coupling". Therefore we should keep in mind the dynamic interactions between tectonics, climate and earth surface processes to focus on the relationship between the Weihe Basin and the Qinling Mountains and the Ordos Plateau. Key topics in this field are:1) the geo-dynamics of the Weihe Basin and Qinling-Ordos system,2) the provenance of Weihe Basin and the history of the unroofing of the Qinling Mountians,3) the Weihe Basin surface subsidence and 4) the tectonics regarding the uplift of Qinling Mountains.Therefore, in this paper, based on a combination of previous studies and field research (mainly focused on the southeast part) we think that the basin began to rift and accept deposits since the Eocene (～50 Ma). Based on previous stratigraphic research, the Cenozoic strata in the Weihe Basin have been recognised and partially revised. The main regional stratigraphic units in the Weihe Basin are compared with the "China Regional Chronostratigraphic (Geo-chronologic) Scale", including the Honghe Formation, the Bauluyuan Formation, and the Lengshuigou Formation, the Koujiacun Formation, the Bahe Formation, the Lantian Formation, the Sanmen Formation and the Quaternary loess-paleosol. We found that the conglomerate at the bottom of Lantian Formation should be part of the upper part of the Bahe Formation. The Sanmen Formation is defined more clearly that it has no vertical relationship with the Lantian Formation. These two formations are the contemporaneous heterotopic facies within the same deposition period, but over different environments, and they have never been found in one section in theWeihe Basin. The "Zhangjiapo Formation" and the "Youhe Formation" were wrongly placed in time before and belong to the "Sanmen Formation". The tectonic rifting and sedimentary evolution of the Weihe Basin has been analysed and reviewed, the Eocene and late Miocene are found as the two most important developmental stages of the Weihe Basin during the Cenozoic era. The Weihe Basin is synergistic and related to the uplift of the Qinling Mountains and the evolution of the Ordos Plateau.In the present study, we focus on the Late Neogene strata at Duanjiapo, in the Bailuyuan Plateau. From the top of the Bailuyuan to the base near the Ba River, there exists a continuous sedimentary record which includes the Pleistocene loess-paleosol sequence, the Lantian Formation and the Bahe Formation. Overall, the entire section examined is 438 m thick and contains 3 lithological units:the lower part is the Bahe Formation with a thickness exceeding 246 m; the central part comprises the 60 m thick Lantian Formation (Red Clay); and the uppermost part is a 132 m thick Quaternary loess-paleosol sequence. The Bahe Formation and the lower Lantian Formation contain a large number of mammal fossils, which are very useful for understanding the age and environment of the Bahe Formation.Our detailed field investigations on fine grained facies of Bahe Formation indicated that the red-brown component of the fine-grained deposits in the studied section typically exhibit a consistently massive sedimentary structure with no bedding, carbonate concretions and clay coatings on grains. Although these alone are not diagnostic features for the eolian origin, they are essentially similar to those in the overlying eolian deposits of the Lantian Formation. This observation raised the question about the origin of the silts in the Bahe Formation.In order to investigate the importance of the Bahe Formation and its sedimentary environmental changes and paleontological characteristics in the Late Neogene period in Northern China, we carried out a detailed magnetostratigraphy study of the typical Bahe Formation called the Duanjiapo section (246 m thick), which is located in the south bank of the Ba River, Lantian County, Shaanxi Province. The magnetic polarity sequence can be clearly calibrated to the Geomagnetic Polarity Time Scale (GPTS). The paleomagnetic results demonstrate that the basal age of the studied interval is about 11 Ma. The result shows that the age of the Bahe Formation is 11-7 Ma, corresponding to the Late Miocene. This result confirms the previously presented age for the base of the Bahe Formation, and provides a robust framework for the detailed sampling in the sequence. The results of the grain size and geochemical analyses demonstrate strong similarities between the Miocene silts and the overlying Red Clay and loess units. This leads us to conclude that both deposits have a similar eolian origin, although the Miocene silts include evidence of fluvial sediments derived from rivers originating from the Qinling Mountains, to the south of the study site. Our results indicate that the southern Chinese Loess Plateau area, or even the entire Loess Plateau region, may have begun to accumulate eolian dust deposits during the Middle Miocene, at least 11 Ma ago, which is 4 Ma older than previously thought. The results raise the possibility to understand the onset of eolian dust in the East Asia. In addition, our findings demonstrate that the onset of eolian accumulation may indicate not only large-scale paleoclimatic changes, but may also reflect the course of local tectonic and geomorphic evolution. It may also provide information on the relation between climate change and tectonics. The Bahe Formation is especially famous for the three-toed equid "Hipparion" and the other mammal fossils, and accordingly it has been established as a standard for the Bahean stage.However, there are still debates about the age of the Bahe Formation. This study believes that the Bahean stage should be a separate stratigraphic unit that is different from the Baodean stage. It could provide a more precise age for the regional mammal fossils. Meanwhile, according to the comparison on sedimentary environment and paleoclimate with other basins, i.e. the Linxia Basin and the Tianshui Basin, the age of the Bahe Formation is also different from the age of the Lantian Formation. The Bahe Formation is characterised by a semiarid steppe climate whereas the Lantian Formation formed under humid forest climate.The summary of innovations in our study including three points.(1)The Cenozoic strata in the Weihe Basin has been recognised and partial revised, from the time and motivation both are synergistic consistent with the Qinling orogeny and the Ordos Plateau.(2) The paleomagnetic results demonstrate that the age of the Bahe Formation is 11-7 Ma. (3) Our results indicate that the southern Chinese Loess Plateau area may have began to accumulate eolian dust deposits during the Middle Miocene, at least 11 Ma ago, which is 4 Ma older than the previous knowledge. The onset of eolian accumulation may be the result of both changes of large-scale paleoclimatic and local tectonic and geomorphic evolution.Meanwhile, there are several disadvantages in this paper, such as the stratigraphic comparison and correlation between the basin margin and the center, the tectonics dynamics and filling patterns of the Weihe Basin still need to be researched in the future.