Magnetostratigraphy Of Late Miocene-Pliocenered Clay In Liulin Area,West Piedmont Of The Lvliang Mountain

Neogene aeolian red clay deposits in the Chinese Loess Plateau(CLP)contain abundant paleoclimate and paleoenvironment information because of its continuity.It is also an important carrier for studying the formation and development of inland aridity in Asia and the evolution of East Asian monsoon.Lvliang Mountain is located on the eastern side of the Chinese Loess Plateau.The red clay deposits on the west piedmont of the Lvliang Mountain were often mixed with sand and gravel units bearing mammalian fossils.These beds are good indicators of the evolution of the Lvliang Mountain uplift and the basin evolution in the Loess Plateau during late Miocene-Pliocene.The uppermost sedimentary sequence of the red clay,which was not interfered by external factors,recorded dust accumulation changes,ecological environment characteristics and climate change events in the eastern Chinese Loess Plateau.In this study,we carry out rock magnetism and paleomagnetism research in the Liulin Fuxing red clay profile at the western piedmont of the Lvliang mountain,to obtain the chronology of the red clay and the interbedded sand/gravel units and fossil mammals.Combined with previous studies in this area,we have reached a new understanding of the uplift of Lvliang mountain and the environmental evolution of the loess plateau during late Miocene-Pliocene.The preliminary results are as follows:1.The results of systematic rock magnetism indicate that the magnetic minerals in the red clay profile are mainly quasi-monodomain(PSD)magnetite and maghemite.The concentration of the hematite is quite low.2.The magnetostratigraphic study indicates that the Liulin Fuxing red clay profile records the polarity zones from C2 An.2n to C3 Bn,with an epoch span of 7.2-3.1 Ma(Pliocene to late Mioncene).The profiles are composed of the Baode Group(about7.2-5.33 Ma)and the Jingle Group(5.33-3.1 Ma).Based on the age control points of magnetostratigraphy,the ages of the two bottom layers bearing mammalian fossilsare calculated to 6.4-6.3 Ma and 5.8-5.7 Ma,respectively.It is highly consistent with the palacontological layer horizon in the red clay section of the surrounding area,which also ensures the accuracy of the paleomagnetic age;3.Three gravel layers are distinguished in the red clay sedimentary sequence.They are the intermittent gravel layer at 19 meters in our profile,the thicker gravel layer at 53 meters in our profile,and the thicker gravel layer at 37 meters in the Fuxing Profile reported by Xu Yong.Their ages are about 6.25-6.2 Ma,5.75-5.5Ma and 3.9 Ma,respectively.The gravel layers are mainly composed of limestone and sandstone.According to the features of the sandstone and previous evidence,two gravel layers at 6.25-6.2 Ma and 5.75-5.5 Ma indicate two rapid uplift events in the Lvliang Mountain area during this period.The gravel layer at 3.9 Ma represents more intense weathering and erosion events in the Lvliang Mountain area during this period;4.By summarizing the profiles of the eastern part of the Loess Plateau,we find that the age of the bottom boundary of the red clay profile in the eastern part is generally around 8.1 Ma.However,the age of the red clay can reach 25-22 Ma in the western Chinese Loess Plateau.In the more western Asian inland areas,such as the Altun region,the Junggar Basin,and southwest part of Mongolia,the Eocene and Oligocene wind-dust red clay deposits were also preserved.The difference in the initiation age is mainly restricted by different deposition conditions.The strong enough wind energy,expansion of arid source area and stable sedimentary environment in the east of Liupan Mountain in the Loess Plateau around 8 Ma contribute to the formation of the red clay.Before 8 Ma,the source area was small and the transport wind was affected by various factors.Thus,the aeolian dust were transported to the close areas for the less dust input and the weak kinetic energy.Later,the uplift of the Tibetan Plateau strengthened the monsoon,drought expansion and differential uplift of Ordos,which provided conditions for the sedimentation of the red clay in the eastern Chinese Loess Plateau.