The Neogene Deposition And Environmental Evolution In Southeast Longxi Basin

The uplift of the Tibetan Plateau not only causes the intense Cenozoic tectonic deformation in Asia, but also has effect on the relief framework and environmental evolution in the surrounding area of the Tibetan Plateau. So, the Tibetan Plateau uplift and its environmental effects become one of the hot spots of the Past Global Change Research. However, there are great differences on the exact time of the Tibetan Plateau uplift, reaching modern height and the impact of environmental evolution process of the surrounding area.Many basins around the Tibetan Plateau formed during the process of the crustal deformation and uplift. And the sediments in a basin is generally a production of rocks bearing the process of weathering, denudation, transition and deposition in river system in orogenic belt. So, the sediments not only reflect the dynamic properties in filling process, but also record the climate and environmental change information in all kinds of geological processes. The southeast Longxi basin, which is located at the convergence of Liupan Mountains and west Qinling on the northeastern margin of the Tibetan Plateau, is the connection between the Tibetan Plateau and the Loess Plateau. And its northern boundary is west Qinling which is the key part in tectonic movement transition from northern to southern China. Its eastern boundary, the east Liupan Mountains, is the line that separates the climatic types and tectonic movements in eastern and western China. As a result, the southeast part of the Longxi Basin is quiet sensitive to climate changes and tectonic movements. So it is an ideal area for studying the process of the Tibetan Plateau uplift, the environmental effects and the process of aridification in northwest inland of China.Based on wide field investigation, data analysis, synthesized various kinds of indice and correlative data, we have mainly obtained the following findings:1. The Paleogene red coarse clastic rocks with the characters of piedmont alluvialfan facies were deposited in southeast Longxi basin. At about 22Ma, the rocks were deformed by the tectonic activity in the Tibetan Plateau and the kamafugitic series lava was formed and unconformably overlaped on the Paleogene sandstone. It showed that the distance effect of collisional deformation between the Indo plate and Eurasia plate has already reached the northeastern margin of the Tibetan Plateau at least on 22Ma. Since the Neogene, a basin has been formed in Southeast Longxi Basin. The fact of the steady finegrain fluvial-lacustrine sediments and abundant fossils suggested that the tectonic activity did not cause rapidly uplift at that time.2. The thickness of the Neogene deposition decreased rapidly (from greater than 1000m to 200-300m) from west Qinling piedmont depression to the broad floodplain in the basin. The sediments become finer and the horizontal difference of sediments facies is obvious. The deposition evolution experienced the following stage since 15.6MaB.P.:15.6-9.23Ma, floodplain and fluvial deposition, the "lower red bedding" consists of red mudstone and calcareous mudstone or calcrate with alternate appearance;9.23-7.1Ma, the lake invasion took place in the basin broadly, the "zebra bed" consists of celadon marlite interbeded with red mudstone;Since 7.1Ma, the lake began to shrink in large scale and floodplain developed in vast area of the basin. 7.1-3.6Ma, "upper red bedding" came into being and consisted mainly of red argillaceous beddings and siltstone beddings, moreover, gypsum-salt deposition has been found at the top of "upper red bedding" in some area;About 3.6Ma, affected by the Tibetan Plateau uplift, the south part near the west Qinling in the basin downfaulted and became lake again;3.6Ma-1.4Ma, "upper green bedding" came into being and consists of celadon mudstone interbeded with offwhite marlite;About 1.4Ma, affected by the intense uplift of the Tibetan Plateau, the development history of the lake in the study area ended. After that, the study area began to experience uplift ,denudation, river terraces development and eolian loess deposition.3. At the end of Paleogene, the climate was dry and huge bedding red calcareous cementation sandstone deposited. The climate bacame humid in Miocene obviously. Finegrain fluvial-lacustrine sediments under low energy circumstance were deposited in Linxia Basin in Miocene. The vegetation changed from woodland steppe at the endof Paleogene to forest in Miocene, which indicated a humid climate. The finegrain fluvial-lacustrine sediments were also deposited in southeast Longxi Basin in Neogene. Large mammals, such as rhinoceros, elephant, deer and antelope, lived in the study area, which showed the climate was humid during this period. But the alternating environment between dry and humid occurred under the humid background. 15.6-9.23Ma, the climate is humid and fluvial process bloomed. 9.23-7.1 Ma, the climate was more humid and the lake expanded. 7.1-3.6Ma, the lake shrunk and climate became dry relatively. Since the climate continued to become drier, gypsum-salt deposition was developed in some areas;After 1.4Ma, the climate became dry and the lake deposition was finished, eolian deposition developed, then, the modern environmental framework came into being.4.Based on the grainsize, geochemistry and microstructure analysis of the sediments deposited during 22-6.2Ma in Guojia, Qin'an, Guo et al (2002) suggested that this set of red beds was eolian loess series. And they deduced that the inner Asian began to dry since 22Ma and this status related to the Tibetan Plateau uplift. But their conclusion is not consistent with other deposition records in the basin at the same period. The detailed contrast and analysis on the QA-I Neogene sediments and Lamashan and Yaodian Neogene lacustrine sediments have been carried out and showed:(1) The comparing analysis on the grainsize characters among QA-I Neogene sediments, Tianshui Neogene fluvial-lacustrine sediments and Quaternary eolian loess showed that the characters of Guojia Neogene sediments is more similar to that of Tianshui Neogene fluvial-lacustrine sediments. Moreover, the frequency and cumulation percentage curves of both sections are more similar, but there is difference between QA-I Neogene sediments and Quaternary eolian loess;In the grainsize scatter figures, two types can be divided obviously: one is QA-I Neogene sediments and Tianshui Neogene lacustrine sediments, the other is Quaternary eolian loess.(2) The geochemistry characters demonstrate that the calcium carbonate content of the yellow-brown "loess" of QA-I Neogene sediments samples may reach 15.76-46.34% or even higher, the silicon dioxide content may reach 29.54-52.26%.However, there are obvious differences in content between the QA-I Neogene sediments and typical Quaternary eolian loess (CaCO3, general lower than 20%;SiOo, general 55-65%). The content of the calcium carbonate and silicon dioxide in the QA-I Neogene sediments is similar to those in Tianshui Neogene fluvial-lacustrine sediments. Moreover, both of the two sections have the same REE distribution patterns, which have the characters that all are moderately rich in Ce family elements, with negative slopes and negative Eu anomaly.(3) There are much sediments produced by fluvial process in the "loess-paleosol " series of QA-I Section, such as the fine sandstones with horizontal beddings or ripple cross beddings in the mudstone sequence and the marlite with red mud block or gravel inside at the bottom of the section, those belts and thick lens composed mainly by marlite and well-grinded gravel at the top part on the middle of the section, the intraclastic texture and diatom observed by microscope at some segments of the section. Moreover, from Tianshui, which is at the south basin, to Qin'an, the horizontal difference of sediments facies is obvious. For example, the celadon marlite in the "zebra bedding" appears in both of Yangjiadazhuang and Jiaowan, which are 20km away from the QA-I Section, but has lighter color and thinner thickness than that in Tianshui. And we also found many well-buried fossils of big vertebrates, such as rhinoceros and elephant, in the neogene stratum near the QA-I Section in Qin'an. The fact suggested that the climate in that period was quiet mild. According to those theories of taphouomy, it is obvious that those big fossils were not able to be buried at QA-I with the local sediments rate (1.67cm/kyr) in Neogene.The results above indicated that although the appearance of QA-I Neogene section is similar to the Loess-paleosol series, it should be an alternative series which is composed of finegrain mudflat deposition on the margin of the basin, floodplain deposition and those products of pedogenensis in the air.5. The article also discussed the aridification problem in inner Asian. The study showed that the early Neogene climate should be warm and moist in inner Asian. This climate change may relate to the Asian palaeo-monsoon formation. And the palaeo-monsoon origin may relate to the expansion of the Asian marginal sea. Notably,the fracture and expansion of South China Sea on a large scale at that time provided an important passway for the Asian palaeo-monsoon;About 8Ma, the climate was dry and the red clay deposited, this may relate to the uplift of the Tibetan Plateau;About 2.6Ma, the uplift of the Tibetan Plateau was rapidly and the Asian monsoon established steadily, the climate was dry and the typical eolian loess began to deposit.