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Дата изменения: Tue Feb 9 21:26:13 1999 Дата индексирования: Tue Oct 2 01:44:36 2012 Кодировка: Поисковые слова: horizon |
THE POST-SIMPOSIUM EXCURSION (
July 31 - August 7,1998)E. Bibus
The six-day excursion took the participants from Lanzhou via the western loess hill country to the north-eastern Tibetan Plateau and via the West Quinling Mountains back to Lanzhou. During the excursion, Prof. Fang (University of Lanzhou) presented highly interesting results concerning loess sequences, terrace sequences, peneplain structures and Tertiary and Quaternary basin fillings as well as the uplift of the north-eastern Tibetan Plateau.
The Beiyuan Loess Section in the Linxia Basin was the first visit on the agenda. This section is 39m thick and contains the loess of the last glacial period as well as a basal complex of flood loam. In the latter, three poorly developed Ah-horizons can be identified. With the help of pollen analyses and TL-datings, these have been dated as belonging to the last interglacial period. The loess of the Wurm Age is subdivided by 16 recognizable, less developed paleosols, which were detected predominantly by means of magnetic susceptibility. The Holocene S0-soil has developed into an Ah-horizon through approximately 600mm of precipitation. Intensive cultivation seems to suggest that it has developed through human interference. The loess sequence is located on the third terrace of the Daxia River, a tributary of the Yellow River. The overall number of seven impressively developed terraces (between 1850m and 2450m above sea-level) in the Linxia Basin corresponds to the number of terraces at the Yellow River. Through the dating of the loess-paleosol surface layers, the terraces T1-T7 can be dated to approximately 1.66, 1.4, 1.1, 0.6, 0.12, 0.055 and 0.01 Ma. The older fillings of the Linxia Basin consist of lacustrine deposits of the early Pleistocene as well as of deposits of the red beds seen during the excursion. Minor parts of the latter extend as far as the north-eastern Tibetan Plateau. Furthermore, these deposits allow conclusion to be drawn about the uplift of the Tibetan Plateau (earliest uplift younger than 4Ma).
Sedimentological analyses showed that the loess of the Hezou Loess Sequence on the Tibetan Plateau comes from the west, from the Plateau itself, whereas the loess of the loess hill country is derived from the Asian interior (Gobi Desert). A poorly developed humic Bv-horizon forms the basis of the Hezou Loess Sequence. In the 16m thick loess profile of the last cold stage, 14 less developed soils could be detected through differences in color and carbonate content. The analysts assume that a complete pedological record of the climatic oscillations of the last cold stage is represented in this profile. They, therefore, draw parallels between the loess profile and the GRIP-ice core project in Greenland. This gave cause for critical discussion. Interestingly enough, a dark brown, poorly developed pseudogleyed Bv-horizon, which is better developed than the S1-soil of the last interglacial period, can be found in the middle section of the profile.
As is the case on the whole of the north-eastern Plateau, two peneplains, one at 4.000-4.200m (P1) and the other at 3.500-3.600m (P2), occur in the Hezou region. The P2-plain, which bears relics of red paleo-regolith, spreads over the youngest layers of the red bed-deposits. A silted-up salt lake, which was dissected by the rivers in the Holocene, can be found on the main plain P2 near Gehai. In the Zoige region there is a large tectonic basin in which two drillings (120m and 310m), which did not reach the ground of the basin, were carried out near Maqu at 3.400m. Various methods have allowed the climatic changes of the last million years to be reconstructed and related to complex Chinese loess profiles and deep-sea drillings.
Near Maqu it could be shown that, as far as fluvial development is concerned, the Yellow River first reached the Zoige Basin between 12.000 and 15.000 BP and that, since then, it incised the ground of the basin, thereby creating two terraces.
The participants of the excursion were fascinated by the beauty of the Tibetan landscape and the naturalness of the Plateau's inhabitants. Furthermore, both the Huanglong National Park (sinter terraces, waterfalls, sinter basins) and the Jiuzhaigou National Park (alpine karst, karst and glacial lakes, V-shaped valleys, waterfalls) were visited and their unique natural beauty greatly admired.
The loess profile of Shaba near Chuanzusi at the eastern front of the Tibetan Plateau is important with respect to loess stratigraphy. Due to the Indian monsoon, mean annual precipitation exceeds 800mm and the [mean annual] temperature is lower than 2-4°C. Here, the S1-soil occurs as a poorly pseudogleyed Bt-horizon complex that contains pieces of charcoal.
The last profile to be visited was the Zhaodaisuagou Loess in Wudu in South West Wuin Ling. 60m of loess with 18 different paleosols forming eight complexes (S0-S7) lie on the fourth terrace of the Bailong Jiang River. The S1-soil of the Eemian Stage was presented as a brown, snail-bearing horizon lacking in a distinct Cc-horizon and rising steeply towards the slope. It is possibly a soil sediment. A closer pedological analysis of this as well as of the other exposures would be desirable. Unfortunately, a preparation of the partly thick loess profile was not possible either. Only very limited sections could, therefore, be seen in all exposures.
Prof. Fang and his colleagues had organized this excursion very well. It conveyed a lasting impression as far as the paleopedology, landscape genesis and culture of a not easily accessible region are concerned. Furthermore, the participants of the excursion had the chance to study the effects of massive intervention into the ecosphere for the sake of cultivation, made necessary by the great population pressure. Disastrous slope erosion, landslides, high floods and mud flows are just some of the consequences of this intervention.