Grottes et karsts de Chine
Sur les traces de Xu Xiake
Gebihe 89 Contents
Introduction: General physical description
Résumé - Abstract - Zusammenfassung-:Carbonated rocks cover a fifth of the surface of China, that’s 2.000.000 km2. The largest karsts areas known lie in south China (Guangxi, Yunnan, Guizhou, Hubei, Hunan) and central China (Sichuan). they are developed in Sinian (Precambrian) to Jurassic limestones and dolomites (fig.2) and belong structurally to the Yangtse paraplatform and to the South China geosyncline (fig. 3). The karst evolution were controled by the late Cretaceous Yanshan tectonic phase and later by the Oligo-Miocene himalayan orogenesis. These polyphased tectonics caused considerable erosion of red formations wich subsequently accumulated in the basins and at the foot-hills (Cretaceous-Eocene red soils).In the cours of different phases, dry valleys and cave levels were cut by lateral erosion while cones and towers resulted.
Subtropical south China karsts is controled by monsoon climate of cool and dry winters alternating with warm and wet summers. Precipitation vary between 1200 to 1800 mm/year while the average temperature ranges from 14 to 16°C at 800 to 1400 m a.s.l.
INTRODUCTION
China is very hilly. Of total area , 65 % is above 1000 m. The country surface dips with three big steps from West to East.
- The upper step is the wide Qinghaï-Xizhang plateau. It extents on 25 % of China. It consists of high plateaus ( Qinghaï, Tibet ) with an altitude above 4000 m and large mountain ranges ( Himalaya ), whose top is the famous Qomolungma: 8846 m ( Everest ) on the Tibet / Nepal border.
- The middle step is built up by the Yunnan-Guizhou limestone plateau in the South, the loess plateaus ( Huangtu, Gaoyuan ) on the north of the Huanghe and in intern Mongolia, and the Sichuan basin in the middle ( the "Red Basin" ). The average altitude ranges between 1000 and 2000 m. With a monsoon climate, this region is subject to a strong fluvial erosion. On the NW we find a separate low area: the Tarim basin. It is very arid and endoreic, with wind erosion.
- The lower step consists in low hills ( less than 1000 m ) and alluvial plains ( less than 200 m ) where the main rivers deposit alluvia. We can distinguish from the North to the South : the NE plains, the N plain and the hilly region in the lower valley of the Changjiang ( Yangtse ). This last region has been often flooded because of the summer monsoon rainfalls ( 20 main floods during the last 500 years, with 3 in the XXth - TAO SHIYAN, 1984 ). The worst floods known happened during the very wet 1839-1855 period. There is maybe a link with the end of the climatic pulsation called "Small Glacial Age".
I. GEOLOGICAL CONTEXT
A. Carbonate rocks
In China carbonates extent over 20 % of the country ( 2 000 000 km2 ) and outcrop on 1 250 000 km2. Here the most extensive and complex karst areas of the Earth are found. There are carbonates in the main geological units of the country ( Yangtse paraplatform, South China geosyncline,etc..). They are dated from the Sinian ( Precambrian ) to the Quaternary. Karst is developed in a great variety of carbonate rocks and in very different climates. It explains a wide range of endokarstic and exokarstic forms.
In the five southern provinces ( Guangxi, Yunnan, Guizhou, Hubei, Hunan ) the geological maps ( 132 sheets with a 1/ 200 000 scale ) show 360 000 km2 of limestone and dolomite. 7064 caves have been located. 2836 underground rivers have been induced from dye tracing and geological study. Their total supposed length would be 14 000 km ( what a job for the cavers ! ).
In the arid western regions of China ( Qinghaï, Xizang ) the carbonate outcrops do not show very typical karstic features. Frost and aridity are the main factors for open air erosion. The influence of corrosion on landscape features is neglectible.
B. Structural Zones
We can distinguish some main tectonic units with specific karstic rocks. The eastern part of China ( where the largest karsts of the country are ) is divided into the Yangtse paraplatform, the folded geosyncline of South China and the China-Corea paraplatform.
1. The China-Korea paraplatform
This region contains North China and the southern part of NE China. The age of the carbonate rocks ranges from the Sinian ( Upper Precambrian : 850 - 570 million of years ) to the Ordovician ( 435 - 500 My ).Their total thickness is generally between 1000 and 2000 m, but it can rise to 7000 m in some regions like the Yanshan Mts, near Beijing. The Precambrian is discordantly covered by :
dolomites and limestones ranging from the Cambrian to the Middle Ordovician ; - coal layers ranging from the Middle Carboniferous to the Lower Permian ( coastal or continental facies ) ; - red beds ( detritic ) ranging from the Upper Permian to the Middle Trias.
The lack of terrains from Upper Ordovician to Lower Carboniferous is specific of this region.
Cretaceous and Jurassic are characterized by intrusions ( granite ) and more or less acidious extrusion., a continental volcanism. The sedimentary cover has been folded and broken during the Yanshan orogenesis.
During the Mesozoic and the Cenozoic an extension tectonic produced wide graben where sediments were trapped ( continental terrains with coal and oil fields ).
2. The Yangtse paraplatform
It extents over most of the Yangtse basin. The base is covered by thick sedimentary layers beginning with molasses, tills and dolomites of the Sinian ( Upper Proterozoic ). They are exposed in the Yangtse gorges. The carbonate rocks are several thousand meters thick and range from the Sinian to the Trias. The Paleozoical and Mesozoical terrains consist mainly in marine limestones, dolomites and shales ( basin and platform facies ). There is also continental basalts.
At the end of Mesozoic the eastern part of the Yangtse platform has been affected by large scale magmatism ( mainly granite and few effusions ). Cenozoic is the time for continental sedimentation ( red beds with oil and gas fields ).
The whole Proterozoic and Lower Paleozoic have been folded at the end of Lower Paleozoic ( Upper Caledonian folding ). In the South China geosyncline these folded layers are covered by clastic sediments of the Lower Devonian. This region belongs to the Pacific margin geosyncline. After the Caledonian orogenesis this geosyncline was transformed into a platform and merged with the Yangtse paraplatform. The Yanshan orogenesis ( Jurassic, Cretaceous and Eocene ) is the lastest structural big event.
3. The South China geosyncline
In this tectonic unit, the thickness of the Upper Paleozoic lies between 2000 and 5000 m. The Middle and Upper Devonian and the Lower Carboniferous extent more widely and are thicker than on the Yangtse paraplateform. During the Trias, the Indochina orogenesis folded the layers. During the Varisc orogenesis ( Upper Precambrian to Upper Paleozoic ) and the Indochina orogenesis ( Trias ), the platform went up and down, the depth of the sea and the carbonate facies varied. During the Cenozoic ( especially during Miocene ), Himalayan orogenesis is followed by a wide regional uplift with local variations ( graben, horst ). The morphology dating from the Jurassic has been modified : alterites erosion, canyon digging, many stages of karstic networks.
C. Geological and climatic zonation of Chinese karsts
The karstic zonation depends on the used criteria. Zhang Shouyue ( 1980 ) distinguished three scales for zonation. Four climatic areas were used ( with specific hydrological characteristics ). Later, the karsts has been divided into the three main tectonic units ( Cf. supra ). Finally some smaller areas are distinguished by specific geological features.
These climatic, geological and morphogenetic features depict the various karst landscapes which directly result from the entrenchement of the karst systems due to the Cenozoic uplift and the subsequent lowering of the base level of erosion. The geological structure substantially influences the karsts subareas. On the flank of the anticline in Eastern Sichuan ( sub-area IA6 ), for example, karst cones are assymetric.
The chinese karst can be divided into 4 regions, 8 areas and 14 sub-areas. The methodological distinction is the scientific foundation for engineering in karstic terrains.
II. CLIMATE
Chinese climate is influenced by the monsoon in the South and East and by a cold , arid to semi-arid climate in the North and the West. The monsoon climate is controlled by the exchanges between the continental and oceanic air masses.
During winter the polar air coming from Siberia and Mongolia influences a large part of the country. During summer the oceanic hot and wet tropical air ( China Sea, Bengal Gulf ) invades the continent. In the South China karsts, the winter monsoon is characterized by a stable weather, soft to fresh. It is a dry season, while the summer monsoon is hot and wet ( rainy season ).
In the tropical and subtropical karsts of South China the annual rainfall ranges from 1200 to 1800 mm/yr. Because of the monsoon cavers are advised well to come during the dry season ( November to March ).
In the middle mountain karsts ( Guizhou, Hubei, Sichuan ) medium annual temperature ranges from 14 °C to 16 °C. The coldest month is January ( 4 °C to 6 °C ) and the hottest July ( 22 to 24 °C ).
Night frost is frequent during winter and the daily thermic amplitude is high, even in the valleys. Frost has been an important geomorphological factor during the coldest periods of Pleistocene ( there are inherited masses of fallen rocks on the feet of the high fault scarps in the Ziyun / Guizhou region, 900 to 1300 m a.s.l. ).
In the low altitude karsts ( Luodian, Guizhou ; 300 m a.s.l. ) medium annual temperature is 19,6 °C ( 10,1 °C in January and 27 °C in July ).
China can be divided into six climatic regions : tropical, subtropical, hot-temperate, temperate, cold-temperate and cold ( China Handbook Series, 1983 )
- the tropical zone s.s. is small ( 1,6 % of China ) ; it is located along the Tropic of Cancer ( South of Yunnan, Guangxi, Guangdong, Hainan ); the tropical region is the only one without frost ; - the subtropical zone is wider ( 26,4 % of China ) and covers the South of the Yangtse and a large part of the chinese karsts ( Yunnan, Guangxi, Guizhou, Hunan, SW Hubei, SE Sichuan ) ; 8 to 12 months without frost ;
III. SOILS
Because it is widely ranging from the North to the South, China contains all the Eurasian soils, excepting those of the tundra and the taïga podzolics gleys. In Eastern China there are two main soil zones divided by a line following the Qin Ling Range and the Huaihe Valley ( 33 - 34 °N ).
On the northern side the soils are calcic, neutral or basic ; the calcrete layers are frequent in semi-arid zones ( pedocals ); here is cultivated corn. On the southern side, the soils are no-calcic ( podzol, fersiallitic or ferralitic ), neutral or acid. This is the rice country. Downstream the Yangtse Gorges, the flood plain soils are neutral and fertile ( CHEN CHENG SIANG, 1984 ).
In the subtropical karsts that we visited ( Guizhou, Sichuan, Hubei ), we saw the thick red continental formations ( Carboniferous and Eocene ). They proceed from accumulation of lateritic soils on slopes ( Guangxi, South Hunan ) and in basins ( Red basin, Yichang ). On the foot of the karstic hills and on some plateaus, we found thick profiles with colluvions, proceeding from the alteration of no karstic rocks ( shales, sandstones,...). On the karstic hills, where the forest disappeared, the soils are cut and the denuded rock ishows old Kryptokarren.
IV. VEGETATION
Everywhere in China, there is a very wide range of vegetation types : mountain, moderate and tropical. This is a consequence of the lack of climatic or topographic barrier in the center and East of the country. As example, in the mountain subtropical karsts of the Guizhou ( Santang county, 27 ° lat N, 1500 m a.s.l. ), there is a mixing of himalayan vegetation ( Leontopodium, Vaccinium, Gentiana, Viola, Asterea, Anemona,..) and moderate and subtropical vegetation.
The quick climatic changes during Plio-Quaternary and the monsoon gave so a very special mosaic of vegetal landscape, with a mixture of species coming from tropical, moderate, nordic or himalayan regions.
The tropical forest of southern China ( South of Yunnan, Hainan ) are quite similar to those one of Indonesia or Indochina. China is also the place where you find many genus and species inherited from the Cenozoic : gymnosperm ( Gincko, Ephedra, Cryptomeria, Metasequoia, "Water pine",...) and angiosperm ( Magnolia,...).
The example of Metasequoia is extraordinary. Paleontologists knew this genus. It can be found in the Cretaceous rocks in the present arctic regions. Until 1941, they thought it has disappeared .Living Metasequoia were discovered in a hilly region at the border of Sichuan and Hubei. This "living fossil" has been confirmed in 1946 by ZHENG WANJUN and HU XIANXIAO. Since this time, it is cultivated in many regions. Older is the Gincko ( yinguo in chinese ). It is the only genus of a family known in the Jurassic ( 200 millions of years ) and who disappeared elsewhere during the Quaternary glaciations. This genus was quoted in 1256 and is, since this time, widely diffused in Asia. It is a fecundity symbol.
