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THE EVOLUTION of KNICK-POINT BELT in a RIVER SECTION in HUANGGUOSHU Yang Hankui Li Po Karst Research Office of Guizhou Academy O

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THE EVOLUTION of KNICK-POINT BELT in a RIVER SECTION in HUANGGUOSHU Yang Hankui Li Po Karst Research Office of Guizhou Academy O IAH 21st Congress XARST HYOROGEOLOGY AND KARST ENVIRONMENT PROTECTION 10-15 October 1988 0U1LIN.CH1NA THE EVOLUTION OF KNICK-POINT BELT IN A RIVER SECTION IN HUANGGUOSHU Yang Hankui Li Po Karst Research Office of Guizhou Academy of Sciences, Guiyang, China THE NATURAL ENVIRONMENTAL FEATURES OF THE REGION This river section is part of the Dabang River which is a first-order tributary of the North Panjiang River. It is 8 km long, with the north end in Doupo and the south end in Hezuilai. There are 5 knick-points. Their total drop of water is 343 meters high. The whole area of Huangguoshu scenic spot is 600 km2. The regional topography spreads down from north to south, and at the same time becomes lower from its east and west sides to the river valley. The river section is located 685-102 3 m above the sea level, lowering down at a rate of 4.3% . Taking Huangguoshu knick-point as the dividing line, its upper reaches are in the géomorphologie form of shallowly-cut hilly valley, while its lower reaches take the shape of deeply-cut mountainous valley-. The top of each knick-point is a rather wide terrace from which waterfalls drop down, and the bottom is a valley. The géomorphologie form of the upper and lower parts of the Huangguoshu knick-point is entirely different from each other, and is regarded as a local datum plane of. erosion in a certain region. Generally, the present average annual temperature is 17 °C and the annual rain­ fall is mere than 1200 mm, and even reaches 1500 mm in its upper part of Luobie. It belongs to a moist sub­ tropical climate. The rainy season is during a period from May to October, which includes 86.9% of the annual rainfall. Since the forests on the ground have been 225 almost destroyed out, the eluvial soil is seriously eroded. Its modulus is 210.9 T • km"2 • a"1, 1.65 times larger than the corrosion rate. Consequently, it results in serious ' desertization ' in that area, and its karsti- fication is characterized by fluvial karst. As the upper reaches of this river section are flowing through the coal measures of the Upper Permian which contains FeS , and there is gypsum in carbonate xock, therefore S0~| exists in the river water, and the *water. is characterized by HC03-S04-Ca~Mg. In the upper reaches of Huangguoshu knick point, there are corrosion fissures, sinkholes and dolines deve­ loped on the terraces of the river bank and in the river bed, but there is no visible spring on the surface. In the valley of the lower reaches, there is a descending spring in Yangjiajing and an ascending spring in Maoshui- tang, both running into the river . This region extends from fault-depression in south Guizhou to fault-uplift.in Guanling. The two geological structUJ.;-:.! units are bounded by the Yadu-Ziyun basement fault. The covering layer is the Balinhe reversed syncline and a series of NW-trending faults. Except the river valley, the covering layers are in the shape of wavy folds with NW, NWW, NNE, and NE faults. Some are. developed into E-W and N-S fractured zones. Besides a small amount of the Lower Permian limestone and the Upper Permian sandstone and shale, most of the area is covered with the lower and middle series of the Triassic, namely, the limestone and dolomitic limestone in the Yongningzhen Group (TTY) and the Guanling Group (T2g) / intercalated with some sandstone and shale. The thickness of a single carbonate rock layer is about 250-12 00 m, and that of the sandstone and shale is varying from 10 m to 130 m. Under certain conditions, they check up.the water flow, resulting in the existence of several layers of karst water in some places. The Yadu-Ziyun basement 'fault is a regional struc­ ture resulting from a number of activities and cycles. Since the Cenozoic Era, the region has risen unevenly several times. Since the Middle Pleistocene Epoch, it has uplifted several hundred to about a thousand meters. 226 According to the data from geodetic leveling and the data about the earthquake in Guanling during the past 3 0 years, more than 20 waterfalls have been formed in Huang- guoshu area because of the'difference between the rising and rebounding. The rejuvenation and change of the rivers got . internal energy. THE FEATURES OF THE KNICK-POINT BELT Along the 8 kilometer long river section, there are 5 knick-points occurring in different time. They are called as Huangguoshu knick-point belt. 1. Doupo Knick-Point It is located 1 kilometer up from Huangguoshu Water­ fall. Its river section occurs in the medium-thick layer of d.olomitic limestone in the Yongningzhen Group (Tjy). There is a NNE upright fault on the right bank, forming the straight fault line cliff. On the left bank in the lower reaches there are remains of a collapsed cave about 100 meters long, and the remains of colluvial rocks, extending to the cave on the left shoulder of the.water­ fall. The cave goes upward ,for 100 meters along the left bank. It is a sinkhole on flood land. Under Doupo Water­ fall, there is an E-W fractured belt about 10 meters wide. In the lower"reaches of the waterfall there are three backward stone steps, on which are scattered gravels. The ^distance of the latest regression is 100 m. When the knick-point withdrew to the fractured belt, it washed out a shoal with broken rocks due to the poor resistance of the rocks against erosion. As a result, a lot of tufa has been piled up there, forming a tufa dam waterfall with a drop of 20.8 m and a width of 105 m. This is a unique type of piled-up knick-point created by the karst river. The headward erosion took place in the early and middle stages of the Late Pleistocene Epoch. The development of the cave on the left bank was an event of the Middle Pleistocene Epoch , while the piling up of tufa dam occurred only in.the past 50000 years. 2. Huangguoshu Knick-Point There are different opinions about the origin of this knick-point. Li Rushen (1980) regards it as the result of 'fault-depression. Someone thinks the collapse 227 of underground stream accounted for its formation, and others regard it as the result of headward erosion on the surface» From the recent data, we find that the formation of this knick-point is,rather complicated. It is located on the medium-thick layer of dolomite limestone in the Yongningzhen Group. In it there are thin layers of argil­ laceous dolomite of a few metres thick. The rock body is cut by 5 groups of faults and joints. In the curved valley in the lower reaches of the waterfall, from the foot/"o"f. the waterfall to Weitang, there is a steep bank of;80~ik00 meters high on the left and Elephant Trunk Stone-bedrock seated terraces on the right. The former goes along the surface of the argillaceous dolomite, forming two precipices. The upper one stretches up to the top of the waterfall while the lower one connects the water-screened cave, indicating that the river water had once stayed there. The latter is covered by mottled sandy clay, containing bean-like iron manganese nodules. The Elephant Trunk Stone is a collapsed rockbody, piled up on the alluvium . There are no obvious beddings m the alluvium. It is similar to the mud piled up in the present caves. The terrace began to form at the same time as the water-screened cave, during the Middle Pleistocene. The Elephant Trunk Stone is covered by platy tufa formed by Splashing water. It is about 46000\C+6000 to ^7000) years old, suggesting that the splashing of- waterfall appeared as early as that time. The Xiniu pool at the foot of the waterfall is 17.7 or 15.2 m deep. This proves that the erosion has continued for a long time. The age of the tufa on the slope behind the waterfall is about 2 0520+420 years, indicating that the knick-point has remained there at least 20000 years. The E-W trending valley located down from Wantang is narrower than the upper reaches. On the left bank there are two steps of bedrock seated terraces. In the alluvium there remain stalactite gravel with ring-like structure usually seen inside the caves. In the river beds, there gather colluvial rocks, forming the foundation of tufa dam. The river bank at the mouth of the valley are obvioualy of collapsed concave type. These facts show that there was once a subterranean stream passing through 228 this section. Along the bank there are still some remains of karst wells, indicating that the slope of the bank is not the result of headward erosion or downcutting. It is considered here that the formation of Huang- guoshu knick-point is like this. When the seepage of the Huangguoshu-Dapingdi surface river occurred at the beginning of the Quaternary Period, it resulted in a sinkhole in the place that we call Tiantan today. Step by step, the hole became a curved tunnel for the subter­ ranean stream. At about the Middle Pleistocene , the water level of the subterranean stream was as high as the ground level at which the water-screened cave and the Elephant Trunk Stone were located. During the Late Pleistocene Epoch, the subterranean stream cut down and became bigger, causing a collapse first in the N-S trending section, then in the E-W section.
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