Geological Exploration and Route Selection in Karst Area of Yunnan- Guizhou Plateau of Chengdu- Guiyang Railway Zhang Yu-jun and Yue Zhi-qin China Railway ErYuan Engineering Group Co.ltd, Chengdu,China [email protected] ABSTRACT Sichuan Yibin City to Guizhou Bijie City section of Chengdu- Guiyang Railway lies in the transitional zone from the southern margin of the Sichuan Basin to the northwestern of Yunnan- Guizhou Plateau. In the transitional zone, karst and karst water are generally controlled by a variety of factors, including lithology, structure, geomorphology, meteorological and water network. With the variation of natural geological conditions, the development degree of karst, the development pattern of karst , karst water distribution and enrichment degree of the karst water change accordingly. Through comprehensive analysis of the factors affecting karst, the study area can be divided into three karst hydrogeological units including the fold zone in the edge of Sichuan Basin, Sichuan Basin - transitional Yunnan-Guizhou Plateau, Yunnan-Guizhou Plateau.Since the end of the Cretaceous, the crust of the study area has experienced two big uplift and three leveled, three ancient karst erosion surface formed. Since the Pleistocene, the earth crust has experienced a number of intermittent rises that result in one to six terraces in the valley and the formation of corresponding multi-cave. The lithologic is the important factor that controls the development of karst; the Qixia Formation and Maokou Formation of Permian are most developed karst in fifteen karst horizons, and the underground river system are in the largest scale, then follows the Triassic; in alternating or interbedded layers of the non-karst and karst, the features of Karst is bedded, and hydraulic connection between layers is weak; The accumulation of karst water is mainly affected by the karst development degree, landform, lithology and geological structures. The results of this study may provide guidance for geology selection of railway and highway in complicated karst mountainous. KEYWORDS: Karst pattern, Hydrogeological unit, Water abundance, Karst partition - 101 - Vol. 24 [2019], Bund. 01 102 OVERVIEWS ON PHYSICAL GEOGRAPHY Geographical location.The section in Chengdu-Guiyang Railway, from Yibin city (Sichuan Province) to Bijie City (Guizhou province), goes through Yibin city, Weixin County, Zhenxiong County, and ends in provincial boundaries between Guizhou province and Yunnan province. This railway section, about 140km long, goes across Nanguang River located in the junction of Sichuan and Yunnan province, Chishui River watershed, Chishui River which lies in the junction of Yunnan and Guizhou province and Wujiang River watershed. The study area is about 12710km2. Geomorphology and evolution. Study area is located in the transitional zone of Sichuan Basin and Yunnan-Guizhou Plateau. The geomorphic conditions in this area from north to south include Yangtze River valley, hills, low mountains, low-medium mountain. The topographical features show significant stratification which has five planation surfaces from a macro perspective. Since the end of the Cretaceous to Oligocene, the crust in the study area is in a relatively stable stage, and landform contour have taken shape. During that time, crustal uplift constituted today's Ⅰ ，Ⅱ planation surface. In Oligocene - Miocene, the Himalayan orogeny occurred; the crust rose sharply, prompted active erosion; gradeⅠ,Ⅱplanation surface suffered severe damage and split; so far, the first large cycle of erosion in the study area completed. Pliocene to Early Pleistocene, the crust which were razed for the second time got into a relatively stable stage. During that time, The second phase of the Karst effect was strong, and non-karst area was subjected to strong chip flow erosion, resulting in early planation height decreasing and its breadth reducing. During the second raze period, the crust uplifted for a time to form Ⅲ，Ⅳ planation surface. In early Pleistocene, the crust again rose strongly; surface erosion revived; canyon began to form; the Karst effect turned to vertical; near the valley karst depressions, dissolved network disintegration generated valley or natural bridge, underground stream. So far, the second largest cycle of erosion in the study area came to an end. Since then, the earth crust has been in a relatively quiet period , and the razing period started again. This district began to form a large number of red hills, karstic erosion effects transferred to horizontal action. Between the valley, many karst depressions, funnel, karst valleys and many vertical slope caves formed. In the meantime, the northern grade Ⅰ，Ⅱ planation surface has been eroded away; the south has become increasingly narrow, only few isolated peak residual surface left. Grade Ⅲ, Ⅳ planation sueface also suffered splitting, destruction and disintegration, but it is still good in the south. In fact, since the Pleistocene, the crust has been rising intermittently on several occasions, resulting in the formation of one to six terraces in the valley and the corresponding multilayered caves. Major canyon developed in deep direction; rivers were subjected to southward traceability erosion; slope gradient increased; the number of splitting points increased, leading to the formation of underground rivers. Vol. 24 [2019], Bund. 01 103 Table 1: The classification system of geomorphic units Microgeomorphology Altitude Relative Slope Geomorphic type Remark unit (m) elevation (m) (°) Accumulation of Terrace river erosion Low mountains valleys ＜1500 200-500 Erosion structure Ridge low-medium ＜1500 500-1000 mountain Hill low mountains ＜1000 ＜200 ＞25 Erosion Trapezoidal medium- Threshold＜ denudation ＜1500 low mountain 200,Mesa＜20 Trough valley ＜50 ＜25 Gentle hill depression ＜200 ＜25 Medium-low mountain 1000-1500 ＞200 series of peaks Series of peaks valley ＞25 Canyon low-medium V-shaped 1000-1500 ＞200 ＞25 mountain valley Dissolution Honey-comb tectonic Series of peaks ＜100 texture Medium-low mountain of Parallel ridge Valley Parallel 1000-1500 500-1000 ,Parallel with river ridge valley valley Parallel ridge valley Parallel 1500-2500 >1000 medium-high mountain ridge valley Low mountain ＜1000 ＜200 Erosion Valley medium-high V-shaped 1500-2500 ＞200 mountain valley Meteorology and hydrology. Research area belongs to subtropical humid monsoon climate with abundant rainfall. The seasonal rainfall is uneven in a year, with average annual rainfall of 800 ~ 1300mm, the maximum of 1341.9mm in Xingwen County, the minimum of 800mm in Weison County. There is a decreasing trend from north to south. The rainfall mostly concentrates in May to September, accounting for about 75% of the annual rainfall. The rivers in the study area belong to Yangtze River region, including Nanguang River, Changning River, Yongning River, Chishui River, Niujie River and Liuchong River. Overall, this rivers flow into the Yangtze River from south to north . These rivers originate in the mountains with mountainous features of steep, narrow channel, deep cutting, big drop, cutting depth, many shoals and rapids. The average total amount of runoff is 178.9╳108 m3,and the average runoff depth is 768mm. Vol. 24 [2019], Bund. 01 104 Figure 1: Schematic of railway plan and distribution of karst in Yibin to Bijie section of Chengdu-Guiyang Railway Stratum and structure Formation lithology.The study area belongs to Yangzi stratigraphic area, partition of the South China stratigraphic area. The exposed strata include Cambrian ( ) dolomite, limestone, sandstone, shale; Ordovician (O) shale, dolomite, limestone; Silurian (S) sandstone, shale interbedded with ∈ limestone; Permian (P) limestone, sandstone, shale; Triassic (T) dolomite, limestone, sandstone, shale; Jurassic (J) sandstone, mudstone; not- well-developed Quaternary (Q) . Where in there are 15 groups of karst (see table 2) and 14 groups non-soluble rock. The cumulative total length of karst is 53.342km, accounting for 38% of the total line length. Of those, the karst In Jurassic, Permian and Triassic developed most well and distributed most widely. Table 2: Karst formations Statistics Erathem System Series Junlian、Xuyong area Zhengxiong area Weixin area Middle T l T f、T g T sh、T s Mesozoic Triassic 2 2 2 2 2 Lower T1j T1y T1m Permian Lower P1m、P1q P1m、P1q P1m、P1q Silurian Lower S1s S1h S1s Upper O3w、O3l O3g、O3w、O3j O3w、O3j Paleozoic Ordovician Middle O2b、O2s O2b、O2s O2b、O2s Lower O1h O1h O1h、O1t Middle-upper ∈2-3ls ∈2-3ls ∈2-3ls Cambrian Middle ∈2g ∈2g ∈2g Lower 1q 1q Geological structure.The study area is located between the∈ Sichuan-Yunnan∈ longitudinal tectonic belt and Sichuan-Guizhou latitudinal tectonic belt. The study area connect with Guizhou Vol. 24 [2019], Bund. 01 105 Central Uplift which lies on the northern edge of Nanling east-west complex structure belt in the south, and with Sichuan Basin which is a primary sedimentation belt of new China structural system in western China in the north. It is the edge of a settlement belt controlled by fold. Since the yanshan orogeny, the earth crust has changed for three times. Repeated stress action caused tectonic deformation. They not only retained their own features, but also interfered, interspersed, used or modified with each other, leading to the formation of various forms of joint and complex relationship, making structural features of the study area tend to be complicated. According to the mechanical properties of the main structural features of the region and generated