1994 Yangtze Gorges Expedition

Geology, Geomorphology and Cave Development

China Caves Project

XIN LONG AREA Kev Senior

The village of Xin Long is situated approximately 35km south of the Jiang Chang (Yangtze River) and the city of Fengjie. The limestone hills around the village rise to nearly 1800 metres and the continental climate is dominated by cold northerly winds in winter and stormy, humid tropical weather from the southeast in the summer.

Average precipitation is between 1500 and 2000 mm with most falling in the summer months often during severe storms. Long periods of drought can occur and we were fortunate to visit the area during one of its longer dry spells because cave exploration was much safer. Temperatures reached 42C in Chongqing with high humidity, but at Xin Long humidity was lower and temperatures a more comfortable 30C.

Sketch Map Of Xin Long Area

From Map: Kev Senior

The landscape around Xin Long reflects two main influences; 1) a geological structure dominated by assymetric folds with northeast-southwest axes and 2) continous lowering of the water table driven by the down-cutting of the Jiang Chang. The structure is responsible for the pattern of relief and the drainage pattern. Ridges are aligned along the limbs of anticlines and cone karst is often well developed on the limbs with moderate dips. The drainage pattern is broadly rectangular with major rivers following synclines or the axes of eroded anticlines. On a regional scale, drainage is to the northeast towards the Jiang Chang and rivers in the Xin Long area ultimately drain to the Jiu Pan He which flows to the Jiang Chang east of Fengjie. Downcutting of the Jiang Chang and of its tributaries have resulted in huge dolines like Xio Zhai Tian Ken and spectacular gorges especially where rivers cut through the limestone ridges at capture points. In turn, cave development is strongly influenced by the effect of dropping base level with the active caves showing predominantly vadose characteristics, controlled by joints and faults and exhibiting evidence of successive river captures. This is particularly evident in the case of the downstream doline cave and its relationship to the Mie Gong He gorge and the parallel gorges further east.

In contrast to present hydrological conditions the earliest phases of cave development were dominated by phreatic conditions predominantly controlled by the bedding. The limestones of the Xin Long area range in age from Devonian to Triassic, although the caves we explored are developed only within the massively bedded Permian and Triassic limestones. These limestones reach a total thickness in excess of 1000 metres and are remarkable for their lack of persistant bedding planes. Within the Permian limestones only one hiatus disrupts the otherwise monotonous sequence of finely laminated calci-lutites. At the hiatus is a bed comprising red and yellow shales with boulders of limestone and sandstone and the chaotic texture is suggestive of an olithostrome. This bed is responsible for the major ledge approximately half way down the Xio Zhai Tian Ken doline and for the 'bench' along either side of the Tian Jing Gorge. It was also the inception horizon for several of the older, fossil caves including the huge phreatic passage in Two winds Cave.

The huge thickness of finely laminated limestones have tended to respond to the tectonic stresses by folding and kinking at the axes and without developing an extensive joint system. For example, along the anticline which houses the downstream Xio Zhai Tian Ken cave, the only persistant joints are those which follow the fold axis and these provide the only route for vadose water. The northern limb of the fold affectively 'seals' the cave waters from the Jiu Pan He which flows in a syncline at a much lower altitude only a kilometre to the north. Where joints and faults do occur across fold limbs, they are the controls for river capture through the limb as has occured at the Mie Gong He gorge.

The expedition investigated two main karst systems:

I) The drainage system comprising the Tian Jing Gorge, the Xio Zhai Tian Ken doline and the Mie Gong He gorge to the north of Xin Long

2) The Tao Yuan He Dong (Peach Forest River Cave) resurgence.

Hydrology of the Hei Yau Dong - Di Feng Dong - Xio Zhai Tien Keng - Mie Gong He Dong System

Kev Senior

North of Xin Long, the Tian Jing Gorge is developed approximately along the axis of syncline S1. At Hei Yau Dong (Black Rock Cave), an important sink, water is presumed to flow north towards Di Feng Dong, short-circuiting the surface river. The gorge exploits joints, J1, to cross anticline A1 whose axis is close to Da Dongzi. The gorge swings to the north down the shallow dip, but is guided mainly by joints, J2, until it reaches the axis of syncline S2 at Di Feng Dong. From Di Feng Dong, water is presumed to follow joints parallel to the axis of syncline S2 as far as the southern end of Xio Zhai Tien cave.

Di Feng is therefore the point at which the Tian Jing Gorge has been captured and north of Di Feng Dong the gorge continues as a dry valley, G4. Joint J3 takes the water along upstream Xio Zhai Tien Keng to the axis of anticline A2, which then links the Xio Zhai Tien Keng doline to the Mie Gong He Dong resurgence. The Mie Gong He gorge, G3 cuts through syncline S3 to join the Jiu Pan He northwest of Yitz.

The surface topography implies that the Mie Gong He once flowed through Yitz to gorge G1 before joining the Jiu Pan He. The dry gorge at G2 records a previous capture. The most recent capture is man made; a tunnel, T, diverts water from Xio Zhai Tien Keng to a hydro-electric power station, HEP, on the Jiu Pan He.

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