大方县九洞天伏流大厅起源初探——兼谈地转偏向力在伏流大厅形成中的作用

用洞穴双旋回演化理论和系统科学思想指导，定性和半定量地讨论了贵州省大方县九洞天伏流大厅的起源。分析研究认为：（1）洪水拥有超过平水期水流几个数量级的动能和溶蚀能力，使季节变化带成为垂直剖面中岩溶发育最为强烈的部位。（2）伏流大厅的形成过程至少是3个子过程耦合作用的结果，即①水流的剥蚀；②崩塌；③化学沉积生长。（3）水流运动的驱动力由重力和地转偏向力耦合而成。在重力驱动下水流对大厅左右壁的剥蚀强度相等，但由于地转偏向力的参与，在北半球驱使剥蚀向右偏蚀。（4）九洞天伏流有明显的右偏蚀形态证据：①左壁化学沉积发育，右壁基岩裸露；②右壁岩块脱落多于左壁；③靠近左壁有一条高角度裂缝状的充水溶洞。（5）地转偏向力在九洞天伏流大厅的形成中扮演着重要角色。伏流的巨大流量能够产生比中小地下河大10至100倍的地转偏向力，使九洞天伏流右偏蚀特别强烈。在洪峰水流的溶蚀和侵蚀下，优先在右壁产生边槽，致使卸荷过程和岩块脱落由左向右不断横向发展，导致大跨度的地下河大厅以及伴生的地表地貌形态生成。（6）对于高角度裂缝状洞穴和缓倾角层面洞穴两种情况，伏流右偏蚀过程有一些差别。对于缓倾斜层面洞穴，当基准面缓慢下降时，形成沿层面倾斜的底板。丰鱼岩就是一个很好的实例。

On origin of the Jiudongtian cave hall in the sinking stream, Dafang county, Guizhou province, China: Evidence for dominant rightward erosion in northern hemisphere by subterranean stream

As an attractive scenic spot, the Jiudongtian cave hall in Dafang County, Guizhou province should be worth world praise and notice. It is thought of great academic significance by foreign and domestic scientists. The Jiudongtian (nine sky holes) sinking stream is located along a nick point in the middle reaches of the northern branch of Wujiang river, Yangtze Basin. It is named "Jiudongtian" by local people because there are nine karst windows along its 2.8 km stream course. In 1987 the sinking stream was first explored by a China-New Zealand speleological expedition led by P.W. Williams and the professors from the Guizhou Normal University. P.W. Williams believed that:"Jiudongtian not only belongs to Guizhou or China but also to the world; It is not inferior to any well-known karst spot in the world......; specifically, it represents a nice natural model for combined formation of surface and subsurface karst landform."Later on, Prof. Yang Mingde from the Guizhou Normal University presented a classic study on the Jiudongtian karst. But both of them did not address much to origin of those cave halls. This paper will add to their ideas with a demonstration showing the role of the Coriolis force and dominant erosion to the wall on the right of the cave hall.(1)This paper give some new ideas about cave hall formation. First, cave hall must originate from a prior phreatic cave,as believed W.M.Davis. Second, owing to that the moving energy and dissolution capacity of flood water are of severel orders of magnitude larger,the zone of seasonal water level fluctuation becomes a place on the walls where the most intensive denudation occurs. Third, as a result notches are forming deeply cutting into both the cave walls. Fourth, at least 3 sub-processes and 5 sub-processes of lower order are subsequintly involved in formation of cave hall. Fifth, coupling of sub-prosesses,3 types of molti-interrelation have already been recognized.(2)Reloading fissures alter the opening state of fissure system, making its permeability more even. This in turn leads to even distribution of speleothems along top and walls of the cave. There are some obvious evidences in morphology of the cave halls for dominant rightward erosion (in Northern hemisphere) by the Tiudongtian subterranean stream. Firstly, there is a unique pattern of bedrock outcrop and speleothem distribution, 90% area on the left wall is covered with speleothems, more than 90% on the right wall with bedrock; while in the middle of the right wall there is a branch cave passage full of speleothems. Secondly, rock collapse mainly appears on the right wall. Thirdly, a vertical fissure-shaped cave can be found on the bottom or at the top of some halls along the wall on the left hand.All of these strongly infer that a vertical phreatic fissure-shaped cave occurred there originally near the left wall. When evolving and entering into the zone of seasonal fluctuation in groundwater table the fissure-shaped cave must has been excavated intensively by flood water, which is estimatedas large as 400 m~3/s in discharge. The Coriolis force generated by such a flood flow would be 10～100 times larger than that of an underground river of 40 m~3/s or 4 m~3/s in discharge respectively. It must force the right wall moves backward to form the huge space of the hall. The rightward erosion for a gently dip bedding plane cave is slightly different to the vertical fissure-shaped one. Direction for cave floor extension is controlled by two factors, namely the rate of rightward erosion and the rate of descending of the local denudation base. When denudation base is descending rapidly, both sorts of caves might form and layered cave system may takes place. While in the case of slow and intermittent tectonic descending, a cave with slight-dipping bedding will form a gently inclined floor along the bedding, as is seen in the Fengyu cave, Guilin. (3) Presented above is a preliminary result and needs to be verified by detailed study in future.