河流裂点迁移对洞穴发育过程影响研究：以中国云南宝牛角洞为例

River incision into bedrock is the visible expression of landscape response to regional, continental, or global changes in base-level due to climate, eustasy, or tectonics. Rapid baselevel lowering is considered to be transmitted upstream via knickpoint propagation, following the stream power law. The rate of knickpoint migration is therefore a key determinant of landscape response to sudden base-level lowering. In fluviokarst areas, due to its unique topography and hydrology features, the drainage is generally interrupted due to part of water diversion into the underlying karst aquifer through fractures, sinkholes, and sinking streams. The process and results of competition for river water between surface and underground drainages will inevitably have great impacts on the rate of knickpoint migration and cave evolution. Can the stream power law be used to model incision by knickpoint migration in fluviokarst basins? In study area, the process of competition between water between surface-river and ground-river has been perfectly presented by Nanni River and BaoniJiaodong cave system. Three dating methods, cosmogenic nuclides 26Al/ 10Be burial dating, AMS 14C and U-Th dating were engaged for constraint ages of events in the process. Based on fully speleogenesis study of cave and sedimentary analysis of cave fluvial sediments on different levels, a cave river incision sequences has been established. Depend on the dating of knickpoint migration through the fossil inlets of cave, rates of knickpoint migration have calculated, and the impact of water losing on knickpoint migration has been discussed based on paleodischarges of fossil inlets. Then, modeling the speeds of knickpoint migration between fossil inlets using stream power model and discuss the influences of water losing on values of exponent m and n in the model. Finally, apply a simple mathematical model based on coupling of flow, dissolution and transport to modelling speleogenesis processes under scenario of sudden hydraulic head lowering. The findings of study will not only help us better understanding the mechanisms of knickpoint migration, speleogenesis, and but also could be also great valuable for the theories of hydraulics and river dynamics applied in fluviokarst areas.

流水喀斯特地区地表和地下水系对河水水量的争夺过程及其结局对河流裂点迁移和洞穴发育有深刻影响，但很少被研究。本研究以南泥河上的宝牛角洞洞穴系统为例地对这一过程开展研究。采用26Al/10Be埋藏测年、AMS 14C和U-Th等四种测年方法对洞穴不同沉积物测年，在洞穴形态测量分析、各种流水沉积物沉积学研究基础上，建立洞穴河流下切时间序列；基于两两化石入水洞口之间裂点后退速率计算和洞穴古水文分析，讨论地表水漏失对迁移速率的影响；以计算的后退速率为约束，模型模拟讨论河流功率定律在喀斯特地区裂点迁移研究中的适用性；探讨河水渗漏以及漏失量变化对模型中m和n指数取值的影响；对比分析地表与地下水系对基准面下降事件响应速率的差异；最后，用计算机模拟补给水头骤降场景对洞穴发育的影响。成果对深入了解喀斯特地区对外因扰动的响应机制、洞穴成因机制有重要学术价值，对流水动力学理论在喀斯特地区的应用也有重要意义。

项目以云南省西双版纳南泥河上宝牛角洞洞穴系统为例，研究地表和地下水系对河水水量争夺过程及其结局对河流裂点迁移和洞穴发育影响。项目通过洞穴系统探测，发现了宝牛角洞出水口段上层洞穴和完好呈现现代裂点迁移对洞穴发育过程影响的实例。对洞穴下切过程具有指代意义的水平延伸槽痕识别与测量结果显示，在垂直方向上水平槽痕数量及其之间相对高差从洞口（下游端）向地下水入口段逐渐减少或减小，条数减少，相对高差由2-3m减小到1-2m，最后到地下水入口段没有可识别水平槽痕，指示洞穴地下河切割是自下游向上游迁移的，地下水系并没有完整地袭夺地表河流，是河床边上的落水洞。旱季时地表河水全部进入地下。6个不同洞段断面扇贝测量数据估算出当时古流速介于1.46-2.65m/s，遵循支流小于主干道规律。基于取自3个不同高度台阶状沉积物的13个OSL年代样品测年，59.25千年左右裂点到达洞穴出水口，34.01±5.03千年至37.12±2.22千年，沉积了第一级“洞穴类阶地”堆积沉积，32.87±4.43千年左右沉积了第二级“洞穴类阶地”，22.28±2.90千年至18.38±2.35千年沉积了第三级“洞穴类阶地”。从“洞穴类阶地”保存的位置和面积大小来看，它们的形成代表了一个进入洞穴的地表水量依次减小的一个过程。洞穴中流水16个观测点水化学研究显示，在河流裂点穿过三个干入水口后，在地下深层又发育了新的管道系统。显然，左侧的管道相对成熟，地表水能快速在洞中流出。研究区范围南泥河段地表河床剖面和区域地质调查资料显示，在晚第四纪时期研究区曾发生过2-3次抬升，在一些河流沿岸形成2-3级2-3m高的阶地。后期的地壳抬升引起的裂点后退在地表河中占优，使洞穴出水口高悬河岸。洞穴成因模拟方面，因研究区地质背景复杂，加上热带雨林的环境，获取当地较详细地质数据较困难，模拟结果不是很理想。成果验证了假设，并认识到洞穴系统空间规模能在短时间内发生巨大变化，对深入了解喀斯特地区对外因扰动的响应机制、洞穴成因机制有重要学术价值。