乐山平羌小三峡河岸壶穴地貌发育条件及作用机制初探

Development Conditions and Formation Mechanisms of Riverbank Potholes in the Pingqiang Small Three Gorges, Leshan

摘要: 壶穴是河流动力侵蚀塑造的典型基岩微地貌，是反演河谷水动力演化与新构造活动的直观地貌标识。岷江乐山平羌小三峡岸坡广泛出露成群发育的河床壶穴，为亚热带丘陵峡谷型河流壶穴研究提供了典型样本。本文以平羌小三峡干流两岸壶穴为研究对象，采用野外测量、形态观察、岩性与水文调查及区域地质资料对比等手段，对河岸壶穴的空间分布、形态特征、成因条件与演化过程进行了系统研究。结果表明：研究区壶穴主要分布于常年洪水位与一级阶地前缘之间的水位变幅带岸坡基岩面上；其形成与扩大以汛期涡流机械磨蚀为主导动力，基岩节理裂隙提供了初始侵蚀突破口。红层砂岩的非均质性、软硬互层结构及节理裂隙系统共同控制了壶穴的初始位置与扩展方向。壶穴演化经历了微凹陷形成、涡流启动、加深扩径、相邻贯通和残余岩体改造等阶段，最终形成联合壶穴、侵蚀槽沟及象形残丘(当地俗称“石鸭子”)等复合地貌。本研究可为岷江流域红层软岩河床差异侵蚀及复合地貌演化研究提供地貌学依据。

Abstract: Potholes are a typical bedrock micro-landform formed by fluvial erosional processes and serve as direct geomorphic indicators for reconstructing river hydrodynamic evolution and neotectonic activity. Numerous clusters of bedrock potholes are widely exposed along the riverbanks of the Pingqiang Small Three Gorges section of the Minjiang River in Leshan, providing an excellent case study for investigating pothole development in subtropical hilly gorge rivers. This study focuses on the riverbank potholes distributed along both banks of the Pingqiang Small Three Gorges reach. Field measurements, morphological observations, lithological and hydrological investigations, together with regional geological data, were employed to systematically examine the spatial distribution, morphological characteristics, formative conditions, and evolutionary processes of the potholes. The results indicate that the potholes are mainly distributed on exposed bedrock surfaces within the fluctuating water-level zone between the perennial flood level and the front edge of the first river terrace. Their formation and enlargement are primarily driven by mechanical abrasion induced by flood-season vortices, while bedrock joints and fractures provide the initial pathways for erosion. The heterogeneous nature of the red-bed sandstone, its alternating soft and hard strata, and the joint-fracture system jointly control the initial location and subsequent expansion direction of the potholes. The evolutionary sequence of the potholes involves the formation of shallow depressions, vortex initiation, deepening and lateral enlargement, coalescence of adjacent potholes, and modification of residual rock masses, ultimately producing compound erosional landforms such as coalescent potholes, erosional grooves, and zoomorphic residual rock forms locally known as “Stone Ducks” (“Shiyazi”).This study provides a geomorphological basis for understanding differential erosion of red-bed soft-rock riverbeds and the evolution of compound fluvial landforms in the Minjiang River Basin.