Geometrical characterization of stream potholes in sandstone from the Sunxi River (Chongqing, China) and implications for the development of bedrock channels

Abstract

Potholes carved into streambeds of the Jurassic and Cretaceous sandstone and mudstone along the Sunxi River (Chongqing, China) have been carefully investigated in order to characterize the geometrical features of the potholes and to explore their implications for river incision into bedrock. The potholes formed by gyratory currents through abrasion are characterized by an average aperture aspect-ratio (a/b) of ∼1.2 and a mean diameter-to-depth ratio (D/h) of ∼1.0. The most parsimonious model for the h-D relationship over the full range of data is a linear equation in which 2 parameters (n is the slope of the line and m is the D-intercept) are essentially controlled by hydrodynamics. Data of the Sunxi River yield that n = 0.71 ± 0.18, indicating that the pothole deepening was faster than the pothole widening by a quasi-constant factor of ∼1.4 during abrasive erosion as the dominant mechanism of pothole development in bedrocks of sandstone interbedded with mudstone. The m value, which varies from 5.9 cm to 11.0 cm, apparently depends on hydrodynamics at each locality, and can be inferred as the dimensional threshold of initial depressions from which potholes grew by deepening and lateral expanding. The potholes with D/h = 1 are prone to entrap and retain coarser sediments which are more effective for pothole growth. The bedrock weakened significantly by coalescence of numerous potholes is easily broken, detached, and removed by flooding (truncation). The cyclic pothole-truncation process is particularly important for incision of bedrock channels and thus landscape evolution of mountain ranges in response to tectonic uplift and climate change.