A Field Study on the Interaction between Bedrock Cracking and Erosion of Various Lithologies

Abstract:

Topography is often thought to reflect the interaction of climate and tectonics, but the complex interactions and feedbacks between erosion processes and material resistance limit our understanding. For example, the erosional resistance of a bedrock channel surface depends on the accumulation of cracks through its history of exhumation and exposure, but the river incision rate limits the time over which geomorphic subcritical cracking processes (weathering and abrasion) can occur. Thus, the erodibility of a given lithology is a spatially and temporally dynamic variable within the channel. Here, we investigate the degree to which different lithologies are damaged by weathering and abrasion in bedrock channels of central Arizona. Our dataset includes channel geometry, laboratory tensile strength, Schmidt Hammer rebound, and P wave velocity to characterize the crack density of channel surfaces. In contrast to model predictions of bedrock behavior, channels with lower sub-surface rock tensile strength tend to have higher stream power. P wave velocities show that channel surfaces are 15-80% slower than core samples drilled from 0.03-0.3 m depth below channel surfaces, which suggests a near-surface gradient in cracking. We find that fine-grained mafic bedrock channels tend to have higher velocities and rebound values on faster eroding surfaces, but we find the opposite trend for coarse-grained felsic bedrock. While the latter trend is antithetical to the idea that faster erosion exposes less damaged rock, these findings demonstrate (1) the influence of microstructural rock properties on subcritical crack lengthening rates and river incision dynamics and (2) the difficulty in characterizing erodibility with rock strength across different rock-types or highly heterogeneous lithologies. Further research on the rock properties and geomorphic agents that resist and drive subcritical cracking within the bedrock channel would advance understanding of topographic evolution.