A FRAMEWORK FOR QUANTIFYING THE RELATIVE IMPORTANCE OF CHEMICAL AND MECHANICAL EROSION PROCESSES IN SOLUBLE BEDROCK CHANNELS

Bedrock channels have been a major area of research activity in the last decade. Improved understanding of the mechanisms of channel incision has played a central role in exploring the interactions between climate, relief, and tectonics. A common assumption within the bedrock channel literature is that chemical erosion processes, such as dissolution, are dwarfed by mechanical erosion, and thus play a minor role. However, this assumption may break down within channels that are incising through highly soluble rocks. In fact, the community of researchers studying cave formation in limestone typically assumes that dissolution processes are dominant. Numerical models of speleogenesis, until recently, have not included mechanical erosion processes. Despite an apparent conflict, each of these two communities is likely correct within a certain range of shear stress, stream chemistry, rock strength, and rock solubility. While these factors should all play a role, the controls on the switch between dominantly chemical and mechanical incision have not been quantified.

Here, I present a theoretical framework that can be used to quantify the relative importance of chemical and mechanical erosion within stream channels. This framework considers how stream chemistry, shear stress, and the resulting erosion rates scale with discharge. While mechanical erosion increases as a power law of discharge, dissolution rates increase to a maximum value and then plateau, such that higher discharges do not increase the rate of dissolution. Therefore, the time distribution of discharge, and its relationship to the threshold for mechanical erosion, plays a crucial role in the relative importance of chemical and mechanical processes. The theoretical framework is illustrated with a field example from Cueva J2 in Oaxaca, Mexico. A set of reaches within this cave were identified where micro-morphological erosion features displayed strong evidence for a dominance of dissolution or mechanical erosion. To elucidate potential controls on the relative importance of erosion processes at these sites, factors such as lithological properties, fracture spacing, and hydraulic conditions were considered. The pattern of erosion processes within Cueva J2 can be explained within the context of the theoretical framework described above.