THE RELATIVE IMPORTANCE OF DISSOLUTION AND MECHANICAL EROSION IN LIMESTONE BEDROCK CHANNELS: A GENERAL FRAMEWORK AND AN EXAMPLE FROM A HIGH-GRADIENT STREAM CAVE, SISTEMA J2, OAXACA, MEXICO

Recent years have seen the development of a substantial body of work on incision processes in bedrock channels and on the role of bedrock channels in the evolution of mountainous terrain. However, there has been relatively little focus on the dynamics of chemical erosion processes, despite the fact that highly soluble strata, such as carbonates, occupy a reasonably large percentage of Earth's surface. Importantly, a disconnect exists between the communities that model bedrock channel processes and speleogenesis. Bedrock channel models typically neglect dissolution, assuming that it is dwarfed by mechanical processes. On the contrary, numerical models of speleogenesis do not typically consider mechanical erosion, and have assumed that dissolution is the dominant process. Despite an apparent conflict, each of these two communities is correct within a certain range of shear stress, stream chemistry, rock strength, and rock solubility. However, the controls on the switch between dominantly chemical and mechanical incision have not been quantified.

Here, I present a framework to quantify the relative importance of chemical and mechanical erosion within karst stream channels, considering how each process scales 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. This framework is illustrated with a field example from Sistema J2 in Oaxaca, Mexico. Sistema J2 is a high-gradient stream cave in the Sierra Madre de Oaxaca, where field evidence of mechanical erosion processes is abundant. For this study, a set of reaches were identified where micro-morphological erosion features displayed strong evidence for a dominance of either dissolution or mechanical erosion. The observed pattern of erosion features within Cueva J2 is explained within the context of the theoretical framework described above.