, Auditorium
The concept of the magnitude and frequency of geomorphic work is central to understanding the action of erosion. Here, I employ this concept to explore the relative importance of chemical and mechanical erosion in carbonate channels. I first develop a conceptual framework to explore how chemical and mechanical erosion rates vary with discharge. Mechanical erosion processes typically activate above a threshold discharge, whereas chemical erosion may be active at all discharges. Mechanical erosion scales more strongly with discharge than chemical erosion. This framework provides insight into the conditions when chemical or mechanical erosion processes will dominate.
I apply this framework to two different settings. In J2, part of the broader Cheve karst system in Mexico, erosion features suggest a dominance of mechanical erosion. The stream is only substantially undersaturated with respect to calcite during flood flows, and mechanical erosion is more effective at those same flow levels. The few sites with clear evidence of dissolution occur upstream of breakdown chokes, where water ponds during high flows.
Next, I examine slot canyons in the Ozarks within a limestone that underlies a sandstone. Channels narrow and steepen as they flow from the sandstone onto the limestone. The mechanical strength of the two rock types is similar. I hypothesize that the most effective discharge for erosion is different within the two rock types because of the contrast in chemical and mechanical erosion processes. A model that incorporates these processes produces slot canyon morphologies in the limestone, similar to those observed in the field.
Matthew D. Covington