78-9 Using erosional features to examine the importance of chemical and mechanical erosion in Sistema J2, Oaxaca, Mexico

Session: New Frontiers in Cave and Karst Science

Presenting Author:

Matthew Covington

Authors:

Covington, Matthew D.¹, Oleson, Ethan William², Cooper, Max³, Hernández Vergara, Rogelio⁴, Verde Ramírez, María de los Ángeles⁵

(1) University of Arkansas, Fayetteville, AR, USA, (2) University of Arkansas, Fayetteville, AR, USA, (3) University of Arkansas, Geosciences, Fayetteville, AR, USA, (4) UNAM Instituto de Geologí­a, Geologí­a Regional, Ciudad de México, MEX, Mexico, (5) UNAM Instituto de Geologí­a, Mexico City, MEX, Mexico,

Abstract:

Most studies of cave erosion have focused on dissolution. However, mature cave streams can transport a substantial sediment load, and, consequently, mechanical erosion processes are likely to be important in many caves. In fact, in studies of bedrock channel erosion, it is typically assumed that mechanical processes outpace chemical processes, even in soluble bedrock such as limestone. Here, we study nine reaches within Sistema J2, a steep stream cave located in Oaxaca, Mexico, which is part of the Cheve karst hydrological system. We identify erosional features that are diagnostic of either mechanical or chemical erosion processes. Mechanical erosion features include potholes, plucked blocks, mechanical erosion flutes, and smooth, polished surfaces. Chemical erosion features include scallops, solution pockets, and sharp, rough surfaces. At each site we quantify coverage by these features and use this as a proxy for the importance of different erosional processes. We measured a variety of other parameters at each site, to quantify potential controls on erosion processes, including: 1) Schmidt Hammer rebound scores, as a proxy for elastic strength, 2) fracture spacing, and 3) percent insoluble material within the rock. We also used data loggers to collect water level, temperature, and specific conductance records at two sites to better understand flood dynamics and variability in calcite saturation state. We find that plucking dominates at sites with closely spaced fractures (< 1 m), but other erosional processes are insensitive to measured parameters. Time series data suggest that the stream only actively dissolves calcite during high flow. In general, we find that mechanical erosion features dominate at all sites. Solutional features were rarely observed. At two sites, there is a dominance of solutional features at elevations >2 meters above the active stream level. Both of these sites are upstream of breakdown blockages that create backflooding during high discharge. We conclude that mechanical erosion dominates the modern incision in this stream cave, except in locations were ponded flood water shuts off mechanical processes but dissolution continues uninhibited. While dissolutional processes undoubtedly drive the initial stages of most cave development, we hypothesize that mechanical processes may often dominate within powerful stream caves, particularly in streams that transport a sediment load and where water is saturated with respect to calcite except during floods.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8838

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