VULNERABILITY OF CARBONATE CRITICAL ZONES TO EXTREME GEOMORPHIC EVENTS (Invited Presentation)

Carbonate critical zones, in comparison with silicate critical zones, tend to be more vertically extensive—on the scale of an order of magnitude deeper—and have much faster travel times for water and therefore for transmission of pollutants. Additionally, karst landscapes, a frequent occurrence in carbonate critical zones, are particularly vulnerable to impacts from extreme geomorphic events on Earth’s surface, such as floods and landslides. According to Goldscheider et al. (2020), “15.2% of the global ice-free continental surface is characterized by the presence of karstifiable carbonate rock.” Because of this relative prevalence of karst and its particular environmental vulnerability, it is important that we understand the ways surface processes affect the subsurface. As an example of a case in which an extreme geomorphic event significantly changed the interior of the karst aquifer in a carbonate critical zone, I present an analysis of a debris flow deposit in the Mammoth Cave System in Kentucky, USA. This was the first study to characterize an in-cave debris flow to this level of detail. The deposit, named Mt. Ararat by cavers, has a maximum thickness of 7 m, a head-to-tail length of 75 m, and a total volume of about 3400 m³, as determined by terrestrial LiDAR and electrical resistivity surveys. The deposit is chaotic, angular, matrix-supported, and roughly inversely graded, with grain sizes, quantified through various grain-size distribution measuring techniques, ranging from clay through boulders larger than 1 m. The clasts are predominantly Mississippian Big Clifty sandstone, which is allochthonous in this part of the cave. The angularity of the blocks in the deposit indicate that they had not experienced significant erosion and therefore are determined to have been transported only a relatively short distance over a short time. The deposit profile is compound in appearance with two heads. This is therefore interpreted as a debris flow deposit resulting from two distinct flow events, and I present a sequence of events leading to the present-day Mt. Ararat in Mammoth Cave. This case study illustrates the vulnerability of carbonate critical zones and emphasizes the intimate connection between the surface and subsurface that must be accounted for in carbonate critical zone analyses.