PECULIAR MUDCRACKS FORMED IN CAVE SEDIMENTS BY AIR-ESCAPE DURING FLOODING

Mudcracks are common features created by the drying of fine-grained sediments at the Earth surface. They are characterized by distinct polygonal fractures in plan view, along with a tapered V-shape of cracks in cross section, and concave-up polygon edges. Mudcracks are also found in caves, where they can develop on clastic sediments that have been deposited within the subterranean setting. Cave mudcracks form in an extreme environment not conducive to the drying necessary to create desiccation structures. High humidity and lack of solar illumination hint that variation from typical (surficial) forms might be expected. We have observed many such features in caves and they are visibly similar to their aboveground counterparts. However, in 3 widely separated locations (Kooken Cave, PA; Union Cave, WV; Camps Gulf Cave, TN) we observed atypical mudcracks that are distinct from the normal morphology. In these cases, the cracks are finely crenulated (wavelength ~ 5 to 30 mm) rather than being straight or broadly curving. In addition, the center of the mudcrack polygon sometimes exhibits significant positive relief, rather than the usual bowl-like depression. We examined the morphology of these features and the character of the sediment. We also evaluated the hydrologic context of the setting. In each of these deposits the fine-grained sediments are underlain by coarser material. In addition, the locations are subject to rapid and deep flooding. We conclude that the peculiar form of these mudcracks has developed due to air escaping from the sediments during the flood pulse. The crenulations develop as bubbles migrating from the lower, coarse-grained, deposits come forth from the cracks during inundation. The center of the polygon develops positive relief because air streaming upwards from the cracks prevents sediment from settling out of the floodwaters near these margins. This leads to accelerated deposition in the center, and draping of mud towards the cracks. The moist and dark environment of the cave does not seem to be responsible for the morphology for these structures. Thus, it appears that the constrained cross-sectional dimensions of cave passages, which promotes very rapid head changes during floods, is critical for the development of these peculiar mudcracks which have not been reported from surface environments.