Hydrogeologic and topographic controls on evolution of karst features in Illinois’ sinkhole plain

Abstract

In the sinkhole plain of southwestern Illinois, the size and morphology of cover-collapse sinkholes can be used as an indicator of the size of the associated underlying crevice/conduit system. Sinkholes that lead to relatively small crevices and small conduit systems (sinkhole volumes >10³ m³, but <10⁵ m³) typically are found in areas with relatively high water tables and, based on storage capacity of conduits and secondary porosity of adjacent bedrock, are incapable of accepting large, rapidly inflowing volumes of surface-water runoff without backflooding. Once flooded, sinkholes become low-energy environments and sites of deposition of fine sediments; consequently, their growth potential (via erosion by runoff within and at the margins of the sinkhole) is limited. Conversely, in those areas where sinkholes lead to relatively large, branchwork-type cave systems, water tables are typically deeper and the storage capacities of the cave system and adjacent bedrock can be one or more orders of magnitude greater. Because these large cave systems can accept greater volumes of rapidly inflowing runoff without backflooding, the sinkholes associated with large cave systems typically become sites of extensive erosion. The aerial extent of a sinkhole draining to large cave systems may continue to increase and incorporate other sinkholes within its drainage area, thereby creating large, compound sinkholes up to 30 ha in area. These sinkholes can have direct or indirect connections to cave passages and have volumes that can be as small as 10⁴ m³ and as large as 10⁶ m³. Beneath the glacial till and loess cover, and in the vicinity of the largest cover-collapse sinkholes, irregularities in bedrock topography appear to have had an effect on the location of the initiation of large caves in the sinkhole plain. The coincidence of bedrock depressions and swales in close proximity to the initiation point of the large caves suggests that focused recharge to fractures leading to bedding-plane partings in bedrock played a major role in the initiation and evolution of the large branchwork-type caves of Illinois’ sinkhole plain.