Formation mechanisms for the largest sandstone sinkhole in Ohio

Abstract

Sinkholes formed in sandstone are not common, but are known in locations worldwide. A mechanical origin (i.e. jointing/slumping) for such features is usually invoked and related to the development of crevice caves. Some researchers have inferred a contribution of dissolution processes to the formation of such caves. In northeast Ohio, there are Pennsylvanian age sandstone/conglomerate-capped hills containing small cave systems; sinkholes serve as entrances in some of these. This study investigates the origin of the largest known of these sinkholes—an eight-meter deep, near-vertical pit on the east flank of Little Mountain (Geauga County, Ohio). Structure-From-Motion photogrammetry models the feature for morphometric analysis, using computer and 3-D printed models. Rock and sediment samples were collected to evaluate disaggregation of the bedrock. Adjoining blocks and joints were mapped to place geometric constraints on potential formation mechanisms. The surface opening is a rough, irregular quadrilateral, about 3.8 m by 6.2 m, and is overhung on three sides. The walls are roughly coincident with joints without distinct fracture surfaces. The floor is sandy and has a few large sandstone boulders apparently emplaced by collapse. At several locations, the walls are spalling off in 3 to 10 centimeter-thick sheets or blocks. Joint-controlled cave passages, some with flowing water, lead into and out of the sinkhole. There is significant variability of induration between samples collected from the host rock. Some samples were poorly indurated, indicating ongoing weathering. The geometry of the sinkhole and adjacent blocks, both in map and vertical perspective, shows that the feature could not have formed solely by simple translation (sliding) of blocks. The overhanging, upper portions do not fit back together if opposite walls of the sinkhole are brought together. Additionally, the major joint to the east of the sinkhole is continuously aligned, which is inconsistent with major motion of the eastern sinkhole wall mass. This sinkhole likely formed and continues to grow through a variety of processes including mechanical joint widening, fluvial erosion, and arenization–cement dissolution, granular disintegration of bedrock and grain transport. Such processes should be considered when evaluating creation of sinkholes in a variety of settings.