PSEUDOKARST SINKHOLE IN THE SHARON CONGLOMERATE CANNOT HAVE FORMED SOLELY BY SLIDING OR SLUMPING PROCESSES

Sinkholes formed in sandstone are not common but are known in locations worldwide. A mechanical (i.e. jointing/slumping) origin for such features is usually invoked and related to the development of "crevice" or "tectonic" (sic.) 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 cave systems. In some cases, there are associated sinkholes. The present study investigates the origin of the largest known of these - an 8 m deep, near vertical pit on the east flank of Little Mountain (Geauga County, Ohio). Structure-from-motion photogrammetry was used to model the feature for morphometric analysis. 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 m by 9 m, and is overhung on three sides. The walls are roughly coincident with joints but do not display distinct fracture surfaces. The floor is sandy and has a few large sandstone boulders apparently due to collapse. At several locations, the walls are spalling off in 3 to 10 cm thick, occasionally curved sheets. Joint-controlled cave passages, some with flowing water, lead into and out of the sinkhole. There is significant variability in the sediments and sedimentary structures found in 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 topographic scarp to the east of the sinkhole is continuously aligned, which is inconsistent with major motion of the eastern sinkhole wall block. This sinkhole formed, and is likely still forming, through a variety of processes including mechanical joint widening, dissolution and disintegration of bedrock, and grain transport. Such processes should be considered when evaluating creation of pseudokarst features in a variety of settings.