MELTWATER CHANNELS IN THE KARST BELLEVUE-CASTALIA PLAIN, SENECA COUNTY, OHIO

The Bellevue-Castalia karst plain in Seneca County, north central Ohio, hosts numerous channel-like depressions previously interpreted to be associated with large sinkholes. The channels are 3–6 m deep, start and stop abruptly, and are generally parallel to ice flow direction, trending uphill from the northeast. Sinkholes of varying sizes are within and outside the channels. A low-density network of existing well-logs prevents determining if the channels are incised into the Columbus Limestone bedrock or glacial sediment. Hence, this study combines electrical resistivity imaging with a geomorphological analysis of the channels’ orientation and distribution to assess the architecture and origin of the channels. Several electrical resistivity tomography (ERT) transects <350 m long were collected across the channels using a dipole-dipole electrode array and a 1 m unit electrode spacing The ERT results shows two main resistivity units with discontinuities within each unit The lower unit with higher resistivity values (250–2500 Ω-m) is interpreted as the limestone bedrock with an irregular upper surface. The overlying less resistive (25–100 Ω-m) unit with thicknesses of 3–5 m inside the channels and ~1–3 m outside the channels is interpreted as till. Soil sampling along the ERT transects by soil auger reached refusal consistently at ~2 m depth. From the ERT results, the bedrock is at a lower elevation beneath the channels than the ground surface adjacent to the channels. This observation indicates the meltwater channels are incised into bedrock, and the till deposit has a sheet-like geometry draping the bedrock surface. The irregular bedrock surface likely reflects a combination of unequal meltwater incision across the channel bottom, and postglacial karstification of the bedrock surface, consistent with the presence of sinkholes within the channels.