PALEOKARST IN CENTRAL AND NORTHWEST OHIO— THE NEWBURG ZONE AS A SYSTEM OF SILURIAN FLANK MARGIN CAVES

In 2011, Ohio State University began drilling wells for a geothermal heating and cooling system to upgrade several dormitories. The drillers encountered problems with collapsing boreholes and incredibly high permeabilities at multiple depths within the Columbus Limestone, Salina Undifferentiated, and the Tymochtee and Greenfield Dolomites. There are two high permeability zones underlying the campus: one within the Columbus Limestone, the another in the lower Salina Undifferentiated down to the top of the Lockport Formation at depths of 280 to 360 feet. The depth of the lower permeable zone coincides with that of the Newburg Zone. Further study of the lower permeable zone based on core descriptions, quarry visits and geophysical logs reveals evidence for extensive karstification in the form of vugs, dissolution chambers, enlarged fractures, spongerock, and missing core. The dissolution chambers are caves that show no evidence of flow channels leading into or out of them, suggesting that they are mixing chambers. These appear to be Silurian analogues to modern flank margin caves that form on the flanks of carbonate islands where the fresh water lens meets sea-water. These flank margin (hypogenetic) caves have been extensively described in tropical regions around the world. Based on the location of Ohio during the Late Silurian period, close to the equator and the tropical climate it would have had, the environment in Ohio would have been similar to modern carbonate environments currently found in San Salvador, Bahamas or Isla de Mona, Puerto Rico that are known for their flank margin caves. The locations of missing core in wells in northwest Ohio coincide with the lateral extent of the Newburg Zone, which is missing on the axis of the Findlay Arch. The Findlay Arch was a positive depositional feature during the Silurian, and was exposed as a series of islands and peninsulas during Silurian sea level changes. We conclude that the paleokarst system that is the Newburg Zone is in fact a system of flank margin caves formed on the rims of carbonate islands whose size would have changed with sea level changes. The different depths of missing core reflect different sea level stands during which there was sufficient time for flank margin caves to form.