| Popis: |
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 very high permeabilities at multiple depths within the Columbus Limestone, Salina Undifferentiated, and the Tymochtee and Greenfield dolomites. The driller reported two high permeability zones underlying the campus: one at 140-150 feet deep, and another at depths of 280 to 360 feet. The shallow zone corresponds to the depth of the Columbus Limestone and the deeper zone corresponds to the Salina Undifferentiated and the underlying dolomite units. The upper permeability zone coincides with the known cave zone within the Delaware and Columbus limestones, which house the Ohio Caverns and the Olentangy Indian Caverns. More in-depth study of this zone’s characteristics through quarry visits and core description reveals traditional continental or epigenetic karst features such as flow channels, dissolution along bedding planes, and missing core. This paleokarst zone was created by groundwater flow at or just below the water table and is a form of epigenic karst. 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, geophysical logs, borehole video, and thin sections reveals evidence for extensive karstification in the form of vugs, dissolution chambers, enlarged fractures, spongerock, and missing core. The dissolution chambers are caves that have no evidence of flow passages leading in 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 seawater. These flank-margin caves have been extensively described in tropical regions around the world. Based on the equatorial location of Ohio during the Late Silurian Period, the environment in Ohio would have been similar to the modern carbonate environment currently in San Salvador Island, 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. I conclude that the two paleokarst zones present under central Ohio have different characteristics and methods of formation. The shallow paleokarst zone is a traditional epigenic karst system. The deep paleokarst system that is the Newburg Zone is, in fact, a system of flank-margin caves formed during the Silurian 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. |
