A MODEL FOR THE ORIGIN OF CHERT AT THE FLINT RIDGE, OHIO

Large chert nodules embedded in thin layer of limestone formation has been known in the Pennsylvanian limestones of south eastern Ohio. This study use the Vanport Limestone exposed at the Flint Ridge, Ohio as an example to explore a possible source of the chert. Previous study indicated that the Vanport Limestone is about 6 feet thick in the area and contains no siliceous fossil. Field observation shows the chert nodules are mostly in 3-D irregular shape and could extend over 20 feet in diameter. In addition to prominent fractures, no concentric or radiating, but thin laminated structure or texture is observed on the nodule. One-foot in diameter or smaller flint nodules and shaly chert nodules found at the bottom of the limestone formation are highly porous and are in direct contact with the underlying sandstone and siliceous shale. Large holes over 50-feet in diameter left on the ground due to historical chert excavation seems punched through the limestone layer. Based on field measurement and estimation, the distribution of chert nodule over an one square mile area at the Flint Ridge National Park is included in a mass balance model made by using the STELLA software on the silica budget of the Vanport Limestone. It indicates the amount of silica existed in the Vanport Limestone is beyond the capacity of an intense subaerial weathering condition could supply, and it needs to call for a significant amount of silica supplied by the groundwater. Furthermore, the major source of silica from the groundwater is less likely coming from the shallow buried Carboniferous sediments, but should be derived from a deeper groundwater in the Appalachian Basin. It implies that the upwelling of silica-rich basin water into the Middle Pennsylvanian siliceous shale and cherty limestone might be a very early response to the initiation of the Allegheny Orogeny.