A MODEL FOR THE SOURCE OF SILICA IN THE PENNSYLVANIAN VANPORT LIMESTONE OF OHIO

The 3-meter thick middle Pennsylvanian Vanport Limestone deposited at the east central Ohio is famous for its inclusion of large chert nodules up to 3 meters in diameter. The limestone is micritic and is fossil scarce. Carlson (1994) interpreted it as a sediment-starved coastal deposit at the front of delta extended from the Appalachian Mountain. However, the origin of the embedded chert nodules was not clearly explained. The siliceous deposit started with small nodules in centimeter size in the Clarion shale beneath the Vanport Limestone, and the silica nodules did not extend into the Kittanning shale above. The chert is known to contain abundant sponge spicules. Since the spicules are not found in the limestone, it is thought to be a major source of silica for the chert precipitation.

The Vanport Limestone and its chert nodule at Flint Ridge, Ohio are observed in this study. Sponge and sponge spicule in the chert could be separated by etching the chert with KCl-KOH saturated solution. The etching texture of chert suggests that most of the silica mass is amorphous and lack of structure by process of precipitation. However, the silica of the sponge and the spicule in the chert have been recrystallized into fibrous quartz. No texture of partial dissolution is observed on either the sponge or the sponge spicule. The occurrence of the sponge and spicule in the limestone and in the chert nodule strongly suggests that the chert cementation took place before the sponge in the host limy deposit was dissolved. The siliceous fossil is then not likely a significant source for the silica of the chert. Alternatively, the most likely source of silica would be the dissolved silica in the basin brine discharged back to the surface. Simple model of mass balance calculation further suggests that if the basin brine were the source of silica, then the emplacement of all silica mass into the thin limestone would take approximately 1.5 million years. Consider that the solidification of silica should take place under a very shallow burial condition, the time duration of one or two million years would roughly match the time needed for the deposition of calcareous sediment of the Vanport Limestone.