GLASS AND GLAUCONY FROM THE OCMULGEE FORMATION ON THE COASTAL PLAIN OF GEORGIA, AGE AND ORIGIN

The Ocmulgee Limestone is a Late Eocene formation that is correlative with the Upper Three Runs Aquifer from the Girard Core in Burke County and the Sandersville Limestone near Sandersville, Georgia. Biostratigraphy of echinoids and bryozoa found in these units shows them to be Jacksonian (Priabonian) in age.

A radiometric age of 33.3 Ma ± 1.0 Ma (2σ) for glaucony from the Ocmulgee Limestone from a sample collected 20 to 22 feet above the base of the formation at its type locality has been determined by the K-Ar method. The glaucony grains were of fine sand size and dark grayish-green to greenish black in color. They had a high percentage (6.72%) of K₂O. This shows that the glaucony grains were highly evolved and were chemically sound for use in radiometric dating. This numerical age is consistent with biostratigraphic data that places the Ocmulgee within the Jacksonian (Priabonian), if one considers that evolution of glaucony requires a period of up to 1 Ma. Glaucony was also found in the Upper Three Runs Aquifer, but due to limited sampling of this core samples did not have enough glaucony for radiometric dating. Glaucony is only found in very low quantities in the Sandersville Limestone.

Besides glaucony fine sand- to silt-sized glass fragments were found in the Sandersville Limestone, the Upper Three Runs Aquifer and the Ocmulgee Limestone. The glass fragments are angular shards. EDS analysis has shown the glass from the Ocmulgee to be high in silicon, aluminum and titanium, while the glass from the other two units lacks titanium. SEM shows paragenesis of authigenic minerals. The glass shards have been altered to zeolite, smectite and opal-CT.

The glass could have formed roughly 35 Ma ago as ejecta from the Chesapeake Bay Impact given the evidence that these formations are Jacksonian (Priabonian) in age. The possibility that it could be from pyroclastic fallout from volcanic activity in western North America, where there were volcanic eruptions in this same time interval, could also be plausible, but is remote.