THE CHARACTER OF GLAUCONITE IN THE TWIGGS CLAY MEMBER ALONG ELKO ROAD, HOUSTON COUNTY, GEORGIA

The glaucony within the Twiggs Clay Member of the Dry Branch Formation that is exposed along Elko Road in Houston County, Georgia, has been viewed as problematic for radiometric age determination. An early K-Ar age value of 33.0 ± 1.7 Ma was too small for Upper Eocene rock according to then-current estimates of the date of the Eocene-Oligocene boundary. Many of the green grains are relatively light in color, and most K₂O contents reported have been substantially less than 6%. More recent work has demonstrated concordancy in K-Ar dates for this glaucony at about 34 Ma, a date that is consistent with the stratigraphic position of the rock according to a current time scale. X-ray diffraction from the <2-µm fraction of the disaggregated glaucony shows the material to be glauconite with d(001) a little less than 10 Å. Ion-exchange experiments show that the long-chained hexadecylammonium ion, in aqueous solution at 35°C, does not displace potassium ions from the glaucony. This is in contrast to its effect on sedimentary illite (from the Conasauga shale), from which a substantial amount of potassium has been displaced in experiments of similar duration at room temperature. The small value of d(001) and the resistance to ion exchange by hexadecylammonium imply that the potassium in the glaucony resides in glauconite having relatively high interlayer cation occupancy. It appears that the clay mineral is a high-potassium glauconite and that the low K₂O values obtained in earlier work are due to the presence of other materials that effectively dilute the glauconite in respect to K-Ar determinations.