Investigators
W. Crawford Elliott
Department of
Geology
Georgia State University
Atlanta, GA 30303
J. Marion Wampler
School of Earth
and Atmospheric Sciences
Georgia Institute of Technology
Atlanta, GA
30030
John T. Haynes
Dept. of Mineral
Sciences
Smithsonian Institution
Washington D.C.
Students
Justin Vickery, Brooke
Fait, Michelle Rupe
Kris Spikes, Michelle Foster, Xuhui Wan
Support: Petroleum
Research Fund (Type B Award for Undergraduate
Research).
Diagenetic illite (mixed layer illite-smectite) from Ordovician K-bentonites was studied to understand the causal factors leading to the formation of illite in the southern Appalachian Basin. The K-Ar ages of these illites range from 260-310 Ma. These K-Ar ages decrease overall from south to north. The K-Ar ages of illite from Devonian K-bentonites in the northern area of study are similar to the ages of illites from Ordovician K-bentonites. These ages are also within the range for the Alleghanian Orogeny. No significant differences were noted in ages of illites in thick K-bentonites.
The K/Rb ratio was measured from these illites as an indication of their formation from saline waters (brines) or from meteoritic waters. Though variable, most illites have K/Rb ca. 255 on average (N = 18, standard deviation = 41), and this was verified by the measurements of other illites from bentonites from the western United States (N = 13, mean = 275, and standard deviation = 114). The difference between these two populations is not significantly different. The mean K/Rb of illites from this study are 325 (N = 20, standard deviation = 91), and the higher K/Rb ratios indicate these illites formed from saline waters versus meteoritic waters. The difference in the K/Rb ratios between the Appalachian illites and non-Appalachian illites was significant at the 1% level.
The combined data indicate these illites from basin brines in K-bentonites during the Alleghanian Orogeny. The effect of burial forming these illites is more pronounced in the northern area of study where the K/Rb is low and the indexes for burial (CAI) are high (4-5). The effect of burial is much less pronounced in the southern area of study as indicated by higher K/Rb, and lower CAI (< 3).