The geochemical and mineralogic expression of sequence boundaries in Mississippian carbonates of the Appalachian Basin, Georgia and Tennessee
Bulger, Daniel Edward
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The clay mineral corrensite is a regularly ordered interstratified chlorite-smectite that is the result of low temperature diagenesis of Mg-rich minerals. Chemically, the smectite to chlorite transition through corrensite involves a decrease in alkali and alkaline earth metals with decreasing Si/(Si+Al), increase in Al+3 for Si+4 substituition in tetrahedral sites and variability in Fe/(Fe+Al) of octahedral cations, which appears to be strongly influenced by whole-rock composition. High resolution transmission electron microscopy has revealed two possible mechanisms for the chloritization of smectite. A comparison of two contrasting depositional sequences was conducted to test the potential for corrensite as a proxy for sequence boundary identification in carbonate sequences deposited in arid-semiarid marine environments. X-ray diffraction analysis of clay minerals within the normal marine sequence of the Tuscumbia and Monteagle Formations reveal the presence of corrensite in association with sequence boundaries and late highstand systems tract (HST) deposits. Transgressive systems tract (TST) and early HST deposits representative of normal marine conditions contain smectite, illite and minor kaolinite. In contrast, corrensite dominates the TST and HST of the restricted marine sequence of the Saint Louis Formation. The Reelsville unit exposed at Jellico, Tennessee is a depositionally complex, well-developed unconformity bounded unit. Field and petrographic observations combined with clay mineral, stable oxygen and carbon isotopes and quantitative bulk rock mineral analysis was conducted on single hand samples. Excursion patterns in mean δ13C and δ18O values and dolomite concentration correlate with features indicative of subaerial exposure observed in the field. In addition, high variance in δ13C and δ18O values occurred at these positions with one exception. Six surfaces of subaerial exposure in the Reelsville unit were identified. Petrographic observations of faunal diversity combined with traditional facies and mineral analysis suggest that the Reelsville unit was deposited in an inner ramp setting. Petrographic analysis and field observation identified three parasequences in the Reelsville unit. Diverse faunal elements suggest the base of parasequence occurred under open marine salinity conditions; however, massive dolomitized mudstone at parasequence tops suggests deposition under restricted conditions. The occurrence of corrensite throughout the Reelsville suggests deposition under high salinity conditions.