WHOLE-ROCK CHEMICAL CONSTRAINTS ON DETRITAL HISTORY OF THE CRETACEOUS TUSCALOOSA FORMATION IN EASTERN ALABAMA AND WESTERN GEORGIA
Geochemical studies comprising eight major elements, sixteen trace elements, and fifteen rare earth elements have been carried out on the sandstones. Concentrations normalized to Post Archean Australian Shale (PAAS) show that the sediments studied are depleted in Ca, Na, Sr, Cs, and Cu and enriched in Nb and Al2O3. PAAS-normalized values for all 8 samples are very similar, probably because this relatively small study area had common felsic source areas.
Several discrimination diagrams such as log(SiO2/ Al2O3) vs log(Na2O/K2O), Zr (ppm) vs TiO2 (wt%), Zr/Sc vs Th/Sc, and La/Th vs Hf indicate a felsic source rock. The Hf vs La/Th pattern indicates a felsic island arc to passive margin environment, while the abundance of SiO2 to K2O/Na2O indicates a passive margin setup.
The chemical index of alteration (CIA) has been used to quantify the degree of weathering. CIA values range between 81 and 97 on a scale of 50-100, indicating a high degree of alteration. On an A-CN-K diagram, the data fall closer to the compositional fields of highly weathered minerals. A high degree of weathering is also indicated by the K2O-Fe2O3-Al2O3 plot.
This study, along with ongoing petrographic analyses, suggests that the source area was geographically close and was likely the upper-grade metamorphic, felsic plutonic, and meta-sedimentary sequences of the Inner Piedmont of the southern Appalachians.