WHOLE-ROCK CHEMICAL CONSTRAINTS ON DETRITAL HISTORY OF THE CRETACEOUS TUSCALOOSA FORMATION IN EASTERN ALABAMA AND WESTERN GEORGIA

The Tuscaloosa Formation (8 to 80m) is composed primarily of poorly sorted, fine to coarse grained, and massive to cross-bedded sandstones. Sands of the Tuscaloosa Formation are primarily sublitharenite but can range between subarkose and lithic subarkose, and plot in the “recycled orogenic” provenance field of Dickinson (1985). This study deals with whole-rock chemical analysis of the Tuscaloosa sands in order to trace their provenance.

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.