THE CAMBRIAN CHILHOWEE GROUP SHALE: A PRELIMINARY STUDY OF THE GEOCHEMICAL VARIATION WITH DEPTH

Geologic mapping of the Borden Springs 7.5 minute quadrangle by Kath and Crawford (2011) reveals a complex relationship between shale of the Chilhowee Group, Rockmart Slate, and a phyllite along the metamorphic front in the hanging wall of the Emerson-Talladega Fault. Because these units are all similar, from a mapping perspective, different interpretations as to the identification of these units exist. Therefore mineralogy and whole rock geochemical studies were conducted, which made it possible to distinguish Rockmart Slate from the Chilhowee Group. Our results showed that the Rockmart Slate is distinguishable from the other rock units by their elevated CaO, and lower K₂O and MgO. Based on geochemistry, we determined that the Rockmart Slate is not a possible protolith for the Blue Ridge phyllites, and that the Chilhowee shales are a possible protolith for the phyllites.

A further study of the geochemistry of the Chilhowee shales sampled from borings was initiated to examine geochemical changes with depth. The samples are drill cuttings from Dobbins Mountain, near Cartersville, Georgia. Samples are from depths ranging from 176 to 236 ft below the surface. Bulk densities were first determined from the rock cuttings, as bulk density changes typically occur during chemical weathering, diagenesis, or just represent the original heterogeneity in the shale unit. This project was initiated to determine if these samples record a chemical weathering front, with increasing bulk density with depth due to a decrease in chemical alteration by weathering. Preliminary results indicate two sample sets showing different increases in bulk density with depth, and different geochemical trends with depth. These geochemical changes are inconsistent with chemical weathering, as one group shows no significant change except lower CaO with decreasing bulk density. More significantly, the other group shows lower SiO₂ with decreasing bulk density, along with increasing Al₂O₃ and K₂O.

Based on this preliminary study, the geochemical variations in the Chilhowee shale drill cuttings with increasing depth are not consistent with chemical weathering. Further work involving mineralogical determination by XRD and petrographic examination in thin section may indicate diagenetic changes as the underlying cause of the chemical variations.