THE USE OF COLOR CATHODOLUMINESCENCE COLOR AND TEXTURE OF DETRITAL QUARTZ GRAINS TO DETERMINE PROVENANCE IN SHALES: AN EXAMPLE FROM THE MIDDLE DEVONIAN OF KENTUCKY AND TENNESSEE

Second to clays, silt-sized detrital quartz grains are a prominent component in many shale and mudstone successions. While traditionally quartz silt has seen little use as a provenance indicator, the use of scanned color cathodoluminescence (SEM-CL) reveals an abundance of features that can be used for that purpose. Color Cl allows the differentiation of authigenic quartz from detrital quartz on the basis of CL intensity and color, and textural details (fractures, granularity, etc.) give additional information about principal source rocks (plutonic, volcanic, metamorphic).

Samples from an outcrop of the New Albany Shale in eastern Kentucky (Appalachian Basin) were used to examine changes in provenance between different members. Quartz grains were compared to a catalog of SEM-CL characteristics in order to group the samples according to CL color and texture and assign a likely provenance. Significant differences were observed between different members of the New Albany Shale with regard to the proportions of plutonic plus high-grade metamorphic quartz vs. low grade metamorphic quartz. Whereas all members contain a low-grade metamorphic quartz component along with authigenic quartz, some members are dominated by plutonic plus high-grade metamorphic quartz. Some quartz grains contain such unique texture-color combinations that they constitute provenance traces that allow correlation over large distances.

The sedimentary record is strongly dominated by mudstones. Whereas paleontological evidence is typically used for intra- and inter-basinal correlations, such studies are untimely and require specialists that are in short supply. In the case of the Devonian epicontinental sea, the use of detrital quartz tracers allows us to quickly test and confirm correlations arrived at on the basis of lithofacies and tracing of sequence boundaries. There is an urgent need to improve internal correlations of shale successions with rapidly growing oil and gas production from shales. An added benefit of SEM-CL of quartz is providing information about the amount and distribution of authigenic quartz. The latter constitutes an important aspect of shale reservoir characterization because changes in the proportions of detrital and authigenic quartz can strongly influence porosity and permeability of organic rich shales.