SPECTRAL GAMMA-RAY CHARACTER OF THE CHATTANOOGA SHALE, TENNESEE AND OKLAHOMA

The Devonian/ Mississippian Chattanooga Shale is a prominent black shale that outcrops across southern North America and has similar-aged equivalents in other Devonian-Mississippian marine depocenters. The spectral gamma-ray profiles of Chattanooga outcrops record changing total and relative concentrations of radionuclides, which reflect lithology and depositional processes. In this study, spectral gamma ray surveys were taken across the Chattanooga Shale at the Hurricane Bridge outcrop in Dekalb County, Tennessee and the No Head Hollow outcrop in Cherokee County, Oklahoma. The Chattanooga at Hurricane Bridge contains two members: the Dowelltown Shale and Gassaway Shale. The Dowelltown rests unconformably on the Ordovician Leipers carbonate. The Dowelltown is further subdivided into a lower and upper unit based on color and weathering properties. Generally, the upper unit is lighter-colored (grey vs. black) and forms a recessed weathering profile. A bentonite marks the boundary between the Dowelltown and overlying Gassaway. The Gassaway is a more resistant and massive black shale without conspicuous subdivisions. The Gassaway is subdivided into lower, middle and upper subunits using subtle weathering characteristics, faunal assemblages and phosphate nodules. The spectral gamma ray identified changes in uranium, thorium and potassium concentrations that corresponded to the lithostratigraphic units. The darker-colored lower unit in the Dowelltown contains higher uranium concentration than the lighter-colored upper unit. Uranium concentrations increase dramatically in the Gassaway and the three units are recognized by varying uranium concentrations. Potassium and thorium concentrations respond inversely to uranium and increase in lighter-colored (grey) shale. Similar spectral gamma-ray responses are recorded for the Chattanooga in eastern Oklahoma as darker-colored units have higher uranium concentration and lower thorium and potassium to uranium ratios.