DEPOSITIONAL ENVIRONMENT OF LOWER PENNSYLVANIAN SEWANEE CONGLOMERATE, LOOKOUT MOUNTAIN, GEORGIA

The Sewanee Conglomerate caps much of Lookout Mountain, located in the Valley and Ridge Province of northwest Georgia. The Sewanee is part of the Crab Orchard Mountains Group, which overlies the Gizzard Group. Multiple interpretations have been made for the depositional environment of the Sewanee, including tidal delta (Ferm et al. 1972) and meandering and braided streams (Churnet and Bergenback 1986). Similar Pennsylvanian deposits of the Breathitt Group in eastern Kentucky were interpreted as major distributary channels and incised valley fills (Aitken and Flint 1995).

Despite its name, these are largely composed of a pebbly conglomerate that fines upward to a medium-grained (mL) sandstone. The Sewanee Conglomerate also contains large-scale trough cross bedding, planar lamination, and lateral accretion surfaces in sets up to 4 m thick. Sigmoidal cross bedding and reactivation surfaces are also present. The Sewanee overlies the Signal Point Shale and is overlain by the Newton Shale; both contacts are sharp. At Cloudland Canyon, the channel bodies are multi-storied. In the section measured at Cloudland Canyon bedding ranges from 10 cm to 3.4 m and forms multi-storied sand bodies.

The presence of lateral accretion surfaces, trough cross bedding, and fining-upward grain size profile indicate a channelized setting with a moderately high shear stress. The presence of sigmoidal cross-stratification and reactivation surfaces are evidence of tidal influence (Shanley et al. 1992). Collectively, these features suggest deposition within tidally influenced river channels. The presence of multi-story channel bodies suggests accumulation during relatively low accommodation rates, likely as an incised valley system.