A MIXED CARBONATE-SILICICLASTIC CHANNEL COMPLEX IN THE MISSISSIPPIAN FORT PAYNE FORMATION, TENNESSEE

The Fort Payne Formation was deposited during the Mississippian on the Borden shelf. Commonly known as the Fort Payne ramp, the system was a continuum of marine shelf-slope features. Recent road construction along TN-52 near Celina, TN, has created a fresh set of Fort Payne exposures. One outcrop, measuring 130 meters wide, shows evidence for a mixed carbonate-siliciclastic channel system, including: 1.) channel incision into a siliciclastic siltstone; and 2.) channel fill consisting of alternating crinoidal grainstone and siliciclastic mudstone. Given that channels are conduits for flow, it seems unlikely that mud could be deposited in the channel and not eroded during subsequent events. This research investigates depositional conditions to make sense of these observations.

Forty meters of stratigraphic section have been measured from the background strata through the channel fill. Seven lithofacies have been identified and described, with emphasis on depositional processes. Samples were collected from grainstones and mudstones for lab analysis. Fourteen thin sections were made from carbonate beds of the channel fill. One grainstone and three mudstone samples were analyzed using x-ray diffraction (XRD). Finally, micron-scale SEM analysis was conducted on a subset of six mudstones.

Thin sections showed grading of disaggregated crinoid columnals, which implies turbulent flow at the time of deposition. Imbrication of stem fragments is present, suggesting unidirectional flow (as expected in a channel). Preliminary SEM imaging results show orientation of clay particles with respect to one another. This was used to determine a pelagic or hemipelagic mudstone origin. Multiple phases of diagenetic alteration are present, evidenced by silica and calcite signatures in XRD results as well as incomplete replacement seen in thin sections.

Better understanding channel fill and geometry within the Fort Payne can lend valuable information to oil and gas exploration in northern Tennessee and south central Kentucky. Future study of porosity and permeability of carbonates within these channels will provide insight into potential reservoirs for oil and gas.