CALCICLASTIC SEDIMENT GRAVITY FLOWS: LESSONS LEARNED FROM OUTCROPS OF THE MISSISSIPPIAN FORT PAYNE FORMATION

Calciclastic submarine slopes are unique settings characterized by the interactions of biological, chemical and clastic depositional processes. While these components are usually treated separately – for example, carbonate vs. siliciclastic classification schemes – the development of mixed margins does not happen in isolation and instead records interactions of biochemical processes (e.g., organism growth, ecology) and clastic transport processes (e.g., erosion, redeposition). Here, we document a slope-to-basin profile of a mixed system developed in Mississippian rocks of Tennessee and Kentucky. The Fort Payne Formation is characterized by interbedded carbonate and siliciclastic facies organized into classic deepwater geometries: erosional paleochannels, levees/overbank deposits, and sheetlike lobes. Sediment transport during the Mississippian was primarily to the west and southwest, sourced from the clastic Borden-Grainger-Fort Payne deltas to the north and carbonate ramps rimming the Illinois Basin.

We use a combination of stratigraphic and petrographic analyses of Fort Payne outcrops to construct cross sections through proximal and distal slope deposits. Detrital components are almost entirely carbonate allochems including disaggregated crinoids, bryozoans, brachiopods, and siliceous sponge spicules. Sedimentary structures include normal grading of skeletal components as well as imbrication of crinoid columnals parallel to bedding. These observations suggest that while carbonate production may have dominated the updip shelf, transport on the slope was more closely linked to sediment gravity flow processes. Thus, we interpret observed lithofacies to reflect a continuum of hydrodynamic conditions resulting in debris flows, high- and low-concentration turbulent flows, and pelagic settling on the seafloor.

In the subsurface, the Fort Payne serves as a reservoir unit hosting oil and gas sourced from the Chattanooga Shale (Devonian). To date, most Mississippian-aged discoveries have been stratigraphic traps attributed to the development of carbonate mud mounds in the lowermost Fort Payne; however, our recent work suggests that porosity and permeability trends may also be controlled by paleochannel geometries and the distribution of proximal-to-distal slope sedimentary bodies.