The Cumberland Plateau thrust sheet is the westernmost thrust sheet of the southeastern Appalachians. Displacement along this thrust has been estimated to be approximately one kilometer. The thrust is exposed in a road-cut northwest of the town of Dunlap, Tennessee. The rocks at this exposure have never been over a major footwall ramp, and therefore represent deformation along a bedding-parallel thrust.
The Valley and Ridge Province of the Appalachians is located between the Blue Ridge Thrust to the east and the Cumberland Plateau Thrust to the west. This province has a trend of approximately N30°E.
The Valley and Ridge is characterized by a series of northeast trending, thin-skinned folds and thrusts that duplicate the Cambro-Ordovician Knox Group (Wilson and Wojtal, 1986). Deformation of the Valley and Ridge is Alleghanian, as evidenced by the fact that Pennsylvanian rocks are deformed throughout the province. The intensity of deformation decreases from east to west. The Cumberland Plateau Thrust, which is the westernmost thrust of the Appalachians, has approximately 1 kilometer of movement directed toward the northwest (Harris and Milici, 1977). The Cumberland Plateau Thrust sheet is dominated by thrust faults, having approximately 10 near the vicinity of Knoxville, Tennessee (Hatcher, et al, 1993). The largest structure in the Cumberland Plateau Thrust sheet is the Sequatchie Valley anticline, a fault-bend fold in this area, which duplicates the Knox Group carbonates (Wilson and Wojtal, 1986). The Sequatchie Valley anticline is the westernmost fold in the southeastern Appalachians.
The Cumberland Plateau Thrust outcrops along Tennessee Highway 8 northwest of Dunlap. The thrust sheet has been divided into two parts by the Sequatchie Valley, and the thrust is exposed along the western side of the Sequatchie Valley (Wilson and Wojtal, 1986). The Sequatchie Valley thrust joins the Cumberland Plateau thrust at the northeastern end of the valley, which suggests that the Cumberland Plateau thrust is an upper detachment for the more steeply dipping Sequatchie Valley thrust (Wilson and Wojtal, 1986).
The lithology of the Cumberland Thrust Sheet exposed at the Dunlap location is the Lower Pennsylvanian Gizzard Group. This unit is comprised of sandstones and shales with at least two laterally continuous coal layers. The detachment fault is located near the base of the Raccoon Mountain Formation, and deformation affects this formation and the Warren Point Sandstone, Sewanee Conglomerate, and the Whitwell Shale (Harris and Milici, 1977). The formations above the Whitwell Shale are unaffected by deformation along the Cumberland Plateau thrust fault. The competency contrasts between the sandstones, shales and coal layers in the Gizzard Group cause refraction of faults as they propagate upward through the formation. Bedding parallel faults tend to form in shale and coal units, and those faults ramp upward in the more competent sandstone beds.
The rocks northwest of Dunlap have never been over a footwall ramp, and therefore represent deformation along a bedding parallel detachment. The detachment formed along the top of a coal bed, which acts as a weak layer (Harris and Milici, 1986). Deformation occurs on discrete, low and high angle faults, which splay from the detachment, and by small-scale folding (Figure B1).
Figure B1. Small-scale detachment fold within the Pennsylvanian Gizzard Group.
Deformation decreases up-section from the detachment level. The lower zone of deformation is dominated by pervasive thrust faulting. Fault imbrication is common in this zone, and probably formed sequentially in duplexes (Wilson and Wojtal, 1986). Low angle thrust faults are commonly cut by high angle thrust faults, indicating that the high angle faults post date the low angle faults. Normal faults are also common in the exposure (Figure B2).
Figure B2. Beka Chace points to normal faults.
Cross-cutting relationships indicate that normal faults post-date some, but not all thrusts. This relationship suggests that the normal faults formed during regional compression. The lower coal unit defines the detachment surface of the Cumberland Thrust Sheet, and an upper coal layer is also a bedding parallel fault with approximately 9 meters of layer-parallel slip. The deformation above this coal layer is quite different from that below, and no structural features cut the coal horizon (Wilson and Wojtal, 1986). Deformation above the coal layer is dominated by high-angle normal faults. Small-scale folds are also common at the Dunlap exposure. These folds are commonly associated with small displacement thrust faults, and verge both in the direction of transport and opposite to the direction of transport. Folds in the sandstone layers generally have thrust faults that ramp upward into the core of the folds (Figure B3), while folds in layers that contain thick shale packages commonly have bedding parallel thrusts that do not ramp upward to cut stratigraphy (Figure B4).
Figure B3. Fault Propagation fold. Diahn Hawkins for scale.
Figure B4. Detachment fold in shale and sandstone.
The rocks at Dunlap have never traveled over a major footwall ramp, and the amount of displacement along the fault is relatively small compared to other thrust sheets within the Appalachians. Deformation decreases in intensity away from the detachment surface and changes character across a layer-parallel detachment surface that parallels a laterally continuous coal layer. The location represents deformation analogous to the very earliest stages of thrust deformation.
An interesting feature of the Dunlap exposure is the presence of both thrust faults and normal faults in the same outcrop. Some, but not all, thrusts are cut by normal faults. Harris and Milici (1977) attributed the presence of the normal faults to alternating periods of extension and compression during deformation. Wilson and Wojtal (1986), however, disagreed with that interpretation. They propose that the normal faults, which are most prevalent in the zone of deformation above the upper coal layer, can be attributed to outer-arc extension around very open folds. They suggest that the normal faults are contemporaneous with thrust faulting, and that no evidence of alternating extension and compression can be found at this location. One of the more widely documented structures at the Dunlap exposure is a fault-propagation fold in sandstones near the eastern end of the exposure (Figure B3). This fold was used as the type example of a fault-propagation fold by Suppe (1985). In the Suppe model, the fold develops above and concurrently with a bedding parallel fault, which ramps upward through stratigraphy. The model is based on constant bed length and constant bed thickness. This fold was also examined by Chester and Chester (1990) and they determined that the fold developed above a fault that does not have a bedding parallel component, as they did not see any folding in the hanging wall that could be attributed to a hinge between a bedding parallel and a ramp fold. They also determined that the fold did not form with constant bed length and constant bed thickness, but rather that the forelimb of the fold thickened during deformation. The problem with both interpretations is that the fault continues below the road surface, and therefore the geometry of the fold, and the location and shape of the lower portion of the fault cannot be accurately constrained (McConnell, et al, 1997).
From Chattanooga, take Highway 127 north to Dunlap. Travel approximately 1 mile to the intersection of Highway 127 and Tennessee Highway 8 (Figure B5). Turn left and drive approximately 2 miles to the top of the hill. Park and walk down section. The shoulders of the road will accommodate large groups of people. Rocks are exposed almost continuously on the north side of the highway for 1.3 miles.
Figure B5. Tennessee highway map showing location of field trip stop.
Chester, J., and Chester, F., 1990. Fault propagation folds above thrusts with constant dip. Journal of Structural Geology, 12, 903-910.
Harris, L.D., and Milici, R.C., 1977. Characteristics of thin-skinned style of deformation in the Southern Appalachians, and potential hydrocarbon traps. U.S. Geological Survey Professional Paper 1018, 40 p.
Hatcher, R.D., Jr., Lemiski, R.J., Hooper, R.J., and Walker, K.R., 1993. Structure and stratigraphy of the southern Appalachian foreland fold and thrust belt in Tennessee. Field trip guide book, 25-35.
McConnell, D.A., Kattenhorn, S.A., and Benner, L.M., 1997. Distribution of fault slip in outcrop-scale fault-related folds, Appalachian Mountains. Journal of Structural Geology, 19, 257-267.
Suppe, J., 1985. Principles of Structural Geology. Prentice-Hall, Englewood Cliffs, New Jersey.
Wilson, R.L., and Wojtal, S.F., 1986. Cumberland Plateau decollement zone at Dunlap, Tennessee. in Geological Society of America Centennial Field Guide - Southeastern Section. 143-148.