Introduction and Purpose

The rocks exposed at Ocoee Gorge near Parksville Dam in southeastern Tennessee contain exposures of the Great Smoky Thrust Fault transition between the Blue Ridge and Valley and Ridge Appalachian provinces. The Great Smoky Fault juxtaposes metamorphic rocks in the Blue Ridge and sedimentary rocks from the Valley and Ridge. The Valley and Ridge carbonates of the Knox Group serve as the footwall of the Great Smoky thrust.Within the actual fault zone there are fault bound slices of Valley and Ridge Chilhowee Group and Blue Ridge Walden Creek Group. The metasedimentary rocks of the Ocoee Supergroup are to the east of the Great Smoky thrust zone and are in the hanging wall. Just east into the Blue Ridge, the Ocoee Series rocks are still low grade metamorphic but are strongly cleaved. The cleaved rocks at Maddens Branch are dominated by folds with amplitudes ranging from a few centimeters to 10 m. The purpose for these field trip stops is to see that the boundary between the Valley and Ridge and Blue Ridge is both a change from sedimentary to metamorphic rocks and also stratigraphic, and to see the nature of the low grade metamorphosed but strongly cleaved rocks beyond the brittle zone of the Great Smoky thrust.

Figure D1. Generalized map of the Ocoee Lake area. Described stops are marked with green numbers.

Geologic Setting

The exposed rocks of the Ocoee Supergroup within the hanging wall of the Great Smoky fault were deposited in deep water on an ancient continental slope and rise to the outer continental platform on a passive margin of the Iapetus Ocean (Rast & Kohles, 1986). When the ocean closed during the Taconic Orogeny, these sediments were thrust eastward onto the North American continent. The proto Atlantic opened and the rocks of the Valley and Ridge were deposited in shallow water as platform clastic and carbonate rocks. Later, during the Alleghanian orogeny, the African plate collided with North America thrusting the Valley and Ridge sediments to the west and carrying the previously deformed Ocoee Supergroup with them. The Valley and Ridge and Blue Ridge Provinces are detached by the Great Smoky thrust fault. The Valley and Ridge sedimentary rocks in the foot wall of the thrust fault have not experienced metamorphic alteration. This indicates one of these things: an insignificant amount of frontal thrust material in the hanging wall, a high angle thrust, or a high erosion rate of the thrust material due to lack of burial of the footwall. The metasedimentary rocks in the hanging wall, juxtaposed to the thrust fault zone, have only been slightly altered. At the Parksville Dam stop, the distinction between the Valley and Ridge and the Blue Ridge is not just "cooked" rocks thrust on top of sedimentary rocks, but there is a significant stratigraphic variance between the two provinces. The rocks of the Valley and Ridge contain near-shore clean quartz sandstone, carbonate and continental shelf shale sequences of Cambrian age through Ordovician. The rocks of the Blue Ridge contain deep sea deposits, including graded turbidite muds, hemipelagic shales, siltstones and occasional sheet sands that are of Precambrian age. This gives good evidence that there was more than one episode of orogenic deformation within the Appalachians. Eastward into the gorge (and further into the Blue Ridge province) at Maddens Branch, the metamorphic rocks are still very low grade but are strongly cleaved and exhibit ductile deformation.

Figure D2. Slices of quartz arenite and shale behind the Parksville Dam. Looking south, footwall is to the right (west).

Regional

The difference between the Valley and Ridge and the Blue Ridge on a regional scale is obvious. The rocks of the Valley and Ridge contain sequences of Cambrian to Mississippian platform clastic and carbonate rocks thrust faulted during the Alleghanian orogeny and exhibiting no metamorphism. The rocks of the western Blue Ridge consist of Precambrian to Lower Cambrian clastic and minor carbonate rocks deposited on a ancient continental slope and have experienced burial and multiple episodes of tectonic deformation, and therefore have been metamorphosed. The two provinces are separated by the Great Smoky thrust fault which exhibits between 350 and 500 km of displacement. (Hatcher et al. 1989)

Figure D3. Large fault bound blocks, 300 meters east of Parksville Dam, looking northeast from U.S. 64. Quartz arenite, top left and shale far right. Possible fault traces drawn in.

Local

At Parksville Dam within Ocoee Gorge, the Great Smoky fault zone, from west to east, begins with a chemically weathered limestone. The limestone has weathered into a valley and the fault zone containing resistant sandstones formed a ridge, this is the reason for the Dam location along the ridge. The limestone is the Knox Group and is the footwall of the GSF zone. There are quartz rich sandstones and shales containing abundant fossil traces, within the fault zone. These rocks exhibit the effects of brittle deformation with abundant fractures and large normal and thrust faults separating the various lithologies. East of the fault zone there are sandstones and pre-slates without fossil attestation that are folded and exhibit minor metamorphic transformation. These rocks are classified as the Ocoee Supergroup and are considered the initial Blue Ridge. Further east into the Blue Ridge, the rocks have undergone more alteration. First, cleavage forms then later metamorphic grades increase. The cleavage is augmented in the deep sea deposits due to the small grains aligning in the same direction. At Maddens Branch, the rocks contain a variety of materials. There are clay layers separated by siltstone and sandstone, the lesser reorientation of the sandstones grains make good variations of cleavage.

Lithologies

In Ocoee Gorge, the footwall of the Great Smoky thrust fault exposes the lower Ordovician Knox Group. The Knox contains dolomite and limestone (Hatcher and Milici 1986). Within the fault zone, there are fault bound blocks of the Chilhowee Group including the Hesse Sandstone and Murray Shale, and the Sandsuck Formation of the Walden Creek Group of the Ocoee Supergroup. The Hesse Sandstone is a clean quartzarenite that contains Scolithus tubes and some crossbedding. The light gray Murray Shale is repeated between blocks of Hesse. The Sandsuck Formation contains dark gray to brown interbedded sand and shale, the clays in the shale / pre-slate have a faint sheen indicating initial metamorphic alteration. The Ocoee Supergroup is exposed to the east of the brittle fault zone. The Sandsuck Formation is present at Maddens Branch. There are sandstone beds up to two meters thick and slate and siltstone units of varying thicknesses that interbed the sands. The clay grains of all these rocks have cleaved perpendicular to bedding.

Figure D4. Generalized Stratigraphic table. (Modified from Hatcher and Milici 1986.)

Structure

At the Parksville Dam, the Great Smoky thrust fault is a zone of fault bounded blocks. (Figures D2 and D3). The Knox Group is the footwall and the Ocoee Supergroup is the hanging wall. Within the fault zone, there is five hundred meters of faulted blocks. If the base of the Knox is exposed as the footwall and Sandsuck Formation at the top of the Ocoee Supergroup is in the hanging wall, then 2600 meters of material has been smeared through the fault zone. This would be the heave of the Great Smoky Fault. Further east into the Blue Ridge, folding and cleavage become the dominate structures. The cleavage is controlled by the varying rock composition. In the finer grained sediments, cleavage takes precedence, but in the sandstones, folding is the dominate feature. The folding occurs in variety with amplitudes ranging from a few centimeters to 10 m (Figures D5 and D6). There also are boudins in the discontinuous layers.

Figure D5. A quarter within small folds at Maddens branch. The flat face of the outcrop is one cleavage plane.

Significance

The structure is controlled somewhat by the lithologies but also by the proximity to the Great Smoky thrust fault. The sandstones around the GSF thrust exhibit brittle deformation whereas the shale dominated blocks are moderately undisturbed. This is probably due to the fortification by the ridged sandstone blocks. The amount of metamorphism experienced by the rocks of the Ocoee Supergroup is due to how they were deformed during earlier tectonic orogenies. The lithologic controls are mainly stratigraphic, in that thinly bedded fine grained slates are more susceptible to cleavage deformation than thickly bedded sandstones.

Figure D6. Large folds at Maddens Branch. The flat face of the outcrop is one cleavage plane. The photo is looking north, there is a vergence of grains to the northwest at about 45 degrees, this is a second cleavage plane.

Problems

There has been some debate of the actual age of the Ocoee Supergroup but the rocks are obviously different than the sediments of the Valley and Ridge and they do not contain abundant trace fossils. The lack of fossil traces could indicate the Precambrian nature of the rocks. The Ocoee Supergroup is classified with the metamorphic rocks of the Blue Ridge but have only been slightly altered. Theoretically, the Ocoee Supergroup sediments deposited before the Taconic orogeny should have been buried during Taconic thrusting, subsided during tectonic hiatus, and brought up by the Alleghanian orogeny, resulting in a good degree of metamorphism to the Ocoee Supergroup. But the rocks exposed within the hanging wall of the Great Smoky thrust are only slightly altered, indicating that they were on the frontal thrust sheet of the Taconic orogeny and were later thrust westward by the Alleghanian orogeny.

Observations

At the Parksville Dam, packages of rocks are both normal and reverse fault bound. (Figures D2 and D3) This zone of high degree brittle deformation is the Great Smoky Thrust fault that separates the Valley and Ridge and Blue Ridge Appalachian provinces. This outcrop is located on a blind curve of U.S. 64. Be careful because the traffic moves pretty fast and there is not much room on the shoulder for rock banging. There is adequate parking across the road from the outcrop in order to view the different fault block lithologies and outcrop can be seen across the river below the dam. The GSF zone extends from just behind the dam to about five hundred meters east of the dam on U.S. 64. As you drive further east into the Blue Ridge Province, more ductile related deformation, such as the formation of cleavage and folds that can be viewed from the vehicle. The cleavage is more prominent in the thinly bedded, fine grained metasediments. At Maddens Branch, there is a variety of fold amplitudes and well developed cleavage. (Figures D5 and D6) Park on the south side of the road across Maddens Branch bridge. There is enough room for a couple of vans. The traffic continues to move fast through this area but you can see the cars coming. There is no shoulder so extreme caution should be taken, but if you walk west about a kilometer there is a greater variety of folding, lithologies and cleavage. For further geologic consideration, the Ocoee Lake is filling with sediments due to the extensive copper mining upstream at Ducktown, North Carolina.

Location

The Ocoee Gorge is located along U.S. 64 in southeastern Tennessee. The gorge was carved by the Ocoee River, of whitewater rafting fame. The Parksville Dam is at the western edge of the gorge further upstream is the location of the 1996 Olympic Whitewater Facility.

Bibliography

Hatcher, R. D. & Milici, R. C., 1986. Ocoee Gorge; Appalachian Valley and Ridge to Blue Ridge transition. Geological Society of America Centennial Field Guide-Southern Section, 265-270.

Hatcher, R.D., Jr., Thomas, W.A., Geiser, P.A., Snoke, A.W., Mosher,S., and Wiltscho,D.V., 1989, Alleghanian orogen, in Hatcher, R.D.,Jr., Thomas, W.A., and Viele, G.W., eds., The Appalachian-Ouachita orogen in the United States: Boulder, Colorado, Geological Society of America, The Geology of North America, v. F-2, Plate 1.

Kish, S. A., 1991. Studies of Precambrian and Paleozoic Stratigraphy in the Western Blue Ridge. Carolina Geological Society. p.161.

Milici, R. C., ed., 1978, A structural transect in the southern Appalachians, in Field trip in the southern Appalachians: Tennessee Division of Geology Department Report of Investigations 37, p. 5-52.

Rast, N. & Kohles, K. M., 1986. The origin of the Ocoee Supergroup. American Journal of Science, 286, 593-616.

Sutton, S. J., 1991. Development of domainal slaty cleavage fabric at Ocoee Gorge, Tennessee. The Journal of Geology, 99, 789-798.