STRUCTURE OF BEDROCK
The bedrock formations of the Great Smoky Mountains and vicinity have been greatly folded and faulted, and those in the mountains themselves and farther southeast have been variably metamorphosed as well. The structures thus produced are illustrated on the four structure sections that accompany the map (pl. 1). Detailed descriptions and interpretations of these structures have been given elsewhere (Hamilton, 1961, p. A35-A46; Hadley and Goldsmith, 1963, p. B74-B96; King, 1964, p. C87-C130; Neuman and Nelson, 1965, p. D45-D68), Here, only the principal faults of the region will be discussed, the bearing which they have on the distribution of the bedrock formations, and their implications in the geologic history.
Three major faults occur, each with a different character and age, and each accompanied by minor related faults:
1. The Greenbrier fault, together with its related faults, occurs within the Ocoee Series of the mountain area, is older than the other two major faults, and is perhaps of earlier Paleozoic age.
2. The Great Smoky fault, however, with its related faults occurs between the Ocoee Series and the Paleozoic rocks at the border between the foothills and the Appalachian Valley, and is of later Paleozoic age.
3. The Gatlinburg fault, together with its related faults, occurs within the Ocoee Series near the northwest foot of the Great Smoky Mountains, is as young as or younger than the Great Smoky and related faults, but is probably also of later Paleozoic age.
Greenbrier and Related Faults
The Greenbrier fault is prominently displayed in the eastern half of the Great Smoky Mountains (Hadley and Goldsmith, 1963, p. B74-B81). It also extends westward nearly the length of the mountains, but in the western half it is much broken up by younger faults and is not continuously exposed (King, 1964, p. C106-C113).
The Greenbrier fault is a low-angle thrust which lies along the contact between the Great Smoky Group and the underlying Snowbird Group. As this relation was their original stratigraphic arrangement, movement on the Greenbrier fault has carried younger rocks over older through the greater part of its length. The actual surface of the fault is exposed in few places, but its existence is indicated by contrasting structures in the rocks above and below it, and by truncation of these structures at the fault, as shown on the geologic map (pl. 1). It is further suggested at the east end of the mountains by close juxtaposition of the thick sequence of Snowbird Group below the fault and the very thin sequence of Snowbird Group above the fault; reconstruction of the original positions of the thin and thick sequences suggests that they once lay at least 15 miles apart and that the rocks above the fault have moved this distance or even farther (Hadley and Goldsmith, 1963, p. B80).
Between the east end of the mountains and the Pigeon River, the fault is warped around the large Cataloochee anticlinorium. On the southeastern flank of the anticlinorium, the basement complex emerges along the upper side of the fault, so that the fault descends into the basement in this direction. Farther west in the mountains, mainly in the Cherokee Indian Reservation, the fault comes to the surface in the Ravensford anticline, and a large area of the rocks beneath is revealed (pl. 1, structure section BB'). Here, as in the Cataloochee anticlinorium, the basement complex is involved. In most of the anticline, only basement complex is exposed beneath the fault, but along Straight Fork at its northeastern end, rocks of the Snowbird Group emerge in a window from beneath the basement. These rocks have been overridden by the basement along another low-angle fault, which is akin to the Greenbrier fault and lies a little beneath it (Hadley and Goldsmith, 1963, p. B85-B88).
Other faults related to the Greenbrier fault occur elsewhere in the mountains, mostly forming the soles of various small slices of formations of the Ocoee Series that lie directly beneath the main fault. Some faults that lie farther north in the foothill area are older than structures adjacent to them and may have formed at the same time as the Greenbrier faultingfor example, the faults beneath the unclassified formations of the Ocoee Series on Webb Mountain, Big Ridge, and near Cades Cove and the Dunn Creek fault which separates the Snowbird Group from the Walden Creek Group in the eastern part of the foothill belt (Hamilton, 1961, p. A40-A42).
The Greenbrier and its related faults evidently formed early in the deformational history of the Great Smoky Mountain region. The Greenbrier fault, for example, was subsequently folded near the Cataloochee anticlinorium and the Straight Fork window, and it was disrupted by the Gatlinburg and related faults that cross it in the western part of the mountains. It is also older than much of the regional metamorphism, for near the Cataloochee anticlinorium the rocks both above and below it pass southward from a low-grade metamorphic facies characterized by biotite into higher grade facies characterized by garnet and staurolite (Hadley and Goldsmith, 1963, p. B106). Near Cove Mountain northwest of Gatlinburg the rocks below and above the fault lie in a still lower grade facies characterized by chlorite. Evidently, these metamorphic facies were imposed on the rocks after the fault and its thrust sheet had been emplaced in their present positions.
Great Smoky and Related Faults
The Great Smoky fault lies at the northern edge of the foothills of the Great Smoky Mountains, on the border of the Appalachian Valley (Neuman, 1951, p. 743-750). Unlike the Greenbrier fault, which is known only in the region of the Great Smoky Mountains, the Great Smoky and its related faults are one segment of a very extensive system of displacement that extends for hundreds of miles along the southeastern edge of the Appalachian Valleyfrom central Virginia to Alabama (Rodgers, 1953, p. 139-147).
Like the Greenbrier fault, the Great Smoky fault is a low-angle thrust, but, unlike the Greenbrier fault, the Great Smoky fault everywhere carries older rocks over younger. Near the Great Smoky Mountains it mostly carries rocks of the Ocoee Series over Ordovician and younger Paleozoic rocks, although near its leading edge, earlier Paleozoic rocks, mainly Chilhowee Group, form part of the thrust sheet. The surface of the fault is exposed in many places, in stream channels and various artificial excavations. It is seen to be mostly a smooth clean-cut surface with little accompanying fault gouge or breccia, which dips at various but generally low angles.
The low dip of the Great Smoky fault applies also to its larger relations, so that it lies at shallow depth for many miles southeast of its leading edge. From this leading edge the fault dips southeastward beneath Chilhowee Mountain, but it rises again in the foothills beyond where it has been exposed by erosion in several places (pl. 1, structure sections CC' and DD') (King, 1964, p. C92-C96; Neuman and Nelson, 1965, p. D45-D56). It thus emerges again around the edges of Wear, Tuckaleechee, and Cades Coves (fig. 3), which are floored by Ordovician rocks that lie beneath the fault. The coves are therefore windows in the thrust sheet of Ocoee Series that overlies the fault. A smaller window of the same kind occurs farther southwest, at Calderwood on the Little Tennessee River, and another one to the northeast, on the Cades Cove road east of Crib Gap. The latter is not well exposed, but its existence is indicated by sinkholes in the valley gravels and by a drill hole that penetrated limestone beneath the gravels.
These windows lie well behind the leading edge of the Great Smoky fault and demonstrate that extensive movement has occurred along it. The window east of Crib Gap lies farthest southeast of the leading edge, a distance of 9 miles. The Ocoee Series of the thrust sheet above the fault has moved at least this far northwestward over the Ordovician and other Paleozoic rocks, and very likely much farther.
Many lesser faults are associated with the Great Smoky fault, most of which are branches that displace the rocks in the northwestern part of the main thrust sheet; some of these have very large displacements of their own. One of them, the Miller Cove fault, borders the southeastern side of the Chilhowee Mountain block for its entire length, generally separating the Chilhowee Group and younger Cambrian strata of the fault block from the Walden Creek Group of the Ocoee Series on the southeast. It mostly forms a single break, but it splits into branches northeastward (King, 1964, p. C98-C100). Similar branch faults occur still farther east, bordering the north and south sides of English Mountain, and these extend beyond the Pigeon River (Hamilton, 1961, p. A43-A44).
Other faults related to the Great Smoky fault form the soles of slices of Paleozoic rocks, which were broken off from the block beneath and were dragged over the main body of Paleozoic rocks for various distances during emplacement of the thrust sheet. Such faults and slices occur at intervals along the leading edge of the Great Smoky fault, but are more conspicuous in the cove, or window, areas. Here are many slices a few feet to several hundred feet thick of Lower Ordovician limestone, and two or three slices of quartzite of the Chilhowee Group, most of which overlie Middle Ordovician shale of the main window areas.
The Great Smoky and related faults formed late in the deformational history of the Great Smoky Mountain region. The Mississippian rocks that lie close against the leading edge of the fault and that were involved in deformation during emplacement of its thrust sheet indicate that this emplacement must have occurred after Mississippian time. Also, unlike the Greenbrier fault, the Great Smoky fault formed after the Ocoee Series had been regionally metamorphosed. Close to the leading edge of the fault the Ocoee Series is little metamorphosed, but farther southeast, near the windows of Wear, Tuckaleechee, and Cades Coves and Calderwood, the rocks of the series contain chlorite, and near the window east of Crib Gap they contain biotite. These metamorphosed rocks lie on Ordovician limestones and shales exposed in the windows which, although deformed, contain none of these metamorphic minerals.
Gatlinburg and Related Faults
The Gatlinburg and related faults lie mostly near the foot of the main Great Smoky Mountains in the southern part of the foothill belt and entirely within the Ocoee Series (Hamilton, 1961, p. A45-A46; Hadley and Goldsmith, 1963, p. B75-B77; King, 1964, p. C113-C117). They extend from the east edge of the mapped area, past Gatlinburg, into the western part of the mapped area. Most of the faults trend east-northeast, with some branching and interlacing. One large fault, the Oconaluftee, branches off southeastward near the middle of the mountains, crosses the mountain crest at Indian Gap and extends down the valley of the Oconaluftee River on the southeastern slope. North of the Oconaluftee fault near Gatlinburg, the east-northeast-trending faults are crossed and partly offset by others trending north-northeast (King, 1964, p. C116). Nearby, another fault extends northwestward down the West Prong of the Little Pigeon River and may be responsible for a conspicuous offset in the trace of the Great Smoky fault near Pigeon Forge.
Unlike the other faults of the region, the Gatlinburg and related faults are well expressed in the topography. They commonly lie in stream valleys and cross from one stream valley to the next through notches in the intervening ridges. The result is a set of trenches or creases which extend nearly straight for long distances and are very evident on any relief model of the mountains. This topographic expression is the result of erosion along the fault lines probably because the rocks near them are much sliced and shattered, hence poorly resistant to erosion.
The straight traces of the Gatlinburg and related faults indicate that they dip at relatively steep anglesgenerally much steeper than either the Greenbrier or Great Smoky fault. The surface of the main east-northeast-trending Gatlinburg fault is exposed at a number of places and dips at an angle of 45° or steeper to the south; older rocks on that side are raised against younger rocks on the north. Slickensides preserved at some of these exposures cross the fault surface at an angle to the dip, indicating that movement had both an updip (dip-slip) and a right lateral (strike-slip) component by which the older rocks on the south moved westward relative to the younger rocks on the north (Hadley and Goldsmith, 1963, p. B77). Strike-slip displacement must be even greater on the other large related fault the Oconaluftee, which in its northwest-trending segment laterally offsets steeply dipping rocks and structures on the two sides by as much as several miles. Westward this displacement was translated to dip-slip displacement on part of the older Greenbrier fault (Neuman and Nelson, 1965, p. D62-D63).
Although the Gatlinburg and related faults involve only rocks of the Ocoee Series, they must be a late feature of the deformation. They have broken and displaced the Greenbrier fault and its thrust sheet, and they extend in discriminately through rocks that have been variably metamorphosed. A late age is also suggested by the marked topographic expression of the Gatlinburg and related faults; the fracturing and slicing of the rocks which has brought this about could not have been preserved if many structures had been superposed later. The Gatlinburg and related faults come into contact with the Great Smoky fault only near Pigeon Forge and at Cades Cove, but the mutual relations between these structures are obscure and their relative ages are unknown. However, the Gatlinburg and related faults seem to be at least as young as the Great Smoky fault and may be younger.
Although the various types of faults previously described are the most influential features determining the present arrangement of the rocks of the area, these rocks were also strongly folded. Effects of this folding range from broad arches and long stretches of uniform dip, such as those seen in the thick-bedded sandstone of the Great Smoky Group on Mount Le Conte and elsewhere along the northern slope of the mountains, to small sharp folds in the shaly, less competent rocks of the Snowbird and Walden Creek Groups in the foothills to the northwest. Most folds in the northern part of the area trend east-northeast and are asymmetric in that their northward-dipping beds are much steeper than the southward-dipping ones and are locally strongly overturned. Farther south where metamorphism is greater, abundant folds of several ages intersect and produce very complex relations as seen in individual exposures. Folding probably began at about the time of the Greenbrier thrusting but continued after the Great Smoky thrusting. In general it reflects relative upward and northwestward movements, in harmony with the major fault movements.
Summary of Deformational History
The earliest structures in the Great Smoky Mountain region are recorded in the basement complex. They formed in Precambrian time by deformation, metamorphism, and partial granitization of ancient stratified rocks, and were eroded and truncated before the Ocoee Series was laid down over them in later Precambrian time.
Deposition of the Ocoee Series seems to have been followed by the deposition of Paleozoic sediments without any conspicuous deformation. Along the northwestern edge of the Ocoee area, at least, the basal Cambrian and younger Paleozoic deposits overlie the top of the series with an inconspicuous erosional break at most, and, although the two assemblages of sediments differ much in origin, their structures and their degrees of metamorphism are nearly identical.
The first notable structures in the Great Smoky Mountain region that are younger than the earlier Precambrian are thus probably of Paleozoic and, perhaps, of early Paleozoic age; they may be related, at least tenuously, to the clastic deposits of Middle Ordovician age in the southeastern part of the Appalachian Valley. Conspicuous among the early structures are the Greenbrier and related faults, which have moved part of the Ocoee Series north westward over another part for a distance of many miles.
At some later time in the Paleozoic, the Ocoee Series and its faulted structures were subjected to regional metamorphism. The rocks on the northwest were almost unaffected, but those on the southeast attained middle-grade metamorphism. During this time, the basement complex was remetamorphosed.
Late in Paleozoic time, at least later than Mississippian, the whole mass of the Ocoee Series, as well as its cover of earliest Paleozoic rocks, was thrust many miles northwestward across the main body of Paleozoic rocks along the Great Smoky fault. Paleozoic rocks overridden by the thrust are now exposed in the cove areas. During this thrusting, the rocks above the Greenbrier fault were carried along with the rest, thus moving them a still greater distance from their original site of deposition.
A very late event in the deformation of the region, as young as or younger than the Great Smoky thrusting and most of the folding, was the breakup of the Ocoee Series by the Gatlinburg and related faults, by some combination of dip-slip and strike-slip movements.
A final topic worthy of discussion is the growth of mountains on the site of the present Great Smoky Mountains. After Precambrian time the first indication of any mountainous lands near the present Great Smoky Mountains is afforded by the clastic rocks in the Middle Ordovician Series, which may be related to the earliest structures in the Great Smoky Mountains. Following these earliest structures, deformation continued, at least intermittently, until late in Paleozoic timeat least after Mississppian time. Continued deformation undoubtedly raised the whole Appalachian region, including the site of the Great Smoky Mountains, into a land area. We can guess that this land area was mountainous, but we cannot tell much about its surface features, because no contemporaneous sediments are preserved nearby.
From Paleozoic time to the present this land area has been continuously eroded, although the erosion may have been accelerated from time to time by renewed uplift without any accompanying deformation of the rocks. The modern Great Smoky Mountains are entirely a product of this prolonged erosion and uplift and are not themselves a direct product of the extensive deformation which is visible in the rocks that compose them.
Last Updated: 20-Nov-2006