Geological Survey Bulletin 587 Geology of the Great Smoky Mountains National Park, Tennessee and North Carolina

Despite the enigmatic nature of the rocks of the Great Smoky Mountains, they are of more than usual geologic interest, as they link the contrasting Paleozoic sedimentary rocks of the Appalachian Valley on the northwest with the metamorphic and granitic rocks of the Blue Ridge on the southeast. Like the rocks of the Appalachian Valley, most of those in the Great Smoky Mountains are of sedimentary origin; moreover, near the Appalachian Valley these rocks are as little changed or metamorphosed as those in the valley. But unlike the latter, which are a varied sequence of fossil-bearing limestones, sandstone, and shales, those of the mountains are a great mass of pebbly, sandy, and muddy sedimentary rocks, lacking in fossil remains. Moreover, by a gradual southeastward increase of shearing, recrystallization, and metamorphism these original sediments acquire more and more the aspect of the rocks of the nearby Blue Ridge.

The intermediate nature of the rocks of the Great Smoky Mountains applies not only to their aspect but also to their age. Relations of these rocks to those adjacent indicate that the rocks of the Great Smoky Mountains were deposited later than most of the rocks of the Blue Ridge, which are of earlier Precambrian age (formed more than a billion years ago), but before the rocks of the Appalachian Valley, which are of early to middle Paleozoic age (formed 600 million to 300 million years ago). Most of the rocks of the Great Smoky Mountains were formed during some part of later Precambrian time (a billion to 600 million years ago).

The bedrock of the Great Smoky Mountains and their surroundings, and the geologic story that can be deduced from it, are thus divisible into three chapters. The metamorphic and granitic rocks characteristic of the Blue Ridge on the southeast (the basement complex) represent the first chapter of the story, that of earlier Precambrian time. The pebbly, sandy, and muddy sedimentary rocks of the Great Smoky Mountains themselves (the Ocoee Series) represent the second chapter, that of later Precambrian time. The sedimentary rocks of the Appalachian Valley represent the third chapter, that of Paleozoic time. In the succeeding account, the rocks and the events which they imply will be treated in this chronological order—which is also roughly their geographic order from southeast to northwest across the mountains.

Basement Complex

The rocks of the Blue Ridge, or basement complex, are the ancient much-altered crystalline foundation on which all the other strata of the region have been laid. They extend along the southeastern side of the Great Smoky Mountains and are extensively exposed in the southeastern part of the area shown on the map on each side of the Pigeon River near Waterville Lake, in the valley of Jonathan Creek, and near the villages of Dellwood and Maggie along U.S. Highway 19W. They also reappear at several places within the mountains themselves where tectonic forces have pushed them up or have thrust them into contact with younger rocks (pl. 1, structure sections A—A' and B—B'. Still farther southwest they form two oval-shaped areas near Ela and Bryson City.

The basement complex consists of a wide variety of gneisses and schists, including both layered gneisses derived from sedimentary or volcanic rocks and non-layered, mostly granitic, gneisses representing intrusive bodies. Layered gneisses are most abundant toward the southeast near Maggie and Dellwood, whereas granitic rocks are dominant in areas toward the northwest. No single type occupies large areas, however; boundary relations between the various types are both gradational and geometrically complex and cannot be shown at the scale of the present map.

The layered gneisses arc foliated crystalline rocks which contain various proportions of biotite, muscovite, quartz, and feldspar. They include also minor amounts of mica schist and of gneisses that contain small to large amounts of hornblende. The identifiable rock types are intercalated in layers and lenses an inch to several tens of feet thick. The granitic rocks are somewhat less diverse in composition and are mostly quartz monzonite and granodiorite in which biotite, primary epidote, and magnetite are the chief mafic minerals. Some of the less granitic rocks range from quartz monzonite to true granite, and a few small intrusive bodies are amphibolite or pegmatite. Although the granitic rocks are not conspicuously layered, most of them have flaser or augen structure resulting from their having been deformed one or more times since initial consolidation of the complex.

The basement complex has undergone so many changes that its origin and age can be stated only in the broadest terms. The mica gneiss and schist were probably sandy and shaly sediments, whereas at least some of the hornblende gneisses may have been volcanic flows or tuffs. The granitic rocks which dominate toward the northwest may have originated partly as a magma that invaded these rocks, but many features, including their gradational relations with the more layered rocks and their bulk chemical composition, suggest that some were converted from originally stratified rocks during deep burial and prolonged metamorphism. During Paleozoic time the basement complex underwent the same deformation and metamorphism to which the younger rocks of the region were subjected, and in many places this has obscured its relations to the younger rocks. Yet, there are many indications that it formed during an earlier geologic cycle. The structures of the basement are more complex and include not only those of Paleozoic age but also metamorphic and plutonic structural features that are lacking in the later Precambrian rocks, and hence are presumably older. Moreover, debris in the basal part of the later Precambrian sedimentary rocks can be matched with constituents of the basement complex; the debris was obviously derived from the basement surface when it was undergoing erosion.

The time at which the stratified rocks of the complex originally formed has not been determined, but the approximate time of their metamorphism and transformation is suggested by age determinations on radioactive minerals. These have yielded ages of about a billion years, as well as a scattering of younger ages down to about 350 million years (Long and others, 1959, p. 588-590; Tilton and others, 1960, p. 4173-4175; Kulp and Eckelmann, 1961; Hadley, 1964, p. 36-39). The earlier ages are believed to express the principal metamorphic and plutonic events to which the basement complex was subjected; the younger ages are believed to represent modifications of these dates by later metamorphic and plutonic events that culminated during the middle of the Paleozoic. Detrital grains from the later Precambrian sedimentary rocks of the Great Smoky Mountains have yielded ages of 820 million to a billion years (Carroll and others, 1957, p. 186-188; Stern and Rose, 1961, p. 609) and express dates in the basement complex from which they were derived, rather than the age of the rocks in which they are now embedded.

Ocoee Series

The later Precambrian sedimentary rocks, which form most of the Great Smoky Mountains and large parts of the adjacent foothills, are known as the Ocoee Series after the river of that name near the southern boundary of Tennessee (Stafford, 1869, p. 183-198). The Ocoee Series extends far beyond the Great Smoky Mountains to the northeast and southwest, along the trend of the ranges—from northeast of Asheville, N.C., at least as far as Cartersville, Ga., a distance of more than 175 miles. Near the Great Smoky Mountains this series extends across the ranges for about 30 miles, and in some other places its breadth is even greater.

Along its northwestern edge, the Ocoee Series has been thrust over the Paleozoic rocks of the Appalachian Valley along the Great Smoky fault (see under "Geologic structure"). The series has also been complexly folded and faulted internally and has been metamorphosed to varying degrees by heat and pressure. Toward the northwest the clay minerals in the sedimentary rocks have been altered to chlorite typical of low-grade metamorphic rocks; southeastward these minerals have been transformed to biotite and garnet typical of medium-grade metamorphic rocks; farthest southeast these minerals are represented by staurolite and kyanite typical of higher grade metamorphic rocks. Rocks with dominant clay minerals thus change southeastward from shales to slates, and these into phyllites and schists. Nevertheless, metamorphic grain sizes are smaller, and obliteration of sedimentary structures is less than in similarly metamorphosed rocks in many regions. Sandstones are recrystallized and their grain sizes modified, but their bedding and other sedimentary structures are clearly preserved nearly everywhere. It is thus convenient in this account to refer to the rocks of the Ocoee Series largely in terms of their original sedimentary nature, rather than in terms of their present metamorphic condition which varies from one part of the mountains to another. Some degree of metamorphism can be assumed for nearly all the rocks of the Ocoee Series.

Throughout great thicknesses, the rocks of the Ocoee Series are of monotonous aspect, with a few types repeated or interbedded, and with the grosser units lensing or grading into one another. Where abrupt contacts are visible between different gross units, these generally have been faulted together. Although the rocks of the Ocoee Series are broadly akin in age and origin, the series includes several differing parts called the Snowbird, Great Smoky, and Walden Creek Groups (King and others 1958, p. 951-954; table 1).

In general, the Great Smoky Group is most widespread toward the southeast, where it forms the main mass of the Great Smoky Mountains; the Snowbird Group occurs in the middle, in the foothills just north of the mountains; and the Walden Creek Group occurs toward the northwest, in the part of the foothills nearest the Appalachian Valley. To some extent, these groups form a sequence. The Snowbird Group lies on the basement complex, and at one place or another is overlain either by the Great Smoky Group or the Walden Creek Group. But the Snowbird Group varies markedly in thickness from one part of the Great Smoky Mountains to another, and the mutual relations between the Great Smoky and Walden Creek Groups are uncertain. Before the relations between the three groups and their probable history are interpreted the rocks of the individual groups will be described.

TABLE 1.—Stratigraphic units of the Ocoee Series of the Great Smoky Mountains

(click on image for an enlargement in a new window)

Snowbird Group

The most informative exposures of the Snowbird Group are east of the end of the Great Smoky Mountains, in the valley of the Pigeon River from Waterville Lake to the village of Hartford, 10 miles to the north. In the valley of the Pigeon River the Snowbird Group forms a northwestward-facing sequence about 13,000 feet thick, which lies on the basement complex near Waterville Lake and is overlain by the Rich Butt Sandstone (pl. 1, structure section A—A'). In this area the Snowbird Group is divisible into four formations that are named, in ascending order, the Wading Branch Formation, the Longarm Quartzite, the Roaring Fork Sandstone, and the Pigeon Siltstone.

The relatively thin Wading Branch Formation, resting on granitic rocks of the basement complex, is a heterogeneous dark-colored assortment of sandy argillite, siltstone, and pebbly feldspathic sandstone. A distinctive basal argillaceous layer, now phyllitic, was probably reworked from clays that accumulated on the weathered surface of the granitic rocks. The overlying and thicker Longarm Quartzite is dominantly light-colored medium- to coarse grained feldspathic quartzite and arkose, many of whose layers are current bedded. Upward, the quartzite is interbedded more and more with sandstone like that in the overlying formation, the Roaring Fork Sandstone, which is darker and finer grained than the Longarm, and forms beds 5-50 feet thick. Sandstone beds of the Roaring Fork are separated by nearly equal thicknesses of argillaceous and silty rocks, mostly phyllitic. The overlying very thick Pigeon Siltstone consists of argillaceous material intermingled with abundant grains of quartz and feldspar that are much smaller than normal sand size. It forms dull-greenish massive layers that are marked throughout by light and dark laminae, straight and even bedded through great thicknesses, but with an irregular or "lenticular-laminate" structure in some parts. The Pigeon Siltstone is mostly very uniform, but in some places it contains a few beds of dark fine-grained sandstone like those in the Roaring Fork; elsewhere, carbonate-rich layers as much as 2 feet thick are present, and are made conspicuous by their rusty-weathered surfaces.

The Snowbird Group is even more extensively exposed along the north side of the Great Smoky Mountains, where it forms a belt of rugged foothills that extends as far west as Cades Cove. There, however, both its top and base are faulted off and are not visible. Moreover, the coarser formations that are well displayed along the Pigeon River fade out westward; the Longarm Quartzite intertongues with the Roaring Fork Sandstone, and the Roaring Fork intertongues with the Pigeon Siltstone. Thus, in the vicinity of Gatlinburg in the central part of the mountains, only the Roaring Fork Sandstone and Pigeon Siltstone are represented; nevertheless, the exposed part of the Snowbird Group here is as much as 17,000 feet thick, or thicker than all four of its formations along the Pigeon River (Hamilton, 1961, p. A9-A10; Hadley and Goldsmith, 1963, p. B33, B37). The transmountain highway (U.S. Highway 441) in its course along the West Prong of the Little Pigeon River traverses the Snowbird Group from 4 miles north of Gatlinburg to the edge of the high mountains 2 miles southeast of national park headquarters; characteristic outcrops of the Pigeon Siltstone may be seen along the highway north of Gatlinburg, and characteristic outcrops of the Roaring Fork Sandstone may be seen southeast of park headquarters.

West of the Gatlinburg area the Snowbird Group is exposed only in strips or slices that are faulted between other units of the Ocoee Series. Here, perhaps because of the faulting and the extreme shearing attendant upon it, sedimentary structures are less evident. This part of the group is termed the Metcalf Phyllite, but is probably equivalent to parts of the Pigeon Siltstone and Roaring Fork Sandstone farther east. The Metcalf is well exposed in many cuts along Tennessee Highway 73 between Elkmont and Tuckaleechee Cove, in the gorge of the main east prong of the Little River.

The Snowbird Group is also exposed in the southeastern part of the Great Smoky Mountains, adjoining the basement complex, in several narrow strips that are structurally separated from the Snowbird Group farther north. Here, characteristic rocks of the Wading Branch Formation, Longarm Quartzite, and Roaring Fork Sandstone are recognizable from one place to another; but the whole group is less than 2,000 feet thick, remarkably thinner than in the northern sequences. Moreover, from Maggie eastward, even this thinned representative of the Snowbird Group wedges out, so that the Great Smoky Group lies directly on the basement complex beyond.

Great Smoky Group

The Great Smoky Group forms the main bulk of the Great Smoky Mountains and extends their entire length, from near the Pigeon River on the east to beyond the Little Tennessee River on the west (fig. 1). It also extends across the mountains from the foothills of Snowbird Group on the north to the areas of basement complex on the southeast. In addition, many rocks in discontinuous areas in the northern foothills resemble the Great Smoky Group, but, for reasons explained on page 6, most of these are considered to be "unclassified formations" of the Ocoee Series.

FIGURE 1. View from Cliff Top on Mount Le Conte, looking southwest across central part of the mountains. State-line divide is defined by Newfound Gap, Clingmans Dome, Silers Bald, and Thunderhead Mountain; valley of West Prong of Little Pigeon River in middle distance, that of main prong of Little River behind the ridge of Sugarland Mountain. The mountains in this view are all formed of the Great Smoky Group of the Ocoee Series. Note the steep-sided craggy pinnacles of Peregrine Peak surmounting Anakeesta Ridge, and The Chimneys, formed of the Anakeesta Formation; the contrasting smoother contours are characteristic of the Thunderhead Sandstone. Drawing by Philip B. King. (click on image for an enlargement in a new window)

The Great Smoky Group is well exposed along the transmountain highway (U.S. Highway 441) from 2 miles southeast of national park headquarters, across Newfound Gap, to Smokemont at the edge of the Cherokee Indian Reservation on the southeastern side. Along the north slope of the Great Smoky Mountains, as shown along the highway, the Great Smoky Group is tilted southeastward at moderate angles, forming a great homocline, which displays in places a thickness of as much as 25,000 feet of strata; elsewhere, especially toward the southeast, its rocks are tightly folded.

Along the north edge of the mountains the Great Smoky Group lies on the Snowbird Group, but the contact is a surface of movement called the Greenbrier fault, along which the higher unit has moved over the lower for a great but undertermined distance (pl. 1, structure section B—B'). Farther south in the southeastern part of the mountains the Great Smoky Group rests conformably on the Snowbird Group. Nowhere in the Great Smoky Mountains is the top of the Great Smoky Group preserved, but south of the western part of the mountains the group is overlain by slate, quartzite, and marble of the Murphy marble belt (for example, in the Nantahala Gorge, south of the area mapped) (Keith, 1907, p. 4-5; Hurst, 1955, p. 45-56). These overlying formations are downfolded into the Great Smoky Group; although the rocks are probably of early Paleozoic age like the older rocks of the Appalachian Valley on the northwest, no fossils have been found.

The Great Smoky Group is a thick monotonous mass of clastic sedimentary rocks, pebble conglomerate, coarse to fine sandstone, and silty or argillaceous rocks, which can be divided into three intertonguing formations—the relatively fine-grained Elkmont Sandstone below, the coarse-grained Thunderhead Sandstone in the middle, and the dark silty and argillaceous rocks of the Anakeesta Formation above. This subdivision is most evident on the north slope of the mountains, as along the transmountain highway and on the slopes of Mount Le Conte (cover drawing). The three formations are less apparent farther east and southeast, where nearly all the Great Smoky Group has the character of the Thunderhead Sandstone, as shown on the map (pl. 1), but where it undoubtedly includes thick bodies of strata higher than any on the north side, including strata at or above the level of the Anakeesta Formation.

Both the Elkmont and Thunderhead Sandstones are somber gray and thick bedded and are composed principally of quartz and potassic feldspar grains, with lesser quantities of plagioclase feldspar and here and there a few pebbles of light-colored granite and quartzite. In the Thunderhead Sandstone many of the quartz grains are conspicuously blue tinted, but such grains are less abundant in the Elkmont Sandstone beneath. Characteristically, the sandstone beds of both formations are graded; that is, the basal part of each layer is coarser than the top part, which is commonly separated from the next sandstone layer by a parting of silty or argillaceous material. The graded layers are a few feet to 25 feet thick, but in each layer the whole textural range of each formation is displayed, from coarse to fine, repeated through thousands of feet of sequence. In the Thunderhead Sandstone the coarse bottom parts of the layers contain pebbles an eighth of an inch to more than half an inch in diameter; in the Elkmont Sandstone, however, the coarsest grains are rarely larger than sand size.

The Anakeesta Formation consists mainly of dark silty and argillaceous rocks altered to slate, phyllite, or schist. It forms steep-sided ridges and craggy pinnacles in the higher parts of the Great Smoky Mountains—such as The Chimneys, the prominent landmark that overlooks the transmountain highway from the west. The silty and argillaceous rocks contain a little free carbon and iron sulfides which produce dark and rusty weathered surfaces. Although the dark fine-grained rocks are the most common components of the Anakeesta Formation, sandstone layers like those of the Thunderhead Sandstone are commonly interbedded with them, and the two formations intertongue extensively. Thus, in places, sandstone replaces the dark fine-grained rocks, and the Anakeesta ceases to be identifiable. The Anakeesta Formation is therefore shown on the geologic map as discontinuous strips and patches; some of these have been isolated from the rest of the formation by faulting or folding, but most of them represent tongues or lenses in a dominant body of sandstone.

Unclassified formations of Ocoee Series

Associated with the Snowbird Group in the foothills north of the Great Smoky Mountains are several areas of coarser sandy rocks. They have some features like those of the Snowbird Group but many more features like those of the Great Smoky Group, although there are some subtle differences, such as absence of blue-tinted quartz grains. Seemingly, they are transitional vertically, and to some extent laterally, from one group to the other, and therefore are not included in any of the groups of the Ocoee Series.

Such unclassified rocks occur on the lower slopes of the northeast end of the Great Smoky Mountains near Mount Cammerer, as well as on Webb Mountain and Big Ridge in the foothills several miles to the northwest and in the hills north and west of Cades Cove much farther west. Those on Mount Cammerer are termed the Rich Butt Sandstone; those on Webb Mountain and Big Ridge are unnamed but are probably equivalent to the Rich Butt (Hamilton, 1961, p. A15); those near Cades Cove are termed the Cades Sandstone. Rocks of similar character occur on the high foothill ridge of Cove Mountain northwest of Gatlinburg, but as these have somewhat more resemblance to the Great Smoky Group of the mountains and physically adjoin it, they are mapped as Elkmont Sandstone and Thunderhead Sandstone. Although these unclassified rocks are exposed in relatively small areas, thicknesses as great as 3,000-4,000 feet are preserved in nearly all their exposures.

The Rich Butt Sandstone and the rocks of Webb Mountain and Big Ridge lie conformably on the Pigeon Siltstone, and there is some evidence that the lower beds of the Rich Butt intertongue laterally with the Pigeon. Relations of the Cades Sandstone to the adjoining Metcalf Phyllite are less obvious because these two units occur in a region of much greater faulting and shearing. The Rich Butt Sandstone is mainly light-colored feldspathic sandstone in thin to thick beds, with sharply contrasting thin argillaceous interbeds, and some laminated silty or argillaceous units much like parts of the Pigeon Siltstone. Some coarse thick-bedded sandstone with graded bedding occurs in places in the Rich Butt, but it dominates the upper two-thirds of the sequence on Webb Mountain and Big Ridge and forms most of the sequence near Cades Cove. The tops of all these sequences are faulted off; the Rich Butt Sandstone is separated from the overlying Great Smoky Group by the Greenbrier fault; the rocks of Webb Mountain, of Big Ridge, and near Cades Cove have been thrust northward on other faults over various formations of the Ocoee Series.

Walden Creek Group

The northern and northwestern parts of the foothills are formed by another assemblage of sedimentary rocks, which are considerably more varied than the remainder of the Ocoee Series and are termed the Walden Creek Group. This group is mostly shale and siltstone, but it includes discontinuous masses of conglomerate and sandstone, as well as minor layers of quartzite, limestone, and dolomite. Especially distinctive are the conglomerates, which are formed mostly of large rounded white quartz pebbles, with minor amounts of pebbles of black quartzite, granite, limestone, and a variety of other rocks. Many of the characteristic rocks and structures of the Walden Creek Group can be observed along Tennessee Highway 73 in its course along the Little River between Chilhowee Mountain and Tuckaleechee Cove, and along U.S. Highway 129 along the Little Tennessee River (Neuman and Nelson, 1965, p. D15, D57).

Structural disorder in the Walden Creek Group exceeds that in any other group of the Ocoee Series, partly because of the weakness of its dominant shales and siltstones during deformation, partly because the group is underlain at shallow depth by the major low-angle Great Smoky thrust fault (pl. 1, structure sections C—C' and D—D'). Its conglomerates and other strong rocks are thus broken into discontinuous lenses, and its silty and argillaceous rocks are intensely folded and crumpled nearly everywhere.

Broad differences in rock types are apparent from one part of the Walden Creek Group to another, but their original sequence and distribution are difficult to ascertain. Relations are clearest in the eastern part of the foothills south of English Mountain and near the main middle prong of the Little Pigeon River, where the group has been divided in ascending order into the Licklog, Shields, Wilhite, and Sandsuck Formations (Hamilton, 1961, p. A18). No continuous sequence of these formations can be found, but they probably have an aggregate thickness of about 8,000 feet. Rocks similar to these formations occur farther west in the outcrop belt of the Walden Creek Group (King, 1964, p. C45; Neuman and Nelson, 1965), and the same classification is used there.

Within the foothills north of the Great Smoky Mountains, the base of the Walden Creek Group is not visible; it is underlain by the Great Smoky fault along which it has been thrust over Paleozoic rocks, and it is downfaulted against the Snowbird Group on the south. Northeast of the map area, however, near the French Broad River, the Walden Creek Group conformably succeeds the Snowbird Group (Oriel, 1950, p. 23-24; Ferguson and Jewell, 1951, p. 16-17).

The Licklog Formation, the basal unit of the Walden Creek Group, now lies on the Great Smoky fault but perhaps was originally deposited on the Snowbird Group. It is dominantly a fine-grained sedimentary rock—shale and sandstone—but it is overlain by the Shields Formation which contains great masses of coarse conglomerate of the type characteristic of the group as a whole; this conglomerate in intermingled with coarse sandstone and intertongues with shale. The Shields is followed by the Wilhite Formation, dominantly a siltstone much like the Pigeon, but containing many lenses of conglomerate like that in the Shields, as well as some lenses of pure white quartzite, and in the upper part, many lenses and beds of limestone and dolomite. The Sandsuck Formation is again dominantly shaly, although, like the other formations, it contains many conglomerate lenses.

South of English Mountain the top of the Sandsuck Formation is not preserved, but in the lengthy fault block of Chilhowee Mountain farther west the Sandsuck is overlain by the Cochran Formation of Early Cambrian (?) age—the basal unit of the Chilhowee Group. In the northeastern part of Chilhowee Mountain the Cochran seemingly lies disconformably on the Sandsuck, that is, upon its eroded and somewhat truncated surface (King, 1964, p. C57-C59); farther southwest, the two may be conformable (Neuman and Nelson, 1965). In any event, the two formations are markedly different—the Sandsuck with contrasting coarse and fine very irregular layers that formed in an unstable environment, the Cochran with less contrasting very persistent layers that formed in a stable environment.

Interpretation of Ocoee Series

All the sediments of the Ocoee Series were deposited under water, very probably in a single large marine basin. Its component groups, however, were obviously formed in different environments. In some places the different deposits of these groups are superposed in stratigraphic sequences, but for the most part they are now preserved in separate fault blocks, juxtaposed by thrusting of undetermined magnitude. The rocks of these fault blocks are mere fragments of the original deposit, the connections between which are now lost. Thus, a definitive reconstruction cannot yet be made of either the original relations of the groups or their history. Nevertheless, some speculations can be offered.

Pertinent to such speculation are the following:

1. The Snowbird Group lies unconformably on the basement complex, and is the oldest unit of the Ocoee Series.

2. In most of the southeastern part of the mountains the Snowbird Group is thin and is overlain conformably by the Great Smoky Group.

3. In places in this part of the mountains the Snowbird Group wedges out, so that the Great Smoky Group is the basal deposit.

4. Farther northwest the Snowbird Group is thick and is overlain conformably by unclassified formations of the Ocoee Series, which may be a vertical and lateral transition from the Great Smoky to the Snowbird Group.

5. In the foothills northwest of the Great Smoky Mountains the Walden Creek Group adjoins the Snowbird Group along a faulted contact, but probably originally overlay it.

6. The Walden Creek Group is overlain on the northwest by the Chilhowee Group, at least part of which is of early Paleozoic age.

7 The Great Smoky Group is overlain on the south by formations of the Murphy marble belt, which are likewise probably of early Paleozoic age.

These facts still leave many questions of interpretation unanswered, for it is clear that the three groups of the Ocoee Series do not form a single stratigraphic sequence. Although they are not themselves in contact, both the Great Smoky Group and the Walden Creek Group overlie the Snowbird Group in different places. What, therefore, is the relation of the great mass of coarse sediments of the Great Smoky Group to the thinner and very different sediments of the Walden Creek Group to the northwest?

Much of the thinning of the Snowbird Group from the northeastern to the southeastern part of the mountains presumably results from overlap of the Snowbird Group against the basement complex, as shown by the increase in amount of the coarse sandy deposits of the Longarm Quartzite in this direction. However, at least some of it is caused by intertonguing of the Snowbird with such coarser formations as the Rich Butt Sandstone, which in turn probably pass southeastward into the Great Smoky Group. The relation of the Great Smoky Group to the Walden Creek Group farther northwest is less clear, because the two are not only very different, but they are separated by a belt several miles wide, which was probably much wider before folding and thrusting. Nevertheless, because both groups overlie the Snowbird, and because both are overlain by strata of early Paleozoic, or probable early Paleozoic age, they are probably laterally equivalent.

The Snowbird Group is a basal and marginal deposit, laid down against a rough surface of the basement complex that projected into the northeastern part of the depositional basin, and from which the coarse sandstones of the group were derived. Its coarser grained rocks give place southwestward along the outcrop to finer grained rocks, and still farther southwest the finer grained rocks may become indistinguishable from similar rocks of the Walden Creek Group. The Walden Creek Group was probably laid down on an unstable shelf that fringed the continental interior toward the northwest. Some of its deposits were laid down in shallow water; others slumped from shallow water into somewhat deeper water at the edge of the shelf. The Great Smoky Group accumulated farther southeast, away from the continent, probably in water of much greater depth than did the other two groups, perhaps in a submarine trench of tectonic origin. Its ubiquitous graded bedding indicates that most of the sediments that accumulated in the trench were transported there by turbidity currents. The source of these sediments cannot be identified, but they decrease in grain size southwestward from Tennessee to Georgia, suggesting that the trough was filled from its northeastern end and that the sediments were derived from the basement rocks well to the northeast. The northwestern edge of the trench must have been the unstable shelf on which the Walden Creek sediments were accumulating. Its southeastern edge is undefined, as it is adjoined in this direction by older rocks of the basement complex, which are in turn adjoined by the highly metamorphosed rocks of the Piedmont province, whose age and sequence are as yet uncertain. It might be that the southeastern edge of the trench in which the Great Smoky sediments accumulated included the site of the northwestern part of the Piedmont province, where its deposits are now greatly metamorphosed and unrecognized.

The time during which the Ocoee sediments accumulated was later than the creation of the basement complex in earlier Precambrian time, and before the deposition of the oldest Paleozoic sediments, hence probably in late Pre-cambrian time. Much searching has failed to reveal any fossil remains in the Ocoee Series. This might suggest that they are of pre-Paleozoic age were it not that large parts of the Ocoee sediments are of types that would be poorly fossiliferous whatever their age. More significant is the fact that the lowest fossil remains in the sequence on Chilhowee Mountain are of Early Cambrian age, and that these fossils occur well above the top of the Walden Creek Group and the Ocoee Series.

Paleozoic Rocks

The third chapter of this account of the bedrock geology of the Great Smoky Mountains and their surroundings, that of the Paleozoic rocks and of Paleozoic time, can be treated more briefly than the preceding two chapters. Most of the Paleozoic rocks involved are northwest of the Great Smoky Mountains and the Great Smoky Mountains National Park, and they have been described at length in other publications (Rodgers, 1953, p. 42-110; Neuman, 1955, p. 146-165; Cattermole, 1955; Neuman, 1960; Neuman and Wilson, 1960; Cattermole, 1962).

The Paleozoic formations northwest of the Great Smoky Mountains are shown on the accompanying geologic map (pl. 1) and are summarized in table 2.

TABLE 2.—Paleozoic formations in the vicinity of the Great Smoky Mountains

The Paleozoic rocks occur in three different structural and geographic situations. The first is in the Great Smoky and related thrust sheets, where they form Chilhowee Mountain and Miller Cove on the west edge of the foothill belt (pl. 1, structure sections C—C' and D—D') and English and Green Mountains on the east (pl. 1, structure section A—A'); here, only the lower part of the Paleozoic sequence is preserved. The second is beneath the Great Smoky fault and northwest of its leading edge in the Appalachian Valley; here, all the formations of the upper part of the Paleozoic sequence are preserved, from the Cambrian Rome Formation, through the Ordovician and Devonian, to the Mississippian Greasy Cove Formation. The third is beneath the Great Smoky fault in Wear, Tuckaleechee and Cades Coves, and at Calderwood where the Paleozoic rocks have been revealed in windows produced by erosion through the Great Smoky thrust sheet; here, a small part of the more complete sequence of the Appalachian Valley emerges—limestone of the Ordovician part of the Knox Group and shales, sandstones, and limestones of the lower part of the Middle Ordovician Series.

The Paleozoic rocks of the first category form long knife-edged ridges, such as Chilhowee Mountain, lined with quartzite ledges of the Chilhowee Group on its northwest face (fig. 2), and with dip slopes on the southeast that pass beneath the Shady and Rome Formations in the low ground of Miller Cove. The Paleozoic rocks of the second category form the low country of the Appalachian Valley, cleared, farmed, and well populated; the limestones of the valley form fertile, rolling country, and the shales form low, knobby hills. The Paleozoic rocks of the third category form low, cleared, fertile coves that duplicate the Appalachian Valley in miniature, surrounded by the dark, forested foothills of the Great Smoky Mountains.

FIGURE 2.—Little River gap looking southeast from crest of ridge supported by Chota Formation. The subdivision of the Chilhowee Group on Chilhowee Mountain are delineated, together with the traces of the Great Smoky and Guess Creek faults. The gap itself follows a minor transcurrent fault, so that the continuity of the formations on its opposite sides is broken. Tennessee Highway 73 is across the river from this viewpoint. Drawing by Philip B. King. (click on image for an enlargement in a new window)

The Chilhowee Group at the base of the Paleozoic sequence is of special interest, for it contains the oldest fossil remains in the Great Smoky Mountain region. The Helenmode Formation at the top has been known for many years to contain fragments of the trilobite Olenellus and other fossil shells, which were collected at Walland at the south end of the gap of the Little River through Chilhowee Mountain (Walcott, 1890, p. 570; Resser, 1938, p. 25). Recently, other fossil shells have been obtained from the Murray Shale lower down in the sequence, from cuts on the Foothills Parkway near Look Rock southwest of Walland; these are shells of the primitive ostracode Indiana (Laurence and Palmer, 1963). Beside these fossil shells, all the formations of the Chilhowee Group above the Cochran contain traces of former life; the quartzites and sandstones contain closely spaced vertical tubes called Scolithus, probably the burrows of a primitive sea worm; the shales and siltstones show various tracks and trails on the bedding surfaces. Because of the occurrence of definite fossil shells in the Murray Shale, this and the higher formations of the Chilhowee Group are classified unequivocally as Lower Cambrian. The underlying formations of the Chilhowee Group are also very likely to be Cambrian, because some of them contain traces of highly organized animal life, and because their rocks are much like, or even identical with, the fossiliferous rocks conformably above them. However, as true fossil shells that would establish an age have so far not been collected from these formations, their assignment to the Lower Cambrian remains questionable.

The succeeding Cambrian and Lower Ordovician formations, from the Shady Dolomite to the top of the Knox Group, are a great sequence of carbonate rocks nearly 7,000 feet thick—mainly limestone and dolomite, with some shaly units like the Rome Formation and the Conasauga Group that are themselves also limy. These indicate a long epoch of quiet deposition in the marine waters of the Paleozoic Appalachian geosyncline—a time when the region was far from any shore, and when even the distant lands in the interior of the continent to the northwest were too low to contribute much erosional debris.

The succeeding Middle Ordovician strata are a sequence of clastic rocks as thick as or thicker than the carbonate sequence—shales and sandstones with little true limestone, ending with red mudrock at the top. These deposits indicate a notable change in the environment of the Appalachian geosyncline, a rising of lands near enough to it and high enough to contribute large quantities of erosional debris (Neuman, 1955, p. 171). These lands must have been southeast of the Middle Ordovician rocks now exposed, for the quantity of shaly and sandy material in the series increases in successive outcrops southeastward across the Appalachian Valley, and the amount of limy material decreases. This would imply that the lands of Middle Ordovician time were in the direction of the present Great Smoky Mountains, although whether they were on the site of the present mountains or somewhere farther away is unkown. The creation of these lands is significant in the interpretation of the history of the Great Smoky Mountain region, as it indicates crustal unrest in the vicinity, thus supporting an inference that some of the structures in the Great Smoky Mountains formed during early or middle Paleozoic time (see below under "Structure of the bedrock").

The highest rocks of the Paleozoic sequence northwest of the Great Smoky Mountains are of Late Mississippian age and are likewise significant in dating the structural history of the mountain region. These Mississippian rocks are preserved in a narrow syncline along the northwestern slope of Chilhowee Mountain which is caught under the leading edge of the Great Smoky thrust sheet (pl. 1, structure section D—D') (Neuman and Wilson, 1960; Neuman and Nelson, 1965, p. D54). The Mississippian rocks are not actually in contact with the Great Smoky fault, as they are separated from it by a narrow fault slice of Middle Ordovician rocks. Nevertheless, their close association with the fault indicates that the emplacement of the Great Smoky thrust sheet in its present position occurred after the end of Mississippian time, probably during some later part of the Paleozoic era.

Igneous Rocks

Except for the granitic rocks of the basement complex, the Great Smoky Mountains and vicinity are conspicuously a region of sedimentary rocks and their metamorphosed equivalents. Few igneous rocks occur, and most of these form bodies too small and too inconspicuous to be shown on the map (Hadley and Goldsmith, 1963, p. B69-B74). The only igneous rock mapped is metadiorite, a rock composed mainly of hornblende and plagioclase. It forms some narrow sills parallel to beds in the Thunderhead Sandstone near the mountain crest between Newfound Gap and Clingmans Dome. These sills extend at least 15 miles southwestward across the unmapped area on the south slope of the mountains, where they have been observed near the old copper mines along Hazel Creek (Espenshade, 1963, p. 119). The sills are believed to have been introduced after the initial metamorphism of the rocks of the Ocoee Series and before the final metamorphism, hence during some part of Paleozoic time.