by Dr.Robert D. Hatcher, Jr, UT Distinguished Scientist and Professor, Tectonics and Structural Geology, Department of Earth and Planetary Sciences, Science Alliance Center of Excellence
The Little Tennessee is a tributary of the modern Tennessee River that has its headwaters in the Blue Ridge of north Georgia and joins the Tennessee in the Valley and Ridge of East Tennessee. This river drains a microcosm of Appalachian geology. The Appalachians extend from the continental margin off Newfoundland some 3,000 km (2,000 mi) southwestward to the subsurface beneath the Coastal Plain of South Alabama and Georgia. The chain was named by the Spanish in the 1500s for a Native American tribe, the Apalachis, who lived far south of the mountains in southern Georgia and northern Florida. The chain reaches its narrowest point in the area immediately west of New York City, and from there widens both to the north and south. This narrowing attribute is not related to lack of exposure because of the Coastal Plain overlap, but is a property of the crust.
Appalachian crust had its beginnings around 750 million years ago (Neoproterozoic time) when a supercontinent called Rodinia that formed ~1 billion years ago began to break up, and the continents of North America and Africa began to separate (Fig. 1). The separation process initially formed a system of rifts similar to today’s East African rift valleys, but the rifting process continued until a small ocean like today’s Red Sea began to develop between North America and Africa. This transition took place between 625 and 565 million years ago, accumulating thick deposits of sand and mud on the southeastern margin of North America that have been estimated to be over seven miles thick that form the high peaks of today’s Great Smoky Mountains. After 565 million years ago (Late Neoproterozoic) Africa and North America continued to separate and the ocean widened, the continental margin was flooded by shallow seas depositing marine shallow-water limestone and shale; the volume of sediments deposited also diminished through time as the older mountains of Rodinia finally eroded. The entire eastern margin of North America was covered by a shallow sea, Africa moved farther away; the sediments consisted of clean sand and mud; and the first carbonate sediments were deposited on the North American margin from Alabama to Newfoundland. Around 535 million years ago (Early Cambrian time) an ocean called Iapetus (Gr., Father of Atlantis) had opened and continental margin sedimentation continued similar to that going on along continental shelf of eastern North America today. All of eastern and central North America as far west as New Mexico and Utah and north into east-central Canada was covered by a shallow sea depositing immense amounts of limestone and dolomite until around 475 million years ago (Early Ordovician). At this time the entire eastern two-thirds of North America was uplifted above sea level, and an immense sinkhole topography developed over this entire region that persisted for several million years. At around 465 million years ago (Middle Ordovician) the interior of eastern North America and the eastern margin subsided again to become covered once more by a shallow sea. This shallow sea inundated the interior of the continent and deposited limestone, but along the eastern margin sand and mud began to pour in from the east because part of the eastern margin of the continent was being uplifted and overridden in response to collision with an island arc that had formed offshore. The Iapetus ocean floor that separated the island arc from eastern North America was being consumed in a subduction zone beneath the island arc. This process continued until all of the ocean crust and part of eastern North America were consumed beneath the island arc and the Iapetus ocean ceased to exist. Around 450 to 445 million years ago (Late Ordovician) the first Paleozoic mountains were built on the eastern margin of North America. The mountain-building event (orogeny) that constructed the island arc and collided it with North America is called the “Taconic orogeny,” and the mountains that were built here shed sediment into the now-filled basin to the west as the mountains were eroded.
The southeastern part of North America was additionally uplifted and was eroded until around 360 million years ago (Devonian-Mississippian boundary). This occurred at the time deposition resumed with in an oxygen-starved environment, blanketing of much of the eastern U.S. of the Chattanooga black shale. This shale is exposed in the lower part of the Little Tennessee River Valley. The Chattanooga Shale represents the southern extent of a huge delta system that has its remnants today in the coarse red sandstones and shales of the Catskill Mountains that spread southwestward into a basin that became starved for oxygen. This delta actually began forming in New York around 400 million years ago (Late Devonian) as an extinct volcanic arc that had formed off Africa around 625 million years ago (Neoproterozoic) began to collide with eastern North America. This ancient arc was no longer active, although today we can find granites and basalt-like rocks to the southeast that formed as a small amount of ocean crust (Rheic ocean) was subducted beneath the arriving arc forming a new arc that was short lived. This mass of ancient volcanic crust arrived obliquely so that it collided with North America first to the north and then rotated so that it destroyed the small Rheic ocean to the south like closing a zipper. Volcanic activity was limited because the amount of subducted oceanic crust was small, similar to today’s subduction of Australia beneath Indonesia. There are up to 15 m (50 ft) thick volcanic ash beds in Virginia that formed during from 400 to 375 million years ago (Late Devonian) that are likely the product of this subduction event. The Catskill delta spread coarse sediments westward and southward into a basin that deepened faster than sediments could be supplied, and this deeper basin became oxygen-starved, which caused organic matter to survive and not be oxidized and destroyed, producing the black mud that became the Chattanooga Shale in Tennessee (and the Marcellus Shale in Pennsylvania and West Virginia). This oblique collision of the Neoproterozoic arc with eastern North America formed another mountain chain called the Acadian-Neoacadian mountains (orogeny), closing the Rheic ocean, and recording the second mountain-building event in the history of the Appalachians (Fig. 1).
Carbonate sedimentation resumed from around 345 to 330 million years ago (Mississippian). At around 325 million years ago, shale became mixed with the limestone being deposited and shortly thereafter (latest Mississippian-early Pennsylvanian) a flood of sandstone, shale, and abundant plant material was deposited in the eastern interior of North America as a great delta spread across the entire eastern interior from the rising mountain chain along the eastern North American margin. This delta is the product of the collision of Africa with North America that formed the Appalachian Mountains (Fig. 1). This last mountain-building event closed the last remaining ocean (the Theic ocean) separating Africa and North America. During collision some 300 to 260 million years ago (late Pennsylvanian-Permian time), the outer crust of eastern North America was broken into a large slab 25-30 km (15-20 mi) thick and was pushed up over the crust farther west. This slab acted like a snowplow pushing snow in front of it. In doing so, the slab formed the large faults and folds of the Valley and Ridge in East Tennessee, southwestern Virginia and northwestern Georgia, producing first one and causing it to move until it locked, then forming another in front of it, repeating the process until there were some 10 smaller slabs composed of sedimentary rocks that moved westward anywhere from 5 to 100 km (three to 60 mi). These faults are preserved across the valley of East Tennessee and are responsible for tilting most of the rocks we see exposed today. Remnants of the delta are preserved in the Cumberland Plateau where sandstone, shale, and coal beds remain today. These sediments are very clean, and the few pebbles that are in them are composed of rounded quartz, indicating that the sediments traveled a great distance before being deposited in the area that is now the Plateau. The “mature” nature of the sediments also indicates that the delta probably extended great distances to the southeast and was deformed, uplifted, and eroded as the great slab of crust moved some 500 km (300 mi), pushing the Valley and Ridge rocks in front of it. This collision that produced the doubling of the crust in the southeastern United States was the third major mountain-building event to produce the Appalachians, and we call this the Alleghanian orogeny. Collision of Africa and North America was similar to the Neoacadian orogeny in that the collision was oblique and occurred first in the north after which Africa began rotating clockwise and sliding along the North America margin closing the Theic ocean like a zipper. Rotation finally produced head-on collision in southeastern North America some 300 to 260 million years ago (late Pennsylvanian to Early Permian) and closed the Theic ocean that would have separated Africa from North America 300 million years before. This last event produced supercontinent Pangea and completed a complex cycle of a breakup of a supercontinent (Rodinia) formation of new oceans and progressively closing these oceans to form supercontinent Pangea.
Some 50 million years later Pangea broke apart and opening the Atlantic Ocean around 200 million years ago (Late Triassic-Early Jurassic), gradually separating Africa from North America to move to its present location during the last 200 million years of geologic time. Eastern North America was initially a very arid region during this breakup period, but as the Atlantic Ocean opened, the climate became more moist and this region became vegetated and populated with the dominant vertebrates, the dinosaurs. No remnants of this history arerecorded in the Valley and Ridge in the southeastern United States, but several basins in the Piedmont from North Carolina northward contain red shales and sandstones, with some coal, that preserve abundant tracks and other evidence that the dinosaurs roamed this area. New ocean crust was generated as Africa moved farther from North America. This cooling denser crust deepened the ocean and caused the continental margin to again be flooded. Today we can see sediments deposited during this event east of the Tennessee River in west-central Tennessee and the eastern Carolinas. Sediments deposited both in West Tennessee, the eastern Carolinas, southern Georgia, southern Alabama, and Mississippi are composed of debris shed from the eroding Appalachians. The present-day topography of mountains and the Valley of East Tennessee and the Plateau to the west probably formed during the recent uplift of the chain beginning only about seven million years ago (late Miocene-Pliocene time).
Glaciers intermittently covered the northern part of North America during the past two million years (Pleistocene time), sculpting the topography of the northern Appalachians to their present shape. The southern Appalachians, despite their height, remained unglaciated, but did develop a timberline above 4000 ft (1200 m). Permanent snowfields, large areas of permafrost and patterned ground, cranberry bogs, and some rock glaciers formed here, and their remnants can be found today. From ~12,000 years ago until today, glaciers have been retreating to the point where there is speculation about when they might totally disappear from the Earth. Note that glaciers have come and gone several times in the geologic past: during the Permian during the existence of supercontinent Pangea (primary evidence in the southern continents), the Late Ordovician (primary evidence in north Africa), and during the Proterozoic nearly 2 billion years ago (primary evidence in the Ontario).
The southern Appalachians contain the highest mountains in the eastern U.S. and Canada. While there is one mountain in New England with an elevation greater than ~2,000 m (~6,200 ft, Mt. Washington, New Hampshire), and none farther north; there are 59 named peaks in the southern Appalachian Blue Ridge of western North Carolina and eastern Tennessee with elevations exceeding 1,900 m (6,000 ft). Topographic relief here can exceed 1600 m (5,000 ft). The intriguing aspect of these high elevations is they do not follow the trend of Appalachian crust, but move from the Blue Ridge in North Carolina and eastern Tennessee northward into the Valley and Ridge of southwestern Virginia and West Virginia. All of the modern rivers that drain westward across the Appalachians have their headwaters on the eastern side of the Blue Ridge and have to cross the high parts of the chain to maintain their courses. The Little Tennessee is one of these rivers.
There is a growing body of evidence that supports uplift during the late Miocene and Pliocene (7-2 Ma). The trace of the Little Tennessee drainage is one of the pieces of evidence that supports this recent event. The headwaters of the Little Tennessee River are in northeast Georgia where Betty’s Creek flows southeastward to join several other streams near Rabun Gap in Rabun County to form the Little Tennessee, which then flows northward into North Carolina, continuing northward past Franklin, and turning northwestward just north of Franklin, and then turning due west near Bryson City, North Carolina, continuing westward into Tennessee, crossing the boundary between the Blue Ridge and the Valley of East Tennessee and joins the Tennessee. Most of the course of the Little Tennessee River flows in the great slab of crust that had moved westward some 400 km (250 mi). The course of the Little Tennessee River is paralleled by almost all of the other rivers that drain into East Tennessee as tributaries of the Tennessee. The extensive and well-dated population of mammals, reptiles, and other organisms at the Gray Fossil Site, an ancient lake deposit that today rests on a hilltop near Johnson City in northeastern Tennessee, provides a link in time to suggest that the uplift of the present mountains and the Valley and Ridge began around seven million years ago. These are but two lines of evidence that Appalachian topography is very young—only a few million years old—and the Appalachians do not comprise “the oldest mountain chain on Earth,” as is frequently stated.