Geologic History of the Southeastern United States
Observations presented here were collected either by direct observation in the field, or—as is the case for zircon ages and metamorphic ages—provided by Dr. David Barbeau.
There are late Mesoproterozoic zircons (1.2–1 Ga) in the Cedar Shoals Gneiss (Site D), the Lake Toxaway Gneiss (Site L), the Great Smokey Group (site S), and in the cores of Silurian and Devonian zircons in the Table Rock Orthogneiss (Sites F, H, and I)
There are late Neoproterozoic zircons (~600 Ma) in the Cedar Shoals Gneiss (Site D) and Cambrian (~500 Ma) zircons in the muscovite schist (PS2) at the Lake Murray Spillway. The ultramafic rocks in the Addie Ultramafic Complex—the dunite and peridotite at the Addie Quarry (Site O) and the protolith to the garnet orthogneissic amphibolite in Sylva (Site Q)—are estimated to have formed in the middle Neoproterozoic through the Cambrian (700–500 Ma), and the silt, sand, carbon, and sulfur that would form the greenschist at Newfound Gap (Site T) and the siltstone in the Rome Formation (Site W) are estimated to have been deposited in the middle Cambrian (520–500 Ma). The sediment that would form the Great Smokey Group (Site S) greenschist was also deposited in the middle Neoproterozoic through the Cambrian (700–500 Ma). The sediment that would form the low-grade schist in the Poor Mountain Formation (Sites G, I, and possibly J) is generally assumed to have been deposited in the Neoproterozoic (1 Ga–541 Ma).
The limestone in the Moccasin Formation (Site W) formed in the Ordivician (485–445 Ma), and the limestone in the Knox Formation (Site U) formed in the late Ordivician (~450 Ma). The sand that would form the sandstone at Clinch Mountain (Site V) was deposited in the Silurian (444–423 Ma), and the beds in the Clinch Mountain sandstone are curved.
The zircons in the quartzite and amphibolite paragneiss (QFG2) at the Lake Murray Spillway are Silurian to early Devonian (450–400 Ma), and the zircons in the granitic orthogneiss there (LMG) are late Silurian/early Devonian (~419 Ga). The zircon age distribution in the Tallulah Falls Formation (Site E) schist (which has some augen) has a fairly tight peak around 440 Ma (early Silurian). There are also some zircons of the same age in the Ashe Formation. The zircons in the Table Rock Gneiss (Sites F, H, and I) range from Silurian to Devonian (450–380 Ma). Some of those zircons have late Mesoproterozoic (1 Ga) cores.
The zircons in the White Side Granite intrusion and its formation range from late Silurian through the Mississippian (430–320 Ma). The formation sits 200-300 miles inland from the current coast. The siliciclastic Ohio Shale (Site X) formed in the Devonian (419–359 Ma), and the sedimentary rock above it at Pound Gap, the Grainger Formation, contains stratified, gently inclined siltstone and sandstone interbedded with shale similar to the Ohio Shale.
Monazite uranium-thorium-lead dating indicates that the Cedar Shoals Gneiss (Site D) metamorphosed in the early Mississippian (~350 Ma). The gneiss also has intrusions of feldspar pegmatite. At Pound Gap (Site X), the Grainger Formation was also forming in the early Mississippian.
In the middle Mississippian, the Newman Limestone (Site X) was forming. The Lower Pennington Formation, middle-to-late Mississippian in age at the same site, features limestone with fossils interbedded with clastic shale in the upper parts of the Lower Pennington, the rock transitions to all clastic shale in tight, periodic light and dark alternating bands and then to sandstone with symmetric fossilized ripples and some cross-bedding.
The large majority of zircons in the low-grade schist at Rainbow Falls (Site M), part of the Ashe Formation, formed at the very end of the Mississippian (325 Ma). In the early Pennsylvanian, the Upper Pennington Formation (Site X) was forming. It features alternating (but non-rhythmic) layers with and without iron oxide.
Argon-argon dating indicates that peak metamorphism at the Lake Murray Spillway occurred near the end of the Pennsylvanian (~300 Ma), and the schist there contains small kyanite and staurolite. The foliation in the gneiss there indicates the maximum direction of compression to be northwest-southeast. The zircons in the Pageland granite at Forty Acre Rock formed at the beginning of the Permian. Fractures in the granite indicate northwest-southeast compression shortly after formation. As well, the Breathitt Group, medium-to-coarse-grained sandstone interbedded with coal, was deposited in the early Permian and contains zircons matching those in Appalachian rocks.
There are early Jurassic zircons in the diabase at Flat Creek Dike. The sandstone at Peachtree, which exhibits fossilized burrows, formed in the Eocene.
The late Mesoproterozoic zircons at sites D, L, S, F, H, and I are the same age as the North American basement and probably indicate that the rocks at each of these sites either are the same rock as the North American basement (as is likely the case for the orthogneiss at Lake Toxaway) or received sediment from North America before they metamorphosed (as is the more likely case for the other sites).
The late Neoproterozoic and Cambrian zircons at Cedar Shoals and the Lake Murray Spillway indicate that there was magmatism in a region from which sediment could be transported to these sites before their rocks’ lithification. To the west, the middle Neoproterozoic through Cambrian ultramafic rocks in the Addie Ultramafic Complex indicate that there was likely a divergent boundary between what is now Lake Toxaway and Waterrock sometime in that range. The formations’ shape suggests that they are obducted crust, indicating that there was a subduction zone there sometime since their formation. The formation of the Rome Formation and the protolith to the Newfound Gap greenschist in the middle Cambrian indicates that there was an ocean there then.
The formation of the limestones at sites W and U in the Ordivician indicate that there was a shallow sea or continental shelf there at that time, and the sandstone at Clinch Mountain with its curved bedding indicates that there were river systems there in the Silurian.
The late Silurian/early Devonian zircons in LMG indicates that there was magmatism at what is now the Lake Murray Spillway at that time. The same is true of the early Silurian zircons at Tallulah Falls. The late Mesoproterozoic cores of some of the Silurian to Devonian zircons in the Table Rock Gneiss indicate that magma intruded sediment or rocks containing late Mesoproterozoic zircons (which likely came from rocks that now form the North American basement) in the Silurian through the Devonian.
The late Silurian through Mississippian zircons in the White Side Granite indicate that there was magmatism there then. The Ohio Shale at Pound Gap indicates that there was likely deep ocean there in the Devonian. The early Mississippian Grainger Formation indicates that there were likely intermittent turbidity currents in a similar environment to the formation environment of the Ohio Shale. This description matches deep ocean near a continental shelf (the turbidity events are collapses from the shelf).
Assuming that the early Mississippian result from the monazite uranium-thorium-lead dating of the Cedar Shoals Gneiss is accurate, this indicates that there was fairly high-grade metamorphism there in the early Mississippian. The gneiss has intrusions of feldspar pegmatite within it. Sodium plagioclase and potassium feldspar melt at about 600°C. Either the rock reached this temperature, or there were volatiles that got carried down with the rock to its site of metamorphism. The burial of the Cedar Shoals protolith is highly unlikely to have been due to a collision with North America, as the Grainger Formation indicates that there was deep ocean to the west of the Cedar Shoals area throughout the early Mississippian. It is also unlikely that the metamorphism occurred in another large continent if the late Mesoprotozoic zircons in the Cedar Shoals Gneiss are in fact of North American origin. Instead, the Cedar Shoals protolith could have been on either a long peninsula off of the North American continent or—more likely—a part of the continent that rifted off of the main continent. If the area that is now Lake Toxaway was also part of this region, this would explain the origin of the Lake Toxaway Gneiss. As well, if it was indeed a rifted-off archipelago featuring a subduction zone to the east and a back-arc basin to the west, this would explain the origin of the Addie Ultramafic Complex (in the middle of the basin), the seemingly marine rocks at Newfound Gap, in the Knox Formation, at Copper Creek, and at Pound Gap (in the basin), and much of the magmatism in the area from the Cambrian through the early Mississippian (from the subduction). This peninsula or archipelago then collided with either some small exotic terrane or possibly the African continent. The former seems more likely due to continued magmatism in this region after the early Mississippian, such as the formation of the late Mississippian zircons that would end up in the Ashe Formation. This other terrane may well have been the origin of the Cambrian zircons—and indeed much of the material—at the Lake Murray Spillway.
The metamorphism, compression, and thrust faults throughout the whole Appalachian region which likely occurred near the end of the Pennsylvanian were probably caused by the Appalachian orogeny. The African continent, either carrying with it the Cedar Shoals peninsula or archipelago from an early Mississippian collision, or having collided with said peninsula or archipelago shortly before, collided with the North American continent, forming the Appalachian mountains and metamorphosing almost all of the rock currently exposed from Cedar Shoals to Townsend, Tennessee. Plutons like that at Forty Acre Rock likely formed as the last bit of magmatism from the subduction zone under Africa right before the orogeny.
The diabase at Flat Creek Dike likely formed in the continental rift as the North American and African continents began to separate to form the Atlantic Ocean. The sandstone at Peachtree indicates that sea levels were likely much higher in the Eocene, as the coast was near where Columbia is today.
What seems the most likely explanation of the evolution of what is now the southeast United States is the formation of what is now the North American basement in the late Mesoproterozoic, followed by the formation of a subduction zone with a volcanic arc and a back-arc basin sometime from the middle Neoproterozoic through the Cambrian, then a collision of a small exotic terrane with the arc in the early Mississippian, and then a collision with the African continent in the late Pennsylvanian, forming the Appalachians, then a continental rift forming in the early Jurassic, separating the two continents again and forming the Atlantic. However, the observations presented here are not hard, and there are many other fairly likely possibilities that would explain the observations even if they are accurate.
© Emberlynn McKinney