Ocoee Supergroup
The Ocoee Supergroup is a thick succession of predominantly metasedimentary rocks exposed in the Blue Ridge Mountains of the southern Appalachian Mountains in the United States. It underlies much of the high-elevation core of the Great Smoky Mountains and adjacent ranges, and is widely mapped in Tennessee, North Carolina, and Georgia.
In the Great Smoky Mountains region, the Ocoee Supergroup is conventionally subdivided, in ascending order, into the Snowbird, Great Smoky, and Walden Creek Groups, though fault repetition and deformation locally obscure their original stratigraphic relationships.
Name and definition
The name Ocoee is derived from exposures along the narrows of the Ocoee River in Tennessee, and is used by the U.S. Geological Survey to refer to the unit as the Ocoee Supergroup in Georgia, North Carolina, and Tennessee.Geographic distribution and exposure
Ocoee Supergroup rocks are widely exposed in the Blue Ridge of eastern Tennessee and western North Carolina, including large areas within the Great Smoky Mountains National Park. The Great Smoky Group forms much of the crest of the Great Smoky Mountains within the park area.Stratigraphy
Subdivision in the Great Smoky Mountains region
In and near Great Smoky Mountains National Park, the Ocoee Supergroup is commonly described as comprising three principal groups : Snowbird, Great Smoky, and Walden Creek.Snowbird Group
The Snowbird Group is reported as more than ~3,900 m thick in the park region and consists mostly of sandstone with interbedded finer-grained sandstone, siltstone, and argillaceous rocks that become increasingly abundant westward. It is commonly divided into the Wading Branch Formation, Longarm Quartzite, Roaring Fork Sandstone, and Pigeon Siltstone; toward the west, the Metcalf Phyllite comprises an increasing proportion of the group.Great Smoky Group
The Great Smoky Group is reported as more than ~7,600 m thick in the park region and is commonly divided into the Elkmont Sandstone, Thunderhead Sandstone, and Anakeesta Formation. The Anakeesta includes dark, silty and argillaceous rocks metamorphosed to slate, phyllite, and schist; oxidation of pyrite and other iron sulfides commonly produces a characteristic rusty weathering color, and its fissile layers help form steep ridges and prominent outcrops.Regional mapping subdivides parts of the Great Smoky Group into formations that are not always distinguished in simplified summaries. One such unit is the Copperhill Formation, which is characterized by massive, coarse-grained metagraywacke and metaconglomerate, locally interbedded with sulfidic, quartz–garnet–muscovite phyllite and schist. The same mapping notes that graphitic and sulfidic slaty rocks occur in the lower part of the Copperhill Formation, with more muscovite-rich schist higher in the section.
More broadly, classic work on the Ocoee succession formalized three major subdivisions—Snowbird, Great Smoky, and Walden Creek—and emphasized complex intertonguing relationships among formations and structural disruption by major thrust faults.
Walden Creek Group
In the park region, the Walden Creek Group is described as a varied assemblage of argillaceous and silty rocks mixed with quartz-pebble conglomerate, locally ~2,400 m thick, and containing prominent quartzite, limestone, and dolomite. It is commonly subdivided into the Licklog Formation, Shields Formation, Wilhite Formation, and Sandsuck Formation.Stratigraphic contacts and regional context
In much of the Great Smoky Mountains region, Ocoee rocks are structurally repeated and juxtaposed by faults, complicating the interpretation of original superposition. Where preserved, uppermost Ocoee strata are closely associated with the latest Neoproterozoic–early Cambrian transition; regional mapping describes the Cochran Formation as transitional between the underlying Sandsuck Formation and the basal part of the Shady Dolomite/Chilhowee succession in the western Great Smoky Mountains foothills.Regional nomenclature
Across the southern Appalachians, mapped subunits of the Ocoee Supergroup include the Snowbird, Great Smoky, and Walden Creek Groups, as well as the Pine Mountain Group in Georgia.Depositional setting
The Ocoee Supergroup is widely interpreted as having been deposited in a long-lived basin that developed along the southeastern margin of Laurentia during Neoproterozoic rifting and early passive-margin evolution. Deposits are commonly described as thick clastic successions built by repeated submarine landslides and turbidity flows in multiple pulses, ultimately reaching very large thicknesses.Metamorphism and deformation
Ocoee rocks in the Great Smoky Mountains region were later deformed by numerous faults that can repeat stratigraphic sections, and they are structurally separated from unmetamorphosed Paleozoic rocks of the adjacent Valley and Ridge province by the Great Smoky thrust fault system.The Great Smoky thrust is part of the broader Blue Ridge–Piedmont fault system and is associated with very large northwest-directed displacement; in the Great Smoky Mountains foothills, lower-grade metamorphism preserves many preexisting structures that help constrain Taconic and Alleghanian deformation histories.
Age and controversies
Determining the age of the Ocoee Supergroup has been historically difficult because parts of the succession were long considered fossil-poor and are strongly deformed. The unit has commonly been assigned a late Precambrian age based on its position above Grenville-age basement rocks and below the lowermost Cambrian strata.Fossil reports in the Wilhite Formation have been cited as evidence for a younger age for at least part of the upper succession. More recent work synthesizing geologic mapping, paleontology, and geochemistry has argued that mid-Paleozoic fossil-bearing rocks occur as structurally isolated thrust-sheet “horses” within a demonstrably Neoproterozoic–Cambrian stratigraphic framework, and that Walden Creek Group rocks on either side are more consistent with a Late Neoproterozoic depositional age of approximately 550–545 Ma.