Stratigraphic and structural relationships of the Ocoee Supergroup, southern Appalachians: Implications for Neoproterzoic rift basin architecture and Paleozoic collisional orogenesis

Author

James Ryan Thigpen, University of Tennessee, Knoxville

Date of Award

12-2005

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Robert D. Hatcher, Jr.

Committee Members

Linda C. Kah, Claudia I. Mora

Abstract

The late Proterozoic-Early Cambrian Ocoee Supergroup (OSG)is the dominant lithostratigraphic sequence of the western Blue Ridge (WBR) province in southeast Tennessee, southwest North Carolina, and northern Georgia. The OSG is divided into the basal Snowbird Group (SG) that nonconformably overlies Grenvillian basement, the thick Great Smoky Group (GSG) that is usually in fault contact with the Snowbird Group, and the Walden Creek Group (WCG) that directly underlies the Cambrian Chilhowee Group and conformably overlies both the Snowbird and Great Smoky Groups. Traditional interpretations suggest that the OSG was deposited during late Neoproterozoic-Early Cambrian rifting along the southeast Laurentian margin, initially deformed and metamorphosed during the Taconic (Ordovician) orogeny, metamorphosed again (locally) during the Neoacadian (Devonian-Mississippian) orogeny, and then deformed and transported northwestward as part of the Blue Ridge-Piedmont (BRP) megathrust sheet during the Alleghanian (Pennsylvanian-Permian) orogeny. Recently reported paleontological evidence, however, suggests a mid-Paleozoic (Silurian-Mississippian) age for all or part of the OSG. Detailed mapping (1:12,000 and 1:24,000 scale) of a relatively small, previously unmapped area (~160 km²) of major structural/stratigraphic incompatibility and geochemical analyses of Walden Creek Group Carbonates Southwest of Great Smoky Mountains National Park (GSMNP) provide new data that are critical for delineating regionally significant stratigraphic and structural relationships.

Detailed mapping in the study area reveals that the Anakeesta-Wehutty, Ammons, Dean, and Wilhite formations form a conformable stratigraphic succession separated from the Elkmont and Thunderhead sandstones only by a minimal displacement Alleghanian fault in the Greenbrier thrust sheet. The lithologic and stratigraphic similarities of the Cades Sandstone and Dean Formation suggest that they are correlatives. The Shields Formation shares many lithologic features with the Dean Formation and lies in the same stratigraphic position beneath the Wilhite Formation in the Miller Cover thrust sheet, although the lack of graded bed suggests that the Shields is time-transgressive with though depositionally distinct from the Cades and Dean in the study area.

Rocks of the study area are deformed by at least three periods of faulting and folding (F₁-F₂) and one regional metamorphic cleavage-forming event (S₂). The only F1 fold in the study area is transected by slaty cleavage (S₂; pressure solution and recrystallization) that is axial-planar to F₂ folds. S₂, S_2a (pressure-solution), and F₂ folds are related to Taconic (480-430 Ma) Barrovian metamorphism based on ⁴⁰Ar/³⁹Ar and ⁴⁰K/³⁹Ar metamorphic mineral cooling ages and the axial-planar relationship of S₂ and S_2a to F₂. Crenulation cleavage (S₃) is suborthagonal to and deforms S₂, suggesting it occurred later. The Greenbrier and Rabbit Creek faults here were initially emplaced prior to Taconic peak metamorphism under dominantly ductile conditions. The Greenbrier fault was folded (F₂) coveval with peak Taconic metamorphism, which prevented late Paleozoic reactivation of that structure. The brittle Hangover Lead fault likely propagated from the hinge or southeast limb of the Ducktown anticlinorium (first-order F₂) during early Alleghanian orogenesis. The brittle Salt Spring Mountain and possibly coeval Oconaluftee fault cut the folded Greenbrier surface to accommodate Alleghanian thrust displacement, while the Rabbit Creek fault was reactivated. During the latest stages of the Alleghanian orogeny, emplacement and bending of the Great Smoky thrust sheet above the Foothills duplex (Valley and Ridge horses) produced second- and higher-order F₃ folds.

Samples from localities 2 and 4 that are interpreted to contain the least altered δ13C values examined in this study temporally delimit the depositional age of the Sanduck Formation to Neoproterozoic intervals interpreted as sthe Varanger and Sturtian glaciations, intervals near the Precambrian-Cambrian boundary (~540 Ma), and to Phanerozoic intervals from the Late Ordovician to the Silurian-Devonian boundary (~400 Ma) and from the Late Devonian (~360 Ma) to the Triassic (~240). While the results of this study seem to conclusively rule out any depositional age from the Sandsuck Formation between the early Middle Cambrian and Late Ordovician isotopic data are inconclusive with respect to differentiating Neoproterozoic

The interpreted stratigraphic position of the Sandsuck Formation beneath the Cambrian fossil-bearing Chilhowee Group and Taconic metamorphic age dates of Walden Creek samples support a Neoproterozoic rather than mid-Paleozoic depositional age. Taken together, the stratigraphic, chronological, and geochemical data are all consistent only with a Neoproterozoic depositional age.

Recommended Citation

Thigpen, James Ryan, "Stratigraphic and structural relationships of the Ocoee Supergroup, southern Appalachians: Implications for Neoproterzoic rift basin architecture and Paleozoic collisional orogenesis. " Master's Thesis, University of Tennessee, 2005.
https://trace.tennessee.edu/utk_gradthes/4605