Cover stratigraphy and structure of the southernmost external basement massifs in the Appalachian Blue Ridge: Evidence for two-stage late proterozoic rifting

Abstract

The southern Appalachian Blue Ridge of north Georgia contains the southernmost Grenvillian crystalline basement massifs in this regionally extensive allochthon. The greenschist-facies Ocoee Supergroup metasedimentary cover stratigraphy of these massifs records two separate periods of ensialic Late Proterozoic extension associated with rifting of Laurentia. The earlier sequence consists of a >1 km thick sequence (Pinelog Formation) of metawacke/ metasandstone and metaconglomerate resting nonconformably on basement. This sequence was deposited as a transgressive sequence, beginning with fluvialalluvial fades, and grading up into shallow-water deltaic/intertidal faciès, and overlying prodelta to delta-front facies. Closest stratigraphie and lithologic correlative of these units is the Late Proterozoic Snowbird Group in the Great Smoky Mountains to the northeast These units are inferred to have been deposited within a growing half-graben, hinged on the southeast and bordered on the northwest by a basement highland uplifted along a major displacement, southeast-dipping, rift boundary extensional fault Sedimentation within this basin was terminated by a second cycle of uplift and extension, which involved tilting and erosion of both Grenville basement and its overlying Pinelog Formation cover sequence. A second basinal sequence, represented by the regionally extensive Great Smoky Group, was deposited unconformably above the earlier basin and bevels across all of the older units. Sense of tilt of the older basin indicates that the second basin also formed within a half graben, hinged on the east, and bounded to the west by a significant displacement extensional fault During subsequent Paleozoic deformation, synchronous with regional low-grade metamorphism, the region was folded into large-scale, northwest-vergent, inclined to recumbent, basement-cored, isoclinal fold nappes. These early structures and fabrics were subsequently refolded by secona- and third-generation folds. During the late Paleozoic, rocks and early-formed structures of the region were detached and thrust northwestward onto unmetamorphosed Paleozoic platform rocks of the foreland fold-and-thrust belt and subsequently refolded by an upright open, map-scale fourth generation fold phase.