Date of Award
Doctor of Philosophy
Steven G. Driese
R. D. Hatcher, William F. Kane, Kenneth R. Walker
The Blue Ridge and Piedmont provinces of the southern Appalachians possess Upper Proterozoic and Lower Paleozoic age sedimentary, metasedimentary, volcanic, and metavolcanic rock thought to represent sedimentation and igneous activity related to the formation of the Iapetos (Proto-Atlantic) ocean. These sequences of strata can be related to the development of the Laurentian - Iapetos margin as seen in the Southern Appalachians. "Rift" phase sequences of the western Blue Ridge, include the Late Proterozoic age Mount Rogers, Catoctin, Grandfather Mountain formations, and the Ocoee Supergroup, which have been interpreted by other workers as representing volcanism and sedimentation in regionally discontinuous, fault-bounded basins. Possible coeval sedimentation on the newly formed continental slope and rise, has been interpreted as resulting in the deposition of the fine-grained Ashe, Lynchburg, and Tallulah Falls formations of the eastern Blue Ridge and Inner Piedmont. The distribution of thickness and facies of upper Proterozoic and early Paleozoic sedimentary and volcanic rock have been interpreted as reflecting the development of an irregular continental margin. Throughout the western Blue Ridge province these sequences are overlain by the Chilhowee Group. The Chilhowee Group is a terrigenous clastic succession that records the stabilization of the Laurentian continental margin following Late Proterozoic rifting and formation of the Iapetos ocean. This stabilization was associated with a change from fluvial sedimentation (lower portions of the Cochran and Unicoi Formations - coeval formations of the basal Chilhowee Group) to marine sedimentation (uppermost Cochran and Unicoi Formation as well as the overlying Hampton and Nichols Formations and the Nebo, Murray, Hesse, and Helenmode Formations and their northeastern equivalent, the Erwin Formation). Examination of the Chilhowee Group at seven localities in East Tennessee (five as part of this study) has resulted in numerous refinements of our understanding of the Late Proterozoic to Early Cambrian evolution of the Laurentian margin.
Based on the recent suggestions of Crimes that trace fossils can be used to assist in correlating the Precambrian-Cambrian boundary interval in stratigraphic sequences in which diagnostic body fossils are lacking, a late Vendian? to early Placentian-equivalent (sub-Tommotian-equivalent) age is assigned to the Cochran and Unicoi Formations. An early late Placentian-equivalent (early to late Tommotian-equivalent) age is assigned to the Nichols and Hampton Formations and the lower and middle Nebo Formation. Finally, a late Placentian-equivalent or younger (Atdabanian-equivalent or younger) age is assigned to the upper Nebo, Murray, Hesse and Helenmode Formations. The Precambrian-Cambrian boundary is probably located somewhere within the uppermost portion of Cochran-Unicoi interval. Because the Cochran-Unicoi is predominantly a coarse-grained, feldspathic terrestrial (braided fluvial/alluvial) sequence, the precise location of the Precambrian-Cambrian boundary may never be determined in the southern Appalachian region.
Variability along strike in Chilhowee facies has been recognized within the confines of the fluvial-to-marine transition. Facies identified across East Tennessee localities cross formational boundaries; thus, a facies analysis provides a .practical basis for studying patterns of Chilhowee sedimentation.
Throughout East Tennessee, six facies were recognized: the conglomerate facies, the interlaminated mudstone-sandstone facies, the sandstone facies, the siltstone-mudstone facies, the hummocky facies, and the quartz arenite facies. The fluvially dominated conglomerate facies represents deposition within a braided stream system, and is typical of the basal Chilhowee Group throughout the outcrop belt. Associated with the conglomerate facies is the interlaminated mudstone-sandstone facies, which represents lacustrine deposition within a braidplain subenvironment of the braided stream system.
Above the fluvially dominated basal Chilhowee, variability along strike increases in the marine-dominated facies. The hummocky, sandstone, and quartz arenite facies consist of interbedded conglomerate, sandstone, siltstone, and mudstone exhibiting fairweather- and storm-wave produced sedimentary structures. Sedimentation occurred in an offshore, storm-dominated shelf, which received progradational pulses of sand (quartz arenite facies) from a craton-ward source.
Variations in relative abundance and stratigraphic position of shelf facies (hummocky and mudstone-siltstone facies), grain-size, and bed-thickness within the Chilhowee Group represent variations in coeval Chilhowee paleoenvironments along strike, attributable to differences in progradation versus transgression at the continental margin. Dispersion of lower Chilhowee Group paleocurrent modes suggest that topographic irregularities, possibly inherited from rifting, may have established initial sedimentary dispersal systems, which influenced later shelf facies variations. With passive-margin stabilization, paleocurrent modes assumed a more uniform pattern of cratonic sediment dispersal to the east. Examination of the available structural data as well as the distribution of facies described above suggest that present day structural strike in the area does not coincide with the latest Proterozoic to Early Cambrian depositional strike. Trends in proximality (with respect to the craton) can be characterized as representing both a northwest to southeast gradient, and a northeast to southwest gradient. This geometry is consistent with previous suggestions of Rankin and Thomas, that the southern strike belts occupied a position within an embayment, while the northeastern strike belts occupied a position adjacent to or within a promontory (Tennessee embayment and Virginia promontory, respectively).
Examination and point-counting of samples (n=112) collected from basal Chilhowee Group strata (Unicoi and Cochran Formations) indicates that the majority of framework grains were derived from underlying Proterozoic rocks. Variation along strike in the relative abundances of the various framework grains, the gross thickness of basal Chilhowee strata, and the restriction of rift-related basaltic volcanism to northeastern exposures are interpreted here as the result of diachronous rifting. Based on regional stratigraphic and sedimentologic patterns, two stages of rifting can be recognized: 1) a Late Proterozoic event giving rise to the numerous Late Proterozoic sequences across the area. This rift stage was followed by a period of tectonic quiescence when sedimentation patterns may have been dominated by thermal subsidence; and 2) a latest Proterozoic to Early Cambrian rifting event which was restricted to the area adjacent to the Virginia promontory.
Recent mapping in the metamorphic core of the southern Appalachians has led to the identification of several internal basement massifs interpreted as windows exposing parautochthonous basement beneath the main thrust sheet. In many instances this parautochthonous basement possesses a metasedimentary cover sequence. One such internal massif is exposed in the Piedmont of North Carolina by the Sauratown Mountains window. Here, the 1.2 Ga basement is overlain by a cover sequence of metaarkose, schist, and quartzite. The westernmost of the quartzite bodies is exposed on Pilot Mountain in Surry County, North Carolina. Lithologic and stratigraphic similarities between the sedimentary sequence at Pilot Mountain and the Chilhowee Group of eastern Tennessee, have prompted some to propose stratigraphic equivalence.
Despite amphibolite-facies regional metamorphism and multiple periods of deformation, the quartzite of Pilot Mountain displays a diverse array of primary sedimentary features. Detailed examination of the primary structures preserved within the quartzite at Pilot Mountain resulted in the delineation of three facies interpreted as representing inner shelf to foreshore marine deposition. In view of the regional west to east gradient in Chilhowee Group sedimentation described above, the quartzite at Pilot Mountain (which possesses a stratigraphic thickness exceeding 45 m) does not appear to represent a distal shelf portion of this passive margin sequence. Palinspastic cross-sections through the Appalachian orogen indicate that the sedimentary sequences exposed within the Sauratown Mountain window and East Tennessee occupy the same relative positions with respect to the Laurentian continental margin today as they did when they were deposited. Two possible paleogeographic- paleotectonic interpretations then seem plausible: 1) the quartzites of the Sauratown Mountains window represent Late Proterozoic, Ashe Formation-equivalent deposition along a sea-floor high associated with the partially or fully rifted basement terrane. In this case subsequent orogenic activity would have resulted in the over-thrusting of the massif and the cover by the finer-grained, offshore deposits of the Ashe Formation, 2) the quartzites of the Sauratown Mountains window represent latest Proterozoic to Early Cambrian (Chilhowee Group time-equivalent) deposition on an isolated, rifted continental fragment. Bathymetric shallowing along the flanks of basement block would result in the deposition of shallow-water sediments derived primarily from the rifted Grenville basement block.
Walker, James Daniel, "The Sedimentology and Stratigraphy of the Chilhowee Group (Uppermost Proterozoic to Lower Cambrian) of Eastern Tennessee and Western North Carolina: The Evolution of the Laurentian - Iapetos Margin. " PhD diss., University of Tennessee, 1990.