ABSTRACT

The cessation of shale and carbonate deposition of the Conasauga Group grand cycles (Middle to Upper Cambrian) and the establishment of the widespread peritidal carbonate deposition of the Knox Group (Upper Cambrian to Lower Ordovician) represent a prominent change in sedimentation along the early Paleozoic passive continental margin in the southern Appalachians. To evaluate the causes for this change, this study focuses on the Maynardville Formation, which is the uppermost carbonate unit of the Conasauga Group. The Maynardville consists of: (1) a lower subtidal facies succession, which is underlain by the Nolichucky Shale and resembles the rest of the Conasauga Group carbonate deposits; and (2) an upper peritidal facies succession, which is conformably overlain by similar peritidal deposits of the Copper Ridge Dolomite (Knox Group). Deposition of shale and subtidal carbonate took place in deep-ramp (Nolichucky) to shallow-ramp, subtidal-shoal, and lagoonal settings (lower Maynardville). The carbonate ramp was westward sloping toward the Conasauga intrashelf shale basin. To the east, the ramp was linked to a broad, semiarid carbonate tidal flat encompassing a variety of peritidal environments (upper Maynardville and Copper Ridge). The Maynardville is a shallowing-upward succession that formed by carbonate platform aggradation and basinward progradation. The transition between the Maynardville and the Copper Ridge Dolomite is within a conformable peritidal carbonate succession that contains common siliciclastic sand-size detritus. This interval is interpreted as a sequence boundary correlative with the craton-wide late Steptoean (Dresbachian-Franconian or Sauk II-Sauk III) unconformity.

The change from a Conasauga to a Knox depositional style may be related to distinct stages in passive-margin evolution. The end of grand-cycle deposition in the early Late Cambrian is coincident with the cessation of tectonic activity along extensional features (an intracratonic graben and other fault systems), and marks the transition into a mature-passive-margin setting. The mature margin was characterized by decreased rates of thermal subsidence, which, coupled with the infilling of the Conasauga intrashelf basin, favored shallow-water carbonate deposition. The final stabilization of the margin is reflected in the deposition of the thick peritidal carbonate strata of the Knox Group.