Carbonate deposition and sequence stratigraphy of the terminal Cambrian grand cycle in the southern Appalachians, USA

The cessation of shale and carbonate deposition of the Conasauga Group gran d 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 P aleozoic passive continental margin in the southern Appalachians, To evalua te the causes for this change, this study focuses on the Maynardville Forma tion, which is the uppermost carbonate unit of the Conasauga Group. The May nardville consists of: (1) a lower subtidal facies succession, which is und erlain by the Nolichucky Shale and resembles the rest of the Conasauga Grou p carbonate deposits; and (2) an upper peritidal facies succession, which i s conformably overlain by similar peritidal deposits of the Copper Ridge Do lomite (Knox Group). Deposition of shale and subtidal carbonate took place in deep-ramp (Nolichucky) to shallow-ramp, subtidal-shoal, and lagoonal set tings (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 e nvironments (upper Maynardville and Copper Ridge). The Maynardville is a sh allowing-upward succession that formed by carbonate platform aggradation an d basinward progradation. The transition between the Maynardville and the C opper Ridge Dolomite is within a conformable peritidal carbonate succession that contains common siliciclastic sand-size detritus, This interval is in terpreted as a sequence boundary correlative with the craton-wide late Step toean (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 deposit ion in the early Late Cambrian is coincident with the cessation of tectonic activity along extensional features tan intracratonic graben and other fau lt systems), and marks the transition into a mature-passive margin setting. The mature margin was characterized by decreased rates of thermal subsiden ce, which, coupled with the infilling of the Conasauga intrashelf basin, fa vored shallow-water carbonate deposition. The final stabilization of the ma rgin is reflected in the deposition of the thick peritidal carbonate strata of the Knox Group.