Sequence stratigraphy of an intrashelf basin carbonate ramp to rimmed platform transition: Maryville Limestone (Middle Cambrian), southern Appalachians

Abstract

Middle and Upper Cambrian strata in the southern Appalachians consist of a sequence of six alternating limestone and shale formations (Conasauga Group). The Conasauga Group constitutes part of a thick pericratonic Cambro-Ordovician passive-margin sequence along the eastern North American continent. The Cambrian carbonate platform was flanked by a high-relief shelf margin toward the east, facing the open ocean, while to the west the carbonate platform sloped into an intrashelf basin. It is this western shelf margin that is the topic of the present study. Detailed lithofacies analysis of the Middle Cambrian Maryville Limestone along a shelf-to-basin depositional transect reveals that the shelf evolved from a gently basinward-sloping ramp to a rimmed platform fringed with steeper slopes. Cyanobacterial buildups ( Renalcis-Girvanella ) dominated the platform margin environments. Progradation of the platform occurred toward the craton. A process-oriented approach has allowed us to define the sequences, sequence boundaries, and the stacking pattern of the Maryville Limestone. The Maryville Limestone sequence consists of two depositional subsequences. The boundary between the two subsequences is not a sequence boundary, because it does not separate rocks deposited in different environmental regimes. The two subsequences within the Maryville sequence consist of a combination of aggradational, retrogradational, and progradational units (with respect to the platform interior). The stacking pattern recognized is the result of variations in sedimentation rate, subsidence, and eustatic sea-level change. Each of the dominantly carbonate units within the Conasauga represents this gradual transition from a ramp-like platform to basin transition into a rimmed platform. The transition between the Maryville Limestone (Middle Cambrian) and the overlying Nolichucky Shale (Late Cambrian) is a sequence boundary. This sequence boundary is both an exposure surface and a drowning unconformity, and it marks a distinct shift in the pattern of sedimentation. Shallow-water carbonate deposition terminated at the boundary followed by the onlap of deeper-water basinal siliciclastics over shallow carbonates. The approach presented here can serve as a useful analogue to other lower Paleozoic and possibly younger passive-margin sequences.