K. Srinivasan et Kr. Walker, SEQUENCE STRATIGRAPHY OF AN INTRASHELF BASIN CARBONATE RAMP TO RIMMEDPLATFORM TRANSITION - MARYVILLE LIMESTONE (MIDDLE CAMBRIAN), SOUTHERNAPPALACHIANS, Geological Society of America bulletin, 105(7), 1993, pp. 883-896
Middle and Upper Cambrian strata in th southern Appalachians consist o
f a sequence o six alternating limestone and shale formations (Conasau
ga Group). The Conasauga Group constitutes part of a thick pericratoni
c Cambro-Ordovician passive-margin sequence along the eastern North Am
erican continent. The Cambrian carbonate platform was flanked by a hig
h-relief shelf margin toward the east, facing the open ocean, while to
the west the carbonate platform sloped into an intrashelf basin. It i
s this western shelf margin that is the topic of the present study. De
tailed lithofacies analysis of the Middle Cambrian Maryville Limestone
along a shelf-to-basin depositional transect reveals that the shelf e
volved from a gently basinward-sloping ramp to a rimmed platform fring
ed with steeper slopes. Cyanobacterial buildups (Renalcis-Girvanella)
dominated the platform margin environments. Progradation of the platfo
rm occurred toward the craton. A process-oriented approach has allowed
us to define the sequences, sequence boundaries, and the stacking pat
tern of the Maryville Limestone. The Maryville Limestone sequence cons
ists of two depositional subsequences. The boundary between the two su
bsequences is not a sequence boundary, because it does not separate ro
cks deposited in different environmental regimes. The two subsequences
within the Maryville sequence consist of a combination of aggradation
al, retrogradational, and progradational units (with respect to the pl
atform interior). The stacking pattern recognized is the result of var
iations in sedimentation rate, subsidence, and eustatic sea-level chan
ge. Each of the dominantly carbonate units within the Conasauga repres
ents this gradual transition from a ramp-like platform to basin transi
tion into a rimmed platform. The transition between the Maryville Lime
stone (Middle Cambrian) and the overlying Nolichucky Shale (Late Cambr
ian) is a sequence boundary. This sequence boundary is both an exposur
e surface and a drowning unconformity, and it marks a distinct shift i
n the pattern of sedimentation. Shallow-water carbonate deposition ter
minated at the boundary followed by the onlap of deeper-water basinal
siliciclastics over shallow carbonates. The approach presented here ca
n serve as a useful analogue to other lower Paleozoic and possibly you
nger passive-margin sequences.