SEQUENCE STRATIGRAPHY OF AN INTRASHELF BASIN CARBONATE RAMP TO RIMMEDPLATFORM TRANSITION - MARYVILLE LIMESTONE (MIDDLE CAMBRIAN), SOUTHERNAPPALACHIANS

Citation
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
Citations number
59
Categorie Soggetti
Geology
ISSN journal
00167606
Volume
105
Issue
7
Year of publication
1993
Pages
883 - 896
Database
ISI
SICI code
0016-7606(1993)105:7<883:SSOAIB>2.0.ZU;2-4
Abstract
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.