Cyclic peritidal facies of a Cambrian aggraded shelf: elbrook and conococheague formations, Virginia Appalachians

The Elbrook-Conococheage Formations (Middle to Upper Cambrian) are a kilometer-thick sequence of cyclic, peritidal carbonates that formed an aggraded, rimmed shelf on a mature, passive continental margin. Sedimentation rates for peri tidal carbonate environments far exceeded long term subsidence of the platform (3 to 5 cm/1000 yrs.); thus, the shelf stayed filled to sea level (i.e., was aggraded). Relative sea level rise did exceed sedimentation for brief periods, causing cyclic transgressions (max. 3 m initial submergence). Average cycle duration was 60,000 years. Cycles (1-7 m thick) are composed of basal subtidal/intertidal limestone consisting of bioherms, grainstone and ribbon carbonate; and dolomitic laminite caps containing minor quartz arenite, shale and breccia. Cycle development was controlled by initial submergence increment and position relative to shelf edge. Large initial submergence produced thick subtidal-based cycles representing shelf lagoon and shoal conditions. Small events resulted in submergence within the intertidal zone, which deposited thick, mudcracked intertidal limestones in outerplatform settings, and thick sequences of laminite in inner platform settings. Slopes on the platform were low (less than 3 cm/km); thus, subtidal facies developed in a mosaic pattern of lagoons and shoals, rather than in shore-parallel belts. When low areas filled, tidal flat laminites prograded seaward over subtidal units. Cyclicity may reflect spasmodic subsidence of the shelf, or uniform subsidence overprinted by small-scale glacio-eustatic sea level changes related to shifting patterns of mountain glaciation. Interior areas of Quaternary carbonate shelves are characterized by incipiently drowned facies and are punctuated by soil/caliche horizons and karst surfaces, that reflect 100 rn glacio-eustatic sea 'level fluctuations. In contrast, many ancient shelves, including the Cambre-Ordovician shelf of the Appalachians, were dominated by cyclic peritidal sequences lacking evidence of major sea level events. Aggraded shelves may represent the typical state of mature carbonate continental shelves in the absence of large-scale sea level fluctuations.

Name thesis:degree_name

M.S.

Level thesis:degree_level

masters

Discipline thesis:degree_discipline

Geology

Department dc:contributor.department

Geology

Grantor dc:publisher

Virginia Polytechnic Institute and State University

Year dc:date.issued

1983

Author dc:creator

• Koerschner, William F.

Statement dc:rights

• In Copyright

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Language dc:language.iso

en

Handle dc:identifier.uri

http://hdl.handle.net/10919/101208

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/101208