Stratigraphy and sedimentology, Cambrian Shady Dolomite, Virginia
Cambrian Shady Dolomite facies exposed northwest and southeast of Austinville, southwestern Virginia, apparently formed in a platform-to-basin setting. Northwest of Austinville, the Shady Dolomite (600 m thick) overlies the Lower Cambrian Erwin Formation and is overlain by the Middle Cambrian Rome Formation. Here the unit consists of ribbon-laminated carbonates (Patterson Member) overlain by massive dolomites and fenestral, cryptalgal carbonates (Austinville and Ivanhoe Members) which pass up into Rome Formation red mudcracked clastics and carbonates. Relict textures suggest that northwestern belts of the Shady Dolomite were deposited in an upward-shoaling sequence on the inner part of a carbonate platform.
The Shady Dolomite thickens southeast of Austinville where exposed (400 m thick) upper Shady Dolomite beds (partly equivalent to Rome Formation) contain calcareous algal bioherms, limeclast grain-stone, monomictic and polymictic carbonate breccia and black laminated limestone and shale. Algal bioherms, limeclast grain-stones and carbonate breccias of southeastern belts suggest seaward, outer platform environments; black laminated limestone and shale units containing local algal bioherms and breccias may indicate local off-platform "deeper water" deposits. Slopes on the carbonate platform margin were probably low (few degrees) as indicated by interlayering of shallow and deeper water lithologies recording considerable migration of facies bands. This southeasternmost facies of the Shady Dolomite exposed in the Valley and Ridge may give important clues as to the lithologic character of equivalent units in the Virginia Piedmont.