Abstract: Geometry, Facies, and Development of Large Nonreefal Shelf and Offshelf Buildups, Middle Ordovician, Virginia

J. F. Read

Geometry and facies of large skeletal buildups (1 to 50 km wide by 30 to 250 m thick) have been documented in outcropping Middle Ordovician platform-to-basin facies rocks (200 to 600 m thick) in Virginia. These Effna Formation buildups overlie and interfinger with dark cherty skeletal wackestone. Outer platform mounds in western belts are overlain by deeper shelf, shaly, skeletal wackestone. Offshelf mounds in eastern belts interfinger with and overlie basinal black limestone and shale.

Many buildups are aggregates of smaller anastomosing masses of carbonate wackestone to mudstone and interfingering pelmatozoan, ramose-bryozoan grainstone. Flanks of buildups are dominated by grainstone facies whereas wackestones dominate buildup interiors. Slopes on buildup margins are generally low, but locally exceed 30° in offshelf mounds. Within buildups, contacts between wackestones and grainstones are sharp to gradational and may be vertical to overhanging. Wackestone and mudstones contain minor encrusting and ramose bryozoans, locally abundant spicules, and mud patches containing fine sinuous spar-filled tubules. The sediments contain large cavity systems with complex internal sediment-cement fills. Marine cements are widespread as intergranular cements and thick crusts w thin stromatactoid cavities. Hardgrounds also are common.

The Virginia nonreefal buildups developed during submergence in shelf and offshelf positions largely as a result of rapid carbonate production, baffling, and pervasive marine cementation. These Middle Ordovician mounds, which have some similarities to late Paleozoic Waulsortian mounds, locally contain solid hydrocarbons suggesting that analogous mounds could be important reservoirs in lower Paleozoic rocks elsewhere.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma