Abstract: "Tideless" Middle Ordovician Sediments

George Grabowski

Sediments of the High Bridge Group in central Kentucky were deposited on a large, shallow platform during Middle Ordovician time. The oldest exposed formation, the Camp Nelson Limestone, records a transition from storm-deposited packstones to mudstones deposited in shallower water. The overlying Oregon Formation and Tyrone Limestone were deposited in a system of tidal flats and lagoons. The lagoonal rocks are fossiliferous wackestones with a few winnowed grainstones. Most of these rocks show no evidence of wave disturbance. They are associated with bioturbated, sparsely prism-cracked mudstones. These mudstones are associated with thin-bedded and cryptalgalaminate tidal-flat mudstones containing abundant prism cracks and intraclasts. The thin-bedded mudstones were formed during occasional storms that reached the tidal flat. The cryptalgalaminate mudstones are similar to those formed in ponded, wet locations of modern tidal flats. Some cryptalgalaminate mudstones contain numerous small mudcracks and were formed in a drier (but not evaporitic) environment. Faint vertical structures in thin sections of cryptalgalaminate mudstone are similar to tufa structures formed in modern freshwater algal marshes. Only a few, shallow (< 20 cm) channels are present.

The sparsity of grainstones and of wave-disturbance features in the lagoonal rocks, the absence of beach ridge or levee sediments, the absence of large channels in the tidal-flat rocks, and the vertical proximity of lagoonal and tidal-flat rocks suggest that tides were non-existent on this large shallow platform.

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