ABSTRACT

The Rochester Formation (Middle Silurian) of southern Ontario consists mainly of grey, fissile, calcareous shale with intercalated thin beds of micrite or biomicrite, the latter differing from the biosparite confined to the basal part of the formation. The Rochester lithology differs significantly from that of the underlying lrondequoit Formation (massive, crystalline, porous, crinoidal limestone) and the overlying DeCew Formation (fine, dolomitic limestone to dolomite). Rochester deposition essentially took place in a calm, subtidal, inner neritic low energy environment with good circulation. The local occurrence of biosparite, intramicrite and intraformational conglomerate, suggests occasional storms, which extended agitation into the subtidal zone.

Comparison with an earlier study of the Rochester Formation carried out by Folk (1962) in Morgan County, West Virginia, shows that conditions were not uniform within the Rochester Sea. While the bulk of the West Virginian Rochester sediments were being deposited in a brackish, stagnant, lagoonal environment devoid of a normal marine fauna, the Ontarian Rochester sediments were formed in an open marine environment with a flourishing benthonic fauna.