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Lower Middle Devonian strata (Eifelian to lower Givetian stages) of New York are identified under the names Onondaga and Marcellus. As has been New York practice for over 80 years, they represent time-significant allostratigraphic units, which to some degree cut across lithologic boundaries. The Onondaga Formation is a relatively tabular, limestone-dominated unit throughout New York. Strata thin from both east and west into more basinward facies in the central part of the state. In contrast, the (revised) “Marcellus subgroup” forms an eastward-thickening and coarsening wedge of siliciclastic-dominated facies. Marcellus-equivalent strata range in thickness from less than seven meters in the western New York subsurface to an estimated maximum thickness of 580 meters in the Hudson Valley, eastern New York.

Few stratigraphic revisions have been proposed for the Onondaga Formation since 1975, beyond minor revisions to two members associated with the abandonment of the informal, former Clarence member, chert-rich facies in western New York. In contrast, the term “Marcellus” has been raised in New York State from formation to subgroup status, with three formation-level units: a lower Union Springs and coeval upper Marcellus Oatka Creek and Mount Marion formations. The latter two represent correlative basinal dark shales and proximal dark shales to shoreface sandstones, respectively. Overall, following Cooper’s classic 1930s stratigraphy of one formation with 11 members, 13 members are now recognized in the Marcellus subgroup; two in the Union Springs Formation and 11 in the upper Marcellus Oatka Creek and Mount Marion succession.

Onondaga and Marcellus strata form three third-order depositional sequences, which feature three very distinct faunas. The sequences, termed Devonian Sequences Ic, Id, and Ie (alternatively Eif-1, Eif-2, and Eif-Giv) consist, respectively of 1) lower to middle Onondaga; 2) upper Onondaga and Union Springs; and 3) coeval Oatka Creek and Mount Marion formations, except where upper Mount Marion strata are not yet clearly distinguished form lower Skaneateles equivalents in eastern New York. The fossil assemblages of the Eifelian to lower Givetian have been subdivided into three “faunas” or ecological-evolutionary subunits. The oldest of the three faunas, the Onondaga Fauna, is succeeded by the Stony Hollow Fauna in shallow facies of the upper Union Springs and lowermost Mount Marion-Oatka Creek formations. The Stony Hollow Fauna is, in turn, succeeded by the classic Middle Devonian Hamilton Fauna throughout the remainder of upper Marcellus strata and Hamilton strata above.

Numerous post-1970 studies have examined the stratigraphy, petrology, sedimentology, basin analyses, paleobiology, and geochemical characters of the Onondaga Formation and Marcellus subgroup. Overviews of these studies are presented herein. In the Appalachian Basin, the correlatives of these units across have been assigned the same names, Onondaga and Marcellus, in eastern Pennsylvania. More argillaceous Onondaga-correlative strata form the upper part of the Needmore Formation from central Pennsylvania to the vicinity of Highland and Pocahontas counties, in Virginia and West Virginia, respectively (Selinsgrove to the informal “calcareous shale and limestone” members). Continuing southwest along the Virginia-West Virginia border, Onondagacorrelative strata are replaced by chert and shale-dominated strata in the upper part of the Huntersville Formation.

South of New York in the Appalachian Basin, the term “Marcellus” is applied lithostratigraphically, not allostratigraphically, so that lowest strata assigned the term Marcellus may variously range from lower Eifelian (middle Onondaga-equivalent, e.g., Frankstown, Pennsylvania) to correlative with the base of the Marcellus in New York. Similarly, youngest strata assigned to the Marcellus in Pennsylvania and southward may range from upper Eifelian Union Springs-equivalent (below proximal sandstones of the Turkey Ridge Member, central Pennsylvania) to lower Givetian, post-Marcellus black shales correlative with at least the Skaneateles Formation of New York in distal, basinward areas. From Highland County, Virginia, and adjacent West Virginia to the southwest, Marcellus strata are assigned to the lower part of the Millboro Shale, Finally, in southwestern Virginia and adjacent West Virginia, strata termed Marcellus in New York occur in lower parts of an interval sometimes termed “New Albany Shale” but shown by their correlation to be equivalent to upper Onondaga or lower Marcellus strata, based upon airfall volcanic tephras.