LATE MIDDLE DEVONIAN (LATE GIVETIAN)TULLY FORMATION IN PENNSYLVANIA: COMPARISON WITH TULLY LIMESTONE AND EQUIVALENT CLASTIC DEPOSITS IN NEW YORK STATE

The late Middle Devonian (Late Givetian) Tully Formation in the classic Taghanic area (Cayuga Valley) of New York State consists of anomalous, clean, micritic limestone which stands in stark contrast to adjacent siliciclastic, often progradational, marine facies of the Catskill Delta Complex. This carbonate facies, recording a major incursion and exit of the "old world" Tully Fauna, not only abruptly grades eastward into a thicker, more basinal, succession of siliciclastic deposits in eastern New York, but it also grades southward into a much thicker succession of unfossiliferous basinal facies in Pennsylvania. These deposits are exposed along part of the Allegheny Front in the central part of the state. The structural trough in east-central New York may connect through the subsurface to the central Pennsylvania depocenter.

The thin, unconformity floored, fossiliferous, carbonate succession in western New York contrasts sharply with thick, barren, ribbon limestone facies in north-central Pennsylvania; the abundance of rhythmic, turbiditic?, limestone beds near Loch Haven and Williamsport illustrate the basins connection to the carbonate factory on the New York platform. South and west of Loch Haven the Tully abruptly thins, owing to condensation, loss of carbonate, and erosional telescoping of the section. East of the Susquehanna Valley, the Tully thickens and passes gradually into siliciclastic facies toward the Delaware Valley. Correlation problems are centered in an area east of Williamsport where either an anomalous siliciclastic lobe is developed at Hughesville or a major unconformity is observed to bevel siliciclastic lower Tully strata east of there. The upper Tully recurrent zone of Hamilton Fauna occurrence, first recognized in New York, is present throughout central and southern Pennsylvania. Unconformities, present within the Tully in New York, show up in some Pennsylvania sections, but close to near-continuity in the depocenter. Current efforts are directed to delimiting the southern erosional? boundary of the Tully near the Maryland border.