EUSTATIC AND FLEXURAL EVENTS RECORDED IN THE LATE MIDDLE DEVONIAN TULLY FORMATION, NEW YORK STATE AND PENNSYLVANIA

The late Givetian Tully Formation, characterized by anomalously clean mid-to-outer shelf carbonates across western New York, is known to have both internal and bounding unconformities that obscure parts of its depositional story. However, in east-central New York and in central Pennsylvania, the Tully thickens markedly as it passes into terrigenous, largely dysoxic facies. In central Pennsylvania dark gray shales of the uppermost Windom Member apparently pass continuously upward into similar facies within the basal Tully; this entire succession yields only a low diversity association of diminutive brachiopods and styliolines indicative of highstand conditions.

Two major lowstand events marked by the regional disconformities occur higher within the Tully. The lower contact coincides with the base of the Smyrna Bed and an equivalent discontinuity within the Carpenters Falls Bed. The upper contact is a disconformity at the base of the Bellona Bed in western New York and the West Brook Shale in eastern New York. Three highstand events are recorded, in ascending order, above the base of the Tully (Cuyler beds), in the upper-medial Tully (Taughannock Falls beds) and in the highest Tully and post-Tully successions (Filmore Glen beds, Geneseo Member). The Tully fauna incursion is coincident with the early Tully highstand. Reestablishment of the Hamilton Fauna within the upper Tully is timed with the widespread second lowstand which allowed diverse brachiopod and large coral biofacies to become established. Local development of basinal depocenters during Tully deposition attests to coincident tectonic effects in areas bordering the carbonate shelf.