RECURRENT FAUNAS REVISTED: THE UPPER DEVONIAN ITHACA SHALE TROPIDOLEPTUS FAUNAS OF CENTRAL NEW YORK

The onset of major deepening in the basal Genesee Formation (late Middle Devonian, Givetian) in New York marks the second phase of the Acadian Orogeny and a significant change in sedimentation and biostratigraphic sequences. This is the position of the global Taghanic bioevent. At the Tully Limestone/Genesee contact, 60-80% of the diverse shallow-marine Hamilton fauna is lost from the Appalachian Basin (Brett, 1998). However, as noted by H.S. Williams (1903, 1913), elements of the Hamilton or “Tropidoleptus fauna” recur in the Upper Devonian, specifically in the Ithaca Formation, long after their apparent extinction near the contact between the Tully Limestone and Geneseo black shales (at the base of the Genesee). These “Lazarus faunas” in the Upper Devonian, mainly brachiopods and trilobites, are a subset of the complete Tropidoleptus fauna of the Middle Devonian. They do not co-occur with the distinctly Upper Devonian Ithaca fauna, but are normally found with taxa of the Tropidoleptus fauna, or in monotypic assemblages.

Events at the Tully Unconformity were more complex than simple sea-level changes and include major faunal migrations, facies changes, and sedimentation response to orogeny. The stratigraphic units above the Hamilton represent responses to Catskill Delta progradation following the second phase of the Acadian Orogeny. Following a period of widespread black, dysoxic shale deposition (Geneseo), the lowest Upper Devonian (lowest Frasnian) Sherburne Siltstone and Ithaca Formation represent a gray shale lithology comparable to the Hamilton, but formed in a different depositional environment. These facies are typified by a generalized, low diversity Genesee fauna, lacking most Hamilton elements. The presence of 1) flaggy laminated siltstones with sharp erosive bases and flute and groove casts, 2) cyclicity at scales ranging from tens of meters to centimeters, and 3) local interbedding with hummocky bedded to bioturbated siltstones and sandstones are all indicative of more storm-influenced deposition in more normally oxygenated shelf settings. It is these beds that contain the recurring Hamilton faunas. These Lazarus faunas are important as they show that certain biofacies may persist across major disruptions and may return to a local basin briefly during times of ameliorating conditions.