STRATIGRAPHIC REVISION OF THE MIDDLE-UPPER DEVONIAN TRANSITION (GENESEE GROUP) IN EASTERN NEW YORK STATE

Rickard's (1975) seminal time-rock chart for New York State has long provided an essential framework for Devonian stratigraphic studies. At the time of publication, however, Rickard noted that the placement of the Middle-Upper Devonian boundary was one of the most uncertain aspects of this work. Subsequently, a number of bio- and sequence-stratigraphic studies have recognized the Middle-Upper Devonian boundary in basinal settings (Genesee magnafacies) of western New York, though revisions of the Portage, Chemung, and Catskill magnafacies of eastern New York Genesee Group have lagged behind. This is in large part because the Genesee Group records the onset of the third tectophase of the Acadian Orogeny, which resulted in a sharp pulse of foredeep subsidence that slightly preceded, and continued through, a dramatic increase in sediment to the foreland basin for the remainder of the Devonian. Indeed, the transformation of the basin profile from a gently sloping ramp to a well-defined shelf-slope system occurs during the late Givetian. Compared to pre-Genesee strata, the excessive sediment influx and rapid progradation of the clastic wedge resulted in anomalously thick, expanded siliciclastic-dominated successions, that have hindered stratigraphic interpretation. Recent identification of the latest Givetian Lodi Limestone in eastern New York sections provides an important datum for correlation within and across diachronous, prograding magnafacies. Additionally, recognition of persistent nearshore taxa of the Middle Devonian 'Hamilton Fauna', and their mixing with the Upper Devonian 'Ithaca Fauna', has allowed refinement of previous mapping efforts. Using the magnafacies concept (Genesee - G, Portage - P, Chemung - Ch, and Catskill - Ca), we recommend the following revised stratigraphic framework for the Middle-Upper Devonian transition (Genesee Group) of eastern New York: 1) in the lowermost, basal Genesee Group, the offshore-onshore transition includes the Geneseo (G), Unadilla (P), Gilboa (Ch), and Oneonta (Ca) formations; 2) in the latest Givetian, the Geneseo (G), Sherburne (P), Otego (new unit replacing un-named units of Rickard, Ch), and Oneonta (Ca) formations; and, 3) in the lowermost Frasnian, the Penn Yan (G), Sherburne (P), Otselic (Ch), and Oneonta (Ca) formations.