A multiproxy analysis of the Frasnian-Famennian transition in western New York State, U.S.A

Abstract

The Late Devonian Frasnian-Famennian transition is well exposed along Walnut Creek in the western New York State region of the Appalachian Basin. The studied stratigraphic interval encompasses approximately 29m of section from the organic-rich Pipe Creek Formation, the equivalent of the Lower Kellwasser carbonaceous horizon of Europe, to a thin black shale layer at the top of the overlying Hanover Formation that correlates with the Upper Kellwasser horizon. The Walnut Creek section comprises five depositional cycles that are likely eustatic in nature, perhaps modulated by tectonic dynamics of the basin. Clastic proxy trends (Ti/Al, Zr/Al) suggest that deposits spanning the Frasnian-Famennian boundary (FFB) accumulated in association with rising sea level, a trend documented from elsewhere in the basin. Increasing Si/Al across the FFB may reflect elevated post-extinction biosiliceous productivity documented from Lower Famennian deposits elsewhere. The Walnut Creek section includes two positive δ13Corg excursions of >3‰, the stratigraphically lower event associated with the Pipe Creek Formation and the highest excursion approximately coincident with the Upper Kellwasser equivalent black shale layer. These isotopic events are interpreted to record episodes of increased global productivity and organic carbon sequestration at the end of the Frasnian Stage. Redox proxy trends (Mo and U enrichment and pyrite framboid size-distributions) documented from black shale associated with the positive excursions suggest sediment accumulation under suboxic to intermittently anoxic or euxinic bottom-water conditions in an unrestricted open-marine setting. The paleoclimate history of the end of the Frasnian based on Th/K data is one of fluctuating atmospheric conditions, including a brief cooling trend at the FFB. Though the Walnut Creek section sheds no light on the triggering mechanism or mechanisms of the Late Devonian extinction, inferred climate fluctuations associated with the FFB reflected in the Th/K data may have contributed to the severity of the biological crisis.