Abstract

The Geneseo and Penn Yann Shale Members of the Genesee Formation in western New York, recording deposition under predominantly anaerobic conditions, contain black shale-roofed discontinuities marked by reworked pyrite accumulations, phosphatic debris and conodonts. In the Honeoye area the discovery of Ancyrodella rotundiloba (late form) in a discontinuity-horizon within a thin black shale in the lower Penn Yan extends the basal Frasnian Lower asymmetricus Zone and the Middle/Upper Devonian boundary downward to a position on top of the main bed of the Lodi limestone submember. In the Bristol and Canandaigua Lake sections where the Lodi has been removed (beveled) by submarine erosion, the discontinuity-horizon with A. rotundiloba and the Middle/Upper Devonian boundary is at or near the contact between the grey shales and siltstones of the Penn Yan and the black shales of the upper Geneseo, which is a well defined lithologic boundary across western New York.

The lower limit of the boundary in the Penn Yan is constrained by the Lowermost asymmetricus Zone age of the main Lodi bed (fauna with Polygnathus norrisi) and, in sections where the Lodi is missing, by the presumably disparilis Zone age of the upper Geneseo (fauna with Polygnathus ordinatus and Polygnathus? caelatus in the Fir Tree limestone submember). The absence of the earliest species of Ancyrodella, A. binodosa and the early form of A. rotundiloba in the discontinuity-horizon at the boundary indicates that parts of the Lowermost asymmetricus (uppermost Givetian) and Lower asymmetricus (lowermost Frasnian) Zones may be missing in the western New York section.