Abstract

Two concretionary limestone units within the Genesee Formation in western New York State show evidence of differential downslope erosional beveling within an anoxic basin. The lower unit, here termed the Fir Tree submember, and a younger unit, the Lodi submember are thin, concretionary limestone layers yielding auloporid corals, brachiopods and molluscs. Each unit is overlain unconformably by dark grey to black laminated mudstone deposits; both limestones are truncated in a basinward direction and are represented in the basin axis only by thin lenses of reworked pyritized burrow fragments, fish teeth and phosphatic shells. Each limestone becomes conspicuously condensed and fossil-rich near its downslope, erosional terminus.

Erosion in the basinal setting produced reworked lag concentrates of pyrite and fish bone which lack associated carbonate grains; erosion is believed to have taken place through a combined process of abrasion and carbonate dissolution. This pattern indicates that dynamic bottom-current processes were present along the basin margin slopes. These discontinuities appear to record transgressive (upslope) migration of a higher energy zone of scour associated with impingement of a rising pycnocline during transgressions. Similar discontinuities associated with transgressive black shales occur in numerous other horizons in the Silurian-Devonian of the northern Appalachian Basin, indicating that the erosion processes were a recurring phenomenon.